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fall 1997 logo1997 MRS Fall Meeting & Exhibit

December 1 - 5, 1997 | Boston
Meeting Chairs:
 Harry A. Atwater, Peter F. Green, Dean W. Face, A. Lindsay Greer 

Symposium L—Complex Fluids and Biomaterials



Nily Dan V. Parsegian, Univ of Delaware National Inst of Health
David Weitz, Univ of Pennsylvania 

Symposium Support 

  • Brookhaven Instruments Incorporated

* Invited paper

Chairs: Alice P. Gast and Philippe Poulin 
Monday Morning, December 1, 1997 
Republic B (S)

9:00 AM *L1.1 

The disorder-order transition is a fundamental process of theoretical and technological interest. Complex fluids can be viewed, in one sense, as complicated multicomponent solutions having structure over varying length scales. On the other hand, some complex fluids such as charged colloidal suspensions or micellar fluids, can be seen as model systems for understanding the richness of the ordering process. The colloidal size of the fluid constituents promotes their study with optical and electron microscopy, light scattering and small angle x-ray and neutron scattering. The ordering behavior is often controlled with simple solution chemistry and the time scales for the crystallization process are in a convenient regime for monitoring their dynamics. In this lecture we discuss three scenarios of ordering in complex fluids that improve our understanding of the crystallization process. Charged colloidal suspensions provide a well-characterized system to investigate ordering of spherical particles interacting through repulsive potential energies. These systems are well-known to form face-centered-cubic (FCC) crystals at moderate ionic strengths and body-centered-cubic (BCC) phases at low salt. Micelles formed from diblock copolymers in a solvent selective for one of the blocks undergo similar crystallization processes with BCC and FCC phases found for long and short polymers respectively. These systems can show singly or doubly re-entrant disorder-order-disorder transitions due to the polymeric nature of the interparticle potential . Two-dimensional (2D) layers of the protein streptavidin order into a variety of lattices depending on solution pH. The resulting crystals take on unique morphologies due to their underlying structure and the dynamics of the crystallization process. These three systems illustrate the variety of ordering transitions that can be explored in complex fluids.

9:30 AM L1.2 
HARD SPHERES INSIDE VESICLES: DEPLETION FORCES AND MEMBRANE CURVATURE. A. D. Dinsmore, D. T. Wong, Philip Nelson, and A. G. Yodh, Univ. of Pennsylvania, Dept. of Phyics and Astronomy. Philadelphia, PA.

We have explored the interplay of membrane curvature and excluded-volume interactions among hard spheres inside vesicles of complex shape. Using optical microscopy, we followed the motions of nearly-hard-sphere polystyrene particles that were trapped inside rigid, multilamellar phospholipid vesicles. By itself, a 0.47-m-diameter sphere diffused randomly inside the vesicle. In the presence of much smaller hard spheres (0.08 m, 0.30 volume fraction), however, this sphere was pinned to the vesicle's surface and was pushed along the wall in the direction of increasing curvature. We measured the force along the wall as a function of the vesicle curvature and found good agreement with the excluded-volume (or depletion) model. We also demonstrate theoretically that adding spheres of two different sizes can induce spontaneous curvature of a flexible unilamellar vesicle wall. The results demonstrate a new mechanism by which the shape of a vesicle can alter the distribution of particles within it and - conversely - by which the particles can alter the shape of the vesicle.

9:45 AM L1.3 
FIELD INDUCED ASSOCIATION OF COLLOIDAL PARTICLES. San Lin and Seth Fraden, Brandeis University, Department of Physics, Waltham, MA.

The rheological properties of colloidal systems undergo dramatic changes in electric fields, with the viscosity of the suspension increasing with electric field. The applied field induces a dipole moment in the colloidal particles and for dilute aqueous suspensions of mono-disperse polystyrene spheres in high frequency AC electric fields, the spheres form pearl chains oriented along the field. Because the binding energies inside each individual chain are much stronger than the interacting energies between neighboring chains, chains of a given length can be treated as a chemically distinct species. Using the law of mass action, we obtain the binding energy of chains from the chain length distribution. The experimental results are compared with theory. More information is available at the Complex Fluids homepage:

10:30 AM L1.4 
VELOCITY FLUCTUATIONS IN DILUTE SEDIMENTING SUSPENSIONS. P.N. Segre, E. Herbolzheimer, and P.M. Chaikin, Exxon Research and Engineering, Annandale, NJ.

Using particle image velocimetry we have measured the sedimentation dynamics of non-Brownian colloidal spheres over a wide range of low concentrations, (), and sample cell sizes W. Fluctuations in the settling velocity show universal large-sacle finite-range correlations, in the form of swirls, which depend simple on volume fraction and particle radius and follow , where . In turn the predicted divergence of the velocity variance with increasing cell size W is cut off in a universal way as W exceeds the swirl size and is well represented by.

10:45 AM L1.5 
CORRELATIONS IN COLLOIDAL SILICA STUDIED BY X-RAY PHOTON CORRELATION SPECTROSCOPY. Lal Jyotsana, Doug Abernathy, Gerhard Gruebel and Loic Auvray, ESRF, Experiments Division, Grenoble, FRANCE.; Loic Auvray, LLB, CE-Saclay, Gif-Sur-Yvette, FRANCE.

We will report the dynamics of colloidal silica suspended in glycerol in thc q wave-vector range of 10-3 to 10-2 Å-1 as studied by the new technique of X-Ray Photon Correlation Spectroscopy (XPCS). At high concentration the dynamics reveals correlations among the silica particles and at low concentration these correlations disappear. These dynamic measurements agree with the results of the static structure factor measured by SmaIl Angle X-Ray Scattering (SAXS). Thus the new technique of XPCS extends the study of dynamics of silica colloids to larger q-range and also, to concentrations not easily accessible by dynamic light scattering methods.

11:00 AM *L1.6 
COLLOIDAL SUSPENSIONS IN ANISOTROPIC FLUID. Philippe Poulin, CRPP/CNRS, Bordeaux, FRANCE; David Weitz, T.C. Lubensky, H. Stark, D. Pettey, M. Zapotocky, Dept of Physics, Univ of Pennsylvania, Philadelphia, PA.

We study the behavior of colloidal particles that are suspended in a nematic liquid crystal. The orientational elasticity of the liquid crystalline material gives rise to a novel class of colloidal interactions which governs the structures and the stability of the particles. By using micron-sized aqueous droplets suspended in a thermotropic liquid crystal, we identify these interactions for different boundary conditions and geometries of confinement. Moreover, we determine the profile of these elastic forces by measuring the closing velocity of attractive ferrofluid droplets; the initial separation between the droplets being adjusted by application of an external magnetic field. Our measurements are in agreement with theoretical descriptions based on an analogy with electrostatics.

11:30 AM *L1.7 
PARTICLE-STABILIZED DEFECT GEL IN CHOLESTERIC EMULSIONS. Martin Zapotocky, Philippe Poulin, D.A. Weitz, T.C. Lubensky, Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, PA.

We report on experimental and theoretical investigations of a material composed of colloidal inclusions embedded in a cholesteric liquid crystal. This material exhibits novel properties due to the presence of a network of ''oily streak'' defects of the cholesteric order, with colloidal inclusions preferentially located at the nodes of the defect network. The network is stabilized by the presence of the inclusions, and vice versa, the presence of the network leads to the stabilization of a 3-dimensional colloidal structure. On appropriate time scales, the described defect structure can be viewed as a cross-linked network of elastic bonds, and gives raise to gel-like rheological behavior. We characterize the bulk rheological properties of the material, and discuss corresponding elastic network models.

11:45 AM L1.8 
DOPED HEXAGONAL PHASES. Laurence Ramos, Physique de la Matiere Condensee, College de France, Paris, FRANCE; Pascale Fabre, UMR CNRS-Elf Atochem, Levallois-Perret, FRANCE.

Magnetic colloidal particles of nanometric size can be incorporated inside the considerably dilated cylinders of a Iyotropic hexagonal phase: a mixed liquid crystal of a new type is thus realized, which is found to exhibit original properties, essentially resulting from the coupling between solid colloid and surfactant matrix. This coupling is first demonstrated by scattering experiments, both static and dynamic. Indeed, small-angle X-ray scattering data enable to show that the unidimensionnaly confined particles are depleted from the inner walls of the cylinders. On the other hand, dynamic light scattering evidences novel hydrodynamics modes for the hybrid hexagonal phase. Moreover, because of the strongly anisotropic spatial distribution of the magnetic particles, the doped system possesses a high anisotropy of magnetic susceptibility, which leads to fascinating behavior of the liquid crystal when submitted to a magnetic field of weak intensity.

Chairs: Randall Kamien and Helmut H. Strey 
Monday Afternoon, December 1, 1997 
Republic B (S)

1:30 PM *L2.1 
LIQUID CRYSTALS: A SELECTED REVIEW. T.C. Lubensky, Department of Physics and Astronomy, University of Pennyslvania, Philadelphia, PA.

Liquid Crystals, as their name implies are materials that have properties, such as the ability to flow, normally associated with liquids, and properties, such as optical anisotropy, normally associated with crystals. Relatively unknown to the materials science and physics communities prior to 1970, they have since had a profound influence on how we think about structure, symmetry, and what kind of materials are possible. Liquid crystalline phases have symmetries that fill the gap between the most disordered homogeneous, isotropic fluids and the the most ordered three-dimensional periodic crystals. Each of these phases provides a new example of the interplay between broken symmetry, elastic rigidity, low-energy hydrodynamic excitations, and topological defects. In addition, liquid crystals provide striking examples of phases, including the three-dimensional smectic phase and the two-dimensional hexatic phase, in which thermal fluctuations convert long-range order into power-law decay. This talk will provide a selected overview of advances in liquid crystals whose impact stretches across disciplines from biology to particle physics.

2:00 PM L2.2 
EXPERIMENTAL STUDIES OF CHIRAL MOLECULAR SELF-ASSEMBLY AND THERMODYNAMICS OF PHOSPHOLIPID TUBULES. Mark S. Spector, Jonathan V. Selinger, and Joel M. Schnur, Center for Bio/Molecular Science and Engineering, Naval Research Laboratory, Washington, D.C.

An interesting class of synthetic phospholipids containing photopolymerizable diacetylenic moieties in the acyl chains has been observed to self-assemble into hollow, cylindrical structures, or tubules, under appropriate conditions. We report on spectroscopic and calorimetric studies of the chiral structure of lipid tubules formed in mixtures of alcohol and water. Theoretical models have suggested a chiral basis for these structures. Circular dichroism experiments provide evidence that tubule formation is based on chiral packing of the lipid molecules. We find that the circular dichroism spectra of tubules formed in mixtures of alcohol and water depends strongly on the alcohol used. The relative spectral intensity of different CD bands correlates with the number of bilayers observed using microscopy. We find significant concentration and temperature dependence of the spectral intensities of the different CD bands. Depending on the type of alcohol, the alcohol/water ratio, and the lipid concentration, the tubules are found to melt either continuously or discontinuously. The CD and specific heat measurements can be explained in terms of two distinct thermodynamic processes: the discontinuous melting is a first- order transition of the lipid bilayers from the ordered phase to the disordered phase, while the continuous melting is a gradual process of transferring lipid from tubules into solution as the lipid solubility increases. These results show that the solvent has an important effect on the thermodynamics of lipid tubules, and that circular dichroism is a sensitive probe of tubule thermodynamics.

2:15 PM *L2.3 
CHIRAL INTERACTIONS AND STRUCTURES. Randall D. Kamien, Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, PA.

We present a microscopic analysis of the instability of the nematic phase to chirality when molecular chirality is introduced perturbatively. We show that for central force interactions the previously neglected short-range biaxial correlations play a crucial role in determining the cholesteric pitch. Finally, we discuss how the local liquid crystalline order can frustrate these correlations leading to weaker than expected interactions.

3:15 PM *L2.4 
CHIRALITY, STRUCTURE AND FREE ENERGY OF BIOPOLYMER LIQUID CRYSTALS. Helmut H. Strey, Rudi Podgornik, Adrian Parsegian, NIH, Bethesda, MD; Sarah L. Keller, UCSB, Santa Barbara, CA; Jin Wang, Eric Sirota, Exxon Research and Engineering, NJ.

Liquid crystals of DNA are the simplest model systems for DNA packing in cell nuclei (e.g. dinoflagellates) or in phage heads. In these systems typically just one extremely long (from up to 1m) DNA molecule forms a macroscopic liquid crystal. We present a structural study on highly oriented DNA liquid crystals measuring the three dimensional structure factor by small angle x-ray scattering using synchrotron radiation (Brookhaven National Labs, X10A). The density of the DNA mesophases was controlled by osmotic stress. This method allows us to measure the equation of state and consequently changes in the free energy in these systems. At high densities DNA orders in a columnar liquid crystal with short range and long range orientational bond order (line hexatic or N+6). The line hexatic phase is the three dimensional analog to the hexatic phase in two dimensions. In contradiction with theoretical predictions, this phase shows no measurable chirality. At lower DNA densities we found an intermediate region in which the DNA molecules show alternate tilts (herring-bone) with respect to the original orientation. This phase also shows two distinct peaks in the x-ray structure factor, a sharper one with a mixed two- and six-fold symmetry and a broader isotropic one. Since we hold all intensive variables (p,T,) fixed, the measured S(q) should come from a single phase. At that point it is not yet clear whether this structure corresponds to a new phase located in between a columnar (non chiral) and cholesteric (chiral) arrangement of the DNA molecules. At even lower pressures, we found a phase transition to a cholesteric phase with a single broad first order maximum in S(q).

3:45 PM L2.5 
OBSERVATION OF A FOUR-LAYER SUPERLATTICE IN A CHIRAL SMECTIC LIQUID CRYSTAL PHASE USING ANOMALOUS X-RAY DIFFRACTION. Ron Pindak, Bell Labs-Lucent Technologies, Murray Hill, NJ; Anne-Marie Levelut, Lab de Physique des Solides, Univ Paris-Sud, Orsay, FRANCE; Philippe Barois, Centre de Recherche Paul Pascal, Pessac, FRANCE; Peter Mach, Dept of Physics, Univ of Minnesota, Minneapolis, MN; Lars Furenlid, National Synchrotron Light Source, Upton, NY.

Ordinary x-ray diffraction cannot distinguish between variants of the chiral Smectic-C (SmC*) liquid crystal phases since the tilted molecules in the fluid layers of these variants have identical z-projected electron density profiles (z axis normal to the layers) with only the tilt direction changing between layers. Nonetheless, by using anomalous scattering, that is, working close to the absorption edge of an atom in the core of the molecule and taking advantage of the polarization coupling of the photons with the atom, classical extinction rules no longer hold and it should be possible to distinguish between the variants. To test this hypothesis, we studied a sulfur containing thiobenzoate compound which exhibits the whole sequence of SmC* variants including the antiferroelectric SmCA* phase. A 200 layer freely suspended film was used in Bragg geometry with a Si 111 monochromator to define the beam and Soller slits before the detector. Measurements were performed at the sulfur absorption edge (Eo=2474.8 ev). Above 103 C, two diffraction peaks were observed in a qz-scan corresponding to the first and second order reflection from the layer planes (qz = qo and 2 qo). Below 103 C, down to crystallization at 90 C, where the bulk is in the SmCA* phase, additional sharp and resolution limited peaks were seen at qz = 0.5, 0.75, 1.25, 1.5, 1.75, and 2.25 qo. These satellite peaks were unobservable further than 7ev from Eo. This diffraction is characteristic of a four layer superlattice with a helical pitch of four layers or pairs of molecules with alternating tilt directions (AF* phase). Further analysis of the polarization state of the anomalously diffracted x-rays is required to select between these alternatives. In summary, we unambiguously demonstrated that anomalous scattering can distinguish between the SmC* variants as well as observed the occurrence of a novel fourfold superlattice.

4:00 PM L2.6 
STERIC AND DIPOLAR TAIL INTERACTIONS IN SUBSTITUTED-ALKYL 4-(4'-DODECYLOXYBIPHENLY-4-YLCARBONYLOXY)BENZOATE AND 3-FLUORO-BENZOATE ESTERS. M.J. Watson, R. Pindak, Bell Labroatories, Lucent Technologies, Murray Hill, NJ; C.J. Booth, Sharp Laboratories of Europe Ltd, Oxford, UNITED KINGDOM; J.W. Goodby, School of Chemistry, The University of Hull, Hull, UNITED KINGDOM; present address: Cavendish Laboratory, The University of Cambridge, Cambridge, UNITED KINGDOM.

The prediction by Meyer et al and subsequent discovery of ferroelectricity in chiral smectic C* liquid crystals has generated a lot of interest both fundamentally and from the point of view of potential applications. More recently, three new modifications of the SmC* phase were observed showing antiferroelectric (SmCA* ) and ferrielectric (SmC* and SmC*) properties. Structurally, the SmC* phase is a modification of the SmC* phase in which the layers adopt an energetically more stable zigzag like structure, and the dipoles associated with the molecular tilt in one layer oppose the dipoles of he molecules in the neighboring layers. Thus, the unwound antiferroelectric arrangement has no net spontaneous polarization Ps, unlike the unwound parent ferroelectric state. The ferroelectric states can be interpreted as a phase which mediates the ferro- and antiferroelectric phases inasmuch as the layer packing resembles a regular pattern of two interpenetrating sublattices with opposite molecular tilt directions. Though there is much discussion on its precise nature, it is generally agreed that dynamic molecular pairing must be present to stabilize the SmC* and SmCA* phases. In study, we are concerned with the subtle molecular interactions which exist in the chiral smectic C* phase and its modifications. The spontaneous polarization, and its magnitude and switching behaviour, is inextricably linked to the molecular dynamics and structural arrangement of smectic layers which therefore makes it an appropriate handle for investigations on molecular interactions. It was concluded from Ps measurements on two related series of materials that the actual nature of the interactions stabilizing SmCA* as against SmC* is different. Moreover, XRD experiments performed on free standing films showed concretely that interlayer permeation increases with increasing SmC* and SmCA* phase stability. This is the strongest experimental support to date for the models where dynamic molecular pairing is essential for the formation of the smectic subphases.

4:15 PM L2.7 
SURFACE ANCHORING AND SURFACE MEMORY IN POLYMER/LIQUID CRYSTAL DISPERSIONS. Karl R. Amundson and Mohan Srinivasarao, Bell Laboratories, Lucent Technologies, Murray Hill, NJ.

Surface anchoring of liquid crystals at interfaces created by polymerization-induced phase separation were studied. Such surfaces are created during the formation of some polymer-dispersed liquid crystal (PDLC) films, for example. PDLC films have micrometer-scale drops of liquid crystal dispersed within a polymeric matrix. They can be switched with an electric field from a scattering to a transparent state, and are of interest for use in flat-panel displays. We found a temperature-induced anchoring transition in a series of polymers in contact with one liquid crystal. These surface anchoring transitions were used to demonstrate a surface memory, where an otherwise isotropic surface is rendered anisotropic by being in contact with the liquid crystal. Surface anchoring, including surface memory, has a dramatic effect on electro-optic properties of PDLC films. Surface memory is dependent on the chemical structure of the side group of the polymeric component. Also, it can be erased by heating the liquid crystal far above the nematic-to-isotropic temperature. Previous reports of surface memory involved imprinting by smectic and crystalline phases. Significantly, the liquid crystal in our studies exhibit only a nematic phase. Molecular mechanisms for surface memory will be discussed.

4:30 PM L2.8 

4:45 PM L2.9 
STUDY OF LIQUID CRYSTALS DROPS AT THE MOLECULAR SCALE. Sebastien Bardon, LCR, Thomson-CSF, Orsay, FRANCE; Marie Pierre Valignat, Francois Vandenbrouck, Anne Marie Cazabat, Condensed Matter Laboratory, College de France, Paris, FRANCE.

The anchoring and wetting properties of liquid crystals are closely related to the organization of the molecules in contact with the solid surface. We study this organization both at the mesoscopic and the molecular scales. We present results obtained with cyanobiphenyl compounds spreading on silicon wafers in the nematic phase. At the mesoscopic scale, thickness profiles measured by ellipsometry show that the equilibrium shape of the drops is a flat film (pancake) whose thickness varies from 15 nm to infinity depending on the temperature. We present a model that describes this shape as the result of a competition between spreading, elastic and anchoring energies. According to this model, the thickness of the pancake is proportional to the extrapolation length of the system. We present a study of the evolution of this thickness versus temperature and compare it to measurments performed in cell. At the microscopic scale, we present a study of the pre-wetting films spreading in front of the drop. Ellipsometry shows that a non dense monolayer spreads in front of a homogeneous film. These two films extend over millimeters after few hours of spreading. X-ray reflectivity allows us to determine the organization of the molecules in the homogeneous film : a smectic like bilayer lies on a monolayer with tilted molecules. The total thickness of this film is 50 angstr– m. Tapping mode atomic force microscopy provides information with a spatial resolution at least three order of magnitude better than the two other techniques. This would be interesting to characterize the new multi-oriented pixels displays. Furthermore, this has allowed us to observe a mechanism of islands nucleation in front of the homogeneous pre-wetting film. To our knowledge, such a mechanism has never been observed before.

5:00 PM L2.10 
BROAD-BAND DIELECTRIC SPECTROSCOPY OF NEMATIC AND SMECTIC LIQUID CRYSTALS CONFINED IN RANDOM POROUS MEDIA. G.P. Sinha and F.M. Aliev, Dept. of Physics and Materials Research Center, University of Puerto Rico, San Juan, PR.

The dielectric behavior of alkylcyanobiphenyls confined in porous glasses (pore sizes - 100 Å and 1000 Å) has been investigated by means of ultra broad-band dielectric spectroscopy in the frequency range from 10-4 Hz to 109 Hz. New features such as two relaxational processes with characteristic frequencies around (1 - 10) Hz and (105) Hz, which are absent in bulk phase, were observed. These processes arise due to the relaxation of interfacial polarization at pore wall - LC interface and the hindered rotation of the molecules near the interface respectively. Two bulk like modes due to the rotation of polar molecules around their short and long axii were also observed in confined geometries and investigated in isotropic, nematic and smectic phases. Confinement strongly modifies these modes. The Debye type relaxational processes describing these modes in bulk is replaced by stretched exponential decay function and broad spectra of relaxation times appear. The temperature dependencies of relaxation times in pores are also different from their bulk behavior. In the temperature range corresponding to the anisotropic phase in pores , where is the relaxation time corresponding to the molecular rotation around it's short axis, is not a linear function of 1/T. However in limited temperature ranges close to bulk nematic range is reasonably well approximated by a linear function. The corresponding activation energy Up is greater than the activation energy of bulk Ub nematic phase. This difference Up - Ub is 0.15 eV, from which we estimate the surface potential of molecule-wall interaction . We also found that all the dielectrically active modes were not frozen even at 25 C below the bulk crystallization temperature.

Chairs: Robert S. Cantor and Igal Szleifer 
Tuesday Morning, December 2, 1997 
Republic B (S)

8:30 AM *L3.1/K3.1 
PROTEIN ADSORPTION ON SURFACES WITH GRAFTED POLYMERS. I. Szleifer, Department of Chemistry, Purdue University, West Lafayette, IN.

The prevention of protein adsorption on surfaces in one of the important requirements in the design of biocompatible materials. In recent years it has been shown that grafting of polymer molecules to the surface of the biomaterial may be very effective in preventing protein adsorption. In this talk the factors that determine the ability of the grafted polymers to prevent protein adsorption will be discussed. A systematic theoretical and experimental study of the ability of grafted poly-ethylene oxide (PEO) to prevent the adsorption of lysozyme and fibrinogen will be shown. The theoretical studies are based on the application of single-chain mean-field theory. The predictions of the theory are in very good agreement with the experimental observations. Furthermore, the theory provides a microscopic understanding of the mechanism of protein adsorption (and/or rejection) on surfaces with grafted polymers. It is found that the most important parameter in controlling protein adsorption is the polymer surface coverage, the polymer chain length has only a minor effect. It is found that in hydrophobic surfaces the grafted PEO also adsorbs to the surface and thus, it prevents protein adsorption by covering adsorbing sites for the proteins. In the cases of surfaces that PEO does not adsorb, like PEO-lipid liposomes, the polymer is less effective in preventing protein adsorption and the mechanism is based on steric repulsion. The time scale for the proteins to reach the surface is predicted to be much faster in hydrophobic surfaces than for the surfaces that do not attract the PEO segments. The effect of changing the structure of the polymer molecules in preventing protein adsorption will be shown for a variety of polymer chemical architectures.

9:00 AM L3.2/K3.2 
ENHANCED BLOOD COMPATIBILITY OF SILICON SURFACES COATED WITH SELF-ASSEMBLED POLYETHYLENE GLYCOL FILMS. Miqin Zhang, Mauro Ferrari, University of California at Berkeley, Biomedical Microdevices Center, Department of Materials Science and Mineral Engineering, Berkeley, CA.

The covalent attachment of Self-Assembled (SA) polyethylene glycol (PEG) coating on silicon surfaces was investigated. The PEG coatings were immobilized by the introduction of silanol groups on the PEG chain ends, which react with the hydroxyl groups on silicon surfaces. The coated films were characterized by contact angle measurement, ellipsometry, and electron spectroscopy for chemical analysis (ESCA). The interaction between treated surfaces and blood components was investigated to assess the performance of the modified surface. The protein adsorption and platelet adhesion were investigated by ellipsometry and optical microscopy, respectively. The PEG modified silicon surfaces showed excellent blood compatibility, including both the reduction of plasma protein adsorption and the suppression of platelet adhesion.

9:15 AM L3.3/K3.3 
PLASMA PROTEIN ADSORPTION AND PLATELET ADHESION ON POLY[BIS(TRIFLUOROETHOXY)PHOSPHAZENE]. Michael Grunze, Alexander Welle, University Heidelberg, Institute of Applied Physical Chemistry, Heidelberg, GERMANY; Dsidra Tur, Academy of Sciences, Institute of Organoelement Compounds, Moscow, RUSSIA.

Poly[bis(trifluoroethoxy)phosphazene] (PTFEP) is a biocompatible material [1] used as bulk material in medical implants. We developed a process to coat surfaces with PTFEP films and performed ELISA experiments designed to understand their blood compatibility. Coatings with different chemical properties (hydroxylated glass, amino-, methyl- and aldehyde- terminated silane films) were used as reference materials. We observed that PTFEP adsorbs preferentially Albumin from blood plasma and only small amounts of coagulation / inflammation stimulating proteins like Fibrinogen, von Willebrand Factor, Fibronectin and Immunoglobulines. In general, there is a good correlation between increasing coverage of adsorbed Albumin and reduced in vitro platelet adhesion. However, there is no direct correlation between the amount of Fibrinogen and platelet adhesion, restricting the use of Fibrinogen as a marker protein for platelet adhesion. An important prerequisite of blood compatibility is the stabilization of the native state of adsorbed proteins, since denaturated proteins stimulate platelet adhesion. Elution of adsorbed proteins by sodiumdodecylsulfate solution was used to quantify the amount of irreversibly attached and presumably denaturated proteins. PTFEP showed, compared to the reference materials, a low amount of irreversibly adsorbed proteins of the coagulation cascade. Circular Dichroism measurements of adsorbed Fibrinogen showed the same trend of protein denaturation at the surface. It was found that the secondary structure of adsorbed Fibrinogen is altered to a lower extent on PTFEP as compared to the reference materials [2]. We conclude that PTFEP has a unique blood compatibility because of the favorable composition and the stabilization of the adsorbed protein layer. We suggest that the trifluoroethanol moieties of PTFEP are responsible for these effects.

9:30 AM L3.4/K3.4 
MELTING AND INTERACTIONS IN MULTILAYERS OF TWO-DIMENSIONAL CRYSTALS OF MEMBRANE-PROTEIN BACTERIORHODOPSIN. I. Koltover, T. Salditt, J.O. Rädler, C.R. Safinya, Materials Department, Physics Department and Biochemistry and Molecular Biology Program, University of California at Santa Barbara, Santa Barbara, CA; K.J. Rothschild, Physics and Molecular Biophysics Laboratory, Boston University, Boston, MA.

Bacteriorhodopsin (bR), a light-driven proton pump, is an integral membrane protein of bacterium Halobacterium Salinarium. In the native bacterial membrane, bR self-assembles into regular hexagonal crystalline arrays in the plane of membrane. This two-dimensional (2D) protein crystal undergoes a fully reversible melting transition as a function of temperature. We have conducted a synchrotron x-ray diffraction study of oriented multilayers of bR-containing native bacterial membrane patches, as well as stacks of novel giant (50 diameter) single-crystal fused bR membranes. The precise in-situ control of humidity and sample temperature combined with the line-shape analyses of x-ray diffraction peaks allowed us to elucidate and control the intra- and inter-membrane protein-protein interactions. The important findings are as follows. First, the ordered 2D self-assembled lattice of proteins is found to exhibit diffraction patterns characteristic of a 2D solid with power-law decay of in-plane positional correlations, which allows to measure the elastic constants of protein crystal. Second, the melting temperature is a function of the multilayer hydration, with two distinct regimes of melting: lower hydration regime of orientationally commensurate membranes and high hydration regime of completely decoupled membranes. Third, preparation of nearly perfect (mosaicity<0.04circ) multilayers of fused bR membranes permitted, for the first time, application of powerful interface x-ray scattering techniques to a membrane-protein system, revealing the membrane corrugations and elucidating correlations of proteins at different hydration levels. Supported by NSF grant DMR 962091, and the Petroleum Research Fund (No.31352-AC7) to CS and NSF grant MCB-9419059 and ARO-SBIR/AmberGen contract to KJR.

9:45 AM L3.5/K3.5 
FLUCTUATIONS OF ACTIVE MEMBRANES. Jean-Baptiste Manneville, Patricia Bassereau, Jacques Prost, Physico-Chimie Curie UMR168, Institut Curie, Paris, FRANCE.

Incorporating pumps or ion channels into lipid bilayers allows us to study the out-of-equilibrium dynamics of fluctuating active membranes. Theoretical calculations (1,2) predict that fluctuations of such `active' membranes should differ both qualitatively and quantitatively from that of `passive' membranes, ie. membranes without any pumping activity. We observe the dynamical properties of fluctuating giant phospholipid visicles containing bacteriorhodopsin (BR), a light activated proton pump. Using micropipets techniques described by Evans et al. (3), we study the effects of BR incorporation and activation on the bending stiffness of the membrane. Modifications of the fluctuation spectrum upon BR activation is investigated by reflexion interference contrast microscopy (RICM) (4).

10:00 AM L3.6/K3.6 
OBSERVATION OF ION CHANNELS IN VESICLES UNDER CONTROLLED TENSION USING A MICROPIPET-ELECTRODE WITHOUT GIGA-SEAL. M. Gouliana, O.N. Mesquitab, D. Kuchnir Fygensona, E. Mosesb, C. Nielsenc, O.S. Andersenc and A. Libchabera,b. aRockefeller University, bNEC Research Inst., Princeton, NJ and cCornell University Medical College, Ithaca, NY.

We observe the effect of membrane tension on cross-membrane dimerization of the membrane-bound peptide gramicidin A. Gramicidin dimers form a simple ion channel when their chains ( 15 amino-acids) align tail-to-tail in the two leaflets of a bilayer membrane. The resulting pore is selective for monovalent cations. Kinetics of gramicidin pore formation have been studied extensively in different membranes, revealing a dependence on bilayer thickness. We observe gramicidin kinetics in large, single-component (DOPC), unilammelar vesicles using a micropipet without the standard ``giga-seal''. Absence of a giga-seal makes the membrane tension a well defined control parameter. We find the rate of channel formation increases with increasing tension in the membrane. We interpret this as a consequence of tension induced thinning of the bilayer, since the length of a gramicidin dimer is less than the relaxed bilayer thickness.

10:45 AM L3.7/K3.7 

Variations in the composition of cell membranes can strongly influence the function of proteins embedded therein. However, in most cases it is not known whether lipids and other membrane components act by binding directly to proteins, or indirectly through changes in a structural or thermodynamic property of the fluid bilayer. In the present work, a simple thermodynamic analysis is developed based on the hypothesis that variations in membrane composition induce changes in the distribution of transverse pressure in lipid bilayers. If protein function involves a conformational transition accompanied by a depth-dependent change in its cross-sectional area, it is predicted that a small redistribution of lateral pressures can induce a large shift in the protein conformational equilibrium. The sensitivity of this equilibrium to the lateral pressure profile arises predominantly from the localization of the large interfacial free energy within a domain of molecular thickness. Lattice statistical thermodynamic calculations are used to predict the effect of cholesterol and lipids of different length, stiffness and head groups on the pressure profile. Of particular interest are small interfacially-active solutes, as a model for general anesthetics: a strong correlation is found between anesthetic potency and increased lateral pressure near the aqueous interface.

11:00 AM L3.8/K3.8 
THE NATURE OF THE INTERACTIONS BETWEEN LIPIDS AND ENGINEERED LUNG SURFACTANT PROTEINS. Ka Yee C. Lee, Michael M. Lipp, Joseph A. Zasadzinski, Dept. of Chemical Engineering, University of California, Santa Barbara, CA; Alan J. Waring, MLK/Drew Medical Center and Perinatal Labs, Harbor-UCLA, Los Angeles, CA.

A mixture of lipids and proteins, commonly known as lung surfactant (LS), lines the pulmonary air spaces and facilitates breathing by reducing the surface tension inside the lungs. A deficiency of LS at birth causes neonatal respiratory distress syndrome, a deadly disease which is difficult and costly to treat. Research on replacement therapy has made significant progress; exogenous LS improves oxygenation when administered to premature infants. However, human LS-specific proteins are difficult to obtain, and concerns over immunological responses and viral contaminants make animal proteins sub-optimal. One LS-specific protein (SP-B), important in replacement therapy, has a net positive charge of 8 and is thought to improve the surface activity of LS by preventing anionic lipids from being squeezed-out from the film during exhalation. It appears that the key to the activity of SP-B protein is a balance between cationic and hydrophobic residues in the form of amphipathic a-helical sequences. To test this hypothesis, we have used fluorescence microscopy and isotherm measurements to study the interactions between anionic lipids and 6 different synthetic proteins: (1) full-length SP-B1-78; (2) truncated SP-B1-25; (3) a 21-residue peptide with repeating leucine and lysine residues in the ratio 4:1 (KL4); (4) a SP-B1-25 analog with charged residues replaced by neutral serine residues; and 25-residue homogeneous sequences of (5) hydrophobic leucine residues and (6) cationic lysine residues. We have discovered that the protein sequences containing amphipathic a-helical sequences (1 and 2) lead to an optimal surface activity upon addition to anionic lipid monolayers. Fluorescence microscopy reveals that this activity is due to the ability of these sequences to form a fluid phase network surrounding the condensed lipid phase domains which is able to persist to low surface tensions. This information may help to facilitate the design of simpler and cheaper peptide sequences for LS replacement therapy.

11:15 AM L3.9/K3.9 
A REVERSIBLE, 2- TO 3-DIMENSIONAL PHASE TRANSITION AT HIGH SURFACE PRESSURE IN MONOLAYERS OF PULMONARY SURFACTANT AND SYNTHETIC CONTROL MIXTURES. W. R. Schief, Jr., V. Vogel , Dept of Bioengineering, University of Washington, Seattle, WA; B. M. Discher, Dept of Biochemistry, and S. B. Hall, Dept of Medicine, Oregon Health Sciences University, Portland, OR; D. W. Grainger, Dept of Chemistry, Colorado State University, Fort Collins, CO.

A thin film floating on a water layer lining the lung interior, pulmonary surfactant is a complex mixture of lipids, proteins, and cholesterol which reduces the work of inhalation and stabilizes the small, spherical air sacs (alveoli) against collapse upon exhalation. Our research focuses on revealing the molecular mechanism(s) by which lung surfactant provides the mechanical stabilization, crucial knowledge for the improvement of synthetic lung surfactant administered to premature infants. Employing Langmuir monolayers as a model system, we have combined light scattering-, fluorescence-, and Brewster angle microscopy (BAM) with a high performance teflon ribbon trough to study the phase behaviour of Calf Lung Surfactant Extract (CLSE) and synthetic lipid mixtures as controls over a wide surface pressure range, from 0 mN/m to a plateau in the surface pressure vs. area diagram at 40-50 mN/m. We document the formation at low pressure, and the persistence over a wide pressure range, of condensed monolayer domains in CLSE. Upon compression onto the high pressure plateau, a new 3D phase nucleates in the liquid-expanded region of the monolayer, coexisting with the condensed domains. The new phase appears as bright discs in both fluorescence and BAM. Quantitative analysis of the BAM images indicates that the discs have bilayer thickness and rest above the monolayer. The redistribution of fluorescent probe from the region of liquid-expanded monolayer to the discs suggests that the degree of chain order is lower in the discs. Expansion and recompression of the monolayer shows that this 2D to 3D phase transition is reversible. Experiments reproducing the transition with binary mixtures of DPPC and cholesterol eliminate the possibility that it is an artifact of sample contamination, and further, demonstrate a strong destabilizing impact of cholesterol. Our observations provide new molecular-level information on the high pressure instability of lung surfactant monolayers. We will discuss areas of agreement and disagreement with existing theories of in vivo lung surfactant function.

11:30 AM L3.10/K3.10 
CAMPYLOTAXIS: CURVATURE DRIVEN MEMBRANE-PROTEIN DYNAMICS. L. Mahadevan, Peter T.C. So, and Seth Newburg, Dept. of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA.

The transport and aggregation of membrane proteins are crucial in regulating cellular functions. However, the detailed interaction between membrane forces with the embedded proteins remains poorly understood. Experimental methods used for such studies include fluorescnt photobleaching recovery, fluorescent single particle tracking and gradient optical trap. The consensus emerged from these diverse methods of examination is that membrane protein motion is regulated by multiple mechanisms. Transient confinement by obstacle clusters and tethering to the cytoskeleton, directed motion driven by molecular motors and free random diffusion all play important roles. In addition to these mechanisms, regions of protein aggregation has been observed to correlate with regions of high membrane curvature. In a physical context, protein mobility may be controlled by the regulation of membrane curvature. While a single molecular motor provides precision control of individual receptors, membrane curvature is a global control mechanism whereby many proteins can be simultaneously directed. The goal of this project is to develop theoretical and experimental models to elucidate the motion of biological molecules driven by local regulation of membrane curvature.

11:45 AM L3.11/K3.11 
STRUCTURAL ANALYSIS OF HISTIDINE-TAGGED CAPSID PROTEINS USING METAL-CHELATING LIPID MONOLAYERS. J. Mc Dermott, E. Barklis, Oregon Health Sciences University, Dept. of Microbiology; S. Wilkens, U. Oregon, Institute of Molecular Biology; Y. Rui, D. Thompson, Purdue University, Dept. of Chemistry, West Lafayette, IN.

We have developed a system for analysis of histidine-tagged (his-tagged) retrovirus core (Gag) proteins, assembled onto lipid monolayers consisting of egg L--phosphatidylcholine (EPC) plus the novel synthetic lipid, 1,2-di-O-hexadecyl-sn-glycero-3-N-(5-amino-1-carboxypentyl)iminodiacetic acid (DHGN). AAS indicates that DHGN binds nickel ions. Mixed DHGN/EPC monolayers specifically bind gold conjugates of his-tagged proteins. Two dimensional arrays of HIV and Moloney leukemia virus capsids were crystallized at the air-water interface and analyzed by cryoTEM. Image analysis of these arrays provides 2D capsid structures that are resolved to 20 and 9.5, respectively. The implications of these structures on viral assembly processes will be discussed.

Chair: A. Campbell 
Tuesday Afternoon, December 2, 1997 
Republic B (S)

1:30 PM *L4a.1/K4a.1 
DESIGN PRINCIPLES FOR CONTROL OF CELL FUNCTION BY BIOMATERIALS DESIGN. D.A. Lauffenburger, Dept of Chemical Engineering & Ctr for Biomedical Engineering, MIT, Cambridge, MA.

It is now well-established that improved design of biomaterials requires understanding of how they will interact with cells. Primarily these interactions occur via interactions of materials-associated biochemical ligands with cell surface receptors, generating chemical and physical processes governing cell behavioral responses such as proliferation, adhesion, migration, and differentation. Hence, a central focus of biomaterials design must be aimed toward controlling these ligand/receptor interactions according to quantitative physicochemical principles relating them to cell responses. Although the identities of receptors, their corresponding ligands, and associated signaling pathways involved in these cell responses are becoming increasingly well-known, merely presenting particular ligands on a materials surface is insufficient for controlling desired cell functions. Rather, quantitative aspects of various ligand/receptor interaction properties have a major influence on cell responses to any given ligand, so principles characterizing these effects must be considered prominently. This talk will present an overview of some principles that are being elucidated, with their relevance to biomaterials design.

Chairs: Eric W. Kaler and David J. Pine 
Tuesday Afternoon, December 2, 1997 
Republic B (S)

2:00 PM L4.1 
PRELIMINARY INVESTIGATION OF THE EFFECT OF POLY(ETHYLENE GLYCOL) ADDITION ON THE RELEASE CHARACTERISTICS OF MODEL DRUG FROM POLY(LACTIDE-CO-GLYCOLIDE) MATRICES. Ahmad R. Hadba, Christopher D. Batich, University of Florida, Dept of Materials Science and Engineering, Gainesville, FL; Gregory S. Schultz, University of Florida, Dept of Obstetrics and Gynecology, Gainesville, FL.

Polylactides, glycolides and their copolymers are the most widely used biodegradable polymers for the controlled release of bioactive agents. These devices are usually in the form of implantable matrix devices or injectable microspheres. The release of bioactive agents from these devices is typically biphasic: a burst of drug release followed by an incubation period after which drug release is resumed. The release rates of drug depend especially on the hydrophilicity of the polymer matrix, the hydrolytic degradation rates of the polymer matrix, the polymer-drug interaction, and the drug solubility in buffer solution. In this work we show the effect of poly(ethylene glycol) (PEG) addition on the release characteritics of a hydrophobic model drug, 4',5'-dibromofluorescein (DBF), from 50:50 poly(d,l-lactide-co-glycolide) copolymer (PLGA). 
PLGA:PEG blend matrices were prepared by solvent casting of drug-loaded polymer solutions on clean glass plates. The resulting polymer films were cut into circular discs and incubated in 50 mM phosphate buffer solution pH 7.4 at 37 C. At predetermined times, samples were collected and analyzed using ultraviolet visual spectrophotometry at = 450 nm. The data show that the addition of PEG to the matrices increases the first burst and reduces the incubation period. The release of DBF from the Medisorb 90:10 PLGA:PEG composition is zero order after the first burst effect. This is not observed for the Purasorb 90:10 PLGA:PEG composition. The addition of PEG to PLGA does not significantly alter the hydrolytic degradation rate of these blends. The trend is to reduce the hydrolytic degradation rate. These results support the heterogeneous hydrolytic degradation mechanism.

2:15 PM L4.2 
KINETICS OF LIPID TUBULE SOLUBILIZATION BY BILE SALTS AND DETERGENTS: A MODEL SYSTEM FOR ORAL DRUG DELIVERY. Paul A. Carlson, Michael H. Gelb, and Paul Yager, Molecular Bioengineering Program, Department of Bioengineering,Departments of Chemistry and Biochemistry, University of Washington, Seattle, WA.

We have shown that lipid-modified peptides and their lipid support matrix create tubules in aqueous solution through the molecular-level self-assembly of constituents. Tight molecular packing in the tubule microstructure protects labile peptide headgroups in harsh acidic environments like the stomach. Subsequent solubilization by bile salts and other surfactants causes peptide release from the microstructure. As a proof-of-principle experiment, we have studied the rate of tubule and vesicle solubilization by non-ionic detergents, fatty acids, and bile salts; acid stability of these microstructures is being tested. Diacetylenic phosphatidylcholines and glutamic acid dialkyl amides with poly-proline headgroups form tubular microstructures in which the molecular packing is crystalline. The peptidic lipid Pro3-Glu-(NH-C16H33)2 is being used to investigate protection from acid hydrolysis conferred by the tubule microstructure. The rate of acid-catalyzed headgroup hydrolysis should be significantly slower for this lipid when it is packed in a tubule microstructure compared to when it is co-dispersed in fluid phospholipid liposomes. Diacetylenic phospholipid tubules and phosphatidylcholine vesicles were used to probe the role of microstructural geometry in the mechanics of solubilization. The kinetics of solubilization should be insensitive to microstructural form and highly sensitive to the state of molecular packing within the bilayer because solubilization involves partitioning of detergent molecules into the bilayer prior to a structural transformation. A spectroscopic assay for micellar phospholipid and turbidity measurements confirmed this hypothesis; phospholipid tubule and vesicle solubilization both proceed by first-order kinetics. The rates of solubilization were highly sensitive to temperature and depended on detergent concentration. The activation energies for tubule and vesicle solubilization were comparable, but the intrinsic rates of vesicle solubilization were 25-fold faster than those measured for tubule solubilization at any given temperature. Preliminary evidence supports the hypothesis that lipid tubules would make effective gastrointestinal peptide delivery vehicles.

2:30 PM L4.3 
AN ATTEMPT TO PROTECT BY MICROENCAPSULATION SOME PHARMACEUTICALS TO IMPROVE THEIR EFFECTS ON THE CHAGAS DISEASE. Jacques Rieumont, Rubén Sánchez, Jaxqueline Siqueira, Center of Science and Technology, State University of North Fluminense - UENF-, Rio de Janeiro, BRAZIL.

The Chagas disease is common in South America and some nitrofurans and nitroimidazols have been used as drugs in its therapeutic treatment but its carcinogenic or toxic effect is well know. Some new pharmaceuticals are being tested with success in order to renew and improve the toxic old ones to eliminate the Trypanosome cruzi, the etiologic agent of the Chagas disease. The new drugs are related to homologues of nicotinamide or nicotinic acid because those compounds are used to synthesize NAD by trypanosomes and also by mammalia, but the latter can produce it from other sources.. Thus, analogues of such compounds can mimic its action inhibiting the production of NAD by trypanosomes. The main problem afforded with these analogues is its degradation in the gastrointestinal tract. With the aim of protecting these drugs a microencapsulation and controlled release study of thionicotinamide (TNA) was carried out using polyhydroxibutyrate (PHB) a well-know biodegradable polymer. TNA shows a partial solubility in some organic solvents and water. Thus, oil in water and water/oil/water emulsions were carried out in order to test the actual percentage of drug loading in the microspheres obtained using these techniques. Scanning and transmission electron microscopy of such microparticles indicates a rough surface morphology and diameters ranging among 5-8 microns. The ones obtained by double emulsion are actually microcapsules. Drug release of TNA was monitored by ultraviolet absortion at 280 nm in chlorhydric acid (pH=1) at 37C. Release profiles of the microcapsules undergoing release and slightly polymer degradation simultaneously resulted in first order plots confirming its capsule like internal morphology. These results encourage us for further in vivo research.

3:15 PM *L4.4 
POLYMERIZATION SETS MICROSTRUCTUR: ORGANIC MATERIALS SYNTHESIS IN COMPLEX FLUIDS. Eric W. Kaler, Carlos Co and John Morgan, University of Delaware, Department of Chemical Engineering, Newark, DE.

The use of complex fluids as templates for new materials is a trendy idea, but there are substantial practical barriers in the way of common methods. This talk reviews the state-of-the-art in synthesis of polymerizable vesicles and microemulsions, and highlights potentially useful directions for future work. Current polymerizations of equilibrium vesicles with ordinary monomers are discussed, and a systematic study of phase behavior, microstructure, and polymerization kinetics of methacrylates in cationic microemulsions reviewed. These experimental results have led to the development of a simple but accurate parameter-free kinetic model of microemulsion polymerization, which has been extended to predict the evolution of the absolute molecular weight and particle-size distribution. Results of on-line small-angle neutron scattering (SANS) studies on the polymerizing microemulsion support the assumptions of a polymer core/monomer shell particle morphology and equilibrium monomer partitioning between the monomer swollen micelle cores and the monomer shell of the particles.

3:45 PM L4.5 
THE INNER STRUCTURE OF COPOLYMER LATEX PARTICLES. Havazelet Bianco-Peled, Moshe Narkis, Yachin Cohen, Technion-Israel Inst of Technology, Haifa, ISRAEL.

Emulsion copolymerization has been widely used in the industry for production of polymeric materials with tailored properties. It is well known that quit often heterogeneity on the molecular level exist in these copolymers. Yet, remarkably little quantitative information is available on heterogeneity on the colloidal scale, determining the morphology of a latex particle. A small angle x-ray scattering (SAXS) study of a model copolymer latex, composed of styrene and pentabromobenzyl acrylate (PBBA, 40 wt%), is presented. The contrast variation method, employed in this study, was shown to be a sensitive probe for inhomogeneity in the particles. The separation of the homogeneous function allows direct calculation of the size distribution of the spherical particles (volume average diameter, 26.7 nm). The SAXS analysis reveals a particle1s inner structure described as a continuos copolymer phase, of composition being slightly richer in PBBA, within which domains of polystyrene are randomly distributed. The volume fraction of the polystyrene domains was estimated as 8 vol%, and their characteristic length as 5.1 nm.

4:00 PM L4.6 
POLYMER REACTION KINETICS: MEASURING SCALING LAWS VIA LASER INDUCED RADICALS. Ben O'Shaughnessy, Department of Chemical Engineering, Materials Science and Mining Engineering, Columbia University, New York, NY; Erdem Karatekin and Nicholas J. Turro, Department of Chemistry, Columbia University, New York, NY.

Although theoretical predictions of a range of universal scaling laws for high polymer reaction kinetics are available, many of these have never been tested experimentally. Here we present novel experimental methods for measuring long time inter-polymeric reaction rate constants kinf and time-dependent short-time rate constants, k(t), for the first time. Consider polymers of N monomer units, each carrying highly reactive end groups, dispersed in a background of unreactive chains. A widely used method for measuring reaction kinetics in such systems is phosphorescence quenching. However, at higher polymer concentrations this method is unsuitable for measuring kinf due to the limited range of phosphorescence lifetimes. The measurement of k(t) is difficult for other reasons: short time reaction rates are determined by the very small number of chain ends which happen to be initially within diffusive range of one another. This leads to weak signals. The novel method presented here for the measurement of kinf involves the creation of macroradical-radical pairs via a laser flash applied to chains carrying photolabile end groups. It is shown that after an initial transient, only the macroradicals survive. The surviving fraction is of order unity. Since macroradical lifetimes due to other processes can be made extremely long, the subsequent decay in macroradical population is determined by kinf. By monitoring this decay, we measure kinf. In a related method, geared to measuring the small time k(t), macroradical pairs are created from photocleavable groups at interior backbone locations. Strong signals result because the pairs are created in proximity to one another.

4:15 PM *L4.7 
ORDERED MACROPOROUS MATERIALS BY EMULSION TEMPLATING. A. Imhof and D.J. Pine, Departments of Chemical Engineering and Materials, University of California, Santa Barbara, CA.

We report the fabrication of macroporous materials of titania, silica, and zirconia by using the droplets of a nonaqueous emulsion as the templates around which material is deposited through a sol-gel process. Subsequent drying and heat treatment yields materials with spherical pores left behind by the emulsion droplets. The pore sizes range from 50 nm to several micrometers. By starting with an emulsion of equally-sized droplets (produced through a repeated fractionation procedure), pores with a uniform and controllable size have been obtained. Self-assembly of these droplets into a colloidal crystalline phase leads to ceramics which contain ordered arrays of pores. This also makes our method of potential use in producing materials with special optical properties, such as optical filters and photonic band gaps, which have strongly wavelength dependent reflection and transmission properties.

4:45 PM L4.8 
SURFACTANT TEMPLATED MESOPOROUS THIN FILMS. Yunfeng Lu, Rahul Ganguli, Celleste A. Drewien, Mark T. Anderson, Gabriel P. Lopez, and C. Jeffery Brinker*, Sandia National Laboratories and The University of New Mexico/NSF Center for Micro-Engineered Materials, The Advanced Materials Laboratory, Albuquerque, NM; Hermes Soyez and Bruce Dunn, Univ of California, Dept of Materials Science, Los Angeles, CA; Michael H. Huang and Jeffrey I. Zink, Univ of California, Dept of Chemistry, Los Angeles, CA.

We report the formation of supported mesoporous films via evaporation-induced surfactant enrichment during dip-coating. The short process timescale, constraint imposed by the substrate, and templating occurring at the vapor-liquid and liquid-solid interfaces enable the preparation of film mesostructures that have no bulk counterparts. For example, we form composites with incipient liquid crystalline order and cubic or 3-dimensional hexagonal liquid crystalline films that form via a lamellar - to cubic - to hexagonal pathway as yet unreported for inorganic mesophases. An in situ optical probe technique, fluorescence depolarization, is used to monitor the progressive evolution of surfactant aggregation (free - to micellar - to liquid crystalline) induced by surfactant enrichment during film deposition. Surface acoustic wave (SAW)-based N2-sorption is used to directly evaluate the pore size and pore accessibility of supported films. SAW results indicate that as prepared mesoporous thin films have a uni-modal pore size distribution. Mesophases exhibiting 3-dimensional porosity are ideal for various applications, such as membranes, catalysts and sensors, because they avoid any unfavorable orientation of the pore channel system.

Tuesday Evening, December 2, 1997 
8:00 P.M. 
Grand Ballroom (S)

CHARACTERIZATION OF COLLOIDAL CdSe AND CdSe/ZnS NANOCRYSTALS: STRUCTURE AND STABILITY. H. Mattoussi, F.V. Mikulec, B.O. Dabbousi, J. Rodrigez-Viejo, and M.G. Bawendi, Center for Materials Science and Engineering, MIT, Cambridge, MA; R. Ober, Laboratoire de Physique de la Matiere Condensee, College de France, Paris, FRANCE.

Nanocrystals of CdSe capped with either organic molecules, or overcoated with a thin layer of ZnS, are made using high temperature solution chemistry [1]. This approach provides narrow distribution of particles with core radius tunable from 10 to 60 Angstroms. In addition, the crystallites can be derivatized with a variety of organic capping molecules that have different electronic affinities to the surface. A microscopic probe of the ZnS growth on the CdSe core, and an understanding of the stability conditions for colloidal dispersions of these nanocrystals provides guidance for post-synthesis manipulation and processing. We use small angle X-ray scattering (SAXS) to probe the structure and the interactions for dispersions of CdSe nanocrystals, with different capping groups. We use SAXS and electron microscopy (TEM) to probe the size increase with ZnS overcoating and the effects on the interactions. We find that interparticle interactions depend on the type of cap and solvent used, but also on the size of the crystallite itself. Furthermore, the effects of ZnS overcoating are clearly reflected in the scattering data. The results are compared to scattering theories from interacting particles within the concepts of steric stabilization developed for classical colloidal solutions.

COLLOIDAL CHARGE DETERMINATION IN CONCENTRATED LIQUID DISPERSIONS USING TORSIONAL RESONANCE OSCILLATION. J. Bergenholtz, N. Willenbacher, Polymer Research Division, BASF AG, Ludwigshafen, GERMANY; N. J. Wagner, Center for Molecular and Engineering Thermodynamics, Department of Chemical Engineering, University of Delaware, Newark, DE.

The high frequency rheology of concentrated liquid charge-stabilized polymer dispersions is studied as a function of salt and surface acid content. A perturbation theory is presented which enables the determination of the effective surface charge from the high frequency shear modulus of the concentrated dispersions. The resulting effective charge increases with both added salt, in agreement with charge renormalization theory, and added surface acid groups. This method, which operates on concentrated dispersions, is found to yield effective charges that agree with dilute measurements of the electrophoretic mobility. Moreover, hard sphere scaling reduces the high frequency viscosity to a master curve independent of salt content.


In a linear alternating electric field, a colloidal particle becomes polarized along the direction of the applied field. Due to relaxation processes, this polarization may be out of phase relative to the applied field. If the particle spins about an axis perpendicular to the applied field, a polarization perpendicular to the applied field develops, and the particle experiences an electric torque as a result. At low frequencies, this electric torque opposes the spinning of the particle and no spontaneous spinning of the particle develops. When the frequency of the applied field is high enough, however, the electric torque, due to the spinning of the particle, can promote spinning. At fields stronger than a frequency-dependent threshold value, the particle goes into a spinning state. The relationship between the spontaneous spinning state and the frequency and strength of the applied field is discussed.


We present the results of photon correlation spectroscopy and static light scattering investigations of the influence of the confinement on dynamical behavior of liquid crystals (LC) dispersed in porous matrices with parallel cylindrical pores of different pore sizes. Confinement has strong influence on the equilibrium and the dynamical properties of liquid crystals. From static light scattering experiments we obtained that the nematic - isotropic phase transition in pores is smeared out and the temperature TNI of this transition is depressed compared to that bulk value. The temperature dependence of the depolarized component of the intensity of scattered light(Isc) in isotropic phase of confined LC is different from that in the bulk. In the bulk isotropic phase Isc is temperature dependent and obeys the mean-field theory, but in pores this intensity is almost independent of temperature. At temperatures below bulk TNI but above TNI in pores, only fast decay due to the order parameter fluctuations is observed. This decay is single exponential with relaxation time s. While decreasing the temperature, but still above TNI in pores, the contribution from order parameter fluctuations decreases and visible second decay due to director fluctuations appears. The existence of this decay in transitional region is an experimental conformation of the formation of ordered phase preceding phase transition in confined LC. This ordered (paranematic) phase has features of nematic phase. However, decay corresponding to the director fluctuations in confined LC is not single but is stretched exponential. In nematic phase fast decay due to order parameter fluctuations vanishes, stretched exponential decay due to director fluctuations dominates, and slow decay in the time range 10ms - 100 s, origin of which is under question appears.


Simulation of nematic texture generation and subsequent coarsening that arise by quenching an isotropic 2-D films into the unstable region of a system displaying the isotropic-nematic phase transition is performed using the Landau-deGennes theory. The dynamics of phase ordering is characterized by the emergence of a defect-filled texture during the early phase-transition stage, where the defects are generated by misalignment of neighboring nematic domains. In the late stage, after the isotropic phase has vanished (except for the defect cores with low scalar order parameter), the defect density evolution enters a coarsening regime. Scaling relations of defect density in the early phase transition stage and in the late coarsening stage are characterized as a function of the ratio of short range elasticity to long range elasticity. The effect of shear flow on coarsening scaling relations is also analyzed and deviations from quiescent coarsening explained using nematorheological principles. Finally, a way to create mono-domain sample is proposed.

ENTROPIC PHASE TRANSITIONS IN ROD-LIKE VIRUS SUSPENSIONS. Marie Adams, Zvonimir Dogic, Seth Fraden, Martin Fisher School of Physics, Brandeis University, Waltham, MA.

Aqueous suspensions of mixtures of rod-like virus with globular macromolecules phase separate and exhibit rich phase behavior. The observed phenomena can be understood within a theoretical framework that considers the phase behavior as being driven by entropy, known as the depletion effect or macromolecular crowding. We have observed two virus systems: fd virus and Tobacco Mosaic Virus. Both are mixed with the globular macromolecules Bovine Serum Albumin and polyethylene oxide. Isotropic, nematic, lamellar, and crystalline phases are observed as a function of the relative size and concentration of the constituents, and the ionic strength. We compare our observations of the fd system with those of the TMV system.

CONTINUUM MODEL FOR SELF-ASSEMBLING RODS: A NEW ANSATZ. Eric M. Kramer, Judith Herzfeld, Brandeis Univ, Dept of Chemistry and Keck Institute for Cellular Visualization, Waltham, MA.

We consider a crowded solution of globular macromolecules capable of reversible self-assembly into rigid rods. Theories of this system seek to determine the thermodynamic equation of state and the equilibrium distribution of rod lengths as a function of orientation. The problem is commonly simplified by restricting the rod orientations to three perpendicular axes. This neglects important details of the distribution. We present (1) a numerical solution for a model with unrestricted rod orientations and (2) a simple ansatz for the solution whose properties agree with the results for unrestricted rods to a few percent.

THE THERMOTROPIC LIQUID CRYSTALLINE BEHAVIOR OF POLY(ARYL ETHER KETONE)S CONTAINING META-BENZENE UNIT. Shanju Zhang, Peng Zhang, Yubai Bai, Yubin Zheng, Zhongwen Wu, Jilin Univ, Dept of Chemistry, Changchun, CHINA.

The thermotropic liquid crystalline poly(aryl ether ketone)s are a new class of main-chain liquid crystalline polymers, which exhibit not only nematic phase but also highly ordered smectic phases. This class of TLCP have melting temperature between 300 droplets in L3 phase. The values are in agreement with the model. 
Moreover the model is not incompatible with an interface transition between tilt anchoring and tangential interface. We have discovered this interface transition in an other lyotropic system (water/SDS/cyclohexane/pentanol), when it is doped with nanometric magnetic particles. The properties of that particular system [2] have been used to confirm the values of the very low interface energy and the mechanism of the transition.

ELECTRO-, MAGNETO-, THERMOSENSITIVE COMPLEX FLUIDS BASED ON POLYVINYL ALCOHOL WITH VARIABLE-VALENCE METALS AND POLYACIDS. Tatiana G. Lazareva, Svetlana A. Prodan, Olga V. Yakovleva, Julian S. Gayduk, Inst of General and Inorganic Chemistry of the National Byelorussian Academy of Sciences, Minsk, BELARUS.

The conditions of formation and variants of application of the hybrid inorganic-organic complexes based on polyvinyl alcohol, variable-valence metals, polyacids, heteropolyacids and orthophosphoric acid are evaluated by a rheological methods with regard for electrophysical, optical and sorption properties. The modification of hydrogen-bonded complexes with the applied electric, magnetic, thermal fields and sorption of water vapor induced structure metastability due to changing the system of inter- and intramolecular hydrogen bonds are discussed. Evaluation is made of an influence of the type and concentration of components on the properties and structure of the complexes. It is established that the applied electric, magnetic, thermal fields and sorption of water vapor induced metastability is associated with the formation of a liquid crystalline phase.

MEASURING THE ANCHORING STRENGTH OF A CAPILLARY USING TOPOLOGICAL DEFECTS. Randall D. Kamien, University of Pennsylvania, Dept of Physics and Astronomy, Philadelphia, PA; Thomas R. Powers, University of Arizona, Tucson, AZ.

We consider a smectic-A* in a capillary with surface anchoring that favors parallel alignment. If the bulk phase of the smectic is the standard twist-grain-boundary phase of chiral smectics, then there will be a critical radius below which the smectic will not have any topological defects. Above this radius a single screw dislocation in the center of the capillary will be favored. Along with surface anchoring, a magnetic field will also suppress the formation of a screw dislocation. In this note, we calculate the critical field at which a defect is energetically preferred as a function of the surface anchoring strength and the capillary radius. Experiments at a few different radii could thus determine the anchoring strength.

LIQUID WATER AND COMPLEX FLUIDS: DENSITY ANOMALY FOR GENERAL DOUBLE WELL POTENTIALS. M. Reza Sadr-Lahijany, Sergey V. Buldyrev, Antonio Scala and H. Eugene Stanley; Center for Polymer Studies and Department of Physics,Boston University, Boston, MA.

We study the properties of general double well models recently proposed to explain the density anomaly of water[1]. We investigate the temperature of maximum density line (TMD) as well as the isothermal compressibility (KT). In addition to exhibiting a density anomaly, the model shows other resemblances to the behavior of water. E.g., the TMD line shows a change of slope from negative to positive in the P-T phase diagram. Also, there exists a line of compressibility maxima which at T=0 intersects the TMD line at a critical pressure, for which KT diverges algebraically as . We investigate the relation between this model and a class of core-softened models which are believed to have a line of first order phase transitions terminating at a critical point[2]. We justify the model by considering the interaction of a cluster of five water molecules with its neighbors.


Nondestructible spectral characterization and certification of bulk silicon-based polymer products and coated silicon surfaces have been developed by spectroscopy methods. In particular the analysis of FTIR spectra of siloxane intraocular lens (IOL) implant makes it possible to determine the degree of polymerization of the material to produce nontoxic lenses for the eye. This FTIR test for biocompatibility of implants with biological media(for example their contact with blood and living tissues) is very important, specially for implants with long explosure in human organisms. We are in the process to develop the most compatible materials such as in polyurethane(used, for example, in leaflet cardiac valves, membranes, etc.). The siloxane materials with low level of polymerization are stable in acid media. In addition they can be used in microelectronics if they have good mechanical and electrical properties, transparency, etc. Similar thin-film silicon-organic compound coatings have been investigated for flat panel display materials. We have shown that the stability of this type of coatings on Si substrates depends on complicated systems of hydrogen bonds, impurities of boron , phosphorus etc. Structural features and stability of these thin films can be tested directly by the FTIR-ATR method with fiberoptical sensors from the surface . The homogeneity of these thin -film polymer composites on Si surfaces can be more efficiently detected by infrared spectromicroscopy using synchrotron radiation in the range of 2- 30 microns. Passivation of bioinert surfaces as biomineral materials, i.e., sapphire allows us to produce more uniform well-structured surfaces on a molecular level.


Dissipative particle dynamics is a mesoscopic simulation technique that allows to address time and length scales not available from molecular dynamics in the study of complex fluids like colloidal and polymeric suspensions. We present recent developments in which shear forces between dissipative particles are included, leading to more realistic interactions between the dissipative particles mimicking lumps of fluid. Also, it is possible to introduce an internal energy variable for the particles in such a way that the algorithm conserves total energy. This allows to study thermal processes in complex fluids with a mesoscopic technique.

APPLICATION OF BIODEGRADABLE POLYURETHANES IN THE CONTROLLED ADMINISTRATION OF ANTIBACTERIAL DRUGS. Erkesh O. Batyrbekov, Rinar M. Iskakov, Bulat A. Zhubanov, Institute of Chemical Sciences, Almaty, KAZAKSTAN; K. Sreenivasan, K. Rathinam, A.K. Kuttiyil, Sree Chitra Tirumal Institute for Medical Sciences & Technology, BMT Wing, Poojapura, Trivandrum, INDIA.

Biodegradable polyurethanes as diffusion-controlled materials for the administration of antibacterial drugs have been studied. Drug delivery systems consists of dispersion of dissolution of drugs in microporous polymeric matrix. Biodegradable polyurethanes were obtained by means a two step procedure using oligoetherglycoles, diisocyanates and branching agents. Drugs were added in polymer at the stage of synthesis in liquid polyurethane prepolymer. The drugs used in the investigation were ciprofloxacin, cloxacillin and rifampicin. The biodegradation period and release profiles of antibiotics in model media and animal organism were studied. The rate of drug release from polymeric matrix can be controlled by manipulating of the percentage drug content in systems. The antibacterial effect of the systems obtained was tested against five different strain of bacteria.

CHANGES IN BOMBYX MORI FIBROIN CONFORMATION INDUCED BY SHEAR AND ELONGATIONAL FLOW DURING FIBER SPINNING. Kimberly Trabbic and Paul Yager, Molecular Bioengineering Program, Department of Bioengineering, University of Washington, Seattle, WA.

Nature has shown that polypeptides can be used to make sophisticated structural materials. Materials such as silk fibers have complex hierarchical structures that can make them stronger and stiffer than steel and tougher than Kevlar. Recent advances in molecular biotechnology and protein engineering have taken great strides toward the production of large quantities of peptide-based biopolymers. However, the production of useful materials, such as fibers, from this potential supply of raw biopolymer requires a detailed understanding of how processing conditions affect the protein conformation. We have studied the fiber spinning of Bombyx mori (silkworm) fibroin ex vivo from 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) and aqueous solutions as model systems for understanding how processing conditions influence the structure of proteinaceous fibers. Fibroin undergoes dramatic conformational changes upon fiber spinning from both these solutions. Fibroin is highly -helical in HFIP solution but is random coil in aqueous solution. The spinning of fibers from either solution causes fibroin to form a polycrystalline, -sheet, polymer/polymer composite structure, and the elongational flow of a post-spinning draw causes the alignment of crystallites and the polypeptide backbone with the fiber axis. Spatially resolved Raman spectroscopy has been used to study the conformation of fibroin in solutions during the shear and elongational flow of fiber spinning. The fibroin secondary structure and orientation have been evaluated quantitatively to determine the effects of shear and elongation under varying solution conditions. The results of these studies yield greater insight into the mechanism responsible for the conversion of fibroin from soluble random coil to insoluble fiber. The continuation of this project to map fibroin conformation onto the synthetic spinning of B. mori fibroin fibers will aid in the practical application of recombinantly produced novel biopolymers as well as yield a deeper insight into the natural B. mori spinning process.

LOCAL DIELECRIC FUNCTION IN BIOLOGICAL AND GEOLOGICAL ARAGONITIC AND CALCITIC MATERIALS USING TRANSMISSION EELS: A COMPARATIVE STUDY. Kalpana S. Katti, North Dakota State University, Department of Polymers and Coatings, Fargo, ND; Daniel W. Frech, Maoxu Qian, Mehmet Sarikaya, University of Washington, Department of Materials Science and Engineering, Seattle, WA.

Previous work on microstructural characterization has shown variations in terms of defects and organization of nanostructures in the two biological and geological polymorphs of calcium carbonate, calcite and aragonite in the prismatic and nacreous sections of mollusc shells. Large variations in mechanical properties are observed between these sections which have been attributed to variations in microstructure. Here we present a local electron energy loss spectroscopic study of calcitic and aragonitic regions of abalone shell. The results are compared to data collected under similar conditions from geologic calcite and aragonite. Local dielectric function is computed for biological and geological phases using Kramer-Kronig analysis. The local dielectric function of the material in the 0-100eV range is representative of picture of the electronic structure of the material and intrinsic material properties. The results show remarkable difference in the aragonitic and calcitic regions between biological and geological materials. Increase in plasmon density, change in the nature of the bound characteristics of the plasmon and decreased contributions to the plasmon from single electronic transitions in biological materials is observed. Implications of these changes is discussed in the context of macromolecular involvement in the making of the microstructures in biogenic phases while this is absent in geological ones.

FREEZING SPHERICAL VESICLES. Gerhard Gompper, MPI fuer Kolloid- und Grenzflaechenforschung, Teltow, GERMANY; Daniel Kroll, University of Minnesota, Minnesota Supercomputer Institute, Minneapolis, MN.

The freezing transition of tensionless fluctuating vesicles is investigated by Monte Carlo simulations and scaling arguments for a simple tether-and-bead model of fluid membranes. In this model, a freezing transition is induced by reducing the tether length. In the case of planar membranes, the model shows a fluid-to-crystalline transition at a tether length , where is the bead diameter. For flexible vesicles with bending rigidities , the reduced free energy of dislocations, , is found to scale for small tether lengths with the scaling variable , where K0 is the Young modulus of a crystalline membrane of the same tether length, and is the average nearest-neighbor distance. This is a strong indication that free dislocations are present, so that the membrane is in a hexatic phase for small tether lengths. A hexatic-to-fluid transition occurs with increasing tether length, or with decreasing bending rigidity.

MICROENCAPSULATION AND CONTROLLED RELEASE OF PEPTIDE HORMONES FOR SUPEROVULATION OF CATTLE: A CHALLENGE. Teresa Eligio, J. Frederico Straggiotti, Jacques Rieumont, Rubén Sánchez, Center of Science and Agricultural Technology, State University of North Fluminense -UENF-. Rio de Janeiro, BRAZIL

The embryo transfer industry has had one limitation above all others: the reliability of superovulation. It is due to activity inconsistency of drugs for the applied procedures. In order to get a stimulation superovulatory activity, the Follicle Stimulating Hormone (FSH), has a high efficiency but it also depends on dose regimens. The classical intramuscular regimen (18-50 mg) of hormones, for a successful superovulation, is twice daily during four days. However, more advanced applications are required to improve the actual method and to avoid the stress produced on the cattle during manipulation. In this context biodegradable polymeric carriers were prepared for hormone controlled release with polyhydroxibutyrate (PHB) and its copolymer with hydroxivalerate P(HBHV)( 22% by water/oil/water emulsion method. The FSH (7 mg) was dissolved in 1 ml of saline buffer and emulsified in 26 mls of chloroform solution of the polymer (0.5 g/100 ml) under stirring with a Turrax IKA-25. The first emulsion was added dropwise to a polyvinyl alcohol water solution (1-3% wt) to obtain a second emulsion. Microcapsules obtained were washed in water (0-4C), collected by centrifugation at 8000 rpm and dried under vacuum. A morphology characterization was performed by Optical and Scanning Electron Microscope. It was observed an irregular cauliflower like surface of microcapsules ranging from 5-8 microns. In vitro release of FSH was carried out using 10 mgs of microcapsules suspended in 1 ml of phosphate buffer solution (pH 7.4) in a separation column. Buffer solution was completely removed and replenished by fresh buffer solution periodically. The elution sample was analyzed by HPLC using the ultraviolet detector at 280nm. FSH controlled release was studied for 10 days. Release profile and microcapsule morphology are discussed.

MOLECULAR DYNAMIC SIMULATIONS OF ACOUSTIC PROPERTIES OF COLLOIDAL SUSPENSIONS. G. Ramirez-Santiago, R. Esquivel-Sirvent, C. Noguez, Instituto de Física, Universidad Nacional Autónoma de México, MEXICO.

Abstract not available.

8:45 AM *L6.1 
CRYOGENIC TRANSMISSION ELECTRON MICROSCOPY (CRYO-TEM) OF COMPLEX FLUIDS: PRESENT AND FUTURE. Y. Talmonh, Department of Chemical Engineering, Technion-Israel Institute of Technology, Haifa, ISRAEL.

Direct-imaging transmission electron microscopy at cryogenic temperatures (cryo-TEM) has become a standard technique, used by an increasing number of complex fluids research groups. While the technique has provided direct images of many systems, and through that has contributed to solving a number of outstanding problems, there are still some open questions related to the technique itself, and to the way it could be extended and modified to make it compatible with a wider range of complex fluids. The first significant extension of the technique was through the controlled environment vitrification system (CEVS), essential for microstructured liquid systems that are sensitive to temperature and concentration changes. More recently on-the-grid processing made it possible to prepare viscous phases directly in the thin liquid specimen, and to follow their formation and intermediate states; that lead to what we call 'time-resolved cryo-TEM'. These techniques are still being developed in many variations tailored for the particular system studied. One of the challenges ahead is direct imaging cryo-TEM of non-aqueous systems. Recent work in this direction is quite promising. Increasing application of digital imaging makes it possible to record usable images of highly electron beam-sensitive and low contrast specimens, and extract more information from them through digital image processing. In my talk I will describe recent application of direct-imaging cryo-TEM to a number of complex fluids, and through those will demonstrate the power and the limitations of the technique and its extensions. I will emphasize on-the-grid-processing, the study of non-aqueous systems, digital imaging, and effects of shear applied to prepare thin liquid specimens. While shear is a possible source of artifacts, it can be made into an additional research tool.

9:15 AM *L6.2 
COEXISTENCE AND PHASE SEPARATION IN SHEARED COMPLEX FLUIDS. Peter Olmsted, University of Leeds, Leeds, UNITED KINGDOM; and C.-Y. David Lu, Polymer and Colloids, Cavendish Laboratory, Cambridge, UNITED KINGDOM.

Many complex fluids, including liquid crystals, polymer solutions, surfactant solutions, and colloidal suspensions, undergo shear-induced transitions between ordered states. These transitions are often manifested in rheological singularities (stress-strain-rate discontinuities). We discuss the general aspects of phase transitions of complex fluids under flow, concentrating on solutions where phase separation is possible. We demonstrate how to construct dynamic phase diagrams in which the conserved composition variable and the broken-symmetry order parameter (nematic, smectic, crystalline) are coupled to shear rate. Our construction relies on a selection criterion, the existence of a steady interface connecting two stable homogeneous states. We use the (generalized) Doi model of lyotropic nematic liquid crystals as a model system, but the method can be easily applied to other systems, provided non-local effects are included. We will discuss relevant experimental signatures of this work.

9:45 AM L6.3 
NON HOMOGENEOUS FLOW PATTERN IN COMPLEX FLUIDS: A NON-EQUILIBRIUM PHASE TRANSITION?. Gregoire Porte, Jean-François Berret and James L. Harden, Groupe de Dynamique des Phases Condensées(UMR CNRS 5581), Université Montpellier II, FRANCE.

When submitted to a high steady shear, wormlike micelles solutions usually exhibit a strongly non-linear viscous response associated with the progressive alignment of the long micelles parallel to the direction of the velocity. In some cases even, shear-banded non-stationary flows have been reported above some characteristic shear rate. A mechanical signature of this behaviour is the onset of a stress plateau at this rate where the stress abruptly becomes independent of the shear rate 1. Similar onsets of a stress plateau have also been reported for complex fluids consisting of totally different self-assembling microstructure: onions in Iyotropic lamellar phases2. Such banded flows are usually interpreted in terms of a mechanical instability of the non-linear flow reqime3. However, in at least two respects, the robustness of the two-state banded flow regime and the kinetics of band formation, they present striking analogies with ordinary field induced phase transitions in equilibrium systems. We propose a phenomenological explanation of these effects in terms of an effective non-equilibrium potential that accounts for the free energy stored in tile visco-elastic medium under steady shear conditions. Within this picture, the shear thinning non-homogeneous flow can be shown to be the manifestation of an underlying hidden structural transition. Some consequences for shear-thickening transitions will also be presented.

10:00 AM *L6.4 
DIRECT OBSERVATION OF SHEAR INDUCED STRUCTURES IN WORM-LIKE MICELLAR SOLUTIONS BY FREEZE-FRACTURE ELECTRON MICROSCOPY. Sarah L. Keller, Philippe Boltenhagen, David J. Pine and Joseph A. Zasadzinski; Depts. of Chemical Engineering and Materials Science, Univ. of California, Santa Barbara, CA and Laboratoire d'Ultrasons et de Dynamiques des Fluides Complexes, Institut Le Bel, Strasbourg, FRANCE.

We present the first direct observation of shear induced structures in low concentration worm-like micellar solutions. The structures consist of micron-sized patches which are much larger than the individual equilibrium micelles. Such morphologies are consistent with recent reports of increased light scattering under shear flow. The structures have a stippled or sponge-like texture and contain more surfactant than the background micellar solution. These patches form aggregates spanning hundreds of microns and are consistent with the shear thickening observed in these systems.

11:00 AM L6.5 
FROM SEMI-FLEXIBLE POLYMERS TO MEMBRANES: ANOMALOUS DIFFUSION AND REPTATION. Rony Granek and Anton G. Zilman, Department of Materials and Interfaces, Weizmann Institute of Science, Rehovot, ISRAEL.

The dynamics of semi-flexible polymers and membranes is discussed. The effect of thermal undulations on both the transversal and longitudinal mean square displacement (MSD) of a tagged ``monomer is studied in free polymers and membranes. The two MSDs are found to be proportional to one another, and behave as for polymers and for membranes on the short time scale. We demonstrate how the latter behavior leads to a strech exponential decay of the dynamic structure factor of bulk membrane phases, , which has been recently observed by Freyssingeas, Roux, and Nallet. The longitudinal motion is shown to be linked to the dynamics of fluctuations of the projected length (area) of the polymer (membrane). We demonstrate how, at long times, these fluctuations lead to reptation motion of the polymer (membrane) in the longitudinal direction. We generalize this approach to investigate the motion of a membrane between two walls and a polymer in a tube. The latter problem is used as a model for polymer motion in semi-dilute solutions in which the persistence length is longer than the entanglement length. Such systems are not suitable for the classical reptation model of de-Gennes and of Doi and Edwards, which was designed for chains that are flexible on the entanglement distance. The reptation diffusion coefficient and relaxation times that we obtain have the same scaling with chain length L as in the classical reptation model, but differ greatly in factors that are dependent on the ratio of persistence length to entanglement length. We also discuss the diffusion of a tagged monomer under imposed tension and liquid crystalline order.

11:15 AM L6.6 
LINEAR VISCOELASTIC MODULI OF F-ACTIN SOLUTIONS. T.G. Mason, A. Palmer, K. Rufener, D. Wirtz, Johns Hopkins University, Dept. of Chemical Engineering, Baltimore, MD.

We measure the motion of thermally-driven colloidal spheres suspended in concentrated solutions of filamentous (F-) actin using Diffusing Wave Spectroscopy and deduce the linear viscoelastic moduli using a frequency-dependent Stokes-Einstein equation. At high actin concentrations, C, the storage modulus, G'(w), exhibits a low-frequency plateau. The C-dependence of this plateau modulus suggests that F-actin solutions formed by quenching the salt concentration of a monomeric solution may resemble a glassy dispersion of dense rod-like filaments.

11:30 AM *L6.7 
VISCOELASTICITY OF CONCENTRATED SOLUTIONS OF SEMI-FLEXIBLE POLYMERS. David Morse, Univ. of Minnesota, Dept. of Chemical Engineering and Materials Science, Minneapolis, MN.

A tube model is presented for the linear and nonlinear viscoelastic behavior of concentrated solutions of semi-flexible polymers in a `tightly-entangled' regime in which the polymers' persistence length exceeds the solutions' entanglement length, which is given here by the distance between collisions of the polymer with the surrounding `tube.' An expression for the stress tensor of a semi-flexible chain is derived, and stress relaxation is assumed to occur by reptation. The resulting stress contains distinct contributions arising from the deformation of the distribution of curvatures and (as in the rigid rod case) of overall chain orientations, allowing the model to interpolate between rigid-rod and random-coil behaviors. The stresses obtained for solutions of long chains under large step deformation or rapid flow deformations are found to be much larger than those obtained for solutions of flexible chains with a similar linear response, due to the dominant contributions of deformation induced 'hairpins', i.e., of atypical but very tightly curved chain segments.

Chair: Andrea J. Liu 
Wednesday Afternoon, December 3, 1997 
Republic B (S)

1:45 PM *L7.1 
OF EMULSIONS. B. Deminiere, A. Colin, F. Leal Calderon and J. Bibette, Centre de Recherche Paul Pascal, Pessac, FRANCE.

Emulsions are metastable systems that are obtained by shearing one phase nto the other in the presence of surface active species. The life time of such metastable systems may considerably vary depending on the chemical nature of both phases and surfactants. By using concentrated monodisperse non ionic oil-in-water emulsions droplets we have inferred some key rules that govern both the life time and the topological transformation of these metastable materials. The emulsion is designed to remain unchanged for months at room temperature, while at a higher temperature a very surprising transformation takes place. Indeed, our monodisperse emulsions coarsens through discrete coalescene events however retaining a unique size throughout the whole sample. Therefore, the droplet size as a function of time may be reliably detected allowing a clean comparison with a simple model that includes only one unknown parameter: the frequency fro nucleating a hole per unit surface () within the flattened parts of adjacent droplets. From Arrhenius plots (Ln as a function of 1/T), we determine the activiation energy (E) that is required for hole nucleation and we also infer the order of magnitude for the bare frequency /(e-^E/kT%%). We discuss the physical basis for these numbers and link the empirical HLB scale with the microscopic parameters that set the hole nucleation frequency scale.

2:15 PM *L7.2 
IS THERE A FOAM TEMPERATURE? S. Tewari, UCLA, Dept. of Physics; S.A. Langer, NIST; and A.J. Liu, UCLA, Dept of Chemistry, Los Angeles, CA.

Viewed as a dense packing of bubbles in a small amount of liquid, a foam is a disordered zero-temperature system because the thermal energy is much smaller than the typical energy barrier required for bubbles to change their relative positions. If a foam is steadily sheared, however, it flows as bubbles change their relative positions in rearrangement events. Our aim is to determine whether the effects of steady shear on bubble motion can be characterized by an effective ``temperature'', T(eef). In a simulation of Durian's model of wet foams [1] we measure T(eef) in several different ways. First, we find that bubble motion in directions perpendicular to the applied shear is diffusive; the diffusion coefficient specifies T(eef) up to a drag according to the Stokes-Einstein relation. In addition, we find that velocity fluctuations of bubbles follow a Gaussian distribution that widens as we increase the shear rate; this specifies T(eef) up to an effective mass, which is also shear-rate dependent. Finally, we measure volume fluctuations of the sheared foam at constant pressure and relate these to the compressibility; the proportionality constant is T(eef).

2:45 PM L7.3 
BEHAVIOR OF A CHARGED VESICLE SYSTEM UNDER THE INFLUENCE OF A SHEAR GRADIENT: A MICROSTRUCTURAL STUDY. M. Bergmeir*, M. Gradzielski*, H. Hoffmann *, K. Mortensen#, *Lehrstuhl für Physikalische Chemie, Universität Bayreuth, Bayreuth, GERMANY; #Condensed Matter Physics and Chemistry Department, Riso National Laboratory, Roskilde, DENMARK.

The influence of shear on multilamellar (onion-type) vesicles has been studied by means of rheology, anisotropy of the electrical conductivity, freeze-fracture transmission electron microscopy (FF-TEM), and small- angle neutron scattering (SANS). The system investigated contained tetradecyldimethylamineoxide (TDMAO), tetradecyltrimethylammoniumbromide (TTABr), and 1-hexanol as cosurfactant. In aqueous solution this system forms spontaneously large multilamellar vesicles (of 1 m and larger) of large polydispersity. In a 100 mM surfactant system they are densely packed which leads to elastic properties and a yield stress. Upon application of shear forces these vesicies become structurally altered. The higher the shear rate the smaller the vesicles become whereby the outer shells of the multilamellar vesicles are stripped off. Finally at very high shear rates small unilamellar vesicles of low degree of polydispersity are formed. This unilamellar system has also elastic properties and its yield stress is even increased in comparison to the original, multilamellar state. The unilamellar state is found to be stable and does not relax back to the original, multilamellar state. From all this evidence it can be concluded that controlled application of shear is a suitable method to control both size and structure of a given vesicle system.

3:30 PM *L7.4 
LIPID BILAYER TRANSFORMATIONS INDUCED BY LASER TWEEZERS. R. Bar-Ziv1, E. Moses1 and P. Nelson2, 1Department of Physics of Complex Systems, Weizmann Institute of Science, Rehovot, ISRAEL; 2Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, PA.

We present the phenomenology of dynamic excitations in lipid bilayers using laser tweezers. A variety of dynamic instabilities and shape transformations are observed including the pearling instability, expulsion of vesicles and more exotic ones such as topological passages. Our physical picture of the laser - membrane interaction is based on the transmission of tension in the bilayer and the actual loss of surface area. While tension is the origin of the pearling instability it does not suffice to explain expulsion of vesicles where we observe opening of giant pores and creeping motion of bilayers. We postulate that area detached from the bilayer gets repacked into an unobserved gas of particles which can in turn store energy producing osmotic pressure and depletion forces.

4:00 PM *L7.5 
PROPAGATION OF THE RAYLEIGH INSTABILITY. Thomas R. Powers (1), Dengfu Zhang (2), Raymond E. Goldstein (1) (3), Howard A. Stone (2). (1) Dept of Physics, Univ of Arizona, Tucson, AZ; (2) Div of Eng and Appl Sci, Harvard Univ, Cambridge, MA; (3) Prog in Appl Math, Univ of Arizona, Tucson, AZ.

Recently, Bar-Ziv and Moses discovered a dynamical shape transformation induced in cylindrical lipid bilayer vesicles by the action of laser tweezers. The most striking aspect of this ``pearling instability is that it propagates out from the laser spot via a front moving at constant velocity, at least for early times. Motivated by this phenomenology, we have posed the following question: can the Rayleigh instability of a cylindrical interface between two stationary viscous fluids develop as a propagating front? We provide numerical evidence that this instability can indeed spread behind a constant velocity front, in some cases leading to a periodic sequence of breakup events. We further demonstrate that the results of these calculations are in reasonable quantitative agreement with the marginal stability criterion, which predicts a front velocity as a function of viscosity contrast. A number of experiments immediately suggest themselves in light of these results.

Chair: Nily Dan 
Thursday Morning, December 4, 1997 
Republic B (S)

9:00 AM *L8.1 
THE SELF ASSEMBLED STRUCTURE OF DNA-CATIONIC LIPOSOMES FROM SYNCHROTRON X-RAY STUDIES. C.R. Safinya, I. Koltover, A. Lin, N. Slack, J.O. Radler, T. Salditt, Materials and Physics Depts and Biochemistry and Molecular Biology Program, University of California, Santa Barbara, CA.

Cationic liposomes complexed with DNA (CL-DNA) are promising synthetically based nonviral carriers of DNA vectors for gene therapy. The solution structure of CL-DNA complexes was probed on length scales from subnanometer to micrometer by synchrotron x-ray diffraction and optical microscopy [1,2]. The addition of either linear or closed circular plasmid DNA to CLs results in a topological transition from liposomes to optically birefringent liquid crystalline condensed globules. X-ray diffraction of the globules reveals a novel multilamellar structure with alternating lipid bilayer and DNA monolayers. We have discovered the -DNA chains form a one-dimensional lattice (i.e. coupled two-dimensional (2D) smectic phase of DNA chains imbedded between lipid bilayers of a 3D smectic phase), with distinct interhelical packing regimes . In the isoelectric point regime, the -DNA interaxial spacing may expand between 24.5 and 60 ngstroms while retaining the finite-sized 2D lattice [2] which is indicative of the presence of long-range electrostatic-induced repulsion. Supported by NSF DMR-9624091, PRF-31352-AC7.

9:30 AM *L8.2 

Although one expects the binding of DNA molecules to a positively charged surface in solution, it is surprising that DNA molecules bond to planar bilayers of cationic lipids assemble into 2-D patterns with some degrees of nematic ordering. We have shown that the binding of DNA molecules to cationic lipids is stronger than 5 kcal/mol per helical turn of DNA. On the basis of our experiments on a variety of systems, we conclude that the assembly process is a 2-D condensation of DNA. The 2-D condensation occurs for both linear and circular DNA molecules and is independent of their lengths. At present, investigations are still required to understand details of the nature of how 2-D condensation of DNA occurs. Our results show support to a recent theoretical model that attributes the diffusive movement of lipids to the rearrangement and ordering of bond DNA molecules. To facilitate direct comparison of our experimental results with some known scattering theories, we will present some data analysis of significant statistics by averaging a large number of images. This will allow us to obtain structural factors and the correlation greens functions for different systems.

10:00 AM L8.3 
DNA COMPLEXES WITH CATIONIC LIPOSOMES. N. Dan, Dept of Chemical Engineering, University of Delaware, Newark, DE.

Studies of DNA complexes with cationic liposomes are prompted by the search >for non-viral DNA carriers for gene therapy. Recent experiments identified a >stable multi-lamellar phase, where ordered smectic layers of DNA alternate >with cationic bilayers. In this paper we examine the forces governing DNA >adsorption on cationic lamellae, as well as compare the stability of lamellar >vs. cylindrical aggregates.

10:45 AM *L8.4 
ELECTROSTATICS OF DNA-CATIONIC LIPID COMPLEXES: ISOELECTRIC INSTABILITY. Robijn Bruinsma, Dept of Physics, University of California, Los Angeles, CA.

We propose a Poisson-Boltzmann electrostatic theory for DNA/cationic lipid complexes modeled as a stack of aligned DNA chains intercalated with lipid bilayers, a structure suggested by the recent x-ray synchrotron studies of Radler et al. Poisson-Boltzmann theory predicts that the isoelectric point - where the DNA and cationic lipid charges are in balance - is unstable against absorption of extra DNA or lipid material. The instability is caused by the entropy gain obtained following the release of small ions inside the complex. The instability is manifested by singular behavior in the rod-rod spacing near the isoelectric point similar to that observed for commensurate-incommensurate transitions of adsorbates.

11:15 AM L8.5 
ELASTICITY OF DNA CATIONIC LIPID COMPLEXES. Corey S. O'Hern and Tom Lubensky, Univ. of Pennsylvania, Dept. of Physics and Astronomy, Philadelphia, PA.

DNA cationic lipid complexes are important as potential vehicles for delivering DNA to cell nuclei in emerging gene therapies. (See J. O. Radler, et. al., Science, Vol. 275 1997.) We consider first an idealized model of these complexes in which DNA molecules form a two-dimensional columnar lattice with lines of columns separated by lipid bilayers in a periodic lamellar structure. We calculate the harmonic elasticity of this model and the resultant fluctuation spectrum of both the DNA columns and the lipid layers. We then consider various lower symmetry melted versions of this idealized structure including a phase in which the DNA lattice looses its long-range periodic positional order but maintains its long-range orientational order.

11:30 AM L8.6 
CONFINEMENT OF POLYSOAPS IN LYOTROPIC MESOPHASES. Y. Yang, R. Prud'homme, Department of Chemical Engineering, Princeton University, Princeton, NJ; K. McGrath, Department of Chemistry, University of Otago, Dunedin, NEW ZELAND; P. Richetti, C.M. Marques, CNRS/Rhône-Poulenc, Complex Fluids Laboratory, Cranbury, NJ.

We study the embedment of hydrophobically modified polymers into lamellar () and sponge (L3) phases of a nonionic lyotropic system. The unmodified polymer is not soluble in the mesophases, but solubility is achieved when enough hydrophobic side-groups are grafted to the polymer chain. The resulting stapled polymr structure induces important changes in the equilibrium and dynamic phase behaviour of the surfactant solution. In particular, we show that insertion of the hydrophobically modified polymer: i) increases the rigidity of the bilayer system and ii) induces a phase separation into two coexisting phases.

11:45 AM L8.7 
CONFORMATIONAL CHANGE OF PARTIALLY NEUTRALIZED POLYACRYLIC ACID INDUCED BY OPPOSITELY-CHARGED SURFACTANTS. Andrew J. Konop and Ralph H. Colby, Dept of Materials Science and Engineering, Penn State Univ, University Park, PA.

Interaction of polyacrylic acid with oppositely-charged surfactants is studied as a function of degree of neutralization (with NaOH), salt concentration (NaBr) and relative concentrations of polyion and surfactant. We use a custom-built transparent PMMA concentric cylinder fixture in a Rheometrics Stress Rheometer to simultaneously monitor viscosity and turbidity. We also measure conductivity, and pH, free surfactant activity and sodium ion activity using selective electrodes. The surfactant lowers the viscosity more than can be explained by the simple salt effect and change in neutralization expected for surfactant binding to the polyion. The apparent dissociation of the polyacid also decreases as surfactant is added. These two observations point to a conformational change in the polyion caused by surfactant binding. The implications of this conformational change on the prediction of the critical aggregation concentration of surfactants in the presence of an oppositely-charged polyion will be discussed.

Chairs: Samuel A. Safran and Michael Schick 
Thursday Afternoon, December 4, 1997 
Republic B (S)

1:30 PM *L9.1 
CALCULATION OF THE LYOTROPIC PHASES OF BIOLOGICAL LIPIDS. Marcus Müller, Institut für Physik, Johannes Gutenberg Universität, Mainz, GERMANY; Michael Schick, University of Washington, Dept. of Physics, Seattle, WA.

We have calculated the phase diagrams of monoacylglycerol/water systems employing a simple model in which the hydrocarbon chain is treated within the rotational isomeric state framework, and the head group as rigid. The calculated phase diagram displays lamellar, inverted hexagonal, and Ia3d cubic phases. Variation of the transition between lamellar and inverted hexagonal phases with chain length and head group architecture is in qualitative agreement with experiment. Detailed composition profiles are also obtained. Within the model, interactions between tail segments and head, and between tail segments and water, are identical and characterized by a single parameter. The volumes of the head group and of the chain segments relative to that of water comprise the other two parameters. The system is incompressible. We employ mean-field theory, use a partial enumeration scheme to evaluate the single-lipid partition function, and solve the self-consistent equations within a space of functions possessing the appropriate space-group symmetry.

2:00 PM *L9.2 
INSTABILITIES OF MIXED-LIPID BILAYERS IN LAMELLAR PHASES. William M. Gelbart, Massimo Noro, University of California, Los Angeles, Dept of Chemistry and Biochemistry, Los Angeles, CA.

We consider two scenarios for the instability of a lamellar phase comprised of two amphiphiles. In the first case, the mixture involves two species which prefer different local curvatures - i.e., planar bilayer and cylindrical micelle - for satisfying the hydrophobic effect; depending on the overall concentration of amphiphile, there is a critical threshold for transition from lamellar to hexagonal phase, reflecting the competition between intra-aggregate ''self-energy'' and inter-aggregate interactions. In the second case, we treat mixtures of neutral and charged lipids (or, equivalently, ones with large and small bending elasticities) and discuss the conditions under which a first-order coexistence can arise between two lamellar phases having different layer spacings and compositions.

2:30 PM L9.3 
LOCAL ADHESION OF MEMBRANES. Rebecca Menes, Materials Research Laboratory, University of California Santa Barbara, CA; Samuel A. Safran, Dept of Materials and Interfaces, Weizmann Insitute of Science, Rehovot, ISRAEL.

Membranes that are locally bound to each other, or to a surface, respond by exhibiting overshooting profiles in the vicinity of the binding site. The overshoot strength will depend on the amount of restriction the membrane experiences both due to the local binding and to the other more uniform intermembrane interactions. Systems which are more constrained can show arbitrarily large overshoots in their profiles; systems which are less constrained will show weaker overshoots, observable only in the profile slopes rather than the actual intermembrane gap. The overshooting response of the membrane to local pinning sites strongly affects inter-site interactions and adhesion site aggregation properties, which are important in collective adhesion problems.

2:45 PM *L9.4 
AGGREGATION OF INCLUSIONS IN A LAMELLAR PHASE. Pierre Sens, Weizmann Institute of Science, Rehovot, ISRAEL; Matthew Tuner, Dept. of Physics, Warwick University, UK.

We study theoretically the effect of inclusions such as colloidal particles or proteins in a smectic liquid crystal. The inclusions are assumed to either locally pinch neighbouring lamellae or to push them away, inducing a long range deformation of the smectic phase. If two particles are close enough to each other, the overlap of their deformation fields leads to indirect interactions between them. We study these interactions, and the possible formation of aggregates of inclusions they may induce. A strong aggregation is predicted in ``stiff'' lamellar phases such as those formed from diblock copolymer melts or ionic surfactant solutions, while inclusions do not interact strongly in more ``flexible'' phases formed by nonionic surfactants. We briefly discuss the influence of the finite size of the lamellar phase, and show that qualitative differences are to be expected for thin lamellar films.

3:45 PM *L9.5 
CURVATURE AND ENTROPY OF MICROEMULSONS. T. Tlusty, S. A. Safran, R. Menes, Dept. Materials and Interfaces, Weizmann Institute, Rehovot, ISREAL; R. Strey, University of Cologne, GERMANY.

A useful approach for understanding the structure, phase behavior, and interfacial properties of microemulsions has been one based on the curvature elasticity of the surfactant interface; this energy typically determines the local shape, while the entropy of the system dictates the size distribution of the water or oil domains and their large-scale spatial organization. Theoretical understanding of microemulsions has evolved beginning with simple globular structures and more recently focusing on sponge-like systems. This evolution is reviewed and attention is then focused on a new model for microemulsions based on a network model that can interpolate between spherical, cylindrical, and sponge-like structures. Being a structural model it naturally predicts size, shape, and network topology; the inclusion of entropy into the model allows the thermodyanmics of the system to be calculated in a unified manner. Among these predictions are novel critical points as the system composition is varied as well as both closed loop and three-phase regions in the phase diagram which compare well with recent experiments.

4:15 PM L9.6 
ENTROPIC NETWORK MODEL OF MICROEMULSIONS. T. Tlusty1, S. A. Safran1 and R.Strey2,1Department of Materials and Interfaces, The Weizmann Institute of Science, Rehovot, ISRAEL; 2 Institut fuer Physikalische Chemie, Koeln, GERMANY.

Microemulsions are modeled by a network composed of cylindrical tubes connected by spherical junctions. The network picture enables a unified structural and thermodynamic predictions of the properties of both dilute systems of compact globules as well as the dense asymmetric sponge. 
The cylinders forming the network interact solely via the curvature energy and entropy of their fluctuating surfactant interfaces separating oil and water domains. An effective attraction due to the entropy of the network junctions competes with the repulsion due to the thermal fluctuations of the cylinders. This balance explains several features of the phase diagrams of microemulsions, including the 2-phase closed loops with their two critical points and the 3-phase triangles, which appear when the two coexisting networks expel an excess phase (emulsification failure). 
The inherent interfacial nature of the model results in universal analytic scaling laws in accord with the experimental data that collapse on a single curve. By introducing the effects of thermal softening on the curvature energy the model predicts a crossover between a regime dominated by the spontaneous curvature and a regime where the structure is that of a symmetric sponge, where thermal fluctuations determine the length scale. The model also predicts the scaling of the ultra-low surface tensions in these various regimes.

4:30 PM L9.7 
PROPERTIES OF ALKYLPOLYGLUCOSIDE MICROEMULSIONS. Larry D. Ryan and Eric W. Kaler, Center for Molecular and Engineering Thermodynamics, Department of Chemical Engineering, University of Delaware, Newark, DE.

Alkylpolyglucosides (CmGn surfactants, where m is the number of carbons in the hydrophobic tail and n is the number of hydrophilic glucose units) are an interesting class of nonionic surfactants. These materials are becoming increasingly important industrially, and benefit from excellent biodegradability and ease of manufacture from renewable resources. However, little is understood about the relationship between CmGn architecture and the phase behavior and properties of mixtures containing CmGn. In particular, we discuss here the properties and phase behavior of isotropic, microstructured solutions (microemulsions) of water, oil, and CmGn. Producing microemulsions with CmGn requires the systematic study of the phase behavior of water - oil - CmGn mixtures as a function of temperature and composition. CmGn are nearly insoluble in alkanes, and ternary mixtures of water - alkane - CmGn produce stable emulsions. By using more hydrophilic oils, alkyl ethylene glycol ethers (CkOC2OCk), and increasing CmGn solubility in the oil, we have been able to produce phase behavior resembling that seen for water - alkane - ethoxylated alcohol mixtures. These water - CkOC2OCk - CmG1 mixtures exhibit tricritical phenomena, and the transition from structured to nonstructured solutions associated with that phenomena is probed by using small angle neutron scattering(SANS) and by measuring the wetting-nonwetting transition of these mixtures as a function of the hydrophobicity of the oil. The results of these experiments are discussed in terms of the amphiphilicity factor, which provides a measure of the strength of the microemulsion.

4:45 PM L9.8 
SINGLE CRYSTAL DIFFRACTION FROM A TERNARY CUBIC PHASE WITH DOUBLE-DIAMOND SYMMETRY. P. Duesing, Imperial College, Dept. of Chemistry, London, UNITED KINGDOM; A.R. Faruqi, MRC Laboratory of Molecular Biology, Cambridge, UNITED KINGDOM; C. Toprakcioglu, Patras University, Dept. of Physics, Patras, GREECE.

Ternary mixtures of water, oil and the amphiphile didodecyldimethylammonium bromide (DDAB) are know to form cubic phases over a certain range of composition. The structure of these cubic phases has been investigated by small-angle x-ray and neutron scattering and found to be well-described by a bicontinous network in which the amphiphilic bilayer decorates a triply periodic minimal surface. Analysis of the x-ray diffraction patterns of the cubic phases allows determination of both the symmetry and topology of their structure. We report, for the first time, single crystal diffraction patterns from a cubic structure formed by the ternary system DDAB/D2)/octane with double-diamond symmetry (space group Pn3m) based on the Schwart-D minimal surface.

Chairs: Jacob Klein and Mark J. Stevens 
Friday Morning, December 5, 1997 
Republic B (S)

9:00 AM *L10.1 
MODIFICATION OF SURFACE INTERACTIONS BY DEMDRIMERS AND BY POLYELECTROLYTES. Xuyen Zhang, Manfred Wilhelm, Erika Eiser, Jacob Klein, Dept. of Materials and Interfaces, Weizmann Institute, Rehovot, ISRAEL.

We have used a surface force balance with extremely high sensitivity in the shear-force measuring mode to examine the way in which different surface attached molecules modify normal and shear interactions between surfaces. Dendrimers are compact, uncharged, hyperbranched polymeric structures, and we have investigated the effect of layers of dendrimers in a good organic solvent on the normal and especially the frictional forces between solid surfaces. A very different system is provided by polyelectrolytes adsorbed to surfaces immersed in aqueous electrolyte. We have found that such adsorbed layers can have remarkable effects on both the overall charge state of the surfaces, the normal interactions between them, and particularly the frictional forces.

9:30 AM L10.2 
CONFORMATIONAL DIFFERENCES BETWEEN QUENCHED AND ANNEALED POLYELECTROLYTE BRUSHES. Phillip Schorr, Matthew Tirrell, Univ of Minnesota, Dept of Chem Eng & Matls Science, Minneapolis, MN; Daniel Cook, Jimmy Mays, University of Alabama-Birmingham, Dept of Chemistry, Birmingham, AL.

The grafting of polymer chains to a surface provides a convenient and powerful means of modifying interfacial properties at a liquid/liquid or liquid/solid junction. To optimize the utility of such films, it becomes important to identify and control the factors which effect the structure of the layer and the interactions this layer has with its environment. Due to the electrostatic forces present in a layer of tethered polyelectrolytes, the conformational physics of a charged polymer brush is complex. Recent SCMF theories have suggested significant differences in the physical behavior of strong (quenched) and weak (annealed) polyelectrolytes tethered to a surface. In particular, the self-regulating behavior of weak polyacids introduces an acute sensitivity to the local pH not found in a strong polyacid. In an attempt to experimentally characterize this behavior, the effects of salt concentration, pH, and molecular weight upon the height of both weak and strong polyelectrolyte brushes has been systematically explored. Self-assembled monolayers of poly(4-t-butylstyrene)-b-sodium poly(styrene-4-sulfonate) (PtBS-NaPSS), a strong polyacid, and polystyrene-b-sodium poly(acrylic acid) (PS-NaPAA), a weak polyacid, were adsorbed onto mica from aqueous solution. In both cases, the hydrophobic block provides an anchor for the much larger polyelectrolyte block. Measurements of the brush height at various experimental conditions were made using the surface forces apparatus (SFA). In this presentation, the experimentally observed differences in conformational behavior will be highlighted with some comparisons to recent scaling and SCMF predictions.

9:45 AM *L10.3 
WETTING BEHAVIOR OF POLYMER ASSEMBLIES. Rachel Yerushalmi-Rozen, Ben-Gurion Univ, Dept. of Chemical Engineering, Beer-Sheva, ISRAEL; Jacob Klein, Tobias Kerle, The Weizmann Institute of Science, Dept. of Materials and Interfaces, Rehovot, ISRAEL.

Polymer additives which self assemble into surface structures are routinely used in a variety of technologies for improving wetting and adhesion, inducing biocompatability, and lubrication. Interfaces which are modified by the presence densely tethered polymer brushes and cross-linked polymer networks present a richness of phenomena, which are not observed at the ideal solid-liquid interface. For example, the selective permeability to solvent allows the system to regulate the width of the interfacial zone. Different scenarios of partial wetting may then develop. We investigated wetting of solvated polymer brushes by polymer solutions and wetting of cross-linked polymer networks by polymer melts. We used light microscopy to characterize the wetting properties, Atomic Force Microscopy (AFM) for characterization of the surface topography, Nuclear Reaction Analysis (NRA) for profiling the depth distribution of polymeric species in thin films, and adhesion experiments (JKR-type) for measuring the surface energy of the cross-linked network. Our observations suggest that surface roughness, interfacial width and conformational entropy dominate the wetting behavior of polymer assemblies.

10:45 AM *L10.4 
IONIC EFFECTS IN POLYELECTROLYTE SOLUTIONS. Mark Stevens, Sandia National Laboratories, Albuquerque, NM.

Results of new molecular dynamics simulations of polyelectrolytes in solution with added salt and systems with multivalent ions will be presented. The simulations with added salt show that in dilute concentrations regime the Debye length determines the structure as a function of added salt. For ratios of the Bjerrum length to charge separation less than one, the Debye-Hueckel (DH) approximations works reasonalby well. However, when the ratio is above one the approximation fails. The ionic density near the chains deviates strongly from DH predictions. Simulations with multivalent ions exhibit several fascinating effects. I will show examples of chain collapse, bundle binding, ion bridging and ion capture.

11:15 AM L10.5 
THE ORDINARY-EXTRAORDINARY TRANSITION OF POLYELECTROLYTES REVISITED: A STUDY IN A HIGHLY POLAR ORGANIC SOLVENT. Amit Sehgal, Thomas A.P. Seery, University of Connecticut, Polymer Program and Dept. of Chemistry, Storrs, CT.

The effect of polyelectrolyte environment on the inter and intramolecular interactions was studied using the model system of Sodium Polystyrene Sulfonate (SPS) in N-Methyl Formamide (NMF) by dynamic light scattering. The dual relaxation modes were observed at higher salt concentrations than the suggested ordinary-extraordinary (O-E) transition at the Drifford-Dalbiez ratio. A drop in relative amplitude of the slow mode is suggested as a more quantitative indicator of the transition which corresponds to disruption of long range ordered domains. The change in the environment was achieved by changing the ionic strength of the solution with monovalent NaCl as the low molecular weight electrolyte or by simply changing the temperature. NMF is a highly polar non-aqueous solvent for SPS with a high dielectric constant. Hydrogen bonding aligns solvent molecules resulting in large dipoles. This is manifested by the strong variation in dielectric constant of the solvent with temperature. The dielectric constant also serves as a means of varying the Debye - Huckel screening length and the Bjerrum length.

11:30 AM L10.6 
ELECTROPHORESIS OF POLYELECTROLYTES AND POLYAMPHOLYTES. Didier Long*, Armand Ajdari, Laboratoire de Physico-Chimie Théorique, ESPCI, Paris, FRANCE; Audrey V. Dobrynin, Michael Rubinstein, Department of Chemistry, University of North Carolina, Chapel Hill, NC; Jean-Louis Viovy, Physique et Chimie, Institut Curie, Paris, FRANCE. *on leave for: Dept of Chemical Engineering & Material Science, University of Minnesota, Minneapolis, MN.

By linearizing the electro-hydrodynamic equations and using general arguments, we have recently described the deformation and drift of a polyelectrolyte in solution under the simultaneous action of an electric field and a non-electric force, in high salt-concentration solutions1,2. The Zimm model can be adapted to obtain a more operational description for such problems3, and allows for the description of the electrophoresis of a polyampholyte4. We show in particular that the electrophoretic mobility of such charged polymers depends not only on the total charge but also on the details of the charge distribution along the chain. The very direction of motion can depend on it. Indeed even globally neutral poly ampholytes can move in an applied electric field.

11:45 AM L10.7 
POLYELECTROLYTE PROPERTIES OF FILAMENTOUS BIOPOLYMERS. Jay X. Tang and Paul A. Janmey, Division of Experimental Medicine, Brigham and Women's Hospital, Boston, MA; Wujing Xian, William H. Braunlin, Department of Chemistry, University of Nebraska-Lincoln, Lincoln, NE.

Polyvalent cations induce lateral aggregation of DNA, F-actin, microtubules, and viruses such as the filamentous phages fd/m13 and the tobacco mosaic virus (TMV). Such general effects are due to the common polyelectrolyte nature of these anionic biopolymers and their nonspecific binding to polyvalent ligands of the opposite charge. In addition to shielding the repulsive charges on the macromolecular surface, an attractive interaction between the charged biopolymers may be mediated by the counterions. A balance of forces thereby allows the reversible formation of bundles containing either a single type of filaments or mixtures of different filament types. Recent studies by 25Mg-NMR for F-actin show that Mg2+ ions bind loosely to actin filaments, suggesting that a fundamental mechanism of counterion condensation traditionally applied to DNA also accounts for some biochemical features of protein biopolymers. The biological implications include the binding and crosslinking of cytoskeletal filaments by cationic proteins, storage and release of metal ions, and perhaps transport of charged ligands in cells.

Chair: Sow-Hsin Chen 
Friday Afternoon, December 5, 1997 
Republic B (S)

1:30 PM *L11.1 
HOW WELL DO POLYMER THEORIES APPLY TO POLYMER-MEDIATED MEMBRANE AND CELL-SURFACE INTEREACTIONS? Tonya Kuhl, Joyce Wong, and Jacob Israelachvili, Dept of Chemical Engineering, University of California at Santa Barbara, CA.

Polymers are widely used in a variety of biomaterial applications ranging from protein separation to cell fusion to creating biocompatible surfaces. In order to optimize these properties, there have been a large number of efforts to understand the role of the polymer in mediating biomembrane interactions. In many cases, existing theories have been quite successful in explaining the interactions of lipid bilayers (both supported and as free vesicles in solution), model membranes composed of lipids and proteins, and cells, when polymers are involved. The polymer we will primarily focus on here is poly ethylene oxide (PEO), either free in solution, adsorbed, attached (end-grafted) or acting as tethers for ligands, at low and high coverage and/or concentration. The resulting forces include attractive bridging and depletion interactions as well as steric repulsion depending on the nature of the polymer-membrane interaction. Both static forces and their dynamics will be considered. Quantitative measurements elucidate the forces involved and this information can be used in the design of biomaterials. In many cases there is surprisingly good agreement between experiment and theory.

2:00 PM L11.2 
AMPHIPHILIC COPOLYMERS IN A LYOTROPIC LAMELLAR PHASE. Francisco Castro Roman, Christian Ligoure, Grégoire Porte, Groupe de Dynamique des Phases Condensées, Université Montpellier II, Montpellier, FRANCE.

We study the effect of an amphophilic block copolymer (PPO-PEO) on the thermodynamic properties of a lyotropic lamellar phase (CPCL/Octanol/Brine). The hydrophobic block (PPO) adsorbs onto the fluid membranes, whereas the hydrophilic tails (PEO) decorate them. Depending on the polymer/surfactant weight ratio, one expects two regimes for the adsorbed polymer: a mushroom-like structure at low polymer/surfactant ratio and a brush-like structure in the opposite case. We have obtained convincing evidences of two different mechanisms leading to lamellar/lamellar phase separations induced by incorporation of the polymer, For dilute lamellar phases, the coupling between curvature and polymer composition of the membrane leads to the formation an onion phase. In the opposite case (concentrated phase) a polymer-rich/surfactant-poor lamellar phase coexist with a surfactant-rich/polymer-poor lamellar phase, to avoid the confinement of polymer layers. At intermediate membrane volume fractions, the lamellar phase remains stable against phase separation, even for a large amount of polymer added. In this case, a quantitative analysis of the small angles neutron scattering patterns show that the polymer strongly enhances the stabilizing Helfrich's interactions between membranes, by increasing the effective bilayer thickness: it now, consists of the sum of the naked membrane and the decorating polymer layers.

2:15 PM L11.3 
COMPLEX PHASE BEHAVIOR IN AQUEOUS SUSPENSIONS OF POLYMERIC SURFACTANTS. D. A. Hajduk, M. B. Kossuth, F. S. Bates, Dept of Chemical Engineering, University of Minnesota, Minneapolis, MN; M. A. Hillmyer, Dept of Chemistry, University of Minnesota, Minneapolis, MN.

We discuss the phase behavior of aqueous solutions of poly(ethylene oxide)-poly(ethylethylene) (PEO-PEE) block copolymers. The molecular weights of these materials are roughly twice that of common poly(ethylene oxide)- poly(propylene oxide) (PEO-PPO) surfactants, leading to microphase separation and a rich mesophase polymorphism in the absence of solvent. Despite the strongly thermotropic nature of the PEO-water interaction, the phase behavior in solutions of high polymer concentration is primarily lyotropic. Mesophase transitions observed in this regime show characteristics similar to those observed in undiluted block copolymers. A crossover to thermotropic behavior occurs at concentrations of order 60 wt% polymer, which corresponds to the formation of a stoichiometric complex of 3 water molecules per ethylene oxide repeat. This crossover is accompanied by changes in the structural dimensions, long-range order, and mechanical characteristics of the mesophases. Transitions in these dilute solutions resemble those previously observed in surfactant systems. These findings suggest that the complex structure of hydrated PEO plays a central role in determining the phase behavior of the system.

2:30 PM L11.4 
SHEAR-INDUCED CRYSTAL STRUCTURES IN AQUEOUS SOLUTIONS OF PEO-PPO-PEO TRIBLOCK COPOLYMERS. Tania M. Slawecki, Charles J. Glinka, Boualem Hammouda, National Institute of Standards and Technology, Gaithersburg, MD.

Aqueous solutions of the amphiphilic triblock copolymer, PEO25PPO40PEO25, were investigated using small-angle neutron scattering (SANS). This triblock exhibits rich phase behavior with increasing temperature due to the increasingly hydrophobic PPO center block, and is known to form a micellar crystalline phase in a particular temperature range. By imposing a small shear, the polycrystallites are known to align to form a single crystal, reported to be cubatic. We have undertaken a more thorough investigation of this crystalline phase and have found that shearing can produce new structures not previously reported. The first appears to be an equilibrium structure to which the sample relaxes upon removal of steady shear. Additionally, two completely different SANS patterns arose as a result of subjecting the sample to oscillatory shear, depending upon the magnitude and frequency of the shear, and hybrid states were found to exist inbetween. The oscillatory shear-induced structures showed no tendency to relax to another state upon cessation of shear on the timescale of the experiment. Rather, they became even more pronounced when shear was removed and could only be disrupted by again subjecting the sample to steady shear. By observing all of these structures with the incident beam both parallel and perpendicular to the flow direction, the SANS data encodes information from which the three dimensional micellar crystal structures can be deduced.

2:45 PM *L11.5 
NEUTRON SCATTERING STUDIES OF THE STRUCTURE, INTERACTION AND VISCOELASTICITY OF TRI-BLOCK CO-POLYMER MICELLES IN AQUEOUS SOLUTION. Y.C. Liu, Department of Materials Science and Engineering,S. H. Chen, Department of Nuclear Engineering, MIT, Cambridge, MA; J.S. Huang, Exxon Research and Engineering Co., Annandale, NJ; F. Mallamace, Physics Department, Univ. of Messina, ITALY.

We investigate the microstructure and their mutual interaction of micellar aggregates formed by two tri-block co-polymers, P84 and P104, belonging to the family of Pluronic. Above the room temperature the co-polymers aquire the surfactant characteristics and self-assemble into spherical micelles in an aqueous environment. The aggregation number of the micelle is highly temperature dependent but practically concetration independent until it reaches the disorder-to-order transition boundary. We proposed a ''Cap-and-Gown'' model for the microstructure of the micelle, taking into account hydration in both the core and the corona regions of the micelle. We treat the inter-micellar correlation in terms of a ''Sticky-sphere'' model. The result of the analysis show excellent agreement with SANS intensity distributions in an absolute scale. The parameters extracted from SANS data includes: the temperature-dependent aggregation number, hydration number in the core and corona regions, dimensions of the spherical micelle, and the stickiness of the micellar surface which increases with increasing temperatue.The structural parametrs and the inter-particle structure factor obtained are then used to calculate the solution viscosity up to volume fraction of 40 according to a recent theory of Cohen et al. We obtain excellent agreement with the measured zero-shear viscosity as a function of temperature and volume fraction. The comparison is being extended to the complex viscosity at finite frequencies.