Meetings & Events


spring 1997 logo1997 MRS Spring Meeting & Exhibit

March 31 - April 4, 1997 | San Francisco
Meeting Chairs: Linda G. Griffith-Cima, David J. Eaglesham, Alexander H. King

Symposium W—Metastability and Critical Phenomenon in Polymer Phase Behavior


Stephen Cheng 
Dept of Polymer Sci 
Univ of Akron 
Akron, OH 44325-3909 

Edwin Thomas
Dept of MS&E
Room 13-5094
Cambridge, MA 02139

Symposium Support
*British Petroleum
*Dutch State Mines
*General Electric Company, Corporate Research & Development
*Hoechst Celanese Corp.
*Philips Petroleum Co.
*Wright Patterson AFB

In sessions below "*" indicates an invited paper.

Chairs: Stephen Z. D. Cheng and Edwin L. Thomas 
Monday Morning, March 31, 1997
Salon 12

8:30 AM *W1.1 

It will be shown that the phase behavior of polymers can only be appreciated and controlled if note is taken of the role played by metastability, real or perceived. The frequent dominance of metastable states has long been recognized in phase transformations in general, finding expression in the century-old Ostwald Stage Rule, which will here be revisited. With polymers in particular, metastability also serves to highlight certain general yet hitherto unrecognized issues. One such situation has arisen in recent studies of polymer crystallization. Here the experimental observations have led to the recognition that the limited size of the newly forming crystal can cause an inversion of the stability of possible phase variants (polymorphs): i.e., a phase which is metastable for infinite size can become the stable one when sufficiently small. This can have a profound influence on phase developments in general, and leads to a unifying picture of chain-folded and chain-extended crystallization for polymers in particular. It also creates a confluence between thermodynamic (stability) and kinetic (rates) considerations setting pointers for explanation and control of polymorphism. Another approach is through the high supercoolings required, especially for polymers, when a metastable phase region may be entered, which by the preceding considerations will dominate the phase development. Examples will be quoted from the field of liquid crystals and physical gels. The latter provides examples of hierarchies of metastability (metastable phases within metastable phases, and so on) and also points to metastable molecular conformational changes while in solution. This could be of consequence for two basic areas: solution behavior of chain molecules and the conceptual approach to crystallization.

9:00 AM *W1.2 
METASTABLE MICROPHASES AND NANOPHASES IN POLYMERS, Bernhard Wunderlich, Univ of Tennessee, Dept of Chemistry, Knoxville, TN.

All types of condensed phases and mesophases can be separated into nine types, making use of order and mobility parameters, namely: liquid, liquid crystals, plastic crystals, condis crystals (i.e. conformationally disordered crystals), crystals, and four glasses corresponding in structure to the liquid and mesophases (glass, LC glass, PC glass, and CD glass) [1]. A schematic phase diagram is then derived for a one-component system which includes all possible phases and combinations of phases in equilibrium and nonequilibrium. Assuming only five condensed phases for the example, namely: crystal, melt, glass, mesophase, and mesophase glass, fifteen different single phases and phase-combinations can exist with simultaneous presences of up to three phases. Examples of all these stable and metastable phases and phase-combinations are documented with results from our laboratory. In many cases the phases have nanometer dimensions, so that the microphase areas are better called nanophases. This scheme can be extended to more components and more than five phases and produces then many more possible phase areas than can be handled experimentally.

9:30 AM W1.3 

The phase behavIor and morphology of blends comprised of two semicrystalline polymers, poly(vinylidenefluoride) (PVF) and poly(1,4-butylene adipate) (PBA) has been investigated. Four phase transitions are observed in this blend system: a glass transition, melting of the PBA phase, melting of the PVF phase, and liquid-liquid phase separation above the LCST of 235C. Both blend components are semicrystalline at room temperature over a wide composition range. These two crystalline phases coexist with a mixed amorphous phase, as is evidenced by the observation of a single glass transition in all blends. The equilibrium melting point of the higher melting PVF component is lowered upon mixing with PBA. These results demonstrate the miscible nature of PVF/PBA blends. Remarkably, no depression of the equilibrium melting point of PBA was observed in its blends with PVF. This behavior appears to arise from morphological rather than thermodynamic effects.

9:45 AM W1.4 

Recent experiments by in-situ electron diffraction, dark field transmission electron microscopy, high resolution transmission electron microscopy, and white-beam synchrotron x-ray topography have provided a detailed view of the radiation-induced solid-state phase transition from monomer to polymer for the carbazolyl-substituted diacetylene DCHD. It has been found that the material remains crystalline throughout the phase transition, with a continuous variation in the crystallographic angle between the monoclinic a and b axes from 94 degrees in the monomer to 108 degrees in the polymer. These results have made it possible to determine the transformation matrix, T, which relates the metric tensor of the monomer crystal to that of the polymer. Here, the results of experiments on DCHD polymerization from our laboratory and from the literature will be critically compared to molecular simulations of crystalline intermediate states in which the polyester content is varied in a systematic manner. This analysis makes it possible to elucidate the various intermolecular and intramolecular contributions to the energetic stability of the system as the polymerization reaction proceeds.

10:30 AM *W1.5 
BETA ISOTACTIC POLYPROPYLENE: A "REENTRANT" PHASE IN CRYSTALLINE POLYMERS, B. Lotz, T. Okihara, M. Schumacher, A. Thierry, J. C. Wittmann, CNRS, ULP, Strasbourg, FRANCE; W. Stocker, Humbolt Universitat zu Berlin, Inst fur Physik, Berlin, GERMANY.

Out of the various crystal phases of isotactic polypropylene, the two most common are the monoclinic and trigonal (hexagonal) modifications. Although iPP is the stable phase, its growth rate is smaller than that of iPP, at least in a temperature ''window'' extending from 100 to 140C, i.e., over most of the ''conventional'' crystallization range of iPP. With regard to growth kinetics, iPP appears to be the only known example of a polymer crystal phase analogous to the so-called ' 're-entrant'' phases of liquid crystals, i.e., a phase the domain of existence of which is bounded at two different frontiers by one and the same other phase. The structure of iPP is a rare case of frustrated packing of chiral helices. Its formation via epitaxy on suitable nucleating substrates and the details of its structure have been determined by means of electron diffraction and, more recently, atomic force microscopy. Conversion of the chiral iPP to racemic iPP phase can only take place via melting and recrystallization. However, to growth transitions are observed at high and low crystallization temperatures, i.e., outside the above-mentioned ''window''. With the availability of both crystal structures, these grown transitions can now be analyzed in molecular terms. Furthermore, they induce a complicated pattern of melting and recrystallization of iPP (first described by Varga et al. ), which is sensitive in particular to the so-called ''post-crystallization'' thermal history, and which has now been illustrated and analyzed for both spherulites and single crystals.

11:00 AM *W1.6 

Syndiotactic polypropylene is particularly suited for studies of the mechanism of polymer crystallization. First, as is known from AFM and EM investigations, we have stacks of laterally extended crystallites and thus a simpler structure as in the case of the ''cross-hatched'' isotactic polypropylene. Secondly, as shown by time-dependent SAXS experiments, in contrast to polyethylene there occurs no crystal thickening during an isothermal crystallization and a successive heating to the melting point. Hence, for syndiotactic polypropylene the relationships between crystallization temperature, crystallite thickness, crystallization rate and crystallite melting point can be exactly determined. We have carried out a comprehensive SAXS and calorimetric study on well-characterized samples synthesized with a metallocene catalyst. In addition to the kinetics of primary crystallization we investigated the secondary crystallization during cooling, which occurs by the ''insertion mode,'' and analyzed in detail melting and recrystallization processes found during heating. The latter processes are of particular interest because crystallization here takes place with a much higher rate and smaller distance to the melting point as for the initial primary mode. It appears very difficult to interpret the results on the basis of the Hoffman Lauritzen scheme with a crystallization process controlled by secondary nucleation. We introduce a different concept, where crystallization rates are determined by the necessary stretching and disentangling of the chain parts in front of the growth face and by relaxation processes in the amorphous layers. The parameters derived from the experiments make sense and thus lend support to our new view.

11:30 AM W1.7 
CONTROLLING FACTORS FOR THE OCCURRENCE OF HETEROEPITAXY OF POLYMERS, Decai Yang, Shouke Yan, Changchun Inst of Applied Chemistry, Changchun, CHINA.

Most heteroepitaxies of polymers have occurred between zigzag chain polymers, e.g., polyethylene (PE), and isotactic polypropylene (iPP). This kind of epitaxy is explained by a one-dimensional chain-row match, i. e., the alignment of the zigzag chains along the rows of methyl groups of iPP. Some experimental phenomena, however, cannot be explained by this pure geometrical model. In fact, polymer epitaxy is a surface-induced growth of one phase (guest crystal) on a substrate phase (host crystal) in strictly defined crystallographic orientation. Unambiguously, secondary nucleation plays an important role in the epitaxial crystallization. The requirement of secondary nucleation is that the dimension of the substrate crystal (the host lamellar thickness) is greater than or at least equal to the dimension of the critical secondary nucleus (similar size to the guest lamellar thickness). Once the necessary requirement is satisfied, subsequent growth is based on the crystallization characteristics of the material itself. According to the view point of the secondary nucleation, the effects of the iPP lamellar thickness and PE recrystallization temperature (supercooling) on epitaxial growth of PE in PE-iPP epitaxial system have been explained satisfactorily. Crystallization rate is another important factor affecting the epitaxial growth of polymers. In contrast to the traditional epitaxy of small molecules, high crystallization rates result in perfect alignment and thicker epitaxial layer of PE lamellae on highly oriented iPP film, while slower crystallization rates lead to poor alignment and thinner epitaxial layer. If the crystallization rate is very slow, no epitaxy occurs at all. It should be pointed out that under the quenching condition, besides the normally observed epitaxial orientations between the c-axes of PE and iPP (50), there are also some PE crystals aligned with their chains parallel to the c-axes of the iPP substrate. Annealing the quenched sample at elevated temperatures, the parallel alignment of PE lamellae disappears, which implies that the parallel PE lamellae exhibit a metastability.

11:45 AM W1.8 
SECTORIZED SINGLE CRYSTALS OF SYNDIOTACTIC POLYPROPYLENE FRACTIONS, Zhengzheng Bu, Wensheng Zhou, Stephen Z. D. Cheng, Univ of Akron, Dept of Polymer Sci, Akron, OH; Eric T. Hsieh, Philips Petroleum Co, Bartlesville, OK.

Highly faceted, regular, lathlike lamellar single crystals of syndiotactic polypropylene (s-PP) fractions have been investigated through transmission electron microscopy (TEM), atomic force microscopy (AFM) and electron diffraction (ED). Single crystals of a-PP over one micrometer in size can be grown from the melt in thin films. ED results obtained from the s-PP single crystals indicate that a unit cell III with = 1.450 nm, = 1.120 nm, and = 0.740 nm, as proposed by Lovinger and Lotz. At high crystallization temperatures, relatively low molecular weight s-PP fractions can grow lamellar single crystals with microsectors. The polyethelene decoration method has been used to identify the chain folding direction and no preferred orientation has been observed on the nonsectorized lamellar crystals. Sectorized lamellar single crystals show two different regions. In the sectors along the long axis (the -axis), the chain folding is found to be parallel to the 010 direction. In the sectors along the short axis (the -axis), little preferred orientation can be found. The deformation method of nonsectorized, high molecular weight s-PP single crystals on a plastic film has also been utilized to determine the chain folidng direction. Microfibrillar structures can be observed in the cracks of the single crystals along both the - and -axes after deformation. This indicates that the folding direction in these nonsectorized, high molecular weight s-PP single crystals may be either along the (110) planes or a combination of the (100) and (010) microfolding and microsectoring. Zigzag shaped edges on the deformed single crystals along the -axis are also observed and the sliding planes can be identified as the (110) planes.

Chair: Andrew J. Lovinger
Monday Afternoon, March 31, 1997
Salon 12

1:30 PM *W2.1 
X-RAY ANALYSES OF THE SUPRAMOLECULAR STRUCTURES FORMED BY POLYMETHACRYLAES WITH HIGH TAPERED SIDE GROUPS, Yong Ku Kwon, J. Blackwell, Virgil Percec, Case Western Reserve Univ, Dept of Macromolecular Sci, Cleveland, OH.

X-ray methods have been used to investigate the supramolecular structures formed by polymethacrylates with highly tapered side groups, and by their monomeric precursors. Typical of the compounds studies are poly [2- (2 methacryloyloxyethoxy) ethoxy] ethoxy ethyl-3,4,5-tris (p-dodecyloxybenzyloxy) benzoate (H12-ABG-4EO-PMA) and 2- [2-(2-hydroxyoxyethoxy) ethoxy ethoxy] ethyl-3 4, 5-tris (p-dodecyloxybenzyloxy) benzoate(H12-ABG-4EO OH). Both of those materials form a three dimensionally ordered hexagonal phase at room temperature, which is converted to a columnar hexagonal liquid crystalline phase (fh) at higher temperature. X-ray patterns for oriented fibrous specimens of H12-ABG-4EO-PMA at room temperature point to cylindrical moleties packed on a hexagonal lattice with dimension a = b = 60.4 Å. The wide angle X-ray data is consistent with possible ''pine tree'' models in which the tapered side groups are stacked with helical symmetry within the columns. With their ''planes'' tilted by 40C there is a transition to a fh columnar hexagonal mesophase. In which the internal structure of the cylinder is more disordered; all that remains are limited correlations due to stacking of the side chains along the axial direction. We have compared the above structures with those formed by monomer precursor H12-ABG 4EO-OH, and homologous polymer and precursors with fully hydrogenated and semifluorinated aliphatic talls. Structural work will be presented on several of these materials, including the precursor with C18 alkyl talls and a tri-oxyethylene spacer (H18-ABG-3EO-OH), which forms two different ordered phases, and the semi fluorinated polymer F6H4-ABG-4EO-PMA, which forms the most ordered structure seen so far for this class of compounds.

2:00 PM *W2.2 
WHAT DO WE MEAN BY "PHASES" IN A SEMICRYSTALLINE POLYMER?, Herve Marand, Srinivas Srivatsan, Ravi Verma, Virginia Polytechnic Inst, Dept of Chemistry, Blacksburg, VA; Vesselin Velikov, Virginia Polytechnic Inst, Dept of Chemisrty, Blacksburg, VA.

Most polymers crystallize under quiescent conditions in the form of spherulites. An isothermal crystallization process is usually thought to consist of a primary stage where primary nucleation, lamellar growth, branching, and splaying result in the development of spherulites and a secondary stage where further crystallization occurs behind the spherulitic growth front. The mere observation at the end of the primary stage (i.e., at the spherulitic impingement time), that the degree of crystallinity is substantially below unity and that the crystal phase exhibits a very high surface-to-volume ratio, makes it clear that the morphology resulting from crystallization is governed by kinetic factors and is not an equilibrium one. During secondary crystallization, the significant driving force to reduce the total crystal-liquid interfacial free energy and to convert the remaining amorphous fraction to a crystal phase must, however, be examined in light of the extreme increase in chain confinement and resulting decrease in molecular mobility that accompanies lamellar crystal formation. The secondary crystallization stage is therefore one where ease of conformational changes and chain transport become key issues. We have previously shown that semiflexible polymers such as PEEK, PPS, PET, PBT, PET, it-PS, nylons display a number of universal features during secondary crystallization (morphology, phase transition, and relaxation behavior). We will focus here on a discussion of the determination of the equilibrium melting temperature and of the crystal and amorphous densities, thus on the metastability of the crystal and liquid fractions. We sill draw from the use of time-resolved small angle x-ray scattering and wide angle x-ray diffraction, dilatometry, and differential scanning calorimetry to show the imapplicability of the Hoffman-Weeks method for the determination of the equilibrium melting temperature and provide compelling evidence against the use of density measurements for the evaluation of the degree of crystallinity in semiflexible polymers.

2:30 PM *W2.3 

Several new insights into isothermal melt crystallization of semicrystalline polymers have been obtained using time-resolved simultaneous synchrotron SAXS/WAXS (SWAX) techniques. The polymers studied included semistiff systems: PAEK, PET, PTT and nylon; and flexible systems: PE, PP and POM. SAXS results indicate a common feature during crystallization in these polymers, i.e., the lamellar long period always decreases with crystallization time. Further analysis using the correlation function reveals that the average crystal lamellar thickness decreases in semistiff polymers but increases slightly in flexible polymers. We ascribe the decrease in lamellar thickness as due to the secondary crystallization which takes place in the restrained melt and produces thinner lamellae. No lamellar thickening is found in semistiff polymers. Comparing results from SWAX measurements, we observed SAXS occurs slightly before WAXD at lower degrees of supercooling. This indicates that density fluctuations may be precursors of crystallization.

3:30 PM *W2.4 

Investigation of molecular weight (M) dependence of crysatl growth rate is one of the most important unsolved problems in polymer science. It is desireable for the investigation to observe it for wide range of M and to observe it on single crystals, because it is necessary to know correctly to which lattice plane the growth rate is related. But it is unfortunate that single crystals can be obtained only for narrow range of M in the case of folded chain crystals, which causes difficulties in solving the mechanism of M dependenc of the growth rate. In this study we have succeeded to observe it on single crystals for wide range of M at the first time using crystallization into the metastabe mobile phase from the melt, which results in formation of extended chain single crystals (ECSCs), as has been predicted by chain sliding diffusion theory presented by one of us (MH). This clearly shows the important role of the metastable mobile phase, such as hexagonal or liquid crystlline phases. Fractionated polyethylene (PE) was used in this study. They were crystallized at a pressure (P) below the triple point pressure, P=0.5GPa, where metastable hexagonal phases can be easily seen. It is shown that single crystals can be formed even for high molecular weight materials, such as number averaged M, Mn = 100 thousand. Reliable experimental formula of the M dependence of the lateral growth rate (V) was obtained. We proposed an expanded sliding diffusion theory which explained well the observed results.

4:00 PM *W2.5 

Phase diagrams of binary crystalline polymer blends involving polyvinylidene fluoride and polybutylene adipate as well as with ethylene vinyl acetate (EVA) copolymer have been established by means of differential scanning calorimetry, light scattering, and optical microscopy. Time evolution of domain structure and crystallization behavior in these blends have been investigated with emphasis on the competition between kinetics of spherulitic growth and dynamics of please separation via spinodal decomposition. Isothermal quenching to higher temperatures results in the development of spherulitic structure, followed by growth of spherulites until impingement takes place among spherulites. The rejected PBA molecules crystallize at the interspherulitic regions while propagating toward to the core of the PVDF spherulites. To account for the coupling between dynamics of phase separation and crystallization kinetics, a theoretical model based on time-dependent Ginzburg-Landau equation coupled with crystallization rate has been introduced. Numerical simulation has been performed on a two-dimensional square lattice by using a finite difference method for spatial derivative and an explicit method for temporal derivative. The predictive capability of our theory has been tested with the experimental results.

4:30 PM *W2.6 

Thin films of poly(styrene-block-methyl methacrylate) copolymer [P(St b-MMA)] with the thickness of ca. 50 nm were prepared on gold substrate by a dip-coating method. The surface chemical composition of the P(St-b MMA( film was investigated on the basis of the temperature-dependent x ray photoelectron spectroscopic (TDXPS) and the angular-dependent XPS (ADXPS) measurements. The temperature, at which the surface composition started to change, was defined as the glass transition temperature, Tg, at the surface region. The surface Tg for the P(St-b MMA) thin film was evaluated on the basis of the temperature-dependent x-ray photoelectron spectroscopic (TDXPS) measurement. It was revealed that the surface, Tg, for the P(St-b-MMA) film was much lower than that for the bulk sample. Tg for the P(St-b-MMA) film increased with an increase in depth distance from the outermost surface. The molecular weight dependence of the surface Tg for the P(St-b-MMA) film was more pronounced than that in the bulk sample, and that was expressed as a function of M. A remarkable lowering of the surface Tg compared with the bulk one can be explained by a surface excess free volume due to the surface localization of chain end groups. The surface segregation of chain end groups was confirmed by dynamic secondary ion mass spectroscopic measurement.

Chair: A. H. Windle
Tuesday Morning, April 1, 1997
Salon 12

8:30 AM *W3.1 
MICROPHASE FORMATION IN COMBLIKE COPOLYMERS OBTAINED BY HYDROGEN BONDING BETWEEN HOMOPOLYMERS AND END-FUNCTIONALIZED OLIGOMERS, Gerrit ten Brinke, Univ of Groningen, Dept of Polymer Science, Groningen, NETHERLANDS; Olli Ikkala, Helsinki Univ of Technology, Dept of Technical Physics, Helsinki, FINLAND.

Comblike copolymer systems obtained by hydrogen bonding between end- functionalized oligomers, like alkyl phenols, and homopolymers, such as poly(4-vinyl pyridine), have been investigated experimentally, with computer sections and theoretically. The phase diagram of these systems is rather complex, because it combines the complexity of ordinary comb copolymers with the thermo reversible nature of the association. Experimental examples of order-disorder transitions will be presented. The relation between the presence of a correlation hole scattering peak at finite non-zero angle in the homogeneous mixtures and the strength of the hydrogen bonding will be discussed. The phase diagram will be addressed in some detail by computer simuations in combination with theoretical modeling based on the chemical equilibria involved and using the Random Phase Approximation. Macrophase separation and microphase separation wil occur depending on the temperature (or interaction strength) and the composition of the system. An excess of either component in terms of the number of functional groups, favors macrophase separation into a homogeneous and a microphase separated phase.

9:00 AM *W3.2 
STABILITY OF ORDERED PHASES IN DIBLOCK COPOLYMER MELTS, An-Chang Shi, Xerox Research Centre of Canada, Mississauga, ON, CANADA; Mohamed Laradji, Univ of Toronto, Dept of Physics, Toronto, CANADA; Jaan Noolandi, Xerox Research Centre of Canada, Mississauga, CANADA; Rashmi C. Desai, Univ of Toronto, Dept of Physics, Toronto, ON.

Due to their amphiphilic mature, diblock polymers form a variety of ordered microphases. The phase behavior of diblock copolymer melts has attracted considerable experimental and theoretical attention, and the equilibrium morphologies of diblock copolymers are well characterized. However, the stability of these ordered states and the processes by which these phases form are relatively unexplored. We have developed a theory of anisotropic fluctuations in diblock copolymers using a self-consistent expansion around the exact mean-field solution [1]. The availability of the anisotropic correlation functions enables us to investigate the stability of the copolymer ordered phases and the fluctuation modes driving the ordered phase into unstable. The stability analysis demonstrates that the classical phases (the lamellar, cylindrical and body-centered-cubic) have a large stable region, and the mean-field one-phase regions of these phases always lie within their spinodals. On the other hand, the haxagonally perforated lamellar phase has a metastable region within the center of the lamellar-cylindrical phase boundaries. The most unstable modes leading to the spinodals are identified, and are used to obtain information about the kinetic pathway of the order-order phase transitions and the epitaxial relation between the ordered phases.

9:30 AM W3.3 
ARCHITECTURAL CONTROL OF GRAFT COPOLYMER MORPHOLOGY, Samuel P. Gido, Chin Lee, Darrin J. Pochan, Univ of Massachusetts, Dept of Polymer Science & Engr, Amherst, MA; Jimmy W. Mays, Univ of Alabama-Birmingham, Dept of Chemistry, Birmingham, AL; Nikos Hadjichristidis, Univ of Athens, Dept of Chemistry, Athens, GREECE.

The effect of well defined graft copolymer architecture on the formation of self assembling morphologies with long-range order has been systematically investigated. Four series of samples across a range of component volume fractions were characterized using TEM and SAXS for different model architectures of polystyrene-polyisoprene single graft and double graft copolymers. The simplest architecture was an symmetric single graft, IS, or Y-shaped molecule. This architecture, formed by grafting a poly(styrene) block onto the center of a poly(isoprene) backbone, is found to shift the volume fraction windows in which specific strongly microphase separated morphologies are observed to higher volume fractions of the PS graft material than in the corresponding linear diblock copolymers. These findings are in good agreement with recently calculated theoretical phase behavior. The effects of asymmetric grafting of the single PS chain of the PI backbone was investigated in a series of II'S materials. Additionally, the effect of multiple graft architecture was explored with S (-shaped) and (Si)I(SI) (p-shaped) materials, each of which has two trifunctional branch points per molecule. It was found that to a good approximation, the behavior of the double graft materials can be mapped onto the behavior of the single graft materials by considering the double graft molecules to be divided into component single graft parts.

9:45 AM W3.4 
PHASE TRANSITIONS OF A RANDOM COPOLYMER, Andrei Moskalenko, Yuri Kuznetsov, Kenneth A. Dawson, Univ College Dublin, Dept of Chemistry, Dublin, IRELAND.

We investigate the equilibrium properties of a random copolymer chain with quenched disorder. We apply a variational approach based on a generic quadratic trial Hamiltonian in terms of the correlation functions of monomer Fourier coordinates in the replica space. This method has the advantage that it allows us to incorporate fluctuations of the density, determined self-consistently, and to study collapse, phase separation transitions, and the onset of the freezing transition within the same mean field theory. The effective free energy of the system is derived analytically and analyzed numerically. The radius of gyration, end-to-end distance, and various order parameters are treated as observables and evaluated by introducing appropriate external fields to the Hamiltonian. We obtain the phase diagram in terms of the model parameters, the scaling and critical composition for the freezing transition, and the dependence of correlation functions on the chain index. We propose a simple Ansatz which enables us to recover the main results without considerable computational efforts. Finally, we compare our results with those from different approaches and discuss the approximations of the method and its possible applications to copolymeric molecules and networks.

10:30 AM *W3.5 
ORDER, DISORDER AND PHASE SEPARATION IN BLOCK COPOLYMER-HOMOPOLYMER BLENDS, Frank S. Bates, Univ of California-S Barbara, Dept of Chem Engr, Santa Barbara, CA; Wayne W. Maurer, Timothy P. Lodge, Marc A. Hillmyer, Univ of Minnesota, Dept of Chem Engr & Matls Science, Minneapolis, MN; Kirstoffer Almdal, Kell Mortensen, Riso National Laboratory, Roskilde, DENMARK.

Block copolymers and homopolymer blends belong to different ''universality'' classes leading to microphase and macrophase separation, respectively. Adding small amounts of a block copolymer to an incompatible polymer-polymer mixture can facilitate the formation of well defined two-phase morphologies that are often required for optimal material mechanical properties. Addition of homopolymer to a block copolymer melt may induce formation of a variety of ordered morphologies, including some not found in the undiluted state. Recent mean-field theories have demonstrated a rich compliment of phase behaviors that depend on the component molecular weights, overall composition, polymer-polymer interaction strengths, and block copolymer symmetry. This lecture will focus on the simplest case: Symmetric blends of equal molecular weight homopolymers (A + B) with a symmetric A-B diblock copolymer. Within mean-field theory, the block copolymer lamellar-to-disorder transition and binary homopolymer mixture critical point are connected by a line of second-order transitions that include a multicritical Lifshitz point. Small-angle neutron scattering (SANS) measurements conducted on high molecular weight A-B/A/B mixtures confirm mean-field Lifshitz behavior in the homogeneous state at the predicted composition. However, fluctuation effects interrupt the lamellar unbinding transition leading to a bicontinuous microemulsion as evidenced by transmission election micrographs. The role of fluctuations has been probed by examining the phase behavior of lower molecular weight systems.

11:00 AM *W3.6 
MANIPULATING SURFACE INTERACTIONS TO CONTROL DIBLOCK COPOLYMERS, Thomas P. Russell, Paul Mansky, Elbert Huang, Lee Rockford, Yun Liu, Univ of Massachusetts, Dept of Polymer Science & Engr, Amherst, MA; Craig Hawker, IBM Almaden Research Center, San Jose, CA; Jimmy W. Mays, Univ of Alabama-Birmingham, Dept of Chemistry, Birmingham, AL.

Diblock copolymers exhibit a wide range of microphase separated structures depending upon the relative molecular weights of the two blocks. In the presence of a strongly interacting surface, the microphase separated morphology is strongly oriented parallel to the surface plane. Mediating these strong surface interactions lifts this constraint on the orientation and affords one the opportunity to alter the orientation of the microphase separated morphology with respect to the surface. These interactions can be modified by altering the interactions uniformly across the surface (as, for example, by attaching a random copolymer brush to a surface) or by using a heterogeneous surface where the interactions vary in a periodic manner. Alternatively, the orientation of the diblock copolymer morphologies can be controlled by use of electric fields. In the case of thin films or where the separation distance between the electrodes is small, then very low voltages can be used to produce large electric fields. Provided the dielectric constant of the two components of the copolymer are different, then an electric field provides a superb means of controlling the orientation of the copolymer microdomains. In this presentation, our efforts in manipulating and controlling the orientation of the nanoscopic domains of diblock copolymers will be discussed for diblock copolymers of poly(methyl methacrylate) and poly(n-butyl methacrylate).

11:30 AM W3.7 
EFFECT OF CONFORMATIONAL AND ARCHITECTURAL ASYMMETRY ON LONG RANGE ORDER IN MICROPHASE SEPARATED BLOCK COPOLYMERS, Samuel P. Gido, Darrin J. Pochan, Univ of Massachusetts, Dept of Polymer Science & Engr, Amherst, MA; Jimmy W. Mays, Univ of Alabama-Birmingham, Dept of Chemistry, Birmingham, AL.

In morphological investigations, using TEM and SAXS, of single graft polystyrene polyisoprene, IS, copolymers and linear, conformationally asymmetric polyisoprene-poly-t-butylmethacrylate (PI-PtBMA) diblock copolymers it was found that there is a striking difference in the degree of long range order of cylindrical structures between the two sides of the morphology diagram. The PI-PtBMA linear diblocks form cylindrical morphologies of tremendous long range order when the stiffer PtBMA block is in the corona of the cylindrical structures and they form poor long range order when the more flexible PI block is in the corona. Similarly cylindrical structures in the IS materials show strong long range order when the two PI block per molecule are in the corona and weak long range order when the single PS block is in the corona. One particular composition of IS cylinders with PS in the corona was found to be totally incapable of forming any long range order at all. This material formed a randomly oriented worm-like micelle structure which was confirmed to be the equilibrium morphology through selective solvent casting and annealing studies. The material was cast from a selective solvent, resulting in the formation of a lamellar structure. Upon removal of the selective solvent and thermal annealing, the unstable lamellar structure transformed itself back into the equilibrium worm-like micelle phase through an intermediate structure of perforated or lacy lamella. The effect of relative chain stiffness and graft architecture on the degree of long range order can be understood by a calculation of the effect of these parameters on the preference for interfacial mean and Gaussian curvatures.

11:45 AM W3.8 
NANOSTRUCTURES AND FLOW ALIGNMENT OF ABC TRIBLOCK COPOLYMERS: EFFECT OF MOLECULAR SIZE, Zhong-Ren Chen, Julie A. Kornfield, California Inst of Technology, Dept of Chemical Engr, Pasadena, CA; Steven D. Smith, Procter & Gamble Co, Corporate Research Div, Cincinnati, OH.

Phase behavior of block copolymers is of vital importance in designing block copolymers for potential applications in nanotechnology. We focus on the effect of molecular size on nanostructures, dynamics, and flow alignment behavior. The model ABC triblock system can be synthesized anionically in all three different permutations, and are composed of styrene (S), isoprene (I), and a random copolymer segment of styrene and isoprene (R), with equal length blocks and 50:50 composition of the R block. Molecular masses range from 15,000 g/mol per block (15K-15K 15K) to 35,000 g/mol per block (35K-35K-35K). SIR triblock forms three microphase lamellae for 35K-35K-35K, and two-microphase lamellae for 25K-25K-25K and below. In IRS, predominantly two-microphase lamellae were observed. For RSI, interpenetrating cubic structures, cylinders, and lamellar structures are observed depending on the interplay of molecular size and alignment conditions. Dynamics and flow-alignment behavior are monitored in-situ using rheo-optical method. Flow alignment behavior varies with molecular size due to changes in morphology and strength of segregation. For example, 15K-15K-15K RSI behaves much like a symmetric PS-PI diblock (slow and fast processes, ''flipping'', two distinct pathways to parallel alignment), while 20K-20K-20K RSI acts like an asymmetric diblock (a hex cylinder structure along the flow direction after shearing, which converts to a twinned interpenetrating cubic phase at elevated temperature in a quiescent state). Blends are prepared for detailed information regarding the effect of molecular size. Nanostructures and long-range order are confirmed by TEM and SAXS.

Chair: Timothy J. Bunning
Tuesday Afternoon, April 1, 1997
Salon 12

1:30 PM *W4.1 

The mechanism of mixing and homogenization of polymer blends under shear flow is an important fundamental problem with vast applications. We have studied the phase separated domain deformation, elongation, string pattern formation and homogenization of polymer blends by phase contrast microscopy and time resolved light scattering. At the same time, the compositions of discrete domains and the matrix are measured by the fluorescence microscopy. The droplet elongation, breakup and the coexistence compositions change can be analyzed and interpreted with macroscopic theories of Taylor Tomotika together with mode coupling calculations of Onuki-Kawasaki. The shear rate dependence of critical temperature shift will be presented, and the homogenization through the corresponding interfacial tension change will be discussed.

2:00 PM *W4.2 
INFLUENCE OF CRYSTAL-AMORPHOUS INTERPHASE ON PHASE BEHAVIOR AND MORPHOLOGY OF SEMICRYSTALLINE/AMORPHOUS BINARY POLYMER BLENDS, Do Y. Yoon, IBM Almaden Research Center, San Jose, CA; A. M. Jonas, Univ Catholique de Louvain, Unite de Physique & de Chimie des hauts Polymeres, Louvain-la-Neuve, BELGIUM.

Binary polymer blends which are fully miscible in melts undergo a phase separation upon crystallization of one of component polymers. For such semicrystalline/amorphous binary blends, the miscibility behavior in noncrystalline regions is distinctly different from that for normal amorphous blends, owing to the appearance of crystal-amorphous interphase in which the crystalline order is not fully dissipated. Experimental results on PEEK/poly(etherimide) (PEI) and PET/PEI blends show that in semicrystalline blends the PEI component is nearly totally demixed from PEEK and PET segments, despite favorable intermolecular interactions. In contrast, previous studies on PVDF/ PMMA, PVDF/PEMA, and PEO/PMMA showed the existence of well-mixed noncrystalline regions as well as demixed interphases. This difference is attributed to the presence of rigid aromatic segments in the chain backbone of PEEK and PET, which makes it extremely difficult to completely dissipate the crystalline order at crystal-amorphous interfaces. Moreover, a steep decrease of chain mobility in polymer melts, due to crystallization induced demixing, is shown to have drastic effects on the morphology of semicrystalline PEEK/PEI blends.

2:30 PM W4.3 
COUPLING BETWEEN THE EVOLUTION OF MORPHOLOGY AND RELAXATION IN A SEMI-CRYSTALLINE BLEND (POLY(ETHER-ETHER-KETONE)(PEEK)/POLY(ETHER-IMIDE) (PEI)), D. A. Ivanov, A. M. Jonas, R. Legras, Univ Catholique de Louvain, Unite de Physique & de Chimie des hauts Polymeres, Louvain-la-Neuve, BELGIUM.

We studied via dynamical mechanical analysis, dilatometry, densitometry, differential scanning calorimetry, x-ray scattering, and atomic force microscopy (AFM) the isothermal and anisothermal crystallization of PEEK/PEI blends. The presence of a noncrystallizable component (PEI) results in a segregation of PEI during crystallization. At low crystallization temperatures the process of expulsion of PEI into the interfibrillar gaps promotes an increase of the glass transition temperature (T) of these regions. This rise of T dramatically retards the kinetics of crystallization. It is sometimes followed by a vitrification of interfibrillar regions in isothermal as well as in anisothermal experiments. The dynamics of cold-crystallization under these conditions is mainly determined by a competition between the crystal growth and vitrification. The evolution of -relaxation and morphology of the semicrystalline structure during subsequent reheating of initially isothermally crystallized samples, leads us to define two regimes of reorganization. The first one consists in a relaxation-controlled crystal perfection occurring at the lamellar scale. The second one corresponds to a relatively rapid melting-recrystallization of whole stacks of lamellae. It is observed only for annealing temperatures higher than about 50 K above T, when the characteristic times of -relaxation are sufficiently short to allow a larger scale reorganization to proceed. The development of the PEEK semicrystalline morphology upon reheating, including the mode of PEI segregation, was examined by AFM. The evolution of a single PEEK spherulite throughout various thermal treatments was monitored by this technique.

3:15 PM *W4.4 

I will discuss 3-dimensional, real-space measurements of the domains in blends of homopolymers and block copolymers obtained with optical confocal microscopy. By measuring the size distributions of the domains, their spatial correlations, and their temporal growth evolution, it is possible to determine the dominant mechanism of domain growth. The addition of a diblock to a homopolymer mixture stabilizes the droplet domains at a finite preferred size. This ternary mixture is a polymeric analogue of a microemulsion, with the diblock being the surfactant, but with much larger length scales than in conventional emulsions.

3:45 PM *W4.5 
KINETICS OF PHASE SEPARATION IN LIQUID-CRYSTAL/POLYMER MIXTURES, A. M. Lapena, Univ of California-Los Angeles, Dept of Chemistry, Los Angeles, CA; Andrea J. Liu, Univ of California-Los Angeles, Dept of Chem & Biochem, Los Angeles, CA.

Networks formed by polymerization of monomers mixed with liquid crystals have been used in electrooptic applications. The morphology of the network depends on the polymerization reaction and on the kinetics of phase separation as the growing polymers phase separate from the liquid crystal, forming an isotropic polymer-rich phase coexisting with a nematic liquid-crystal-rich phase. Here we present a numerical simulations of a simple model for the kinetics that govern the formation of the network, and show how the kinetics can amplify thermodynamic factors and lead to highly anisotropic domains. Because the two phases differ both in composition and in orientational order, we allow both the local concentration and orientational density to evolve with time. We replace the distribution of polymers formed by the reaction with linear polymers of a single molecular weight that grows with time due to the reaction, and describe the time evolution of composition and orientation by generalized Cahn-Hilliard equations.

4:15 PM *W4.6 
BLOCK COPOLYMER NANOPHASES: KINETICS, FLUCTUATIONS, TANDEM ORDERING, AND FRUSTRATIONS, Zhen-Gang Wang, California Inst of Technology, Div of Chemistry & Chemical Engr, Pasadena, CA.

This talk discusses several issues of current interest in block copolymer nanophases. The talk consists of two parts. The first part deals with kinetics of order-order and order-disorder transitions in AB diblock copolymers. Results from direct numerical simulation as well as a simplified mode-analysis using a time-dependent Ginzburg-Landau approach will be presented. A focus here is the existence and nature of certain kinetic states during the phase transitions. In particular, we highlight pseudostable and transient states-corresponding respectively to saddle points and ridge-like features in the free energy surface. Linear stability analysis on an ordered structure reveals rather interesting anisotropic fluctuations the largest of which become the fastest-growing mode when the structure reaches its spinodal. As an example, the structural nature, stability and mechanism of formation of the perforated lamellar structure are elucidated. In the second part of the talk, we discuss some novel aspects associated with nanophases of ABC triblock systems. Using numerical self-consistent field methods, we address the effect of block sequence on the phase behavior of these systems, and how the segregation between one pair of blocks affects that between the others. Since different domain structures can often involve different topology in the conformations of the polymer (e.g., loops, bridges), transformation from one type of domain structure to another (e.g., by lowering the temperature) can be hindered by the inability of the polymers to change their conformation topology. As a result, the system may become locked in a topologically frustrated state.

Chair: David C Martin
Wednesday Morning, April 2, 1997
Salon 12

8:30 AM *W5.1 
METASTABILITY AND PHASE BEHAVIOR IN POLYMER-DISPERSED LIQUID CRYSTALS, Andrew J. Lovinger, Karl R. Amundson, Bell Labs, Lucent Technologies, Murray Hill, NJ.

Polymer-dispersed liquid crystals (PDLCs) are um-sized dispersions of nematic droplets within a polymer matrix. They are prepared by UV induced phase separation of a compatible blend of a liquid-crystal (LC) mixture and a prepolymer. As the latter undergoes cross-linking, a variety of interesting morphologies and complex phase behavior is obtained, depending upon composition and temperature. Within a compositional window of ca. 20-80 LC by weight, lower UV irradiation temperatures and higher liquid crystal contents favor phase separation in the form of discrete nematic droplets embedded within the polymer matrix. The droplets are bipolar and their sizes and shapes depend upon the viscoelastic characteristics of the polymer as the droplets nucleate and grow. However, for low LC contents and high irradiation temperatures, an unusual space-filling spherulitic morphology is obtained. We have shown that these spherulites involve large-scale organization of the LC molecules in a tangential orientation within the amorphous polymeric matrix. Radial growth is accompanied by proliferation of surface inversion wall defects, which are initiated at s = +l/2 (and terminated at s = -l/2) disclinations. Heating studies show that a ''memory''of the spherulitic structure, including the inversion walls, survives above the isotropization temperature, causing reappearance of these features upon cooling.

9:00 AM *W5.2 
NANO- AND MICROPHASE MORPHOLOGY OF LIQUID CRYSTAL DOMAINS DISPERSED ANISOTROPICALLY WITHIN A POLYMER MATRIX, Timothy J. Bunning, Lalgudi V. Natarajan, Vincent P. Tondiglia, Richard L. Sutherland, Science Applications Intl Corp, Dayton, OH; W. W. Adams, Wright Patterson AFB, Materials

Directorate. The formation of complex ultrastructures of phase separated liquid crystal domains within a polymer host using spatially varying curing conditions is described. The relative balance among polymerization and diffusion is discussed using trends in the morphology obtained from scanning and transmission electron microscopy. Specifically, differences in the morphology between reflection and transmission gratings are discussed. Droplet anisotropies and the average director configuration (under an applied e-field) within the films relative to the grating vector are reversed in reflection versus transmission gratings. Coupled with these morphology results are real-time optical monitoring of the anisotropic phase separated structures using photocuring induced changes in the refractive indices as a probe. Key challenges in characterizing such nanoscale organic-based structures are also addressed.

9:30 AM W5.3 
TIME RESOLVED STUDY IN PHASE SEPARATION DYNAMICS IN POLYMER/LIQUID CRYSTAL MIXTURES, Jill B. Nephew, Taryn Nihei, Sue A. Carter, Univ of California-Santa Cruz, Dept of Physics, Santa Cruz, CA.

The dynamics of addition polymerization induced phase separation in a liquid-crystal-polymer mixture is examined via confocal microscopy in systems where the final morphology consists of spherical nematic domains suspended in a polymer matrix. We observe in polymerization induced phase separation two distinct mechanisms: one in which liquid crystal domains develop via nucleation and slow growth that dominates at early and late times during polymerization, and a brief hydrodynamically active period in which phase separation accelerates and domains rapidly coalesce occurring part way into the polymerization process. We speculate that the hydrodynamics are caused by the segregation of the polymer at a critical degree of polymerization. We present a time resolved study of the phase separation as a function of cure temperatures, polymerization rates, and solvent concentrations and compare these results to phase separation of the same system induced by cooling below the isotropic nematic phase boundary. At late times, we also observe the predicted double phase separation giving polymer balls inside the larger liquid crystal domains.

10:15 AM *W5.4 
ELONGATIONAL FLOW OF A TWO-DIMENSIONAL POLYMER NEMATIC, Gerald G. Fuller, Takayuki Maruyama, Curtis W. Frank, Channing C. Robertson, Stanford Univ, Dept of Chemical Engr, Stanford, CA.

The orientational dynamics of a nematic formed from a Langmuir film of a rigid, "hairy rod" polymer subjected to extensional flow are examined. Using dichroism to measure the order parameter during flow, it is demonstrated that the simple molecular model of Marrucci et al. can quantitatively describe the results. A strong flow/weak flow criteria is also described. Below a critical velocity gradient, the hydrodynamic forces are unable to overcome the nematic potential. Above this rate, the flow can orient the director independently of the initial condition.

10:45 AM *W5.5 
MESOPHASE FORMATION IN AROMATIC POLYESTERS, B. Freeman, North Carolina State Univ, Raleigh, NC.; Michael Jaffe, Hoechst-Celanese Corp, Summit, NJ; E. Samulski, T. Dingemans, Univ of North Carolina, Chapel Hill, NC.

Moieties leading to mesogenicity in aromatic polyesters are being defined in terms of dihedral angle between ester linkages and overall size of planar units. It has been found that three-ring-based moieties such as 2.5 diphenyoxidiazole form excellent mesogenic cores, even at relatively sharp dihedral angles (i.e., 134 which is nonmesogen forming in single-ring systems). The three-ring moieties also demonstrate biaxial mesophases in low molar mass compounds and may lead to biaxial LCPs as well. Compositionally equivalent polymers synthesized via melt and solution techniques show marked differences in phase nature and transition temperatures, illustrating the importance of primary structure on phase behavior and ultimate performance. Transport of small molecules is very sensitive to the phase nature and backbone chemistry of aromatic polyesters. In cases where the polymer can be quenched from an isotropic melt or solution to an isotropic glass, major changes in transport (1-2 orders of magnitude decrease) can be engendered by the solid state conversion of the metastable isotropic glass to the more stable mesogenic organization. Acetone permeation is being utilized to isolate the effects of backbone chemistry and process history on the phase nature of aromatic polyesters and the mechanism of transport through these structures.

11:15 AM W5.6 
PECULIAR MESOPHASE OF A GROUP OF NOVEL LIQUID CRYSTAL COMPOUNDS, Qi-Feng Zhou, Dong Zhang, Yu-Guo Ma, Xin-Jiu Wang, Peking Univ, Dept of Polymer Science & Engr, Beijing, CHINA; Xinhua Wan, Hong Kong Univ Sci & Tech, Dept of Chemistry, Kowloon, HONG KONG; Xin-De Feng, Peking Univ, Dept of Chemistry, Beijing, CHINA.

A group of novel aromatic amides with 1,4-bis(benzamido)phenylene as the common rodlike core was found to form peculiar liquid crystalline phases. Studies of the mesophase by using different techniques lead to different suggestions for type of the phase. For example, the DSC of 2 bromo-4,4-bis)p-methoxybenzamido)benzene showed endotherms at 249, 252, 255C, respectively, with an enthalpy of 10, 50, and 1 kj/mol. The transition at 255 was probably a nematic-to-liquid for the enthalpy was so small. With POM, three transitions were also found: a solid-to-solid transition at 242C, a solid-to-solid crystal transition at 246C, a transition to isotropic liquid at 250 C. However, in the heating from 246 to 250C, the texture first formed was the ''sanded,'' probably of a smectic C phase, which became highly birefringent with schlieren textures showing some singularities with two brushes. Homotropic domains were also observed. This phase became isotropic at 249.5C. It showed a small degree of supercooling of 0.5 degrees in cooling. Schlieren textures with singularities of m = 1 and m = 1/2 started to form at 249C, together with homeotropic regions. Also observed were inversion walls. Thus, the mesophase was more likely to be nematic, even though a sanded texture was also observed. On the other hand, x-ray studies showed at 249C a very sharp diffraction at 2 = 5.94 (corresponding to a spacing of 14.86 ) and a diffuse halo at 220, for an average distance of 5.2. The scattering became of an amorphous matter at 252C, showing only a diffused halo at around 20, but no diffraction at small angles. Since the calculated length for the extended conformer of this molecule is 19.98 , the phase at 249C can be assigned smectic-C with an average tilt angle of 41.9 . An interpretation for the discrepancy showed by this and other compounds is attempted.

11:30 AM W5.7 
STRESS INDUCED ORDERING IN META-STABLE POLYMER/LIQUID CRYSTAL MIXTURES, Taryn Nihei, Jill B. Nephew, Sue A. Carter, Univ of California-Santa Cruz, Dept of Physics, Santa Cruz, CA.

We observe the formation of labyrinthine patterns in liquid-crystal polymer mixtures induced by elastic stresses of the polymer matrix. An isotropic mixture of monomer and liquid crystal is fully polarized to initially form a metastable suspension of liquid crystal domains in a crosslinked polymer matrix. Minutes after the polymerization, channels of liquid crystals form with a characteristic length scale and eventually branch and break up to form a stable ordered labyrinth over the course of a few days. We examine the stability of the suspension and the final morphology of the labyrinth for different polymerization rates, monomer concentrations, and degree of crosslinking; study the dynamics of the forming labyrinth using time resolved optical microscopy; and examine the interfacial interactions and structure of the liquid crystal in the formed labyrinth using fluorescence and polarizing microscopy.

11:45 AM W5.8 
PHASE IDENTIFICATIONS OF A DISCOTIC LC HPT USING ELECTRON DIFFRACTION, Donghang Yan, Changchun Inst of Applied Chemistry, Polymer Physics Lab, Changchun, CHINA; Tao Wang, Changchun Inst of Applied Chemistry, Lab Polymer Physics, Changchun, CHINA; Enie Zhou, Changchun Inst of Applied Chemistry, Dept of Polymer Physics, Changchun, CHINA.

This paper presents crystal and liquid crystal (LC) structures of 2,3,6,7,10,11-hexakispentyloxy triphenylene (HPT) and the LC structure of its main chain polymer, HPTm, in molecular level in order to understand the nature of formation and their mutual transitions. HPT liquid crystal phase D (''h'' for hexagonal bidimensional lattice, ''o'' for ordered molecular spacing in each column) is usually found in the past two decades. HPT in two liquid crystal phases different in column diameter, heat up from crystalline state and cool down from isotropic state, were found in this study, and the diameter of columns in these two LC are close to a parameter of two stable crystals separately. These two crystal structures were further analyzed by using electron diffraction and molecular simulation in unit cell corresponding to low energy condition, and the structures in crystal and liquid crystal states are given in molecular level. HPT main chain polymer was found in a same manner as close packed liquid crystal phase of HPT. The D phase is, therefore, classified to be D (hexagonal bidimensional lattice with disordered core orientation) and D (hexagonal bidimensional lattice with ordered core orientation) phases in order to distinguish these two D phases.

Chair: J. Blackwell
Wednesday Afternoon, April 2, 1997
Salon 12

1:30 PM *W6.1 
POLYMER ALIGNMENT LAYERS: PREPARATION, STRUCTURE AND APPLICATIONS, J. C. Wittmann, CNRS, ULP, Strasbourg, FRANCE; S. Meyer, Univ of Southampton, Dept of Physics, Southampton, UNITED KINGDOM.

The controlled formation of highly oriented thin layers of liquid crystalline (LC) or crystalline materials is important for both scientific and technological aspects. Unidirectionally rubbed polymer layers are widely used in liquid crystal displays to provide a defect-free alignment of the LC material, required for high performances. Although many studies have addressed the problem of polymer alignment mechanism, a deeper knowledge of the structural changes induced by the rubbing process is still needed. We are presently exploring the rubbing of polymers such as isotactic polystyrene or aromatic polyesters, which can be solvent-cast as thin amorphous layers, rubbed, and subsequently crystallized by thermal annealing or exposure to solvent vapors The morphologies observed after annealing clearly demonstrate that the polymer chains are uniaxially oriented at or near the layer surface by rubbing. Indeed, the as oriented chains act as very efficient nuclei leading to a shish-kebab-type morphology and the formation of a lamellar transcrystalline layer which, under appropriate growth conditions, extends over the whole film thickness. Friction-transferred PTFE layers represent another type of efficient polymer alignment or orienting substrates. In contrast to rubbed polymer layers, their structure is well defined and they have an equivalent or even superior LC alignment capacity. Annealing of a side-chain LC polymer and a fluorescence dye containing copolymer deposited on PTFE substrates is found to yield macroscopically aligned glassy films. As shown by electron diffraction, the smectic glass adopts a nearly perfect bookshelf geometry in which the smectic layers are oriented edge-on and perpendicular to the alignment direction, I. e., the PTFE chain axis direction. The UV-visible dichroic ratio and the order parameter of the aligned copolymer layers increase as the annealing temperature approaches the smectic to nematic phase transition temperature.

2:00 PM W6.2 
THIN-FILM SURFACE-INDUCED STRUCTURE AND MORPHOLOGY FROM HIGHLY ORDERED MAIN-CHAIN LIQUID CRYSTALLINE POLYMER, Yeocheol Yoon, Stephen Z. D. Cheng, Univ of Akron, Dept of Polymer Sci, Akron, OH; Rong-Ming Ho, Univ of Minnesota, Depr of Chem Engr & Material Sci, Mineapolis, MN; Virgil Percec, Case Western Reserve Univ, Dept of Macromolecular Sci, Cleveland, OH; Pelhwel Chu, Case Western Reserve Univ, Dept of Macromolecular Sci, Clecveland, OH.

A series of liquid crystalline polyethers has been synthesized from 1-(4 hydroxy-4-biphenylyl)-2-(4-hydroxyphenyl) propane and -dibromoalkanes [TPP(n)]. TPPs show multiple phase transitions during cooling and heating. In bulk and fiber samples, multiple liquid crystalline phases have been identified: nematic phase, smectic F, smectic crystal G, and smectic crystal H phases. The detailed structures and morphology of TPP thin films (with a thickness ranging from 10 nm to 100 nm) have been studied by electron diffraction and transmission electron microscopy experiments on three different types of substrates. These include silane grafted, amorphous carbon coated, and clean glass surfaces. Both silane grafted and amorphous carbon coated surfaces can induce structural ordering In TPP(n = 7) to form an orthorhombic lateral packing which does not exist in the bulk and fiber samples, and has only appeared in TPP(n 11). This phase has been identified as a smectic crystal H phase. It has been found that the monodomain morphology of the highly ordered smectic crystal phases with the homeotropic molecular alignment depends strong on the structural symmetry. The clean glass surface does not induce orthorhombic packing and only polydomaln structures can be found in which an in-plane homogeneous alignment of the chain directors exists. Electron diffraction results on the TPP(n = 12) thin films confirm the phase structures identified In bulk materials. When the thin film samples are quenched from the isotropic melt to lower structural formation temperatures, the samples no longer show homeotropic molecular orientation. This indicates that the amorphous carbon surface induced alignment process is very sensitive to the type ot phase In which the structural formation occurs. It is not possible to access the homeotropic orientation when the molecules have been in an ordered structure higher than the S phase. In the smectic crystal G phase, molecular mobility along the chain direction can still be expected, which is evidenced by an observation of significant morphological layer thickening during annealing in this phase.

2:15 PM W6.3 
FRUSTRATED PHASE BEHAVIOUR OF CHIRAL SIDE-CHAIN LC POLYMERS, Mikhail V. Kozlovsky, Inst of Crystallography, Darmstadt, GERMANY; Michael Darius, Wolfgang Haase, Technical Univ Darmstadt, Inst of Physical Chemistry, Darmstadt, Germany.

A frustrated phase behaviour has been observed for a chiral siloxane side-chain polymer. It shows either the sequence of mesophases (gl) 31-35 Sm C* 44 Sm A* 47 Iso, or (gl) 31-35 TGB-C 47 Iso, depending on cell prehistory. Both phases are kinetically stable and reproducible. The polymer shows extremely low driving voltage values of optical switching in the Sm C* phase, ca. 0.2 MV/m, and low untwisting voltages, 0.5 - 1.5 MV/m. Electroclinic switching in the Sm A* phase and ferroelectric switching in the Sm C* phase has been studied, as well as dielectric relaxation (soft mode) and pyroelectric effect. For the TGB-C phase, X-ray data and UV-VIS spectra are reported. Similar peculiarities have been found for the phase behaviour os a chiral side-chain acrylic copolymer.

2:30 PM W6.4 
SYNTHESIS, STRUCTURE AND PROPERTIES OF MESOMORPHIC LINEAR AND CYCLOLINEAR POLYORGANOSILOXANE POLYMERS, Natalia N. Makarova, Elena V. Matukhina, Inst of Organic-Element Compounds, Moscow, RUSSIA; Eri E. Boda, Univ of Sheffield, Dept of Engeneering Matls, Sheffield, UNITED KINGDOM; Tatiana V. Timofeeva, New Mexico Highlands Univ, Dept of Physical Science, Las Vegas, NM.

Alkyl-phenyl substituted organosiloxane polymers of linear structure and alkyl-phenyl substituted cyclolinear polimers with a cycle size fro 3 to 6 (Si))-units and various bridging atoms were synthesized. Structure and phase transitions (including mesophase transitions) of these polymers were investigated using DSC and x-ray analysis combined with molecular modeling. On the base of experimental investigations and computational data model of Lengmuir film formation by cyclolinear polymers was proposed. Properties of linear linear polyorganosiloxanes, such as high film durability and elasticity are discussed in connection with mesomorphic properties of these polymers.

3:30 PM *W6.6 
THERMAL TRANSITIONS AND STRUCTURE FORMATION IN POLYMER SOLUTIONS, Hugo Berghmans, J. Arnauts, F. Deberdt, R. De Cooman, A. Jacobs, T. Roels, P. Vandeweerdt, Katholieke Univ Leuven, Dept of Chemistry, Heverlee, BELGIUM.

Structure formation in polymer solutions by thermally induced phase transitions is an interesting route towards the formation of polymeric materials like membranes and porous fibers. The process that leads to such materials is based on the interference of a liquid-liquid demixing and a solidification of the concentrated domains so that metastable solutions are frozen in. The porous materials are obtained after elimination of the solvent. Different mechanisms, their possible interferences, and the resulting morphologies will be discussed on the basis of the corresponding temperature concentration phase diagrams and will be illustrated by typical examples. Vitrification leads to amorphous, porous materials with a final morphology that depends on the demixing mechanism (binodal or spinodal) and the factors that affect the growth and collapse of the domain structure. The combination of this type of structure formation and extrusion leads to porous fibers with m size, oriented porosity, extending like channels along the fiber axis. The use of mixtures of noncompatible polymers with different solubility in a common solvent results in a complex, two-step demixing process that can be fully understood from the detailed investigation of the ternary phase diagram. Extrusion of such a complex system leads in one step to a porous fiber in which the less soluble polymer forms a fibrillar matrix that will be coated with the better soluble polymer. The use of crystallizable polymers generally leads to crystalline porous materials because crystallization proceeds at much higher temperatures than vitrification, unless the melting point and the glass transition temperature are not too far apart, as illustrated by poly(2,6-dimethyl 1,4-phenylene ether) (PPO). Structure formation will then be controlled by the competition between the rate of these thermal transitions and the cooling rate during the processing. This structure formation can become even more complex and eventually be completely suppressed when phenomenon like gelation can take place. This is illustrated by the solutions of the stereo isomers of polystyrene in decalin in which the formation of the most stable crystal phase can be suppressed for kinetic reasons in favor of the formation of the metastable gel phase which in turn can suppress the liquid-liquid demixing and therefore the corresponding formation of porous materials.

4:00 PM *W6.7 

Multilayer structures incorporating polymers can be built using the Langmuir-Blodgett technique. The structures formed may reflect the imposition of ordering due to interaction of the polymer with the subphase prior to deposition and subsequent constraints in mobility. The issues of structural change upon annealing of LB multilayer structures and of mobility in such systems has been a focus of investigations pursued by the speaker and his collaborators. Information on the structure and how it changes with annealing has been garnered primarily using x-ray and neutron reflectometry. Examples to be discussed will include multilayers made using both ''hairy-rod'' polyglutamates and polyimides.

4:30 PM W6.8 

The binary phase diagrams of hydroxypropylcellulose (HPC) in water and in thiocyanate saline water have been determined with data such as cloud point temperatures (T), isotropic-mesophase transition temperatures (T), and critical concentrations of mesophase formation. The ternary phase diagram of mesogen/coils/solvent system, the HPC/polyethylene glycol(PEG)/HO system, was determined upon the observations of polarizing microscopy and deuterium nuclear magnetic resonance (D-NMR). Binary systems in pure water, T are not observable because liquid-liquid phase separation temperatures (T) occurs at 44C. Inclusion of salt-in salt, the T reappear and a different phase diagram takes place, because salt-in effect lifts T to temperatures higher than T. For ternary mixtures of HPC/polyethylene glycol(PEG)1/HO, the mesogen and coiled polymers are immiscible as predicted in theory. A region in the phase diagram shows the coexistence of two isotropic phases and one mesophase. Interestingly, the microscopic observations suggest that the immiscibility is mitigated by introduction of thiocyanate salts such that the three-phase area disappears. The phase diagrams and salt-in effect will be discussed and compared with theoretical phase diagrams.

Chair: Herve Marand
Thursday Morning, April 3, 1997
Salon 12

8:30 AM *W7.1 
CONTROLLING PATTERNS OF PHASE SEPARATION IN POLYMER FILMS, Edward J. Kramer, Jan Genzer, Jakob Heier, Cornell Univ, Dept of MS&E, Ithaca, NY.

In thin films of polymer blends or block copolymers, the interfaces of the film can strongly influence the phase separation. In this paper we still discuss how one can ''tune'' the interfaces so as to produce regular patterns in the phase separated structure of the film, both through the thickness and in the lateral dimensions. Particularly useful is the ability to ''tune'' the interface using self-assembled monolayers of thiols on gold as the substrate on which to cast the film. Using mixed monolayers of the thiols it is possible to observe a reversal in the phase that wets the interface as the quench depth from critical temperature is increased. Microcontact printing also permits one to produce a lateral pattern of different thiols on the substrate, and such a patterned substrate can cause dramatic changes in the lateral structure of overlying ordered block copolymer films.

9:00 AM *W7.2 

It has long been known that polymer microspheres formed in water can be stabilized by 1) small surfactant molecules; 2) ionic groups on the particle surface; and 3) water-soluble macromonomer/polymer grafted/adsorbed on the particle surface. Generally, the particle size decreases as the concentration of the stabilizer increased. Experimentally we demonstrate that the surface area(s) per stabilizer for a given system is the physical parameter to govern the final hydrodynamic radius (R) of the particles. On the basis of our results, simple structural models are proposed and formulated to link microscopic parameters, such as R and S to macroscopic parameters, such as the weights of stabilizer and polymer. Using these models, we are able to control and predict the particle size for each given system by doing only a few experiments.

9:30 AM W7.3 
THE MECHANICAL RELAXATION BEHAVIOR OF POLYBUTADIENE CRITICAL GELS, Michael E. DeRosa, Air Force Wright Laboratory, MLPJ, WPAFB, OH; H. Henning Winter, Univ of Massachusetts, Dept of Chemical Engr, Amherst, MA.

During the chemical crosslinking reaction of a polymer, the material undergoes a phase transition from a viscous liquid to a rubbery solid. At this liquid-solid transition the mechanical properties are intermediate of the two states and the material is called a critical gel to distinguish it from other materials commonly called gels. Rheological studies have shown that these materials exhibit power law relaxation behavior of the form G(t) = St where G(t) is the shear relaxation modulus, S is the gel stiffness, and n is the critical relaxation exponent which is representative of the self-similar molecular structure. This study focuses on investigating the effects that physical entanglements have on the gelation process and on the rheological properties of critical gels. Dynamic mechanical techniques are used to investigate the kinetics of gelation and relaxation behavior of high molecular weight polybutadiene precursors which are crosslinked at random sites along the polymer backbone. A presentation will be made on how entanglements affect the relaxation time spectrum and how this led to the discovery of a new scaling relationship between the gel stiffness and the precursor molecular weight. Results on how entanglements affect gel point detection by dynamic mechanical methods will also be presented.

10:15 AM *W7.4 
STRUCTURE AND ASSOCIATED TRANSITIONS OF POLYMER ADSORBED TO AIR-WATER INTERFACES, Shaw Lin Hsu, Sophie Riou, Bert Chien, Huijuan Chen, David A. Tirrell, Univ of Massachusetts, Dept of Polymer Science & Engr, Amherst, MA.

External reflection-absorbtion Fourier transform infrared spectra of polymers adsorbed to air-solution interface were monitored as a function of molecular area on the surface of a microcomputer-controlled Langmuir trough interfaced to the interferometer. This combination of vibrational spectroscopy with surface tension measurements allows the possibility to analyze the microstructure and associated transitions of polymers adsorbed to an interface as a function of surface pressure, interaction with liquid subphase, rate of compression, temperature, and time. The spectra obtained can only be analyzed by assuming that adsorbed chains exist in a structure substantially different from the ones in the bulk solution. In addition, we found that the helix-coil or crystal-crystal transitions are severely perturbed by the presence of an interface. For example, the periodic polypeptide (poly-(AG)EG) with 36 repeats of octapeptide (AlaGly)GluGly prepared by bacterial experssion of an artificial gene exists in a random coil conformation under various pH conditions in bulk solution. However, it self-assembles into a sheet conformation at low solution pH and a random coil conformation at high pH at the air-water interface. In addition, if was found, depending on the liquid subphase used, that adsorbed poly-aspartate exhibits an unusual transition from a right-handed helix to a left-handed one through an intermediate state. This transition can only occur because of specific interaction between polymer and the liquid subphase.

10:45 AM *W7.5 
STRUCTURE DEVELOPMENT IN POLYMER NANOFOAMS, Jeffrey S. Fodor, Univ of Maryland, Dept of Materials & Nuclear Engr, College Park, MD; Tom P. Russell, Kenneth R. Carter, James L. Hedrick, Robert D. Miller, IBM Almaden Research Center, San Jose, CA; Robert M. Briber, Univ of Maryland, Dept of Materials & Nuclear Engr, College Park, MD.

Polymer Nanofoams are produced from triblock copolymers where the end blocks are thermally less stable than the center block. Nanometer-sized voids are formed by thermal degradation of the microphase-separated domains formed by the end blocks of the copolymer. Characterization of this process by in-situ small angle neutron scattering (SANS) and neutron reflectivity (NR) has shown that during spin casting of a thin film of a polypropylene oxide (PO)-poly(amic ester) (endblock and centerblock, respectively) copolymer, the microphase-separated morphology develops during the imidization step. The film is homogeneous when first spun and that microphase separation occurs during subsequent imidization in an inert atmosphere. Upon further heating in an air atmosphere in order to degrade the PO block, the morphology is relatively unchanged with the microphase separated PO domains converting to voids. MR shows that the void porosity remains constant during heating from room temperature up to the glass transition of the polyimide with no apparent collapse of the voids.

11:15 AM W7.6 

It is believed that iodine molecules penetrate into the crystalline phase of nylon 6(N6) and interrupt hydrogen bonds. This facilitates the rearrangement into new hydrogen bonds between the chains during the subsequent removal of iodine. In consequence, the iodine treated N6 provides a method for changing the crystalline morphology of N6. In this study we have investigated the structure and deformation behavior of iodine treated N6 made from aggregates of solution grown and melt grown crystals. Polymer chips (IV = 1.0 dL/g) were dissolved in hexylene glycol at 190C, and the crystals were precipitated from the solution during slow cooling from 190C to room temperature. The crystal suspension was slowly filtered at room temperature, produced a thin film (SGC). Films of melt grown crystals (MGC) were obtained by a compression molding of polymer chips at 240C, followed by a slow cooling to room temperature. The crystal structure of both SGC and MGC films was -form. Both films were immersed in an aqueous solution of KI-I at room temperature. After the treatment, the films were soaked into an aqueous solution of sodium thiosulphate. Elemental microanalysis revealed that there was no trace of iodine in the treated films. The treated films from both SGC and MGC showed an anisotropic crystal orientation. Further, the crystalline morphology, as well as the deformation behavior, was greatly affected by the conditions for both iodine and sodium thiosulphate treatments.

11:30 AM W7.7 
THIN-FILM STRUCTURE AND PROPERTIES OF SULFUR-DERIVATIZED AMPHIPHILIC BRUSH COPOLYMERS, John F. Rabolt, Mei-Wei Tsao, Univ of Delaware, Dept of Materials Science, Newark, DE; Yen Ren, Seagate Corp, Milpitas, CA; Catherine L. Hoffmann, Charles Evans & Associates, Redwood City, CA; David G. Castner, Univ of Washington, Dept of Chemical Engr, Seattle, WA; Shaw Lin Hsu, Univ of Massachusetts, Dept of Polymer Science & Engr, Amherst, MA.

A specifically designed amphiphilic copolymer with long hydrocarbon side groups, hydrophilic spacers, and disulfide sticky side groups was synthesized and its thin films were produced by self-assembly and Langmuir-Blodgett techniques on gold surfaces. The films were studied by ellipsometry, x-ray photoelectron spectroscopy, contact angle measurements, and infrared spectroscopy. The self-assembled film on gold is disordered and porous with a film thickness of 38 as determined by ellipsometry. The Langmuir-Blodgett film structure has been studied in detail, both before and after transferring the film from water surface to gold substrate. The copolymer side chains have been found to be highly ordered with an all-trans conformation on the water surface, but this crystalline order is completely lost after the film is transferred onto gold surface. The strong affinity between the disulfide units and gold atoms causes structural rearrangement (as confirmed by XPS) which consequently disrupts the side chain packing order which had been present on the subphase. To confirm this hypothesis, a noninteracting substrate ZnSe was used to transfer the film and it has been shown that the copolymer side chain order is preserved after the film is transferred. Thus these results have illustrated the importance of the molecular architecture and the film-substrate interface on the thin film structure and its properties.

Chair: Gerrit ten Brinke
Thursday Afternoon, April 3, 1997
Salon 12

1:30 PM *W8.1 
METASTABILITY IN THE CRYSTALLIZATION OF POLYMERS FROM MELTS, BLENDS, AND SUPERCRITICAL SOLVENTS, M. Mutukummar, Y. Akpalu, P. Ehrlich, P. Whaley, Univ of Massachusetts, Amherst, MA; Richard S. Stein, Univ of Massachusetts, Polymer Research Inst, Amherst, MA.

Polymers crystallize by forming branched structures which grow until they either exhaust the supply of crystallizing material or affect their surroundings such as to prevent further crystallization from occurring. The crystals grow by branching the degree of which depends upon temperature, chain branch structure, concentration, and nature of surrounding material (solvent or melt). At low concentration, these may interlock, forming a space-filling structure. Any remaining crystallizable polymer then fills in the spaces between crystals within this network. Noncrystallizable material is rejected and remains in domains within this structure. If this material is a volatile solvent, its evaporation leads to a foam. In the case of a blend of linear and branched polyethylene, the resulting structure is often a spherically symmetrical interconnected array of linear polyethylene crystals containing a ''filler'' of branched polyethylene which may be nucleated by the linear polyethylene crystals. The growth and morphology of these structures may be followed by the combined use of wide-angle x-ray diffraction, small-angle x-ray scattering, and small-angle light scattering. For following kinetics, synchrotron x-ray sources are employed. In the case of foams, surface areas may be measurements using gas adsorption techniques. In addition to the polyethylene blend studies, results of preparing high surface area foams from supercritical n-alkane solvents will be presented.

2:00 PM *W8.2 

Recent developments in polymer synthesis-in particular metallocene catalysis-provide opportunities for the commercialization of copolymers of ethylene with various comonomers which have densities intermediate between those of conventional copolymers such as LDPE, LLDPE and VLDPE on the one hand, and rubbers such as EPDM on the other. It has been found that, as the comonomer content of homogeneous copolymers increases, their morphology changes from a lamellar base morphology-with crystallites in which the ethylene sequences are mainly folded-into a granular morphology and ultimately (possibly via a fringed micelle morphology with bundled sequences) into a morphology characteristic of low-crystalline rubbers which, according to Monte Carlo simulations, probably consists of clusters of loosely packed ethylene sequences. This concept will be discussed with reference to the results of TEM, dynamic WAXS, SAXS and DSC experiments. The results also show that the above-mentioned changes are of a continuous nature.

2:30 PM *W8.3 
BLIND ALLEYS IN THE PATHWAY OF CRYSTALLIZING CHAINS, Goran Ungar, Univ of Sheffield, Dept of Engr Materials, Sheffield, UNITED KINGDOM; Eru Bodz, Felix Diaz, Xianbing Zeng, Chi-Ming Chen, Univ of Sheffield, Dept of Engineering Matls, Sheffield, UNITED KINGDOM; Paul G. Higgs, Univ of Manchester, School of Biological Science, Manchester, UNITED KINGDOM; Gerald M. Brooke, Univ of Durham, Dept of Chemistry, Durham, UNITED KINGDOM.

Monodisperse long n-alkanes continue to expose new details of crystalline polymers hitherto obscured by the polydispersity intrinsic to real polymers. We have focused on the so far poorly understood processes of chain deposition on a growing polymer crystal, on nucleation, lamellar thickening and thinning, as well as on the nature of the fold layer in as formed and mature crystals, and on chain tilt. In addition to pure monodisperse chain alkanes up to CH, we have been studying the effect on crystallization behavior of chemical modification of n-alkanes such as introduction of short-chain branching or carboxylation. Furthermore, binary mixtures of selected alkanes were studied, where the alkane components varied in length by different amounts. The latter studies are being carried out partly in order to close the gap between the monodisperse models and real-life polydisperse systems. Perhaps the most intriguing effect found with the long n-alkanes is the crystallization rate minimum as a function of crystallization temperature occurring at the transition between the extended-chain and folded-chain growth regimes. This unprecedented anomaly was first found in alkanes CH and CH [1] and subsequently similar effects were reported for some aromatic poly(etherketones) [2] and PEO fractions [3[. We have now confirmed this effect in a series of 7 n aLkanes covering the range from CH to CH, i.e., the range in which rate measurements were practicable [4]. The original qualitative interpretation was in terms of "self-poisoning," i.e., blocking of the extended-chain crystal growth face by frequent nearly-stable but unproductive folded-chain depositions slightly above their melting point [1]. The experimental rate curves have been reproduced very well by both a rate equation approach and by Monte Carlo simulation using a very simple model with two-segment chains [5]. More realistic simulations are currently underway in Manchester using a bond fluctuation lattice model with multisegment chains. The self-poisoning effect is a striking illustration of the high entropic barrier against chain extension which dominates growth kinetics and morphology of polymer crystals.

3:30 PM *W8.4 
MESO MODELLING OF TRANSFORMATIONS AND STRUCTURES IN LIQUID CRYSTALLINE POLYMERS, J. Hobdell, Cambridge Univ, Dept of Matls Sci & Metallurgy, Cambridge, UNITED KINGDOM; D. Gonin, ESPCI, PCSM, Paris, FRANCE; A. H. Windle, Cambridge Univ, Dept of MS&M, Cambridge, UNITED KINGDOM.

Thermotropic liquid crystalline polymers have great potential as structural materials. They have high strength and stiffness in the direction of molecular alignment and their low melt viscosity facilitates processing. Without resort to large aligning fields, molded samples show inhomogeneities either in the form of variations in the direction of alignment across the sample or topological defects. An understanding of these textures and defects is crucial to the further utilization of these promising materials. The microstructures observed in LCPs are different from those observed in small molecule nematics because of the anisotropy of the Frank elastic constants in the polymeric case. In polymeric nematics, the splay constant is the highest and the twist constant is the lowest. Splay distortion requires chain ends to segregate to maintain a constant density and this organization is entropically unfavorable. A modeling technique has been employed to investigate the effect of different elastic constants on microstructural form. Previous models and simulations have worked primarily in the single constant approximation which is valid in small molecule nematics but not for polymeric liquid crystals. The new technique separates the contributions from splay, twist and bend by discretizing Frank's equation onto a 3D array of cubic cells. It is found that the character of the microstructures which occur during the evolution vary a great deal with the values of the Frank elastic constants put into the simulations. When elastic constants appropriate for polymeric liquid crystals are used, simulated microstructures show good agreement with experimentally observed microstructures. The nematic-isotropic phase transformation can be modeled by the simple expedient of the Monte-Carlo approach, where the cell dimensions of the model approach the molecular dimensions and the internal thermal energy is handled as random "kicks" to the director orientation The prediction of the loss of nematic order as the phase transition is approached is remarkably accurate, bearing in mind the simplicity of the interparticle energy function, at least for the equal constants case. Such a model, known as the Lebwohl-Lasher model, can also be used to examine the microstructural changes which occur in the region of the transition itself. A key aspect of the observations is the onset of orientational percolations as the more ordered phase permeates across the model.

4:00 PM *W8.5 

This presentation will describe the rational design of selected examples of monodendrons, dendrimers and polymers containing dendritic building blocks which self-organize into nematic, smectic, columnar hexagonal and cubic thermotropic liquid crystalline phases. These examples will demonstrate that dendritic building blocks self-organize via principles which are common both to conventional calamitic thermotropic mesogens and microphase-segregated block copolymers and, therefore, provide an ideal architecture which can bridge between these two subdisciplines.

4:30 PM *W8.6 
SUPRAMOLECULAR POLYMERS, Samuel I. Stupp, Univ of Illinois-Urbana, Dept of MS&E, Urbana, IL.

An interesting target in polymer science is to find pathways to highly regular supramolecular units with dimensions similar to those of high molar mass linear polymers. These units could serve as precursors to shape invariant covalent polymers analogous to folded proteins. We are pursuing access to these polymers with designed molecules programmed to assemble into nanostructures of regular shape and dimension. Space filling requirements for shape invariant nanostructures, and the great structural diversity that can be achieved by molecular synthesis, can lead to polymeric materials with unique potential to be multifunctional and easily processable. Another interesting aspect of polymers is the possibility of forming 2D or 3D networks by self assembly of these nanostructures. This lecture will describe the discovery of systems that yield supramolecular polymers and discuss their phase transitions into mesophases. The main system to be discussed is one in which mushroom-shaped supramolecular polymers self assemble into macroscopic materials that integrate several functions, related in this case to polar stacking of the units. The mushroom nanostructures are formed by which are believed to form highly regular supramolecular units as a result of a frustrated crystallization mediated by repulsive forces.