Anna Balazs Jeffrey Koberstein, Univ of Pittsburgh Univ of Connecticut
Sanat Kumar, Pennsylvania State Univ
National Science Foundation
* Invited paper
SESSION N1: NANOCOMPOSITE MATERIALS
Chair: Sanat K. Kumar
Monday Morning, December 1, 1997
8:30 AM *N1.1
THE NATURE OF NANOSCOPICALLY CONFINED POLYELECTROLYTES. Evangelos Manias, Emily Hackett, David Zax,* Emmanuel P. Giannelis, Department of Materials Science and Engineering, Cornell University, Ithaca, NY; *Chemistry Department, Cornell University, Ithaca, NY.
We present a combined Molecular Dynamics (MD) and experimental investigation of the sructure and dynamics of poly(ethylene-oxide) (PEO) intercalated in mica-type silicates. The PEO is confined between atomically smooth surfaces separated by nm distances. The resulting nanocomposites were structurally characterized by X-Ray Diffraction and commensurate MD simulations were employed to investigate the molecular structure of these systems. A variety of atomistic and molecular information, such as the polymer conformations, polymer dynamics -relaxations and mobilities-, and the probability of finding Na+ and Li+ across the interlayer gallery will be presented. Correpsonding 7Li and 2H NMR studies explore the dynamics of the intercalated polyelectrolyte and of the Li+ cations, as well as the position of Li+ inside the silicate gallery. The simulation and NMR results are in very good agreement and together provide a detailed picture of the nature of nanoscopically confined polyelectrolytes.
9:00 AM *N1.2
STRUCTURE AND DYNAMICS OF POLYMER LAYERED-SILICATE NANOCOMPOSITES. Ramanan Krishnamoorti, University of Houston, Dept. of Chemical Engineering, Houston, TX; Emmanuel P. Giannelis, Cornell University, Dept. of Mat. Sci. and Engr., Ithaca, NY.
Polymer based layered-silicate nanocomposites are model systems to probe the statics and dynamics of confined polymers and polymer brushes. Confinement over length scales of 1 nm to 100 nm or more are acheived using these nanocomposites. The static and dynamic properties of these confined polymer systems and polymer brushes (synthesized by tethering the polymers to the silicate layers) have been studied by bulk experimental techniques such as melt-state rheology, inelastic neutron scattering, small angle neutron scattering, and neutron diffraction. Unique features associated with the statics and dynamics of these systems were observed and will be reported in the talk.
9:30 AM N1.3
DIRECT FABRICATION OF MULTI-FUNCTIONAL COMPOSITES VIA SUPRAMOLECULAR SELF-ASSEMBLY. C.Jeffrey Brinker, Anh Nguyen and Alan Sellinger, Advanced Materials Laboratory, University of New Mexico/Sandia National Laboratories, Albuquerque, NM.
The extraordinary toughness of 'chalk' when constructed by nature into shell, a layered organic-inorganic composite, has inspired a new area of materials science termed biomimetic processing. Unfortunately after years of research no efficient means of synthesizing such bio-inspired composites exists. Thus, weíve commenced a program towards the direct fabrication of organic-inorganic nanocomposites in a continuous process by supramolecular self-assembly. Beginning with an inexpensive homogeneous solution of an amphiphile and silica, we recently demonstrated the formation of a family of mesostructured liquid-crystalline films (hexagonal, cubic, lamellar) via evaporation-induced co-assembly of silica and surfactant. By using polymerizable amphiphiles and introducing functionalized photo or thermal-polymerizable monomers, we extend this versatile state-of-the-art work to the preparation of liquid-crystalline organic-inorganic composites with a wide variety of potential applications including abrasion resistant coatings (automotive finishes), radiation-hard barrier coatings and ultra-high capacitors to name a few.
10:15 AM N1.4
IN-SITU STUDY OF THE GROWTH OF THE NANOSTRUCTURED SURFACTANT/SILICA FILMS. Linbo Zhou, Paul Fenter, Peter M. Eisenberger, Department of Physics and Princeton Materials Institute; Ilhan A. Aksay, Department of Chemical Engineering and Princeton Materials Institute, Princeton University, Princeton, NJ.
In-situ x-ray studies have been performed on the self-assembled mesoscaled surfactant/silica thin films which are synthesized with C19H42ClN (CTAC)/(C2H5O)Si (TEOS) under aqueous conditions at pH= 1 on freshly cleaved mica substrates. The films which consist of a lattice of hexagonally packed tubules are highly ordered and well oriented. The film growth is characterized by an induction time followed by a nonlinear growth rate after nucleation. The induction time is independent of the nature of the substrate, but it depends on the concentration of both TEOS and CTAC. Our experiments show that there are different growth regimes. One is the low surfactant concentration regime, at which the induction time only depends on the TEOS concentration, while at the relatively higher surfactant concentration, the induction time depends on the ratio of TEOS to surfactant linearly. These data suggest that the growth proceeds in a very unusual fashion, and raise questions concerning the fundamental nature of the initial nucleation of the film and the microscopic processes which result in non-linear growth rates and different growth regimes. We discuss these results in the context of a model of the film nucleation, growth kinetics, and growth mechanism.
10:30 AM N1.5
ULTRATHIN ORGANIC FILMS IN THE INTERLAYER OF BONDED SILICON WAFERS. Gertrud Kräuter, Ulrich Gösele, Max-Planck-Institut für Mikrostrukturphysik, Halle/Saale, GERMANY; Thomas Jaworek, Gerhard Wegner, Max-Planck-Institut für Polymerforschung, Mainz, GERMANY.
Smooth and clean surfaces adhere to each other when brought into contact. This is due to van-der-Waals forces and hydrogen bonding between the molecules of the opposing surfaces. At room temperature the bonding strength between the two surfaces is rather low; however during an annealing step at elevated temperatures (typically 800 - 1100C) covalent bonds are formed and the bonding becomes irreversible. The phenomenon can be exploited for the bonding of mirror-polished silicon wafers. In fact, the so-called wafer bonding technique has emerged to be an indispensable tool in microelectronics for the preparation of silicon-on-insulator layers as well as in micromechanics for the fabrication of sensors. For the bonding of wafers containing temperature-sensitive structures a bonding technique which leads to high bonding strength at low temperatures is desirable. One approach to low temperature wafer bonding is the chemical modification of the wafer surfaces prior to bonding. We have deposited ultrathin layers of organic compounds on the wafer surfaces which serve as interlayers after the bonding. The films were deposited both by the Langmuir-Blodgett technique and by self-assembly. Ultrathin LB-films of hairy rod-like polymers were found to increase the bonding strength by a factor of 10. Alkylchlorosilanes can be covalently bonded to the surface of a hydrophilic silicon wafers. When two wafers which have been treated with alkylchlorosilanes are contacted, bonding occurs due to the formation of van-der-Waals interactions. The bonding strength is weak under these conditions; however it can be increased dramatically during an annealing step at moderate temperatures. The bonding of wafers having ultrathin organic films on their surfaces and the properties of organic interlayers formed during the bonding process will be discussed.
10:45 AM N1.6
PROPERTIES AND BEHAVIOR OF POMA CONFINED IN RANDOM POROUS MATRICES. F.M. Aliev, G. P. Sinha and B. Batalla, Dept. of Physics and Materials Research Center, University of Puerto Rico, San Juan, PR.
We report the results of investigations of equilibrium and dynamical properties of poly(octylmethacrylate) (POMA) confined in porous matrices by means of static and dynamic light scattering as well as broad band dielectric spectroscopy (frequency range 10-3 - 109 Hz). The polymer was synthesized directly in the randomly distributed and thoroughly interconnected pores of the porous glass matrices by free radical polymerization of the monomer. We used porous matrices of two sizes: 1000 Å (volume fraction of pores 40%) and 100 Å (27%). In a narrow temperature range, an anomalous change in the temperature dependence of the intensity of scattered light (Isc) was observed for POMA confined in 1000 Å pores. As a consequence of which the composite switches from an opaque state to a transparent state when the temperature is increased from 327 K to 332 K, and it returns back to the opaque state while cooling from 327 K to 322 K. The character of the temperature dependence and observed temperature hysteresis in Isc(T) suggest that in the temperature range in which Isc changes significantly, a smeared first order phase transition is observed. These changes are explained by assuming that at low temperatures (opaque state) the orientational order of relatively long linear aliphatic side groups is induced by the inner pore surface. The transition into the state with disordered arrangement (transparent state) of these groups is a first order phase transition which is absent for bulk POMA. The composite POMA - 100 Å porous glass was transparent in the optical wavelength range. The influence of pore size and temperature on dynamical behavior investigated by dynamic light scattering and dielectric spectroscopy are discussed.
11:00 AM N1.7
SYNTHESIS OF SELF-ASSEMBLED PHOTOCONDUCTIVE MOLECULES IN MESOPOROUS MATERIALS; OPTICAL AND ELECTRICAL PROPERTIES OF ORGANIC/INORGANIC NANOCOMPOSITES. Itaru Honma and H.S. Zhou, Electrotechnical Laboratory, AIST, Tsukuba, Ibaraki, JAPAN.
The functional opto-electrical materials with molecular-to-nano size structure has attracted much attention for devices with excellent properties. We present , in this paper, a new synthetic route of self-organization process to produce self-assembled photoconductive molecules doped in mesoporous materials. The synthesis was examined in two ways. The first one is that the dye-molecule such as Porphyrins was doped in mesoporous inorganic materials such as silicate V2O5, WO3, MoO3 and TiO2 MCM. The hydrophobic Porphyrin molecules were desolved in C16TMA/H2O solution and hydrolized with a silicate precursor or other metal oxides precursor to form dye-doped mesoporous materials, where the dye molecules are doped and self-assembled in a nanochannels. The colored product shows a clear hexagonal phase by XRD analysis and d-spacing of the product was slightly shortened from that of pure silicate MCM. Only phdrophobic molecules were doped within C16TMA micelles and self-assembled in the channels.Similarly, several transition metal oxides were used as a MCM frame work to have carrier conductivities for the self-organized ceramic/polymer nanocomposites.Optical and electrical properties of these products were examined in relation with a mesostructure of the materials. In a second way, we developed acetylene molecules involved trimethylammonium surfactant and synthesize polydiacethylene-doped silica mesoporous structures. The PDA/SIO2 co-assembled products are formed in lamellae form and polymerized to produce polydiacethylenes extended along with a mesoporous channels. The products are colored and have an optical absorption band in a visible regions. The optical absorptrion spectra, electrical conductivities and photoconductivities of those polymer/inorganic nanocomposite mesoporous materials are investigated and will be reported.
11:15 AM N1.8
INJECTION THRESHOLD FOR A STATISTICALLY BRANCHED POLYMER INSIDE A NANOPORE. Cyprien Gay, Pierre-Gilles de Gennes, Elie Raphaèl, Laboratoire de Physique de la Matiére Condensée, Collège de France, Paris, FRANCE; Françoise Brochard-Wyart, Laboratoire PSI, Institut Curie, Paris, FRANCE.
A nonadsorbing, flexible polymer (in dilute solution with a good solvent) enters a pore (of diameter D smaller than its natural size, R) only when it is sucked in by a solvent flux, J, higher than a threshold value, Jc. For linear polymers JkT/ (where T is the temperature and the solvent viscosity). We discuss here the case of branched polymers, with an average number, b, of monomers between branched points. We find that there are two different regimes, depending on the tube diameter. By measuring Jc in both regimes, we should determine both the molecular weight and the number b.
11:45 AM *N1.9
THE SYNTHESIS, STRUCTURE AND PROPERTIES OF POLYMER/CLAY NANOCOMPOSITE FILMS. Elaine R. Kleinfeld, Barbara A. MacNeill, Irwin Chen and Gregory S. Ferguson, Department of Chemistry, Lehigh University, Bethlehem, PA.
Sequential adsorption of a cationic polyelectrolyte and single sheets of an anionic clay-silicate mineral from aqueous solution onto various surfaces produces optical transparent films that are sufficiently ordered in the direction normal to the substrate to diffract x-rays. These films have lattice spacings of nanometer dimension with the polymer and clay stacked in an ABABAB arrangement. The stepwise method used to form these films allows precise control over their thickness, an important parameter for optical applications. This talk will include a discussion of the experimental parameters important in the synthesis of these multilayers, for example the relationship between the concentration of the polymer solution used in the absorptions and the structure of the film produced.
SESSION N2: DYNAMICS OF CONFINED SYSTEMS-I
Chair: Georg Krausch
Monday Afternoon, December 1, 1997
1:30 PM *N2.1
TOWARD UNDERSTANDING POLYMER/WALL INTERFACES IN MELT FLOW. J.R.Barone, X. Yang, C. Deeprasertkul, N. Plucktaveesak, S.Q. Wang, Dept of Macromolecular Science, Case Western Reserve University, Cleveland, OH.
This presentation introduces the latest findings concerning polymer dynamics confined by presence of a solid wall. Detailed experimental data will be described to elucidate the molecular origins of and mechanisms for interfacial flow instabilities of linear polyethylenes (PE) during capillary extrusion. Specifically, at sufficiently high flow rates, molecular conformational transitions lead to removal of chain entanglements at PE/wall interfaces and to global/local oscillation of the boundary condition. These molecular instabilities cause flow oscillation and sharkskin like extrudate distortion and have seriously challenged any efforts to improve processing of polyethylenes and PE-based materials. The recent advances in this field will hopefully result in new and fundamental solutions for the world-wide polyethylene processing industry.
2:00 PM *N2.2
SPECTROSCOPIC CHARACTERIZATION OF MOLECULES IN SHEAR FIELDS WITHIN CONFINED SPACES. Yoon-Kyoung Cho, Iwao Soga, Ali Dhinojwala, and Steve Granick, Univ of Illinois, Dept of Materials Science and Engineering, Urbana, IL.
Seeking to augment force measurement by ancillary spectroscopic probes, we have developed a new instrument to measure concurrently molecular orientation (by time-resolved infrared spectroscopy) and dynamic viscoelastic responses in a shear field (with a homebuilt rheometer) between parallel optically-flat windows. The window spacing can be varied continuously, using piezoelectric devices, from submicron thickness to 500 µm. The window spacing is monitored, with submicron accuracy, either by optical interferometry or by capacitance sensors. Shear can be applied with frequency 1 to 700 Hz with amplitude 0.l nm to 30 µm. Molecular alignment at separate points during sinusoidal shear cycles can be determined using step-scan time resolved infrared spectroscopy with 5 µsec time resolution. In progress are studies with confined polymers as well as with liquid crystals.
2:30 PM N2.3
SHEAR IN NANOSCALE CONTACTS TO POLYVINYLETHELENE. W. N. Unertl, LASST/SERC, University of Maine, Orono, ME; K. J. Wahl, and S.V. Stepnowski, Tribology Branch, Naval Research Laboratory, Washington, DC.
We describe viscoelastic effects in shear deformation during the making and breaking of nanometer scale contacts to polyvinylethelene (PVE) using a scanned force microscope (SFM). PVE samples were films cast from toluene and vacuum dried. The bulk glass transition temperature is 273 K. Typical shear modulation amplitudes applied to the PVE sample were several angstroms at frequencies between 50 and 1200 Hz. We describe contacts formed by approaching the sample toward, then retracting it from, the SFM tip at constant speed while measuring the amplitude t and phase of the torsional response of the SFM cantilever. Maximum applied loads were less than 5 nN resulting in estimated contact radii in the range 20-50 nm. At jump-to-contact, both t and increased suddenly and then changed continuously during the approach and retraction. t continued to increase even after the maximum load was reached and the tip had started to retract. The maximum t and the maximum change in did not occur until the contact was under tension, even for speeds as slow as 0.5 nm/s. Both t and showed significant changes well before the contact ruptured at pull-off. The characteristics of the response were similar at all frequencies except that the magnitudes of t and varied as expected for a viscoelastic material; i. e., at higher frequencies the contact was more glassy. However, the characteristic relaxation time of the contact at maximum load was approximately 3 ms, which was much lower than for the bulk. This relaxation time decreased by more than an order of magnitude as the sample aged and became more elastic. The shear response was heterogeneous across the surface and was dependent on the history of the contact; e. g., speed of contact formation, number of contacts, and time interval between contacts.
3:15 PM N2.4
THIN POLYMER LIQUID FILMS UNDER SHEAR: EVOLUTION TO STEADY-STATE SLIDING. Carlos Drummond and Jacob Israelachvili, Chemical Engineering and Materials Departments, University of California, Santa Barbara, CA.
Using the Surface Forces Apparatus thin films of different branched hydrocarbon lubricants suc as poly alpha olefin (PAO) were confined between molecularly smooth mica surfaces and subjected to shear by moving the surfaces with respect to each other at constant velocity. Film thickness, contact area, load and friction forces were measured in these model single asperity systems. The evolution of the systems to steady-state sliding after a change in the sliding velocity was thoroughly studied, and the presence of different length and time scales could be identified. It was observed that the time necessary to reach steady-state sliding increase at slower shear rates, suggesting the idea of a characteristic sliding distance as the important parameter to describe the evolution of the friction forces to steady-state sliding. The film thickness under different sliding conditions was also measured. It was observed that upon starting sliding from rest the lubricant film thickness increased (``dilated'') with a corresponding decrease in the friction forces (``shear thinning). A quantitative description of these findings is presented, and the results are interpreted in terms of the changing conformational states of the lubricant molecules between the surfaces.
3:30 PM N2.5
NONEQUILIBRIUM POLYMER DIFFUSION IN INHOMOGENEOUS FLOWS OF ENTANGLED POLYMER FLUIDS. David Morse, Univ. of Minnesota, Dept. of Chemical Engineering and Materials Science, Minneapolis, MN.
The average velocity of an entangled polymer relative to the motion of the matrix of entangling polymers within a rapidly flowing, spatially inhomogeneous flow field is calculated via the reptation model. Motion of the polymer center-of-mass relative to the motion of the enclosing `tube' may be driven either (as in equilibrium) by inhomogeneities in the polymer chemical potential, leading to a diffusive flux that may be characterized here by an anisotropic non-equilibrium diffusion tensor, or by inhomogeneties in the rate-of-deformation tensor, which may drive relative flow by imposing unbalanced frictional forces upon a chain. These effects can lead to molecular-weight dependent transport of polymers across flow lines, and thus to some fractionation of polydisperse melts and blends in sufficiently tightly confined flows.
3:45 PM *N2.6
THE ROLE OF INTERFACIAL MOBILITY ON THE ORGANIZATION AND ORDER IN THIN POLYMER FILMS. John F. Rabolt, Department of Materials Science, University of Delaware, Newark, DE.
Polymers deposited on fluid interfaces have been shown to exhibit a high degree of orientation and order due to the inherent mobility of the polymer/fluid interface. When these thin films are transferred to solid supports the extent of organization and order which is retained will depend on the strength of the interaction between the polymer and the substrate. Several illustrative examples will be given.
4:15 PM N2.7
INTERFACIAL ``SLIP'' IN PHASE-SEGREGATED POLYMER BLENDS. J. L. Goveas, UCSB, Materials Research Lab, Santa Barbara, CA; G.H. Fredrickson, UCSB, Dept. of Chem. Eng., Santa Barbara, CA; L. Leibler, Elf-Atochem/CNRS, Paris, FRANCE.
We consider a strongly-segregated symmetric homopolymer blend, under weak steady shear parallel to the blend interface. When there is a repulsive interaction between the homopolymers (characterized by the Flory- Huggins interaction parameter ), a low viscosity region is created at the interface where the blend appears to ``slip''. For chains obeying Rouse dynamics, the interfacial viscosity is found to be in agreement with a scaling prediction due to de Gennes, where is the bead friction coefficient and b is the monomer length. Such slip phenomena directly affect the bulk processing of polymer blends and laminates. This also serves as a model problem in the development of a constitutive relation for stress relaxation in strongly inhomogeneous polymeric systems.
4:30 PM *N2.8
POLYMER THIN FILM AND INTERFACE STUDIES BY BEAM-POSITRON ANNIHILATION LIFETIME SPECTROSCOPY*. David Gidley, Gregory DeMaggio, Albert Yee, William Frieze, Dept. of Physics and Dept. of Materials Science and Engineering, The University of Michigan, Ann Arbor, MI.
A new technique: beam-PALS, and the results of its application on nm-thick polymer films are described. In conventional PALS the positrons have residual energy from the decay process and must be dissipated before they can be trapped by electron-deficient regions in the solid. This effectively limits their usefulness to bulk specimens. In beam-PALS a focused beam of low energy positrons is implanted into the surface of a material. Measurements indicate that the mean probe profile is given mainly by the average positron implantation distance, Z(E) . The thermal expansion behaviors of thin, Si-supported polystyrene films near the Tg were measured. The results on PS can be fitted using a 3-layer model incorporating a 5 nm constrained layer at the Si interface and a 2 nm surface region with reduced Tg . The constrained layer has a thermal coefficient of expansion equal to that of the Si substrate. Polycarbonate (PC) thin films and bulk interfaces were also studied. The surface of a bulk PC sample that had been in contact with Poly(ethylene terephthalate) film at a high tempearture is found to be well-represented by a 50/50 random copolymer of PC/PET, probably formed by an exchange reaction. Moreover this copolymer is confined within a very thin (nm) region, suggesting that the copolymer cannot diffuse beyond the original interface. PC thin films deposited on Si suggest that the constrained layer is just under 20 nm. The larger constrained layer thickness of PC is ascribed to the greater stiffness of PC chains in comparison with PS chains.
SESSION N3: DYNAMICS OF CONFINED SYSTEMS-II
Chair: Thomas A. P. Seery
Tuesday Morning, December 2, 1997
8:30 AM *N3.1
POLYMER INTERDIFFUSION NEAR ATTRACTIVE WALLS. Wen-li Wu, Eric K. Lin, Rainer Kolb, Polymers Division; Sushil K. Satija, Reactor Radiation Division, NIST, Gaithersburg, MD.
Neutron reflectometry was used to study the interdiffusion between bilayers of protonated and deuterated poly (methyl methacrylate) (PMMA) on silicon single crystal wafers with surface covered with either native oxide or a monolayer of silane capped with methyl end groups. The PMMA-silicon oxide interaction is strongly attractive due to the polar nature of the PMMA molecule. This interaction is expected to be weakened by the presence of a silane monolayer. The effect of substrate surface on the interdiffusion rate as a function of the distance from the substrate is investigated. Lower layer thickness of the deuterated PMMA is ranging from 0.4 to 4 times the radius of gyration of the molecules. The rate of interdiffusion is found to be strongly dependent on the lower film thickness. Films less than one radius of gyration in thickness have effective diffusion rate two orders of magnitude smaller than that of the thickest films.
9:00 AM N3.2
NANOPROPERTY FLUCTUATIONS IN POLYMER FILMS NEAR THE GLASS TRANSITION. N. E. Israeloff, L.E. Walther, and H. Alvarez Gomariz, Northeastern University, Dept. of Physics, Boston, MA.
Using novel scanning probe microscopy (SPM) techniques, we have measured dielectric and visco-elastic properties on ultra-small, mesoscopic, length scales in glassy polymer (PVAc) films near the glass transition. The effective length scale sampled by these techniques is 50 nm. This is large enough to probe subsurface properties, but small enough to observe deviations from bulk-like behavior in the form of large fluctuations of measured properties, such as mechanical dissipation, near the glass transition. These fluctuations are argued to arise from intrinsic inhomogeneities and cooperative dynamics on somewhat smaller length scales.
9:15 AM N3.3
MECHANICAL PROPERTIES AND MORPHOLOGY OF FREELY-STANDING SiOx/PS/SiOx TRILAYER FILMS*. Kari Dalnoki-Veress and John R. Dutcher, Dept of Physics and Guelph-Waterloo Program for Graduate Work in Physics, University of Guelph, Guelph, Ontario, CANADA.
Large reductions in the glass transition temperature Tg are observed for thin freely-standing films of polystyrene (PS) [J.A. Forrest et al., Phys. Rev. Lett. , 2002 (1996)]. For temperatures T > Tg, the films thin and are unstable to the formation of holes. We have investigated the thermal stability of thin PS films which are symmetrically confined by thin silicon oxide (SiOx) layers to form freely-standing SiOx/PS/SiOx trilayer films. Using Brillouin light scattering, ellipsometry and optical microscopy, we have measured the mechanical properties and morphology of the freely-standing SiOx/PS/SiOx trilayer films. Specifically, we have compared the Tg values obtained for the trilayer films with those obtained for freely-standing PS films, and studied the morphology of the trilayer films as the temperature is increased above Tg. * Research supported by NSERC of Canada
9:30 AM N3.4
SMALL-MOLECULE PROBE DIFFUSION IN ULTRATHIN POLYMER FILMS: STUDIES VIA FLUORESCENCE AND NONLINEAR OPTICS. David B. Hall, John M. Torkelson, Northwestern Univ., Dept. of Chemical Engineering, Evanston, IL.
Interesting phenomena involving alpha-relaxation dynamics and polymer diffusion in ultrathin (less than 200 nm thick) polymer films have recently been observed. The underlying molecular causes for this behavior are still being debated. Molecular probe techniques are well-suited for studying dynamics in these confined systems. In this study, translational diffusion coefficients of small-molecule probes in thin and ultrathin polymer films near the glass transition are measured using fluorescence nonradiative energy transfer. The probes employed are large enough so that their motion is coupled to the cooperative segmental motion of the polymer a-relaxation as determined via second harmonic generation studies; therefore, changes in probe diffusion coefficients with polymer film thickness reflect changes in polymer alpha-relaxation dynamics. Initial results of probe diffusion in poly(isobutyl methacrylate) films on fused silica substrates have shown only slight changes in diffusion coefficients as compared to bulk for films as thin as 50 nm. This is consistent with previous studies on a similar system which indicated very little change in the glass transition temperature with film thickness. Systems with strong polymer / substrate interactions which should show larger deviations from bulk behavior in ultrathin films are the subject of current study along with an examination of the role of polymer molecular weight.
9:45 AM N3.5
POLYMER DIFFUSION IN SEMICRYSTALLINE POLYMER MEDIA. Steve Lustig, John Van Alsten, Central Research, DuPont, Wilmington, DE.
The transport of monodisperse, atactic polystyrene-D (aPS-D) into atactic polystyrene (aPS) melts and isotactic polystyrene (iPS) semicrystalline matrices has been characterized by a quantitative ATR-FTIR technique. Values of the aPS-D tracer diffusion coefficients in aPS melts agree very well with previous literature indicating the effects of (i) diffusant molecular weight holding constant the matrix molecular weight and (ii) matrix molecular weight holding constant the diffusant molecular weight. Semicrystalline morphologies of iPS were characterized extensively by DSC, TEM, SEM, microscopy, WAX and SAXS as functions of temperature. Non-Fickian transport kinetics are observed during the imbibition of aPS-D into semicrystalline iPS at low temperatures and/or high penetrant molecular weights. Analogous observations are also reported for poly(aryl ether imide) transport in semicrystalline poly(aryl ether ketone ketone). There are two apparent time regimes of non-Fickian kinetics. The first regime of imbibition kinetics occurs at early times where the uptake can be described by Fickian mutual diffusion kinetics. Later, there is a decrease in the imbibition rate and the rates of continued imbibition are no longer Fickian. Theory and simulation of diffusion in random, disordered media indicate the transition to a slower transport kinetics regime can be identified with a time scale over which penetrant macromolecules in amorphous pockets explore bottlenecks between crystalline, dendritic obstacles.
10:30 AM N3.6
FRICTIONAL BEHAVIOR OF BLOCK COPOLYMER BRUSHES. S. Michael Kilbey II, M. Tirrell, F.S. Bates, Dept of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN.
The shear response of polymer brushes immersed in both a good solvent and a near-theta solvent was studied using the surface forces technique. The opposing brushes were created through preferential adsorption of polystyrene - polyvinylpyridine block copolymers from toluene. In this good solvent, the PS blocks stretch away from the tethering surface, and the height of the layer is a few times the free solution radius-of-gyration of the tethered chain. When contacting brushes in a good solvent were sheared, no frictional forces were detected even when the brushes were compressed to a fraction of their equilibrium height. Changing the solvent to cyclohexane near the theta temperature produced the expected contraction of the brush, but the layer remained stretched a few times the free solution radius-of-gyration of the PS block. This solvent change drastically altered the shear response of the system. Frictional forces were measured at ranges comparable to the distance at which the opposing brushes contacted one another, and these forces increased rapidly as the surface separation was decreased. The pattern of behavior exhibited in the shearing experiments can be explained by frictional interactions between the brush chains and the solvent.
10:45 AM *N3.7
ADJUSTED FRICTION AT POLYMER/SOLID INTERFACE THROUGH CONTROLLED GRAFTED LAYERS. Liliane Léger, Emmanuel Durliat and Hubert Hervet, Laboratoire de physique de la matière condensée, Collège de France, Paris , FRANCE.
We shall present series of experiments conducted on a model system (polydimethylsiloxane (PDMS) melt against silica surfaces covered with grafted PDMS chains of controlled length and surface densities) which allow to identify the molecular mechanisms responsible for polymer-wall friction in melt flows. From the measurements of both the local fluid velocity at the wall and the friction force, three different friction regimes can be identified when increasing progressively the shear rate, which are characteristic of a mechanical coupling between the polymer melt and the grafted chains through chains entanglements. When the shear rate is increased, the surface chains are progressively elongated under the effect of the friction force, and finally disentangle from the melt, leading to strong wall slip. Data showing how the corresponding weak to strong slip transition is influenced by the molecular parameters of the system (molecular weights of the surface and bulk chains and grafting density) will be presented and discussed.
11:15 AM N3.8
STRUCTURE AND Li-ION TRANSPORT PROPERTIES OF POLYANILINE INTERCALATED LAYERED MANGANESE DIOXIDE (-MnO2) NANOCOMPOSITE. Kecheng Gong, Wen Zhang, Polymer Structure & Modification Res. Lab., South China Univ. of Technology, Guangzhou, CHINA.
Polyaniline intercalated -MnO2 nanocomposite was successfully achieved by multi-step method in our laboratory. THe FTIR, powder XTRD, thermal analysis and elemental analysis results indicate that the repeat distance of nanocomposite crystal increases by 15.4 from 7.1 (-MnO2) and the interlayered separation is consistent with bilayer of polyaniline chains in -MnO2 lattices. The performance of battery using nanocomposite as cathode shows that Li-ion transfer in nanocomposite is greatly enhanced due to polyaniline chains propping open the -MnO2 crystal layers effectively reducing the electrostatic and steric effect which hinder Li-ion diffusion in the interlayer space as well as electron transfer in polyaniline chains inducing Li-ion transfer. The specific capacity of nanocomposite cathode is twice as large as that of -MnO2 at 5mA/cm2 current density and has excellent stability with rising temperature. The specific capacity preservation of nanocomposite is 95% after ten charge-discharge cycles at 1mA/cm2 current density.
11:30 AM N3.9
MOBILITY OF HIGH POLYMERS IN NANOMETER CONFINEMENTS. Evangelos Manias, Jan Genzer,* Hua Chen, Edward J. Kramer,* Emmanuel P. Giannelis, Department of Materials Science and Engineering, Cornell University, Ithaca NY; *Department of Materials Science and Engineering, University of California at Santa Barbara, Santa Barbara, CA.
The intercalation kinetics of high polymers in layered silicate inorganic compounds and the mobility of the polymer coils in dimensions of 2-3nm was studied by X-Ray Diffraction. High molecular weight Polystyrene (PS) is used with fluorohectorite, a synthetic 2:1 layered silicate. In order to investigate the efect of the wall-polymer energetics, different surfactants are used to modify the silicate surface, and/or strongly interacting groups are randomly attached along the PS chain. Diffusion is dramatically slowed down with increasing wall affinity. Moreover, the effective polymer diffusion constant scales with the molecular weight as , whereas in absolute numbers is comparable and even faster than the ocorresponding polymer self-diffusion. In order to unveil the origins of the polymer dynamics in the vicinity of an adsorbing surface, computer simulations using Molecular Dynamics of polymer systems confined between parallel walls are also carried out and the results are correlated with the experimental findings.
11:45 AM N3.10
PARTITIONING OF NONELECTROLYTE POLYMERS INTO A NANOMETER-SCALE PORE. J.J. Kasianowicz, NIST, Biotechnology Div., Gaithersburg, MD; S.M. Bezrukov, DCRT, NIH, Bethesda, MD.
Size-selected linear nonelectrolyte polymers (e.g. poly(ethylene glycol)) are commonly used to estimate the diameter of nanometer-scale pores in ion channels. Polymers that are sufficiently small partition into the pore, thereby causing a decrease in the channel's ionic conductance. We show that PEGs smaller than a characteristic cut-off size freely partition into single channels formed by Staphylococcus aureus alpha-hemolysin. However, the dependence of the single channel conductance, the variance of the polymer-induced current noise, and the correlation time of the polymer in the pore (which is estimated from the corner frequency of the current spectral density) suggest that poly(ethylene glycol) interacts with this channel and that the pore is lined in part by hydrophobic residues. We also show that an estimate of this pore's diameter by the single channel conductance is not in good agreement with the results of the PEG partitioning measurements. For example, a decrease in pH causes an increase in the channel's current carried by sodium and chloride ions but leads to a decrease in the characteristic PEG cut-off size. The latter result is rather surprising because it suggests that changing the electrostatic potential of a surface can alter its ability to interact with an uncharged moiety.
SESSION N4: GRAFTED CHAINS
Chair: Anna C. Balazs
Tuesday Afternoon, December 2, 1997
1:30 PM *N4.1
COLLECTIVE DYNAMICS OF TETHERED POLYMER CHAINS PROBED BY EVANESCENT WAVE DYNAMIC LIGHT SCATTERING#. S.H. Anastasiadis, G. Fytas, D. Vlassopoulos, R. Seghrouchni, C. Toprakcioglu, A. Likhtman, and A.N. Semenov, Foundation for Research and Technology-Hellas, Institute of Electronic Structure and Laser, Crete, GREECE.
The thermal fluctuations of the segment density profile of a polymer brush formed by end-attached polymer chains onto a solid surface from solution are probed by evanescent wave dynamic light scattering; in this set-up, the evanescent light illumination, produced under total reflection conditions, is utilized as an incident beam in the scattering experiment, and the time correlation functions of concentration fluctuations with wavevector q are obtained using photon correlation spectroscopy. By utilizing a set of terminally-attached layers with thicknesses (L) from 45 to 130 nm, it is found that there is a preferred wavelength of O(L) of these fluctuations with a concurrent slowing down of their dynamics. These observations indicate that the fluctuations observed may be associated with the small-amplitude deformation of the free surface of the brush. We present a theory for the deformation free energy of a parabolic brush in solution and show that a maximum exists in the respective structure factor of concentration fluctuations at q* due to a balance between the chain stretching energy and the standard gradient term accounting for the conformational entropy loss due to the inhomogeneous monomer distribution; in agreement with the experiment, the predicted q*L O(1).
2:00 PM *N4.2
SURFACE INITIATED POLYMERIZATION: A NEW TOOL FOR THE DESIGN OF POLYMER SURFACES. Thomas A. P. Seery, Preeti Dhar, Dale L. Huber and Fatma Vatansever, Polymer Program and Department of Chemistry, University of Connecticut, Storrs CT.
To date, new polymeric surfaces on organic or inorganic substrates have been generated primarily via the adsorption of previously synthesized polymers. This method has found wide use and formed the basis for experiments that have tested theories of polymer brushes. There are, however, limitations to the adsorption approach, and surface initiated polymerizations (SIP) can address many of these issues. SIP requires an initiator group covalently bound to a surface that can be susequently utilized for a chain growth polymerization. The steric constraints for the initiator are far less than that of a polymer, so that the areal graft density can, in principle, be much greater from SIP than from adsorption or grafting of chains. Using organometallic catalysts in a SIP scheme has provided access to a number of chain backbones that have not previously been studied at surfaces. In particular, surface bound polyhexylisocyanates have been prepared so that the effect of chain stiffness on surface properties may be observed. ROMP catalysts and anionic polymerization routes are currently being explored.
2:30 PM N4.3
STRUCTURES OF SHORT ALKYL CHAINS AND POLYETHYLENE MONOLAYERS GRAFTED ON A SILICA SURFACE. Jannick Duchet, Jean-Paul Chapel, Bernard Chabert, Jean-Francois Gerard, Laboratoire des Materiaux Macromoleculaires,CNRS, INSA, Villeurbanne, FRANCE.
Monochlorosilane-terminated polyethylenes of different molar masses, i.e. different chain length (from 1,700 to 32,500 g/mol) were synthesized and grafted by a solvent process on a silica surface (oxidized silicon wafer, nano-particles). The structure and the grafting density of these grafted monolayers are studied by various techniques such as ellipsometry, atomic force microscopy, wettability measurements,a nd 29Si NMR. Monolayers prepared from alkylchlorosilanes with different chain lengths (from C4 to C30) and by using two method (Sagiv and Kovacs) were also considered. Depending on the chain length and by comparing with the Flory radius of the unperturbated chain, the polyethylene grafted chains display from a brush-like or a mushroom-like structure to a structure for which the chains are spread-out onto the surface.
2:45 PM *N4.4
POLY(ETHYLENE OXIDE) CONTAINING BLOCK COPOLYMERS IN AQUEOUS SOLUTION AT SURFACES AND INTERFACES. PREDICTIONS FROM MEAN-FIELD LATTICE THEORIES. Per Linse, Phys Chem 1, Center for Chem and Chem Eng, Lund Univ.
During the last decade, ABA triblock copolymers, where one block being poly(ethylene oxide) and the other poly(propylene oxide) or poly(butylene oxide), have been detailed investigated . In aqueous solution, these polymers display a rich spectrum of ordered phases, and similarly, at surfaces and interfaces they are self-organized in a way which resembles that in the lamellar phase. The rapid decrease of the hydrophilic nature of the EO and PO blocks with increasing temperature of the copolymer in solution is an important factor affecting the phase behaviour and the adsorption. By using a lattice mean-field theory, the adsorption as well as the phase behaviour of ABA triblock copolymers in aqueous solutions has been theoretically investigated. The approach incorporated internal monomeric states important for describing the reverse temperature dependent displayed by EO and the interaction parameters used were generally obtained by fitting properties of simpler systems to experimental data. After presentation of some experimental key findings, the theoretical model and its major predictions regarding the adsorption of ABA triblock copolymers at solid surfaces and interfaces will be given. The role of the polymer architecture, polymer composition, and polydispersity will be discussed.
3:15 PM N4.5
NEUTRON REFLECTIVITY STUDY OF ADSORBED POLYMER BRUSHES: INFLUENCE OF SOLVENT QUALITY AND SHEAR FLOW. D. Anastasopoulos, C. Toprakcioglu, A. Vradis, Patras University, Dept of Physics, Patras, GREECE; S.M. Baker, Harvey Mudd College, Dept of Chemistry, Claremont, CA; G.S. Smith, LANSCE, Los Alamos National Laboratory, Los Alamos, NM; D. Bucknall, RAL, ISIS, Didcot, UNITED KINGDOM.
Highly asymmetric polystyrene-polyethylene oxide (PS-PEO) block copolymer chains were terminally adsorbed onto quartz from d-cyclohexane (marginally good solvent at 45C) and d-toluene (athermal solvent). The resulting adsorbed layers were studied by neutron reflectometry at various temperatures both above and below the theta point (22C, 33C and 45C). The layer thickness was found to increase with improving solvent quality, the volume fraction profile of the polymer brush being well-described by self-consistent mean field theory. In the athermal solvent toluene, only the PEO block adsorbs while the PS chain remains non-adsorbing and extends into solution reaching the maximum brush height attainable by the system for a given value of the polymer adsorbance. In cyclohexane, however, the reflectivity profiles indicate the presence of a dense surface layer which is attributed to adsorbed PS segments as a result of the poorer quality of this solvent. When the brush is subjected to shear flow in toluene, its height and volume fraction profile remain essentially unchanged up to shear rates of ca. 10000 s-1. However, at shear rates of ca. 19000 s-1 there is evidence of substantial polymer desorption.
3:30 PM *N4.6
END GRAFTED POLYSTYRENE BRUSHES IN A CRITICAL BINARY MIXTURE. S.K Satija, P.D. Gallagher, A. Karim, NIST Center for Neutron Nesearch, NIST, Gaithersburg, MD, and L.J. Fetters, Exxon Corporate Research Laboratories, Annandale, NJ.
We have carried out neutron reflectivity measurements on end-grafted deuterated polystyrene brushes grafted on silicon surfaces. The brushes were immeresed in binary mixtures of methanol and cyclohexane of various proportions including the critical composition. For single phase mixtures with compositions rich in methanol, the brush is very collapsed at high temperatures. Upon lowering the temperature, the brush height increases indicating that excess cyclohexane is adsorbed near the brush as cyclohexane-methanol phase separation temperature is approached. By changing the deuteration of the components of the binary mixture we have determined the excess amount of cyclohexane adsorbed near the brush surface as compared to the bulk binary mixture. On the other side of the binary mixture phase diagram, single phase mixtures with a small amout of added methanol show a surprising stretching of the polymer brush, with brush heights that exceed those measured in pure cyclohexane at the same temperatures. This suggest that the adsorption of methanol to the surface near the brush plays a role in the stretching of the end-grafted chains.
4:00 PM *N4.7
POLYMERS GRAFTED FROM SOLID SURFACES. Juergen Ruehe Max-Planck-Institute for Polymer Research, Mainz, GERMANY.
Both from a theoretical and from an experimental point of view much attention has been directed to systems where macromolecules are terminally attached to a solid substrate. Several such systems were developed in the past, mostly in which functional groups of the polymers are reacted with appropriate surface sites (grafting to technique). To avoid some problems associated with this technique, we have generated the polymer molecules directly on the surfaces of the substrates by using self-assembled monolayers of initiators and polymerisation in situ (grafting from). Following this grafting from approach polymer monolayers with precisely controlled film thicknesses in the range between 2 and 1500 nm can be prepared. By adjusting the chemical composition of the attached polymer, surfaces with tailor-made properties could be prepared. The obtained polymer brushes were characterized by a variety of techniques including x-ray photoelectron spectroscopy (XPS), FT-IR, x-ray reflectometry, atomic force microscopy (AFM), neutron reflectometry and surface plasmon spectroscopy. In order to permit spatial control over the composition of the attached molecularly thin films and to allow the preparation multifunctional polymer layers with high lateral resolution several new photolithographic techniques have been developed, which take advantage of the covalent bonding of the polymer molecules to the surfaces. It will be shown, that the patterned, molecularly thin films can be used to control the adhesion of biological cells to surfaces.
4:30 PM N4.8
MONTE-CARLO STUDY OF INTRAMOLECULAR INTERACTIONS AND CONFORMATIONS OF MOLECULAR BOTTLE-BRUSHES. Mika Saariaho, Olli Ikkala, Helsinki Univ of Technology, Materials Physics Lab, Espoo, FINLAND; Igal Szleifer, Purdue Univ, Dept of Chemistry, West Lafayette, USA; Igor Erukhimovich, Moscow State Univ, Dept of Physics, Moscow, RUSSIA; Gerrit ten Brinke, Univ of Groningen, Materials Science Center, Groningen, THE NETHERLANDS.
Molecular bottle-brushes are comb copolymers consisting of relatively long flexible side branches close spaced along the equally flexible main chain. We consider the effect of intramolecular interactions between side groups on the conformations of the bottle-brush backbone. Our results show that the ratio between the persistence lentgh and the bottle-brush diameter, which determines the possible lyotropic behavior, remains a constant as a function of the side-chain length. However, the ratio displays a strong dependence on the topology of the side groups. Therefore, by the proper choice of the chemistry of side-chain molecules, lyotropic behavior of bottle-brushes can be observed in certain real systems. Recent experimental observations are discussed in terms of these findings.
4:45 PM N4.9
ADSORPTION, GEOMETRY AND SHEAR EFFECTS OF POLYSTYRENE-POLYETHYLENE OXIDE DIBLOCKS ADSORBED ON QUARTZ IN POOR AND NEAR THETA SOLVENT. Shenda Baker, E. Torgerson, M. Perri, Harvey Mudd College, Dept of Chemistry, Claremont, CA; C. Toprakcioglu, A. Vradis, Patras University, Dept of Physics, Patras, GREECE; G. Smith, LANSCE, Los Alamos, NM.
An asymmetric diblock copolymer of polystyrene-polyethylene oxide (PS-PEO) was adsorbed to quartz from fully deuterated cyclohexane at 20 C (below the theta point of PS). The adsorbed film was examined by neutron reflectivity at 20, 29, 36, 42, 52 and 60 C. The polymer segment density near the surface decreased and the total film thickness increased as a function of T as predicted by SCF and Monte Carlo calculations. Minimal shearing near the theta point caused a dramatic and irreversible change in the density profile. Adsorption kinetics of the polymer were determined as a function of temperature by FTIR in the attenuated total reflection mode. At temperatures below the theta point, the polymer adsorption was characterized by a diffusion limited process. At higher temperatures, the adsorption approached, but did not reach a more classical first order process. As expected, the half-life of adsorption and the surface excess decrease as function of temperature. As the solvent quality improves, the effect of volume exclusion and entropy result in less adsorbed polymer and thicker.
SESSION N5: POSTER SESSION
Tuesday Evening, December 2, 1997
Grand Ballroom (S)
TOWARDS NEW METALLO-SUPRAMOLECULAR ASSEMBLIES AND POLYMERS. Ulrich S. Schubert, Christian H. Weidl, Markus K. Eigner, Lehrstuhl fuer Makromolekulare Stoffe, Technische Universitaet Muenchen, GERMANY; Jean-Marie Lehn, Laboratoire de Chimie Supramoleculaire, Universite Louis Pasteur, Strasbourg, FRANCE.
The controlled arrangement of metal ions into specific arrays and patterns is one of the major goals in modern metallo-supramolecular chemistry. Such two- or three- dimensional superstructures, formed by programmed self-assembly from mixtures of organic ligands and metal ions, potentially may display new magnetic, photochemical or redox properties. As a result, they can be used as building blocks for novel functional supramolecular assemblies and polymers. A recent investigation into synthesis and properties of tetranuclear [2 x 2]-grid complexes using 4,6-bis(6-(2,2'-bipyridyl))pyrimidine and its derivatives as ligands and metal ions such as cobalt(II), nickel(II), Copper(II) or zinc(II) has been extended to include peripherally functionalized analogues. These ligands and grids open avenues towards polymolecular assemblies and supramolecular polymers with novel material properties. The hydroxy-functionalized coordination arrays were found to form extended molecular assemblies. Furthermore, the incorporation of this supramolecular systems (ligands and/or complexes) in synthetic polymers opens a new class of materials.
NANOMETER SCALE MEASUREMENTS OF NOISE AND DISSIPATION IN POLYMER FILMS NEAR THE GLASS TRANSITION. H. Alvarez Gomariz, L. E. Walther, and N. E. Israeloff, Northeastern University, Dept. of Physics, Boston, MA.
Non contact atomic force microscopy is employed to measure dielectric polarization noise and viscoelastic dissipation in a glassy polymer, poly-vinyl-acetate. A custom-built variable-temperature high-vacuum atomic force probe with exceptional thermal stability was used. Thermal dielectric polarization noise (1/f noise) was observed in a non-contact capacitance measurement. The noise peaked at the glass transition and can be used to study nanoscale dynamics. Force modulation techniques were used to measure local mechanical disspation. This showed large fluctuations as a function of applied force near the glass transition.
COMPATIBILIZATION VIA POLYMER AND COPOLYMER ADDITION/ PRODUCTION AT INTERFACES IN PHASE-SEPARATED POLYMER BLENDS: INVESTIGATION OF COARSENING OF MICROSTRUCTURE AND END-USE PROPERTIES. Naomi Furgiuele, Klementina Khait, John Torkelson, Depts of Chemical Engineering and of Materials Science and Engineering, Evanston, IL.
Compatibilization of phase-separated polymer blends is of long-standing interest from both technological and basic scientific standpoints. It is of concern not only for the production of novel products from virgin polymers but also for implications in recycling commingled plastics. Here we consider several possibilities for compatibilization of phase-separated blends, i.e., the arresting of coarsening or growth in dispersed-phase size scales: (a) addition of polymers containing low levels of polar or other groups with a strong preference for one of the blend components, (b) addition of gradient copolymers made by living free radical polymerization, and (c) production of block copolymers at interfaces in process with the use of solid-state shear pulverization. In particular, methods (b) and (c) may provide solutions to problems currently encountered by using block copolymer addition (formation of block copolymer micelles, reducing the level of compatibilizing copolymer which may be active at the interfaces) and reactive processing employing polymers with functional groups, thus promoting interfacial block copolymer formation (cost of the functionalized polymers). Blends studied include polystyrene/polymethylmethacrylate and polystyrene/polypropylene. Effects of compabilization will be described at both the microscopic (via microscopy) and macroscopic scales (via mechanical property characterization).
POLYPHENYL THIOL MONOLAYERS ON GOLD(111) STUDIED BY SURFACE X-RAY DIFFRACTION. T. Y. B. Leung, F. Schreiber, P. Fenter and G. Scoles, Princeton Materials Institute, Princeton Univ, Princeton, NJ.
Polyphenyl thiol monolayers on the (111) face of single crystal gold have been characterized by surface x-ray diffraction. In particular, we have focused on monolayers made from 4-methylbiphenyl-4-thiol which has a biphenyl unit as backbone. As it occurs for alkanethiol monolayers, polyphenyl thiol monolayers are found to form two phases of different packing density. In the high density phase, the measured unit mesh suggests that the molecules pack hexagonally and the molecular backbones are oriented away from the surface. The low density phase is instead believed to have a unit mesh composed of 4 molecules with a head-to-head orientation and the molecular axes are parallel to the surface. The growth kinetics of the biphenylthiol monolayers is quite different from that of the alkanethiol system. In contrast to the alkanethiol monolayers, in which the hydrocarbon backbones easily ''stand up'' at full coverage, our observations suggest that the barrier for transition from the low density phase to the high density phase is significantly larger for this polyphenyl thiol system. Indeed we have had a difficult time in producing the high density phase and the quality of these data is poor. We attribute these observations to the lack of conformational freedom in the 4-methylbiphenyl-4-thiol and a strong adsorbate-substrate interaction. Also, the thermal properties of the low density phase have been studied in detail. These monolayers are found to be thermally very robust and remain as ordered layers up to 170C. Lastly, we are presently exploring other growth modes such as liquid phase growth and coadsorption assisted growth and will present the results at the meeting. As some of the liquid crystals and non-linear optical materials have similar polyphenyl units, a detailed understranding of the growth kinetics of these polyphenyl thiols monolayers is technologically significant.
ELECTROACTIVE AND PHOTOACTIVE ROD-COIL BLOCK COPOLYMERS: SELF-ORGANIZATION AND PHOTOPHYSICAL PROPERTIES. X. Linda Chen, Samson A. Jenekhe, Departments of Chemical Engineering and Chemistry, University of Rochester, Rochester, NY.
Rod-coil block copolymers are known to exhibit complex phase behavior and rich nanoscale morphologies. Our work aims to exploit some of the novel features introduced into a rod-coil block copolymer when its rodlike block is electroactive and photoactive: (1) tailorable solid state electronic, optoelectronic, and optical properties arising from control of nanoscale morphologies; and (2) the possibility of using optical and photophysical probes to elucidate the phase behavior, self-organization, and morphologies of rod-coil block copolymers. We have synthesized, characterized, and investigated two series of new electroactive and photoactive rod-coil-rod and coil-rod-coil triblock copolymers: poly(2,6-benzoxazole)- block-poly(benzobisthiazole decamethylene)- block-poly(2,6-benzoxazole)(RCRA) and poly [oxy(1-oxohexadecamethylene)]- block - poly(p-phenylene benzobisthiazole) - block - poly[oxy(1-oxohexadecamethylene)] (CRCA). Differential scanning calorimetry and polarized optical microscopy revealed that RCRA was liquid crystalline with a Smectic texture in the temperature range 120 C - 228 C. Steady-state and time-resolved photoluminescence studies of thin films of RCRA and CRCA showed the effects of self-organization, annealing at 110 C, block lengths, and composition on photophysical properties. The progressive band narrowing of the absorption spectra of CRCA thin films confirmed the effects of spatial confinement with increasing coil block size. Bright green electroluminescence has been observed from thin films of RCRA which suggests the potential fabrication of nanoscale light sources from rod-coil block copolymers.
DEWETTING DYNAMICS OF PS ON PMMA AS A FUNCTION OF dPS-PMMA DIBLOCK COPOLYMERS CONCENTRATION AT THE INTERFACE*. Shichun Qu, Mirian H. Rafailovich, and Johnathan Sokolov, Department of MS&E, SUNY at Stony Brook, Stony Brook, NY.
We have studied the dynamics of dewetting at the interface between polystyrene (PS) and poly(methyl methacrylate) (PMMA) in the presence of symmetric diblock PS-PMMA copolymers. We have used optical microscopy and atomic force microscopy to follow the dewetting process and image both the surface and interfacial structure in the vicinity of the growing holes. SIMS experiments have been performed on selected samples. We observed that the dewetting process was drastically changed from homopolymers. We determined the scaling behavior of the dewetting velocity as a function of the weight percentage of the diblock copolymer dPS-PMMA (40k-40k). On 330k PMMA, as the concentration of block copolymer increases, dewetting velocity first decreases and reaches its minimum at 30wt% of block copolymer, and then rises as a result of increasing brush density. Within the error of the experiment, the dewetting velocity was found to be constant in time on all PMMA substrates. The experimental data is interpreted in term of slip-stick dynamics.
DETERMINATION OF DIFFERENT PHASES ON A PS-PEA GRAFT COPOLYMER FILM BY AFM STUDY. L. Guo, S. Ge, M. Rafailovich, J. Sokolov, SUNY, Dept. of Materials Science and Engineering, Stony Brook, NY; D. Peiffer, Exxon Research and Engineering Company, Annandale, NJ; S. Schwarz, Queens College, Dept. of Physics, Flushing, NY; R. Overney, University of Washington, Seattle, WA.
The dewetting of PS or PEA homopolymer on the ordered PS-PEA graft copolymer film is quite unique. The phase distribution behavior can be readily determined by the AFM force mode. Our work has shown that after annealing the bilayer samples the moduli of different phases can be readily determined, therefore characterizing the ``hardness'' of the ordered PEA backbones of the PEA-PS graft copolymers and identifying different components in the copolymers and homopolymers. It was found that the graft copolymer with more PS grafts per chain has higher modulus than that with less PS grafts per chain, which falls between that of PEA and PS homopolymer. Furthermore, this method also enabled us to see the phase behavior of the graft copolymers after dewetting, which visualizes the re-distribution of components in the copolymer during the dewetting process. For example, at the edge of the covered and uncovered area of the graft copolymer film with 3 grafts per chain the ordered surface structures undergo a sheer force by the dewetting upper layer of PEA. The island or hole structure disappears and is transformed into a totally flat ``transit area'', whose phase can be identified by AFM. On the other hand, graft copolymer surfaces covered by PS homopolymer undergo a totally different process and the islands/holes were expelled from the junction of the upper PS layer. The dewetting process for different copolymers will be discussed with regard to mechanism of the re-shaping of the ordered surface structure of copolymer and surface tension.
EXCIMER LASER INDUCED PHOTOCHEMICAL MODIFICATION OF POROUS PTFE FOR DESIRED PROPERTIES. Shinsuke Ito* and Masataka Murahara**; *Tokai University, **Faculty of Electrical Engineering, Tokai University, Kanagawa, JAPAN.
Fluorocarbon resin has excellent properties such as heat, chemical and electric insulation resistance. Therefore, this material is widely used in some fields. If small cavity of porous PTFE is successfully modified to be hydrophilic, it makes possible to produce a one-way liquid permeable filter and to apply to a liquid separation film or a membrane filter. It is difficult, however, to modify a fluorocarbon resin to be functional. We tried to demonstrate modification of porous PTFE, substituting desired functional groups not only on the surface but also small cavity to improve permeability. The porous PTFE was modified to be hydrophilic by using an ArF excimer laser as a light source and reaction solutions. In order to modify the small cavity to be hydrophilic, it is imperative for a reaction solution to be permeated into the inside. Therefore, we used an ethanol solution, whose critical surface tension is lower than that of porous PTFE, to modify the small cavity of porous PTFE. Ethanol solution dropped on the porous PTFE to form a thin layer. With vertical irradiation of ArF excimer laser light, F atoms on the exposed area were extracted to start a substitution reaction of desired functional groups. Then, the substituted functional groups were controlled by energy density. When the density was too low, a small amount of OH groups were substituted. On the contrary, when too high, no OH groups but lots of CH3 or C2H5 groups which were dissociated from ethanol were substituted. When further higher, nothing was substituted, and C-C bonds were formed. This reveals that by controlling energy density, only desired functional groups can be substituted on an exposed area of porous PTFE. Consequently, the contact angle with water of the modified sample was 40 degrees, whereas that of non-treated sample was 140 degrees. And the sample penetrated water about 40cc/cm2min. at 700 Torr.
ECONOMICAL SURFACE MODIFICATION OF FLUOROCARBON RESIN FOR HYDROPHILIC PROPERTY WITH PRELIMINARY MODIFICATION BY USING 172nm Xe2* EXCIMER LAMP. Ken Hatao* and Masataka Murahara**; *Tokai University; **Faculty of Electrical Engineering, Tokai University, Kanagawa, JAPAN.
Fluorocarbon resin is chemically a high stable polymer that consists of C F bonds. It has excellent properties of electric insulation, thermal and chemical resistance. With these properties, it is noticed for its possibility of applications as compound materials such as a high voltage resistance compound material and high frequency print circuit board. However, its surface repels both oil and water and has poor adhesion. We have photochemically modified a fluorocarbon resin surface into hydrophilic by using ArF excimer laser in order to improve its adhesive strength. In this paper, for simpler and safer modification, we successfully demonstrated a more efficient surface modification of fluorocarbon resin with a preliminary treatment. Water or B(OH)3 solution dropped on a fluorocarbon resin surface, which was covered with a fused silica glass to form a thin layer by a capillary phenomenon. Then, 172nm Xe2* excimer lamp light irradiated perpendicular to the interface between the fluorocarbon resin and the. reaction solution to modify preliminary. The fused silica glass was removed, and water or B(OH)3 was exclusively dropped onto the irradiation area; which was irradiated with ArF excimer laser (wavelength =193nm) perpendicularly. The hydrophilic property of the modifled surface was evaluated by measurements of contact angle with water. With preliminary modifcation time of three minutes, ArF excimer laser fluence of 10mJ/cm2 and laser shot number of 3000, we successfully achieved a contact angle of 50 degrees on the preliminary modified surface after the excimer lamp irradiation and a contact angle of 30 degrees on the surface further irradiated with ArF excimer laser, whereas the non-treated fluorocarbon resin surface has a contact angle of about 120 degrees.
ULTRATHIN FILMS OF POLYSILANES PREPARED BY FRICTION-TRANSFER TECHNIQUE. Nobutaka Tanigaki, Momoyo Wada, Akira Kaito, Yuji Yoshida, Kiyoshi Yase, Yoshikazu Tanabe, National Institute of Materials and Chemical Research, Tsukuba, JAPAN.
Polysilanes, whose main chains are composed of Si atoms, have attracted much attention as promising materials for opto-electronics. Aligning of molecular chain of the polysilane is very valuable because the electronic functions of the polysilane originate from delocalization of -electron along the Si-backbone. We prepared highly oriented thin films of polysilanes by the friction-transfer technique. Especially, poly(dimethylsilane) (PDMSi), which is known as an intractable polymer, provided ultrathin films with extremely high orientation. Pressing a piece of compressed polymer onto a smooth substrate, whose temperature was controlled, with a constant pressure and dragging it straight with a constant velocity afforded a film on the surface of the substrate. The film thickness was estimated to be several nm. The properties of films strongly depended on the preparation temperature. In the case of PDMSi, the films deposited at temperatures above 210C were uniform and highly oriented. Around the temperature PDMSi is known to undergo a phase transition. The analogous phenomenon was observed in the case of poly(diethylsilane). The friction-transfer of the polysilanes was closely related the order-disorder transition. It was found that polymer chains were aligned along the direction of the friction. The orientation was detected by polarized light absorption and fluorescence spectroscopy and X-ray and electron diffraction methods. X ray diffraction study of the ultrathin film of PDMSi revealed crystallites had a preferred orientation toward the substrate surface. In other word, the friction-transferred film has three-dimensional order. The highly oriented PDMSi has an abnormally large absorbance at 345nm. Moreover, it emitted fluorescence at 350nm, but the powder specimen emitted at 360nm. This suggests the oriented thin film has different electronic state from bulk polymer.
SURFACE INITIATED POLYMERIZATIONS OF CYLIC OLEFINS USING TETHERED ROMP CATALYSTS. Fatma Vatansever, Preeti Dhar, and Thomas A. P. Seery, Polymer Program and Department of Chemistry, University of Connecticut, Storrs CT.
Thin polymeric films can be formed from chain growth polymerizations when the initiator is covalently bound to the surface. Several novel systems of this type have been previously reported where the initiator is bound to a glass or gold surface. The previous work has utilized titanium alkoxide catalysts to prepare thiol covered gold nanoparticles coated with polyhexylisocyanate. Nanometer scale silica particles have been functionalized with silane coupling agents. The coupling agents have been chemically transformed into surface bound alcohols that also serve as initiating ligands in a titanium alkoxide polymerization. The results of these syntheses are characterized with GC-MS, NMR, IR, TEM and light scattering. This current effort utilizes ring opening metathesis polymerization (ROMP) catalysts for surface initiated polymerization. Pendant vinyl groups may be located at a glass surface commercially available silane coupling agents. These vinyl groups are then functionalized with a metal ROMP catalyst as the volatile products of that metathesis are removed under vacuum. Polymer layers can then form upon addition of strained cyclic olefins.
THIN POLYMER COATINGS ON SILICON OXIDE SURFACES FROM SURFACE INITIATED POLYMERIZATION. Preeti Dhar, Fatma Vatansever, and T.A.P. Seery, Polymer Program and Department of Chemistry University of Connecticut, Storrs, CT
The surface initiated polymerization (SIP) approach has been used to make thin polymer coatings directly on silicon oxide (glass) surfaces. The general technique of SIP involves attachment of an organic functionality on the surface of silica and then the generation of an initiating group through various organic transformations. Nanometer scale silica particles have been functionalized with silane coupling agents. These coupling agents have been chemically transformed so as to provide a hydroxyl group at the free end. This hydroxyl moiety then allows the attachment of a transition metal complex giving rise to an active polymerization catalyst, which in our case is the titanium trichloride alkoxide. Monomers are then polymerized through a chain growth insertion mechanism mediated by a transition metal catalyst. Since this is a living polymerization, the catalyst remains bound to the polymer until an end capping step cleaves the catalyst. The final step provides an additional degree of control over the surface composition. Due to the surface bound nature of the catalyst, synthesis of free polymer is avoided and excess monomer can be removed by rinsing with an appropriate organic solvent. All the stable intermediates have been characterized by ordinary spectroscopies, GC-MS, 1H-NMR, IR as well as by TEM and light scattering experiments.
ALUMINUM OXIDE MEMBRANES WITH MESOSCOPIC CHANNELS FORMED THROUGH SELF-ASSEMBLY. Hiroaki Imai, Ilhan A. Aksay, Department of Chemical Engineering and Princeton Materials Institute, Princeton, NJ; Nan Yao, Princeton Materials Institute, Princeton University, Princeton, NJ.
Anodic aluminum oxide membranes have been filled by infiltration of an aqueous solution of aluminum nitrate nonahydrate Al(N03)39H20, sodium dodecyl sulfate Me(CH2)11OSO3Na. This solution has been shown to yield a mesoscopic hexagonal phase of alumina through the self-assembly of the surfactants.l The infiltration process results in hexagonally packed mesoscopic tubules with an average diameter of 4 nm within the micro channels of the anodic aluminum oxide. Electron microscopic characterization of the tubules has shown them to be oriented along the channels of anodic alumina and thus providing a continuous path of nanoscale channels across the membrane.
FORCES BETWEEN BLOCK COPOLYMER BRUSHES AT SOLID-FLUID INTERFACES. S. Michael Kilbey II, M. Tirrell, F.S. Bates, University of Minnesota, Dept of Chemical Engineering and Materials Science, Minneapolis, MN.
The surface forces apparatus was used to measure structural forces between two polymer brush covered surfaces in both a good solvent and a near-theta solvent. The brushes self-assemble through preferential adsorption of polystyrene - polyvinylpyridine block copolymers from toluene. These amphiphilic block copolymers adsorb strongly through the PVP block causing the well-solvated PS block to swell away from the surface in order to relieve lateral crowding. The equilibrium force profiles (intermolecular force versus separation distance) in toluene show that the height of the brush layer is several times its free solution radius-of-gyration of the tethered chains. The force profiles in the good solvent coalesce onto a universal profile based on Alexander - de Gennes scaling arguments. Changing the solvent to cyclohexane near the theta temperature produces the expected contraction of the PS cilia as evidenced by the shorter ranges of the force profiles. Despite the contraction, the brushes in near-theta solvent remain stretched a few times the free solution radius-of-gyration of the PS chain. These equilibrium force profile data show that brushes in a near-theta solvent obey the same scaling as brushes in a good solvent, and coalesce to their own unique universal profile.
EXPLORING THE LIMITS OF SELECTIVE ADSORPTION IN THE PATTERNING OF LAYER-BY-LAYER ASSEMBLED POLYION FILMS. Sarah L. Clark, Paula T. Hammond, Massachusetts Institute of Technology, Dept of Chemical Engineering, Cambridge, MA.
We have establisted micropatterned self-assembled monolayers (SAMs) as a method for templating layer-by-layer ionic assembly of polyions. Polydiallyldimethyl ammonium chloride (PDAC) and sulfonated polystyrene (SPS) have been used as polyion pairs for these investigations. Previous studies have shown that patterned SAMs of COOH and (OCH2CH2)3OH terminated alkanethiols direct the adsorption of SPS and PDAC to deposit on the COOH regions. At an alternate set of solution and processing parameters the adsorption is directed onto the (OCH2CH2)3OH regions. We explore the limits of how this potentially useful ''reverse deposition'' can be manipulated to control the final architecture of features deposited as ionic multilayers. We also will explore the limits of the templating technique as it relates to the number of patterned layers, the size and shape of the patterned regions, and the surface functionality of other patterned SAMs.
ADSORPTION KINETICS AND GEOMETRY OF POLYSTYRENE-POLYETHYLENE OXIDE DIBLOCKS ADSORBED ON SILICON IN POOR SOLVENT CONDITIONS AS A FUNCTION OF CONCENTRATION. Shenda Baker, Mark Perri, J. Johannes, M. Bishop, J. Smythe, Harvey Mudd College, Claremont, CA.
An asymmetric diblock polystyrene-polyethylene oxide (PS-PEO) was adsorbed on a silicon reflection element from cyclohexane at concentrations ranging from 0.01 to 0.2 mg/ml at 22 C. The half life of adsorption and surface excess were determined by FTIR in the attenuated total reflection mode. The same concentration cyclohexane solutions were used to adsorb the diblock onto silicon wafers from cyclohexane. Additional films were prepared by adsorption from toluene solutions of 0.1 mg/ml. These films were imaged in cyclohexane and air by atomic force microscopy (AFM). For increasing concentrations of the diblock, the half-life of adsorption decreases; however, the higher concentration isotherms require a fit including diffusion limited process. This rearrangement and subsequent PEO adsorption appears to be greater at higher concentrations although the overall time required to reach an equilibrium surface coverage decreases. AFM images suggest that the polymer adsorbs in mounds consistent with minimal aggregation or limited phase separation of the PS from the solvent.
CHEMICAL STRUCTURE OF AMORPOUS MODIFICATORS AND PHASE TRANSITION IN THE THIN FILMS OF POLY(ETHYLENE OXIDE). Albina H.Hasanova, Anna I. Suvorova, Irina S.Tyukova, Alexei B.Burdin, Ural State University, Ekaterinburg, RUSSIA.
Melting temperatures (T(m)) and phase transitions of poly(ethylene oxide) (PEO) mixed with amorphous poly(butyl methacrylates) having an isomeric structure of alkyl-radical in the monomer unit were determined by DTA, small angle light scattering and polarization microscopy. PEO has a dense molecular packing and crystal structure. The glass transition temperature of the poly (n-, iso-, tert- butyl acrylates) in the series of these polymers increases regularly with increasing volume of the butyl-radical. In accordance with increasing of the molecular volume of the amorphous component its compatibility with PEO in the blends becomes worse. The compatibility parameter chi(23) based on the Flory-Huggins theory was estimated using the value of T(m) by the Nishi-Wang method.
8:30 AM *N6.1
ADHESION OF SOFT GELS TO RIGID SURFACES. Kenneth R. Shull, Cynthia Mowery, Alfred J. Crosby, Dongchan Ahn, Northwestern University, Dept of Materials Science and Engineering, Evanston, IL.
One important characteristic of low modulus materials is that their shapes can be significantly distorted by relatively weak forces associated with equilibrium surface interactions. We have developed an ideal model system for studying these effects. Our approach is based on the use of a very low modulus material in conjunction with a linear elastic fracture mechanics analysis based on the treatment of Johnson, Kendall and Roberts (JKR.) Because the deformations are quite large in our case, modifications to the original JKR theory are required. We have used finite element methods to develop a set of equations from which the adhesion energy between a soft elastic solid and a rigid surface can be obtained. Adhesion experiments have been performed on low modulus lenses formed by diluting a triblock copolymer with 2 ethyl-hexanol. The triblock copolymer has poly(methyl methacrylate) end blocks and a poly(n-butyl acrylate) midblock, with the midblock being preferentially dissolved by the solvent. Rheological studies of this swollen copolymer indicate that the material is completely elastic at room temperature and undergoes a rapid, thermally reversible gelation, thus making it an excellent model system. For this low modulus material, the applied loads are too low to measure directly. Instead, we obtain expressions for G/E, the adhesion energy normalized by Youngís modulus. Comparisons to rheological data show that this analysis provides an accurate, yet simple method for obtaining this information. Our approach has great potential for quantifying the adhesion of a variety of soft materials, including, for example, hydrogels and biological tissues.
9:00 AM N6.2
CONNECTOR CHAIN AGGREGATION EFFECT IN ELASTOMER-ELASTOMER ADHESION PROMOTION. Christian Ligoure, James L. Harden, Groupe de Dynamique des Phases Condensées, Université Montpellier II, Montpellier, FRANCE.
We consider the effects of chain aggregation during pull out on the adhesion of elastomer-elastomer surfaces fortified with interfacial connector chains. For the case of monodisperse end-grafted A homopolymer chains, at an A/B elastomer interface, we show that lateral aggregation of connector chains in the gap into tethered micelle-like structures occurs at characteristic length scale 1/ set by the grafting density . This is a first-order transition characterized by a metastable single-fibril state separated from a state of arbitrarily large aggregates by a gap-size dependant energy barrier As a result, the threshold fracture toughness Go as a function of is predicted to have a plateau reflecting a regime of spontaneous connector chain extraction induced by aggregation. Experimental access to this plateau depends on the rate of A/B elastomer separation, due to the slow kinetics of the aggregation instability. For the case of polydisperse connector brushes, aggregation processes are predicted to occur on many length scales, leading to a possible enhancement of adhesion promotion compared with monodisperse brushes. We briefly highlight this effect with several examples of polydisperse connector brushes and pseudobrushes.
9:15 AM N6.3
A REVERSIBLE EFFECT OF A RANDOM COPOLYMER MIXTURE ON ADHESION OF IMMISCIBLE HOMOPOLYMERS. L. Guo, M. Rafailovich, J. Sokolov, SUNY, Dept. of Materials Science and Engineering, Stony Brook, NJ; D. Peiffer, Exxon Research and Engineering Company, Annandale, NJ; S. Schwarz, Queens College, Dept. of Physics, Flushing, NY; A. Eisenberg, McGill University, Montreal, CANADA.
Recent work has shown that random copolymers can be effective in enhancing the interfacial toughness of immiscible homopolymers by segregating to the interface, depending on the composition of the copolymer. A layer of two mixed PS-PVP random copolymers with compositions at two extremes was shown to increase significantly the interfacial toughness (Gc) of PS/PMMA interfaces when annealed at T=180C. Lowering the annealing temperature to 120C reduced Gc to Gc=12 J/m2. This reversible effect was observed to have at least two cycles before it weakens out, which is caused by the diffusion of the random copolymer into the homopolymer bulk. The reason for this effect is that the mixture of the two copolymers undergoes a phase separation process when the temperature decreases. At a higher temperature (180C) the mixture of copolymers is in the one-phase region and the homopolymers are bonded together by the two well connected copolymers since they segregate to corresponding side of homopolymers. At a lower temperature (120C) the mixture is in the two-phase region and the homopolymers are debonded as the two copolymers undergo phase separation. This unique effect makes it possible to design a temperature sensitive mechanical switch on a molecular level. Measurements of Gc using a double cantilever technique were made and correlated to SIMS and Neutron Reflection measurements of the phase distribution in the mixture of the two copolymers at different temperatures using one copolymer with a deuterated component.
9:30 AM *N6.4
LOCATION AND MECHANISM OF RUPTURE IN THIN ADHESIVE FILMS. Arlette R. C. Baljon, Dept of Chemical Engineering, Cornell Univ, Ithaca, NY; Dilip Gersappe, Mark O. Robbins, Johns Hopkins Univ, Dept Physics and Astronomy, Baltimore, MD.
Molecular dynamics simulations have been used to study the rupture of a thin adhesive film between rigid solid walls. The film contains linear chain molecules whose length N is varied. Rupture may occur either at an adhesive/solid interface or within the adhesive film. Equilibrium arguments predict that the final state should minimize interfacial free energy, but we find that failure occurs where the yield stress is smallest. This location is nearly independent of N, but the strength of the bond rises rapidly with N. The work needed to rupture the bond increases linearly with the amount of dissipation during rupture. We find that the velocity dependence of the work is directly correlated with the viscous response of the adhesive film. The work increases linearly with velocity in simple fluid films and is independent of velocity in low temperature, glassy films. In glassy films, molecules move through a sequence of rapid rearrangements with high instantaneous velocities. We find three distinct stages of rapid motion. When the film is stretched beyond a critical value, it becomes unstable against density fluctuations and cavitation occurs. Next, the cavities grow and merge through a series of rapid structural rearrangements as the local yield stress for plastic flow is exceeded. In the final stage, the few remaining bridges spanning the junction rupture. These types of rapid motion have direct counterparts in rupture of practical adhesives: cavity nucleation, plastic flow and crazing. When the chains are shorter than the entanglement length, the final bridges are simple single strands that span directly across the junction. Chains that are four times the entanglement length develop a complex craze structure. A network of interconnected fibrils forms. Each is a few nanometers thick and contains several chains. Fibrils merge and split to form crosslinks that are spaced by a length that is comparable to the entanglement length.
10:30 AM N6.5
ADHESIVE FAILURE IN THERMOSETTING POLYMERS. Mark Stevens, Sandia National Laboratories, Albuquerque, NM.
The failure of thermosetting polymer adhesives is an important problem for which there is little understanding particularly from the molecular viewpoint. I have performed molecular dynamics simulations of network polymers that model epoxy systems. The polymers are modeled as bead-spring chains. The network structure is dynamically formed by crosslinking from a melt. This is important since the structure is inhomogeneous especially in the interphase region. A crack initiation occurs by applying a tensile stress to the system. The local stress is calculated and compared to the continuum stress determined by fracture mechanics. The connection between the network structure and the stress distribution will be discussed.
10:45 AM N6.6
MODEL OF CRAZE PROPAGATION IN A THIN POLYMER FILM. Tom N. Krupenkin and Glenn H. Fredrickson, University of California, Santa Barbara, CA.
The problem of crazing of a thin polymer glass film is theoretically investigated. A simple model of craze propagation based on the assumption about scale dependence of polymer surface tension is presented. Craze propagation velocity, fibril size, and fibril separation are calculated as a function of applied stress, film thickness, temperature and model parameters. The obtained results are in good qualitative agreement with experiment.
11:00 AM N6.7
ADHESION AND DEBONDING OF POLYMER/SILICON INTERFACES IN MICROELECTRONIC PACKAGING APPLICATIONS. Jeffrey Snodgrass, Irwin Liu, Reiner Dauskardt, Department of Materials Science and Engineering, Stanford University, Stanford, CA .
The increasing complexity of microelectronic packages means that product reliability is often dependent on the integrity of interfaces between the chip (silicon) and the package materials (polymers). These interfaces may often be exposed to high thermomechanical stresses during both manufacturing and use. Unfortunately, little quantitative data about silicon/polymer interface adhesion exists. Our research characterizes the adhesion of bimaterial interfaces using a fracture mechanics approach. Critical interface adhesion values are measured by driving a stable debond along the polymer/silicon interface. A sandwich structure is utilized for the sample geometry, thereby constraining the polymer of interest between two rigid silicon substrates, which prevent the relaxation of residual stresses during testing. Techniques for enhancing interface adhesion, including both chemical adhesion promoters and tailored interface morphology, are evaluated.
11:15 AM *N6.8
ADSORPTION OF HIGHLY CHARGED POLY(STYRENESULFONATE) ON HYDROPHOBIC SURFACES. Olivier Theodoly, Claudine E Williams, Condensed Matter Laboratory, CNRS and College de France, Paris, FRANCE.
It has been found that random copolymers of styrene and sulfonated styrene (PSS) form an adsorbed layer on hydrophobic surfaces even though the polyelectrolyte chain is highly charged (linear charge fractions between 0.4 and 0.8, well above the onset of counterion condensation). We will first present surface tension measurments at the air-solution interface which allow us to examine the surface excess and the kinetics of adsorption and desorption as a function of charge density for polymers. We will then show a characterization by AFM of various stages of formation of wet films formed on Si wafers coated with a self-assembled hydrophobic monolayer. The amount of polyelectrolyte adsorbed is a strong function of the ionic strength of the solution. The solution is then removed, the film is washed with pure water until no chain desorbs and the remaining stable film characterized again. The unique behaviour of PSS will be discussed in terms of strong hydrophobicity of the backbone and compared to the unusual features of the polymer in solution.
11:45 AM N6.9
LATERAL SEPARATION OF POLYELECTROLYTES IN MICROLITHOGRAPHIC ARRAYS. Deniz Ertas, Department of Physics, Harvard University, Cambridge, MA.
A new approach to separation of a variety of microscopic and mesoscopic objects in dilute solution is presented. The approach takes advantage of unique properties of a specially designed separation device (sieve), which can be readily built using already developed microlithographic techniques[W. D. Volkmuth and R. H. Austin, Nature 358, 600 (1992).] Due to the broken reflection symmetry in its design, the direction of motion of an object in the sieve varies as a function of its self-diffusion constant, causing separation transverse to its direction of motion. This gives the device some significant and unique advantages over existing fractionation methods based on centrifugation and electrophoresis.
SESSION N7: POLYELECTROLYTES & BIOPOLYMERS
Chair: Kenneth R. Shull
Wednesday Afternoon, December 3, 1997
1:30 AM *N7.1
SPONTANEOUS FORMATION OF LIPOSOMES WITH GRAFTED POLYMERS. I. Szleifer, Department of Chemistry, Purdue University, West Lafayette, IN.
Liposomes are closed spherical bilayer, formed by lipid molecules, that have potential use as drug delivery systems. Liposomes containing polymers grafted on their surfaces have been shown to have extended longevity in the blood stream. The increased longevity is believed to be due to the steric repulsion that the grafted polymer layer presents to proteins in blood and to cells. These liposomes are in general prepared by extrusion or sonication methods, i.e. they are formed by the insertion of energy on the system. The phase diagram of spontaneous forming liposomes in mixtures of lipid and lipid-poly ethylene oxide (PEO) molecules has been calculated using the single-chain mean-field theory. The theory predicts the minimal composition of lipid-PEO necessary to have stable liposomes as a function of the chain length of the polymer (PEO) chain. The predictions of the theory have been tested with experimental observations and there is very good agreement between the two. The driving forces for spontaneous liposome formation will be discussed. The spontaneously formed liposomes are predicted to have an asymmetric distribution of polymer chains between the inner and outer monolayers. The theory has been applied for a variety of polymer chain chemical architectures. In each case the range of spontaneous forming liposomes will be shown, and the use of different polymer structures for targeted drug delivery and their ability to provide the necessary steric barrier will be described. Thermodynamic stabilization of metastable liposomes by grafting polymers on the outer surface of the liposome will be shown for a variety of conditions. The ability to control the size of the spontaneous forming liposomes by the choice of the polymer molecule will be discussed in detail.
2:00 PM *N7.2
COMPARISON OF GRAFTED LINEAR AND STAR POLYMER LAYERS FOR BIOMATERIALS APPLICATIONS. A.M. Mayes and D.J. Irvine, Department of Materials Science and Engineering, M.I.T., Cambridge, MA; L.G. Griffith, Department of Chemical Engineering, M.I.T., Cambridge, MA.
Grafted polyethylene oxide (PEO) chains are systems of growing interest to the biomaterials community for the preparation of implant surfaces resistant to nonspecific protein adsorption. However, the ability to prepare protein resistant surfaces using brushes of linear PEO chains is limited by the inherent difficulty in creating a dense layer of these chains due to steric exclusion when grafted from solution. One means to increase the grafted layer density may be to modify the molecular architecture of the grafted chains. Self-consistent field calculations comparing grafted star polymer layers to linear chains at equal grafting density suggest that grafted stars could provide a denser surface coverage and a high degree of arm extension into the solvent. Experimentally, however, the achievable grafting density for star systems was observed to be an order of magnitude lower than that of linear PEO. Neutron reflectivity, atomic force microscopy and protein adsorption studies were performed to examine the relationship between protein resistance and grafted layer density for both star and linear systems.
3:00 PM N7.3
PLANAR LIPID BILAYERS (BLMs) AS A CONFINED SPACE REACTOR FOR BIOMEMBRANE FUNCTIONS. H.T. Tien and A.L. Ottova, Membrane Biophysics Lab, Michigan State Univeristy, East Lansing, MI.
The approach of molecular design has been around for some time, the goal of which is to draft the structure of a compound with desired properties on paper before attempting actual synthesis. This approach differs from that which simply mimics natural products and improves upon them . Some of the compounds, for example, such as TCNQ, TTF, and modified fullerenes are cases in point. We have been embedding the aforementioned compounds into experimental BLMs (bilayer lipid membranes or planar lipid bilayers) and investigated their properties, mainly electrical, using cyclic voltammetry . It is worth noting that a planar BLM whose thickness is about 5 nm has been extensively used as a model of biomembranes. A planar BLM is not merely a physical barrier separating two aqueous phases (like a cell membrane interposed between the outside and inside of a cell), but serves as a ''reactor'' or transducer of a narrow confine for vital physicochemical reactions. Much work needs to be done in this field .
3:15 PM N7.4
TRANSPORT OF POLYNUCLEOTIDES THROUGH A PROTEIN ION CHANNEL. J.J. Kasianowicz, S.E Henrickson, NIST, Biotechnology Div., Gaithersburg, MD; E. Brandin, D. Branton, Dept. of Biological Sciences, Harvard U., Cambridge, MA; M. Akeson and D.W. Deamer, Dept. of Chemistry, UC Santa Cruz, CA.
Ion channels are proteinaceous pores which are well known for their ability to temporally and spatially regulate the flow of ions through nerve and muscle membranes. Some pore-forming proteins, like the alpha-hemolysin secreted by Staphylococcus aureus, are relatively large ion channels (diameter 2 nm) which can remain open virtually indefinitely. We recently showed that single-stranded homopolymers of poly[U] RNA and single stranded heterooligomeric DNA, but not double stranded DNA, can be forced through this channel by an applied electric field. The passage of individual polymers through the pore causes transient blockades in the ionic current. The duration of the blockades is proportional to the polymer length, which suggests that the polynucleotides traverse the pore as linear, unfolded chains. We will illustrate the effects of the applied potential, the solution viscosity and the type of homopolymer on the characteristics of the polynucleotide-induced single channel current blockades. In addition, the ability of polynucleotides and nonelectrolyte polymers to partition into this channel will be compared. Potential applications of this technology for polynucleotide characterization, including DNA sequencing, will be discussed. Supported by the NAS/NRC(JJK), NSF(DB), NASA (DWD) and the NIH (DWD).
3:30 PM N7.5
STRUCTURE OF CONFINED DNA-MEMBRANE COMPLEXES IN MICROCHANNEL ARRAYS. G.C.L. Wong, Y. Li, G. Subramanian, C.R. Safinya, Materials Research Laboratory, University of California, Santa Barbara, CA; E. Caine, E.L. Hu, Dept. of Electrical & Computer Engineering, University of California, Santa Barbara, CA.
We report a study of confined DNA-membrane complexes in microlithographic channel arrays using micro-focussed synchrotron x-ray diffraction techniques and optical microscopy. DNA and cationic liposome mixtures spontaneously self-assemble into a multilamellar structure where a periodic 1D lattice of parallel DNA chains is confined between stacked lipid sheets . The DNA lattice in such DNA-cationic lipid complexes are polycrystalline, with no positional or orientational correlations between the DNA chains across adjacent layers. Microlithographic channel arrays can be used to induce such correlations, due to confinement effects and surface interactions. We have found that DNA-cationic lipid complexes form fiber bundles with diameters ranging from 1 to 100 ,m, which can be manipulated and oriented using microchannels. Moreover, Bragg-Fresnel optics have been used to produce sub-micron x-ray beams for micro-diffraction studies of such DNA-membrane complexes confined within single microchannels. Potential applications of such biomolecular composites include the development of molecular sieves with controlled pore sizes (1-10 nm), nanolithography, and novel electrophoretic media. This work is supported by NSF grant DMR-9625977 and Office of Naval Research grant ONR N00014-93-1-0269.
3:45 PM *N7.6
BIOMOLECULE ADSORPTION AT SELF-ASSEMBLED SYNTHETIC SURFACES. Mark D. Foster, The University of Akron, Institute of Polymer Science, Akron, OH.
Adsorption of blood proteins at surfaces created by self-assembly has been studied using X-ray and neutron reflectivity (XR, NR) as well as scanning probe microscopy (SPM). Self assembly has been used to create surfaces of various functionality and character while the structure of the adsorbed layers has been probed with techniques which are truly capable of resolving gradients in structure on the length scale commensurate with the size of a single protein. With NR the dimensions of the adsorbed layer can be resolved in situ. In companion work the kinetics of the adsorption to these self-assembled surfaces has been studied using total internal reflectance fluorescence. Human serum albumin is found to adsorb more tenaciously to aliphatic SAMs of lower packing density than to more nearly perfect, high density aliphatic SAMs.
4:15 PM N7.7
SPONTANEOUS DENSITY INHOMOGENEITIES IN CONFINED POLYELECTROLYTE SOLUTIONS. M.A. Carignano and N. Dan, Dept of Chemical Engineering, University of Delaware, Newark, DE.
The density profile of dilute polyelectrolyte solutions confined between neutral parallel walls is studied using Monte Carlo simulations. We find distinct oscillations in the density profile of the polyelectrolyte chains. The wavelength of these oscillations corresponds to density correlations observed in bulk polyelectrolyte solutions.
4:30 PM *N7.8
PATTERN RECOGNITION BY POLYMERS. M. Muthukumar, Univ. of Massachusetts at Amherst, Dept. of Polymer Science and Engineering, Amherst, MA.
Theoretical analysis of efficiency of recognition of surface patterns by heteropolymer molecules at molecular length scales will be presented. The thermodynamic and kinetic issues relevant in macromolecular recognition will be compared with those of commensuration between surfce pattern wavelength and natural length of spontaneously organized microphase separated morphologies in bulk.
SESSION N8: PHASE TRANSITIONS IN CONFINED GEOMETRIES-I
Chair: Mark D. Foster
Thursday Morning, December 4, 1997
8:30 AM *N8.1
KINETICS OF PHASE SEPARATION IN THIN POLYMER BLEND FILMS BY ATOMIC FORCE MICROSCOPY. Alamgir Karim, Brett Ermi, Jack Douglas, Polymers Division, National Institute of Standards and Technology, Gaithersburg, MD.
The phase separation of ultrathin polymer blend films ( 100 nm) is examined for critical and off-critical blend compositions by atomic force microscopy (AFM). Ultrathin blend films were prepared by spin casting mixtures of deuterated poly(styrene)(dPS) and poly(vinylmethylether) (PVME) from toluene solutions onto HF etched Si substrates to obtain films of varying dPS composition in the range 0.1 - 0.55. Phase separation within the films results in undulations of the free boundary caused by in-plane variations of the surface tension between the fluid phases. These topographic changes allow for the observation of spinodal decomposition and nucleation-type growth at near critical and off-critical compositions, respectively. The kinetics of growth in intermediate - late stage phase separation in these films is examined and compared with bulk phase separation kinetics data and theoretical expectations.
9:00 AM N8.2
DYNAMICS OF PHASE TRANSITION AND SPATIAL CORRELATION IN PATTERN FORMATION OF POLYMER FILMS. Tao Huang, Tomohiro Tsuji, M.R. Kamal, A.D. Rey, McGill Univ, Dept of Chemical Engineering, Montreal, CANADA.
The dynamics of phase transition and spatial correlation in 2D semicrystalline polymer films during free solidification experiments have been investigated via direct imaging in real-time in-situ observations. A new model has been developed for the time evolution of the structure factor and Voronoi characterization of domain growth. The predications of the model agree with experimental results from the time-resolved FFT transform of digital image analysis. We present the growth dynamics and the direct geometrical measurements on experimental images, and provide insights regarding the fundamental mechanisms that control the inter-domain dynamics, nearest neighbor correlation of domain core centers, and the domain size distribution. The presented novel methods are suitable to characterize any static or dynamic droplet patterns and process in a wide range of polymer fields.
9:15 AM N8.3
COARSENING IN PHASE-SEPARATED, 2D POLYMER-SOLVENT SYSTEMS: THE ROLE OF INTERFACIAL TENSION IN THE CROSSOVER OF COARSENING MECHANISMS. Christopher K. Haas, John M. Torkelson, Northwestern Univ, Depts of Chemical Engineering and of Materials Science and Engineering, Evanston, IL.
Coarsening, or the growth in size scale of the dispersed phase domains, has been studied in two-dimensional (2D) phase-separated polymer-solvent systems. These experiments, based on polystyrene-diethyl oxalate, are the first to confirm expectations based on theory for coarsening in which the dispersed-phase domain sizes are comparable to or larger than the specimen thickness. In particular, for off-critical compositions, the growth-rate is seen to scale as the 1/3 power of time, while for near-critical compositions the growth-rate scaling is consistent with an initial 1/3 power in time followed by a crossover to an ultimate 2/3 power in time. This crossover in coarsening mechanism for near-critical systems, from one dominated by coalescence/Ostwald ripening to one dominated by hydrodynamic flow, is predicted theoretically to have a strong dependence of crossover domain size on interfacial tension. By measuring interfacial tension of polystyrene-diethyl oxalate via sessile drop methods, we have determined that the crossover domain size scales as the - 0.9 +/- 0.1 power of interfacial tension, in agreement with theory which predicts a -1 power. Other factors seen to affect crossover domain size, such as system thickness and composition, will also be discussed.
9:30 AM N8.4
KINETIC AND TOPOLOGICAL CONSTRAINTS ON MICROPHASE SEPARATION IN SOLUTION-CAST SBS TRIBLOCK COPOLYMER THIN FILMS. G. Kim and M. Libera, Stevens Institute of Technology, Hoboken, NJ.
Microphase separation in block copolymers are under intense study, because of the rich polymer-physics questions they raise and because of the need for better understanding required by adhesive, compatibilizer, and template applications. This paper describes the evolution of bulk and surface microstructure in -100nm films of polystyrene (PS) - polybutadiene (PB) polystyrene (SBS) cast from dilute toluene solutions under controlled solvent-evaporation rates. The 30% PS composition produces the expected morphology of PS cylinders arranged on a hexagonal lattice in a continuous PB matrix. The relative orientation of the cylinders with respect to the film plane is controlled by the solvent-evaporation rate. Vertical cylinders form at higher rates and in-plane cylinders form at lower rates. Deviations from a pure hexagonal array occur in proximity to the free surface. Quantitative analysis of digital images shows an increase in intercylinder spacing and cylinder radius while maintaining constant PS volume fraction in response to post-cast annealing. This finding is consistent with ideas proposed by the Hashimoto group concerning kinetic limitations during the final stages of room-temperature solvent evaporation which prevent the microstructure from achieving the cylinder spacing and size predicted by equilibrium thermodynamics. Similar quantitative analysis indicates that the PB surfaces layers formed are not pure but contain some amount of dissolved PS.
10:15 AM N8.5
SUBSTRATE TOPOGRAPHY-MEDIATED LATERAL PATTERNING OF DIBLOCK COPOLYMER FILMS. M.J. Fasolka, D.J. Harris, A.M. Mayes, Massachusetts Institute of Technology, Dept. Materials Science and Engineering, Cambridge, MA; M. Yoon, S.G.J. Mochrie, Massachusetts Institute of Technology, Dept. of Physics, Cambridge, MA.
We study the morphology of symmetric diblock copolymer films for film thicknesses below the bulk equilibrium period supported by both flat and corrugated substrates. In this thickness regime, the film morphology is characterized by the formation of lateral domains. On flat substrates, these domains are of uniform size, but randomly arraged. In contrast, similiar films on corrugated substrates form lateral domains which decorate the peaks of the substrate corrugations. Our observations suggest a novel and simple scheme for the general nanometer scale patterning of diblock copolymer films. Such phenomena show potential for use in the development of nanoscale thin film devices.
10:30 AM N8.6
MICRO- AND MACROPHASE SEPARATION IN COMB BLOCK COPOLYMER-LIKE COMPLEXES CONSISTING OF HOMOPOLYMERS HYDROGEN BONDED TO AMPHIPHILES. Janne Ruokolainen, Olli Ikkala, Helsinki Univ of Technology, Materials Physics Lab, Espoo, FINLAND; Mika Torkkeli, Ritva Serimaa, Helsinki Univ, Dept of Physics, Helsinki, FINLAND; Gerrit ten Brinke, Univ of Groningen, Materials Science Centre, Groningen, NETHERLANDS.
Block copolymers with chemically connected repulsive blocks are known to self-organize into different microphase separated nanostructures. We show that block copolymer-like microphase separated structures can be obtained even if the repulsive moieties are only physically connected, for example by hydrogen bonding. In poly(4-vinyl pyridine) hydrogen bonded to alkyl phenols we demonstrate order-disorder transitions using time resolved SAXS and dynamic rheometry. If charges are additionally incorporated to form a hydrogen bonded complex between pentadecyl phenol and poly(4-vinyl pyridinium methane sulphonate), a complex phase diagram is observed with the regions of microphase- and macrophase separation as well as the disordered phase.
10:45 AM N8.7
PHASE TRANSITIONS OF CHIRAL SMECTIC C* BLOCK COPOLYMERS: INTERPLAY BETWEEN MESOPHASE AND MICROPHASE ORDER. Wen Yue Zheng, Mitch Anthamatten, and Paula T. Hammond, Dept. of Chemical Engineering, MIT, Cambridge MA.
The juxtaposition of molecular and supramolecular ordering in liquid crystal block copolymers is expected to provide the key to a number of novel applications in electro-optical memory, mechano-optical sensors, smart materials, and displays. In this paper, we examine the influence of the liquid crystalline (LC) mesophase on block copolymer ordering, and the reciprocal effects of the block copolymer interface on LC properties. Chiral side chain liquid crystalline diblock copolymers have been synthesized and fully characterized; these materials are monodisperse, anionically polymerized block copolymers with a smectic C* side chain mesogen. The lamellar domains of the block copolymers appear to stabilize the layered smectic C* liquid crystalline phase, and destabilize the higher temperature smectic A and chiral nematic mesophases. These findings imply that the block copolymer interface plays an important role in the arrangement and the stabilization of the LC phase. On the other hand, lamellar morphologies are found in these polymers at assymmetric compositions more appropriate for cylindrical morphologies, perhaps due in part to the presence of the smectic phase and the nature of the polymethacrylate backbone. For the first time, we have observed that the presence of the LC phase can actually induce microphase segregation for LC side chain block copolymers. When the molecular weight is low, and the liquid crystal block content is high, the order-disorder transition and the LC clearing point occur at the same temperature. The block copolymer morphology in these cases is arranged to accommodate the focal conic superstructure seen in optical micrographs of this LC block copolymer. The superposition of order in oriented samples, including the relative arrangement of the smectic layers with respect to block copolymer lamellae will also be discussed.
11:00 AM N8.8
CRYSTALLIZATION WITHIN CONFINED E/EP/E TRIBLOCK COPOLYMER MORPHOLOGIES. Peter L. Drzal, and Peter Kofinas, Univeristy of Maryland, Dept of Materials and Nuclear Engineering, College Park, MD; John D. Barnes, National Institute of Standards and Technology, Gaithesburg, MD.
The lamellar morphology and crystallization of spatially confined ethylene chains within microphase separated block copolymer morphologies was investigated in a series of semicrystalline E/EP/E triblock copolymers and their blends with E homopolymers. The pure semicrystalline triblocks had a total molecular weight of 100,000 g/mole and form heterogeneous melts. The triblock copolymer systems and their blends with E homopolymers were subjected to high levels of plane strain compression using a channel die. The die was maintained at a selected constant temperature during the compression flow, and the load was applied continuously until the desired compression ratios was achieved. The compressed specimens were quenched under load to room temperature, followed by load release. The final compression ratio was determined from the reduction of the thickness of the samples. Two dimensional small angle x-ray scattering (SAXS) was used to determine the domain spacing and lamellar orientation relative to the specimen boundaries. The variation of the long period in the phase separated morphology with molecular weight, composition, and degree of orientation was also measured by SAXS . Wide angle x-ray diffraction (WAXS) pole figure analysis was used to determine the lattice unit cell orientation of the crystallized E chains with respect to the lamellar superstructure. The morphologies produced for the E/EP/E block copolymer systems were attributed to the proximity of the order-disorder temperature (ODT) to the processing temperature. The SAXS and WAXS experiments indicated that changes in the temperature and cooling rate of plane strain compression processing can be used to force lamellar populations to orient either perpendicular or parallel to the plane of shear, and then upon cooling the E block chains crystallize within the confined space of the amorphous lamellar microdomains. Semicrystalline systems offer the advantage that the crystallographic texture, which is eventually locked into the materials when cooled below Tm provides an independent set of clues regarding the orientation of the lamellae at the point when crystallization takes place. These results demonstrated clearly that rather dramatic morphology control is possible through changes in melt processing of semicrystalline block copolymers near the ODT.
11:15 AM N8.9
THIN FILMS OF DIBLOCK COPOLYMERS ON STRIPED SURFACES: COMMENSURATE TO INCOMMENSURATE TRANSITIONS. Gerald Pereira, David R.M. Williams, Australian National University, Canberra, AUSTRALIA.
It has recently become possible to produce flat striped surfaces with a periodicity which is close to the lamellar spacing in bulk diblock copolymer systems. In general each stripe will have a preference for one of the diblock components, thus coupling the diblocks to the surface modulation. We study a system composed of a thin diblock melt placed upon such a surface both analytically and by numerical computation. For stripes which are thicker than the bulk lamellar spacing a tilted phase is expected. For stripes which are thinner than the bulk lamellar spacing an interesting series of transitions takes place where the film spacing changes from the bulk value to the value imposed by the stripes. In between these regions is a region where the the spacing is not uniform but changes across the sample. This comptetition between the bulk spacing and the spacing favoured by the stripes is reminisecent of the Frenkel-Kontorova model of solid state physics. We thus expect that this system will also show commensurate to incommensurate transitions.
11:30 AM N8.10
LATERAL MICROPHASE SEPARATION OF DI- AND TRIBLOCK COPOLYMERS IN ULTRATHIN FILMS FORMING HYDROPHOBIC-HYDROPHILIC SURFACE ARRAYS. Joachim P. Spatz, Peter Eibeck, Stefan Mössmer, Martin Möller, University of Ulm, Organische Chemie III-Makromolekulare Chemie, Ulm, GERMANY.
Diblock copolymers show microphase separation in bulk and in thin films. In thin films (approx. 100nm) the highly ordered domain structures (e.g. lamellae) organize parallel to a substrate. Strong effort is directed to the question if such domains can be oriented perpendicular to a substrate in order to address the chemical different microdomains on the polymer-air interface for, e.g., molecular recognition processes or for lithography. We will demonstrate the lateral phase separation of an amphiphilic diblock copolymer in ultrathin films (5-10nm) at the polymer air interface. In ultrathin films, a polystyrene-b-poly(2-vinylpyridine) diblock copolymer on a polar substrate like mica or silicon oxide shows selective wetting of the substrate by P2VP forming a layer of 1-3nm and dewetting by the PS blocks. The PS blocks agglomerate to hexagonally ordered clusters of 5-30nm in height depending on the molecular weight. The periodicity and the number of agglomerated PS chains per cluster is controlled by the molecular weight of PS and the surface energy of the substrate, i.e., the unfavorable interaction between the hydrophobic blocks and the polar substrate is the controlling parameter of the lateral structured microdomains. In correlation to the diblock copolymer behavior in bulk, the lateral microphase separation in ultrathin films show an order-disorder transition by decreasing the molecular weight of the PS block and by increasing the P2VP layer thickness. The latter demonstrates the strong influence of the substrate on the microdomain organization. The variation of the cluster size, the variation of the cluster periodicity and the lateral order-disorder transition will be demonstrated by a series of different molecular weights of PS-b-P2VP diblocks. Triblock copolymers (PS-P2VP-PMMA) show a similar lateral microphase separation behavior in ultrathin films under the influence of polar substrates. The influence of the third block on the microdomain structure will be demonstrated, resulting in the exposure of three different polymer domains to the polymer-air interface.
11:45 AM N8.11
CONTROLLING PATTERNS IN THIN DIBLOCK COPOLYMER FILMS. J. Heier, J. Genzer, E. J. Kramer, University of California at Santa Barbara, Santa Barbara, CA; F. S. Bates, University of Minnesota, Minneapolis, MN; S. Walheim and G. Krausch, University of Konstanz, Konstanz, GERMANY.
It has been well established that the morphologies in thin films of symmetric diblock copolymers can be tuned by controlling the polymer/substrate interactions. We extend this idea to probe the behavior in thin films of poly(styrene-b-2-vinylpyridine) diblock copolymers on chemically heterogeneous substrates. Laterally patterned substrates consisting of stripes of hydrophobic and hydrophilic regions are fabricated via a microcontact printing technique by depositing self-assembled monolayers of H3C- and HO-terminated alkanethiols on gold. Scanning force microscopy shows that for initial film thicknesses of (n+0.5)L0, where L0 is the lamellar period of the block copolymer and n is an integer, the films are flat. Cross-sectional and plan view transmission electron microscopy reveals that the block copolymer forms well ordered lamellae parallel to the HO-terminated SAM with the PVP block wetting the substrate. The block copolymer layers on the H3C-terminated SAM are frequently oriented perpendicular to the substrate. For film thicknesses slightly larger than (n+0.5)L0, excess material accumulates only on layers formed above the H3C-terminated SAM. These effects can be enhanced by first confining the copolymer film to a constant thickness during an initial ordering anneal and subsequently reannealing the sample after removing the top confinement layer. We discuss the driving forces that lead to such morphologies.
SESSION N9: PHASE TRANSITIONS IN CONFINED GEOMETRIES-II
Chair: Alamgir Karim
Thursday Afternoon, December 4, 1997
1:30 PM *N9.1
SURFACE INDUCED SELF-ASSEMBLY IN THIN FILMS OF HIGHLY INCOMPATIBLE POLYMER BLENDS. Georg Krausch, Kevin C. Phelan, LMU Muenchen, Institut fuer Physikalische Chemie, Muenchen, GERMANY; Stefan Walheim, M. Boeltau, Ullrich Steiner, Universitaet Konstanz, Fakultaet fuer Physik, Konstanz, GERMANY.
We study the influence of confining surfaces on the phase segregation of strongly incompatible polymer blends. In the presence of a laterally homogeneous surface, the domain structures depend mainly on the interfacial energies and on the quality of the solvent for the respective phases. We give indication that a surface driven, highly anisotropic bilayer structure is formed in the early stage of film preparation. This layered structure then becomes unstable and transforms into a lateral domain structure via rapid dewetting of the top layer. We show that the lateral domain structure can be controlled by use of pre-structured substrate surfaces. We discuss the conditions under which such an alignment is possible.
2:00 PM N9.2
WETTING REVERSAL TRANSITION IN PHASE-SEPARATED POLYMER MIXTURES. Jan Genzer and Edward J. Kramer, Department of Materials, University of California at Santa Barbara, Santa Barbara, CA.
We investigate a wetting reversal transition in thin films of two-phase mixtures of poly(ethylene-propylene) (PEP) and its deuterated analogue (dPEP) on a substrate covered by a self-assembled monolayer (SAM) whose surface energy, , is tuned by varying the SAM composition. As increases from 21 to 24 mJ/m2, a transition from a 3-layer (air/dPEP/PEP/dPEP/SAM) to a 2-layer (air/dPEP/PEP/SAM) structure occurs at increasing Tc - Ttr, where Tc and Ttr are the critical and transition temperatures, respectively. We find that with decreasing the quench depth the 3-layer to 2-layer transition takes place at lower . As the system structure changes from 3-layer to 2-layer, the thicknesses of the dPEP-rich wetting layers at the air/mixture and mixture/SAM interfaces are found to smoothly increase and decrease, respectively, while the thickness of the PEP-rich layer (ca. one half of the total film thickness) does not change. In addition, the response of the system to subsequent quenches from the 3-layer to 2-layer and from the 2-layer to 3-layer regions, respectively, in samples cast on substrates with constant is also investigated. The dependence of the transition temperature on is predicted by a simple model using the experimental data on the surface energies of PEP/dPEP and estimates of the interfacial energy between PEP and dPEP.
2:15 PM N9.3
MOLECULAR AGGREGATION STATE IN ULTRATHIN FILMS OF POLYMER BLENDS. Tisato Kajiyama, Keiji Tanaka, Atsushi Takahara, Kyushu Univ, Dept of Materials Physics & Chemistry, Fukuoka, JAPAN.
The molecular aggregation states of immiscible [polystyrene/poly(methyl methacrylate)] (PS/PMMA) thin and ultrathin films were investigated on the basis of atomic force microscopic observation and X-ray photoelectron spectroscopic measurement. In the case of the (PS/PMMA) film of 25 micrometer thick, the air-polymer interfacial region was covered with a PS rich overlayer due to its lower surface free energy compared with that of PMMA and well defined macroscopic phase-separated structure was formed in a bulk phase. Also, in the case of the (PS/PMMA) thin film of 100 nm thick, the phase-separated structure that the PMMA rich domains sticked out of the PS rich matrix was formed at the film surface. The formation of the surface structure for the (PS/PMMA) thin film can be explained by the reason that chain conformation or chain aggregation structure is frozen at the air-polymer interfacial region before the formation of a PS rich overlayer due to the fairly fast evaporation of solvent molecules. On the other hand, the two-dimensional (PS/PMMA) ultrathin film of 10.2 nm thick did not show distinct phase-separated structure. When the film thickness became more thinner than 10.2 nm, the two-dimensional (PS/PMMA) ultrathin film of 6.7 nm thick showed fine and distinct phase separated-structure with the domain size of a few hundred nm. This structure can be designated as "mesoscopic phase-separated structure". The surface phase state for the two-dimensional (PS/PMMA) ultrathin films can be explained by the film thickness dependence of both interaction parameter and the degree of entanglement among polymer chains.
3:00 PM N9.4
COMPRESSION OF WATER-SOLUBLE POLYMER BRUSHES: PHASE CHANGES INDUCED BY CONFINEMENT. Edith M. Sevick, Research School of Chemistry, The Australian National University, Canberra, AUSTRALIA; Avi Halperin, Institut de Chemie des Surfaces et Interfaces (ICSI), Mulhouse, FRANCE.
Polymers which are capable of hydrogen bonding with a solvent are of fundamental interest and practical importance, particularly as water is our most abundant and natural solvent. A particular and recent application is the adsorption of water-soluble polymer onto blood cells to hide blood type, rendering universal blood. We are interested in the compression of hydrosoluble polymer layers and, in particular, how compression of the adsorbed layer changes the solvation of the polymer and impacts the colloidal stabilisation. Using the deGennes n-cluster model, we show that the compression of a hydrosoluble polymer brush leads to interesting phase behaviour within the body of the brush. With compression, poor solvency conditions can be induced in a brush which is otherwise swollen with solvent. The compression force profiles of such n-cluster brushes provide a signature of the phase behaviour. This suggest that surface force apparatus experiments might detail the applicability of hydrosoluble solution models to specific water soluble polymers.
3:15 PM N9.5
SHEAR INDUCED CONFORMATIONAL CHANGES OF TETHERED MACROMOLECULES. Phillip Schorr, Matthew Tirrell, Univ of Minnesota, Dept of Chem Engr & Matls Science, Minneapolis, MN.
In order to develop effective protective polymer coatings for materials or specialized lubricants, it is important to understand the effect of shear upon the structure and properties of tethered chains. Although the (compressive) normal forces between two neutral polymer layers is fairly well understood, numerous conflicting theories exist to explain the dynamic behavior of polymer brushes. We have recently explored the effects of high frequency shear upon grafted copolymer conformation using a modified surface forces apparatus. Monolayers of polystyrene-b-poly(2-vinylpyridine) (PS-PVP) were self-assembled from selective solvent onto a mica substrate. The force profiles between two such brushes are shown to become steeper and longer-ranged with increasing shear velocity. These findings suggest the polymer chains are stretched beyond their equilibrium brush height and that the segment density profile has become more uniform or ëstep-likeí. The phenomenon is described in terms of a shear-induced diffusion of polymer segments away from the surface.
3:30 PM N9.6
A MONTE CARLO STUDY OF THE CONFINEMENT EFFECTS ON COPOLYMER PHASE MORPHOLOGIES. R. S. Pai-Panandiker and J. R. Dorgan, Department of Chemical Engineering, Colorado School of Mines, Golden,CO.
Block copolymers have the unique ability to self assemble into a wide array of microstructures. The self assembly, which may be related to interfacial activity is affected by the geometrical constraints under which the microstructure is subjected. The incompatibility between the length scale of the microstructure and the confinement length scale induces frustration. In this study, the effects of confinement on the microstructure morphology of diblock copolymers are studied using Monte Carlo techniques. The problem is investigated through the calculation of conformational properties such as the end-to-end vector and radius of gyration, order parameters, and thermodynamic properties such as the heat capacity. Molecular visualization is used to show the process of microstructure formation. The microphase separation transition is studied for diblocks in a thin film geometry between hard parallel walls. In the case of symmetric diblocks, the stable lamellar morphology with its interface parallel to the walls is observed below the microphase separation temperature. However, the interplay between the two characteristic lengths of the problem ( length scale of the microstructure and the confinement length scale ) lead to ordering of the lamellae. In the case where the two lengths are compatible, lamellar order parallel to the surfaces is observed whereas when the two lengths are incompatible, a tilted lamellar structure is observed. Effects of composition ( asymmetric diblock copolymers ) and degree of confinement on the morphology of the microstructure in the weak as well as the strong segregation limit are presented.
3:45 PM *N9.7
POLYMERS ON HETEROGENEOUS SURFACES. L. Rockford, Y. Liu, T.P. Russell, Pllymer Science and Engineering Department, University of Massachusetts, Amherst, MA; S. Soong, M. Yoon and S. Mochrie, Physics Department, MIT, Cambridge, MA.
Using surfaces with a periodic crystal faceting in combination with a glancing angle evaporation technique, periodic, heterogeneous surfaces were prepared with characteristic length scales ranging from several tens of nanometers to hundreds of nanometers. The lateral size scale of the heterogeneities range from the dimensions of single polymer chains and greater. The amplitude of the surface roughness was 2 nm, much smaller than the size of a polymer chain. By varying the metal used in the evaporation, the nature of the surface can be easily altered. For the studies discussed here a periodic array of stripes were prepared with alternating polar and nonpolar interactions. For a polar surface, silicon oxide was used, whereas, for the nonpolar surface stripes of gold were used. In the case of polymer mixtures, the size scale of the phase separated morphology was reduced to the molecular level. In the case of diblock copolymers, an orientation of the microdomains normal to the film surface was achieved. Finally, it is shown that the periodic surface cold easily be replicated by common, glassy polymers which, in turn, provides a novel means of studying surface relaxation behavior.
4:15 PM *N9.8
FORMATION AND PROPERTIES OF NANO-CRYSTALS OF FLEXIBLE CHAIN ASSEMBLIES AT SURFACES. D.L. Allara, K. Seshadri, G.D. Hietpas, S.V. Atre, Depts. of Chemistry and Materials Science, Pennsylvania State University, University Park, PA;Y-T. Tao, Institute of Chemistry, Taipei, TAIWAN.
Common interfacial phenomena involving flexible chains, e.g., lubrication, wetting, adhesion and mass transport through barrier films, depend significantly upon the nano-scale structural and dynamic characteristics of the chains when constrained at the requisite solid surface. In order to probe these phenomena in detail, we have been developing methods for preparing nanocrystalline assemblies of flexible chains bound at solid surfaces and in this paper report on two systems: 1.) Formation (often reversible)of discrete molecular crystallites via spontaneous dewetting of self-organized monolayers upon sudden chemical changes at the film/substrate interface and 2.) metastable structural states of kinetically trapped polymethylene nanocrystals formed at gold substrate surface defects during surface-catalyzed polymerization of reactive methylidene intermediates. Interesting properties of these two systems and some of their relevant connections will be discussed.
4:45 PM N9.9
SURFACE ROUGHENING OF POLYMER GELS UNDER MECHANICAL CONSTRAINT. Atsushi Suzuki, Yasuhiro Kobiki, Hiromichi Suzuki, Gang Bai, Yokohama National University, Dept of Materials Science, Yokohama, JAPAN.
We report the surface roughening of polymer gels in submicrometer scale during the bulk volume phase transition. The gel was synthesized in a thin disklike shape, and one of the gel surfaces is chemically adhered on a glass plate. The spongelike domains were directly observed in water by tapping mode atomic force microscopy (AFM). The surface domain structure under mechanical constraint was found to be strongly affected not only by the nature of system (e.g., the degree of inhomogeneities of polymer networks introduced at gelation, etc.) but also by the bulk phase transition in response to the change in the environment conditions. The surface structure was reproducibly observed at exactly the same position during the phase transition, and the roughening was found to be reversible. The surface structure characterized by the spongelike domains was discussed in terms of the auto-correlation function, the root-mean-square roughness, and the power spectral density which were calculated from the AFM images. The present polymer gel surface is smooth and flat for the larger scale with millimeter size, while for the smallest scale limit it should be reflected by the random chain structure. As for the intermediate size scale from 10 nanometers to microns, it was found to depend on the characteristic domain size in the observed area. The present results suggest that the gel surface structures can be classified into three categories: the macroscopic pattern, the intermediate spongelike domain, and the molecular conformation. The respective structure may be introduced by the mechanical constraint (mechanical instability), the nature of system (e.g., polymer-solvent interaction at gelation, etc.) as well as the environment conditions, and the nature of molecules (molecular interactions), which can determine the characteristic length. The second intermediate structure could have an important role to explain some smart functions of polymer gel surfaces.
SESSION N10: ADSORPTION
Chair: Spiros H. Anastasiadis
Friday Morning, December 5, 1997
8:30 AM *N10.1
AMPHIPHILIC POLYMERS AT THE AIR - WATER INTERFACE. S. K. Peace, R. W. Richards, Interdisciplinary Research Centre in Polymer Science and Technology, University of Durham, Durham, UNITED KINGDOM.
When amphiphilic polymers are spread at the air-water interface there are two aspects of interest. Firstly, how does the polymer arrange itself and what changes in the organisation take place as the concentration of polymer increases? Secondly, are changes in surface viscoelastic properties attributable to organisational changes? The results of a neutron reflectometry and surface quasi - elastic light scattering investigation of graft copolymers of polymethyl methacrylate (backbone) and polyethylene oxide (grafts) at the air - water interface will be presented. The grafts are only of modest molecular weight, having 54 ethylene oxide units per graft. As the polyethylene oxide content in the surface region increases a more complicated model is needed to reproduce the observed reflectivity data. At high surface concentrations a three layer model of the surface region is needed. Greater depths of the aqueous subphase become penetrated by the polyethylene oxide grafts as the surface concentration increases. Surface quasi - elastic light scattering allows us to extract surface tension, transverse shear viscosity, dilational modulus and dilational viscosity at the capillary wave frequency. The dilational properties ( i.e. the in-plane viscoelasticity) show quite unique dependence on the surface concentration of copolymer which indicates that energy is being supplied to the dilational modes. An abrupt change in these properties at a defined surface concentration is attributed to penetration of the aqueous subphase by the polyethylene oxide grafts to an appreciable extent.
9:00 AM *N10.2
SURFACE ARCHITECTURES FROM LONG, FLEXIBLE POLYMERS ADSORBED STRONGLY FROM THE MELT. C.J. Durning, Columbia University, Department of Chemical Engineering and Materials Science, New York NY; J. Majewski and G. Smith, Los Alamos National Lab, Manuel Lujan Jr. Neutron Scattering Center, Los Alamos, NM.
Immobilized layers of poly-(methyl methacrylate) (PMMA) were produced by spin-coating moderately concentrated solutions onto hydroxylated quartz and annealing at melt conditions to erase the casting history. The annealed layers were then quenched to room temperature and unbound material was leached away in good solvent (benzene) to leave a residual, strongly-adsorbed layer. The architecture of such layers, either swollen in good solvent or collapsed and dried, was studied by neutron reflection. The effects of varying annealing time and molecular weight on the layer structure were studied. The subsequent penetration of the layer by a bulk PMMA melt was also studied. Spin-coating without annealing, followed by leaching in good solvent produces very thin layers, collapsed relative to those expected from ''trapping'' chains in the melt, with simple random-walk statistics. Annealing relaxes this initial non-equilibrium structure. After sufficiently long annealing before rinsing, the dry residual adsorbed layer thicknesses scale h = N0.6, close to the law expected from an immobilized random-walk model. Dried adsorbed layers from melts equilibrated against the wall showed a ``core'' density adjacent to the substrate somewhat above the bulk value, in agreement with recent data for D-PMMA thin films spin-coated on glass and annealed. At the higher molecular weights, dried films also exhibited depleted density at the free surface. The residual adsorbed layers, swollen in good solvent exhibit a highly extended structure, consistent with a power-law profile phi (z)=const. z-n, with n>0, rather than a step-profile characteristic of grafted brushes. Significant penetration of residual adsorbed layers by bulk melts of equal or lower molecular weight is also observed.
9:30 AM N10.3
HAIRY-ROD POLYIMIDE POLYMERS CONFINED AT THE AIR-WATER INTERFACE. Hyun Yim, Mark D. Foster, Kevin McCreight, Xiaoming Jin, Stephen Cheng, Frank Harris, The Univ of Akron, Dept of Polymer Science, Akron, OH.
Alkyl side chain substituted preformed polyimides exhibit unusual behavior when confined to a monolayer at an air-water interface. This behavior is thought to arise from the particular way in which the alkyl side chains attached to the biphenyl are arranged in a two-dimensional film. The pressure-area behavior of these monolayers is strongly dependent on compression rate and temperature, indicating interesting dynamic behavior. A marked side chain length dependence of the pressure-area curves suggests that the side chains can not readily project away from the water surface, but rather occupy considerable space at that surface at lower temperatures. Monolayers made from octadecyl substituted (C18) polyimide show two step transitions as temperature is increased or compression rate decreased. For C18, a sharp reduction in zero pressure area occurs between 20 and 24 oC. With a decrease in side chain length from octadecyl to hexadecyl (C16) to heptyl (C7), the change in zero pressure area with temperature becomes smaller.
10:15 AM N10.4
COMPARING THE PREDICTIONS OF THE SCALING THEORY OF POLYMER ADSORPTION TO EXPERIMENTAL RESULTS. Jacob Klein, Weizmann Institute of Science, Rehovot, ISRAEL; and Giuseppe Rossi, Ford Research Laboratory, Ford Motor Company, Dearborn, MI.
In the good solvent regime the scaling theory of polymer adsorption predicts that the sign of the force between surfaces carrying irreversibly adsorbed polymer depends on surface coverage. Specifically, saturated surfaces repel each other at all distances, while "starved" surfaces experience attraction at large separations, and repulsion when the surfaces are brought sufficiently close to each other. While, it has been known for some time, that experimental data collected using the surface force apparatus are in qualitative agreement with these predictions, no detailed quantitative comparison between theory and experiment has been presented thus far. This tally is devoted to such a comparison for surfaces bearing adsorbed polyethylene oxide in an aqueous solution. We are able to evaluate all parameters entering the theoretical description directly from bulk and single surface experimental measurements. The surface force profiles computed using these parameters compare very well both in magnitude and spatial scale with the experimental results.
10:30 AM *N10.5
POLYMER ADSORPTION AND DEPOSITION FROM SUPERCRITICAL FLUIDS. Shawn E. Conway,John H. van Zanten, Mark A. McHugh, Dept of Chemical Engineering, Johns Hopkins University, Baltimore, MD.
We are currently exploring polymer adsorption and deposition from supercritical fluids. We are particularly interested in relating polymer interfacial properties to the process conditions (composition, temperature, pressure) at which the polymer films are formed. These 'coating' experiments are monitored in situ with surface plasmon resonance spectroscopy followed by additional film characterization via Fourier transform infrared spectroscopy, scanning probe microscopy and contact angle measurements. Two potential benefits of working with supercritical fluids are (1) the ability to gently manipulate barely processable polymeric materials and (2) the replacement of environmentally hazardous solvents.
11:00 AM N10.6
ADSORPTION OF COPOLYMERS IN SELECTIVE SOLVENTS ON SURFACES COATED WITH COPOLYMERS, Chandralekha Singh, Galen T. Pickett and Anna C. Balazs, Department of Chemical Engineering, University of Pittsburgh, Pittsburgh, PA.
Using the self-consistent field calculations and the scaling theory, we investigate the adsorption pattern and the amount adsorbed for polymers in selective solvents adsorbing onto surfaces coated copolymers. In particular, we explore the effect of the sequence distributions of the grafted and free copolymers and the grafting density of the tethered copolymers. The amount of the free copolymer adsorbed near the surface shows a strong nonmonotonic dependence (the adsorbed amount vanishes at the minima) on the grafting density of the adsorbed copolymers when the solvophobic block is grafted. As a function of grafting density, the number of maxima in the amount adsorbed depends only on the number of solvophobic blocks in the grafted copolymer and is independent of the architecture of the free copolymers. For high grafting density the adsorbed amount either saturates at a large value or vanishes depending upon whether the grafted copolymer has an odd or even number of blocks respectively. The sequence distribution of the free copolymers determines the magnitude of the copolymer adsorbed and this adsorbed amount increases as the block length of the polymer increases. The results provide guidelines for forming patterned surfaces and for selective adsorption of copolymers.
11:15 AM N10.7
MOLECULAR ASPECTS OF TAILORING THIN FILM DEWETTING BY ADSORBING BLOCK COPOLYMERS. Robert Oslanec, Ana C. Costa, Russell J. Composto, Dept of Materials Science and Engineering, and Laboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, PA; Petr Vlcek, Institute of Macromolecular Chemistry, Prague; William A. Hamilton, Oak Ridge National Laboratory, Oak Ridge, TN.
The adsorption of poly(deuterated styrene-block-methylmethacrylate) (dPS-b-PMMA) at the polymer matrix/silicon oxide (SiOx) interface is studied using neutron reflectivity (NR) and forward recoil spectrometry (FRES). The short MMA block adsorbs to SiOx, whereas the long dS block (NB segments) extends into the matrix. We find that upon increasing the matrix/copolymer interaction parameter, the amount of adsorbed copolymer decreases. In contrast, as the number of segments (P) of a neutral, polystyrene (PS) matrix increases, the adsorbed amount increases rapidly and becomes constant for P > 2NB. At low P the layer thickness and interfacial width are large and decrease as P approaches 2NB. Because the matrix/adsorbed layer interpenetration is large at low P, we expect the adsorbed copolymer to strongly influence the macroscopic properties of the polymer film. We use optical and atomic force microscopies, to study dewetting kinetics of thin, low P polystyrene films from SiOx with and without PS-b-PMMA block copolymer. As in the FRES studies, the block copolymer contains short MMA and long S blocks. The dewetting kinetics of PS films is investigated by measuring the hole diameter as a function of annealing time. We find that the addition of adsorbing block copolymer slows down the dewetting rate by anchoring the polymer film to the substrate. These macroscopic results are correlated with the molecular aspects of block copolymer adsorption, such as the volume fraction profile and the coverage of the adsorbed layer measured by NR and FRES.
11:30 AM *N10.8
CRYSTALLISATION OF ADSORBED POLYMER MONOLAYERS. Günter Reiter, Institut de Chimie des Surfaces et Interfaces (ICSI-CNRS), Mulhouse, FRANCE; Jens-Uwe Sommer, Theoretische Polymerphysik, Univ Freiburg, Freiburg, GERMANY.
We present AFM results and corresponding computer simulations on the crystallisation patterns obtained from quasi 2-D monolayers of polyetheleneoxid adsorbed onto bare silicon wafers. The monolayers have been prepared by a dewetting process in the molten state. They were then crystallized at variable temperatures Tc below the melting point Tm. A characteristic width w of the finger-like branched patterns has been determined. w decreased exponentially as (Tm - Tc) increased. The patterns are explained by a simple model considering the interplay of transport on the surface and the probability of attachement to the crystal. Computer simulations based on this model could reproduce the experimentally observed features. Our results provide direct information on the crystallisation process of polymers and have also implication for cystallisation in the bulk. It has to be emphasized that w can be changed in a controlled fashion continuously from a nanometer to a milimeter scale by simply varying Tc. This opens fascinating possibilities for the fabrication of patterned substrates by simple means and at ambient conditions.
Chair: John H. van Zanten
Friday Afternoon, December 5, 1997
1:30 PM N11.1
THE SMART MATERIALS CHALLENGE. Robert R. Reeber, Army Research Office, Materials Science Division, Research Triangle Park, NC.
Hierarchical ordered arrays of atoms and molecules; metals, ceramics, polymers with or without biological origin; organo-metallic or otherwise chemically bonded, that provide stimulus sensing, signal processing and a controlled and useful, often non-linear, response have been defined as smart materials. Their material problems are delineated from those involved in the design and improvement of macroscopic or even MEMS-like devices and smart structures. Mesoscopic smart materials processing work at ARO and targeted technology will be described and a possible future program strategy for enhanced smart materials work will be outlined. The objective is to identify research and development impacting the autonomous action desired from new complex high entropy configurations of matter. These configurations have molecular modules that perform specific functions at some liquid, solid or vapor interface. Sensing by photoactive, chemically reactive, magnetic, biological and electromechanical activation; switching by surface and solid state phase transitions, negative expansion, entropic changes such as compositionally induced diffusion, and electrochemical differences are just a few of the possible avenues for stimuli detection, amplification and control. Hierarchical control, templating, self-assembly, understanding symmetry, size effects and critical dimension scales can be important for providing crystalline polymeric and ceramic textures as well as ordered arrays with controlled porosity. These could be of uniform size with uniformly controllable group variations. Such flexible and in other cases rigid ''skeletons'' would conceivably provide percolation paths for transduction mechanisms through organic electroactive materials yet unknown. Finally nano-scale controlled motion, induced color changes and chemical degradation in addition to lessons learned from evolutionary biology will continue to guide this challenging field.
1:45 PM *N11.2
POLYMERS & POLYMER MIMICS AT INTERFACES. G. Whitesides, Harvard University, Department of Chemistry, Cambridge, MA.
Abstract not Available.
2:15 PM *N11.3
DIFFUSE REFLECTANCE FROM ROUGH SURFACES. S.Y. Hobbs, S. Feldman and H. Hatti, General Electric Research and Devlopment Center, Schenectady, NY; J.T. Bendler, University of South Dakota, Rapid City, SD.
Injection molded thermoplastics are often painted to achieve precise control over color and gloss. Such coatings also provide resistance to abrasion and weathering. Because of increasing pressure to reduce costs and emissions associated with secondary finishing, however, it has become desirable build these characteristics into the base resin. The appearance of pigmented blends is typically affected by the optical characteristics of the outer 150 m of the sample although some qualities, such as gloss, may be dominated by features having dimensions of 1 m or less. With the advent of atomic force microscopy (AFM), it has become possible to make precise surface profile measurements in a routine fashion. The methodology for relating such analytical data to perceptual qualities such as gloss, haze or distinctness of image is much less well developed and represents a major technical challenge. In addition to developing an appropriate mathematical formalism for describing these relationships, it is also desirable to know more about the character of the roughness itself and how this changes under environmental exposure, etc. In this paper we discuss progress in developing a properly normalized theoretical expression for describing the overall reflectance from roughened surfaces. The calculated results are compared with data obtained on injection molded samples having a range of roughness scales. The surfaces were characterized by AFM and mechanical profilometry and relevant mathematical descriptors of the surface topographies were extracted by digital image analysis. The calculated and measured reflectance values were found to agree well for incidence angles of 20 and 60 over a wide range of reflected intensities. At higher angles (82), the measured reflectance values were observed to be more specular. The results of some preliminary attempts to model surface roughness development during accelerated weathering and its effect on surface reflectivity will also be discussed.
3:15 PM *N11.4
SIMULATION OF THE FORMATION,PROPERTIES, AND SWELLING OF POLYMER NETWORKS. Carol K. Hall, Nirupama R. Kenkare, Saad A. Khan, North Carolina State University, Department of Chemical Engineering, Raleigh, NC.
We use equilibrium discontinous molecular dynamics to simulate the formation and relaxation of endlinked polymer networks constructed from melts of tangent hard-sphere chains. Trifunctional networks of strand lengths 20,35,50,100,and 150 are constructed and relaxed at a volume fraction of 0.43. The mean-squared displacement of the crosslinks and chain inner segments,the dynamic structure factors, and the Rouse modes are analyzed to provide information about network structure and elastic properties. The networks are subjected to isotropic expansion/contraction to obtain pressure/volume data and the thermophysical properties are compared with those of melt systems. Simulation of network swelling in the presence of a hard-sphere solvent is performed and the results are compared to the predictions of the Flory-Rehner theory. The effects of network irregularities are also discussed.
3:45 PM N11.5
THERMOPLASTIC ELASTOMER GELS: A DIRECT BRIDGE BETWEEN POLYMER AND COLLOID SCIENCE. Jonathan Laurer, James Mulling, Richard Spontak, NC State Univ, Dept of Materials Science & Engineering, Raleigh, NC; Saad Khan, NC State Univ, Dept of Chemical Engineering, Raleigh, NC; Steve Smith, Procter & Gamble Co., Corporate Research Div, Cincinnati, OH; Jon Samseth, Inst for Energy Technology, Dept of Physics, Kjeller, NORWAY; Kell Mortensen, Risø National Lab, Dept of Solid State Physics, Roskilde, DENMARK.
Thermoplastic elastomer gels (TPEGs) consist of a triblock copolymer and a nonvolatile, low-molar-mass oil that is chemically compatible with the midblock of the copolymer. Copolymer self-organization occurs in a low-viscosity medium, which greatly facilitates both morphological study and material development. In this work, we examine the effects of composition, thermal history and shear on the morphological and property development of model TPEGs based on a compositionally symmetric poly[styrene-b-(ethylene-co-propylene)-b-styrene] (SEPS) triblock copolymer, as well as its unsaturated analog. Transmission electron microscopy and small-angle scattering have been employed to investigate the characteristics of swollen copolymer morphologies, copolymer micelles and grain boundaries, while dynamic strain rheology has been performed to ensure gel behavior and probe the dynamics of the copolymer microstructure at elevated temperatures and from different quench states.
4:00 PM N11.6
REACTION KINETICS AT POLYMER-POLYMER INTERFACES. Ben O'Shaughnessy, Columbia Univ, Dept of Chemical Engineering, Materials Science and Mining Engineering, New York, NY; Dimitrios Vavylonis, Columbia Univ, Dept of Physics, New York, NY.
In a large class of chemically reacting polymer systems, reactions between polymer chains carrying chemically reactive groups occur at an interface separating two bulk polymer phases. Of particular technological significance is the process of reactive blending where copolymers generated by reactions promote the mechanical mixing of the melts and reinforce the interface. We have developed a systematic theory of these interfacial reacting systems. Three classes of reaction kinetics are identified: mean field (MF), second order diffusion-controlled (DC2) and first order diffusion-controlled (DC1). MF and DC2 reaction kinetics are second order, with a second order rate constant k2. That is, the reaction rate per unit area Rt = k2 n2 is quadratic in n, the far field bulk density of reactive polymers. For the MF case k2 is a constant. For the DC2 case, k2 scales as the first time derivative of xt4 where xt is the rms displacement of a polymer reactive group after time t. By contrast, DC1 reaction kinetics are first order, Rt = k1 n with first order rate constant k1 = dxt/dt. This leads to various power laws for Rt with exponents which depend on whether entanglements are absent (Rouse dynamics) or present (reptation dynamics). For a given system, the behavior depends on the density n and the local reactivity, Q, of the reactive groups. We present a "phase diagram" in the Q-n plane: different regions correspond to different realised sequences of kinetic behavior. For example, for large Q and high density, the sequence is: MF,DC2,DC1. Physically, the short time DC2 kinetics are governed by reactive pairs which happen to be initially in close proximity. A diffusion hole of size xt develops in the 2-body correlation function. By contrast, in the long time DC1 regime a diffusion hole of size xt develops in the density field itself at the interface.
4:15 PM N11.7
LITHOGRAPHY WITH A MASK OF BLOCK COPOLYMER MICROSTRUCTURES. Christopher Harrison, Miri Park, Paul Chaikin, Dept of Physics; Richard Register, Dept of Chemical Engineering; Douglas Adamson, Princeton Materials Institute; Princeton University, Princeton, NJ.
Block copolymer microstructures are becoming increasingly useful for the fabrication of nanostructures. Using the spontaneous microphase separation of a block copolymer, a length scale ( 20 nm) is created that is difficult to access with conventional lithographic techniques. With an ordered lattice of spherical microstructures as a template and with suitable chemical modification, we have fabricated a periodic array of holes in silicon, silicon nitride, and germanium, with a density of 1 1011 holes per cm2, over regions as large as a three inch silicon wafer. By selecting an alternate chemical modification of the template, we have fabricated an ordered lattice of posts. We find that the substrate can greatly inhibit the ordering of a PS-PB block copolymer template by pinning the diene phase to the substrate, but this can be overcome by choosing a system with appropriate wetting constraints or alternatively, modification of the substrate by attaching a layer of polymer brushes.
4:30 PM *N11.8
POLYMERS CONFINED AT SURFACES. M. Rafailovich, J. Sokolov, Dept. of Materials Science, SUNY, Stony Brook, NY.
Abstract not available