Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session U24: Phase Transitions in Polymeric Systems I |
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Sponsoring Units: DPOLY Chair: Alamgir Karim, National Institute of Standards and Technology Room: Colorado Convention Center 201 |
Thursday, March 8, 2007 8:00AM - 8:36AM |
U24.00001: Influence of Phase Separation and Shear on the Crystallization of Polyolefin Blends Invited Speaker: The correlation between liquid-liquid phase separation (LLPS) and crystallization at several compositions in statistical copolymer blends of poly (ethylene-co-hexene) (PEH) and poly (ethylene-co-butene) (PEB) has been examined by optical microscopy (OM), atomic force microscopy (AFM) and differential scanning calorimetry (DSC). The overwhelming change in the crystallization kinetics due to the density fluctuation caused by the spontaneous spinodal LLPS is observed. This coupling mechanism suggests a new mechanism in the nucleation-crystallization process. All evidences are pointing to a cross-over mechanism from the spinodal fluctuations (of liquid-liquid phase separation) to the nucleation and than crystallization. Also, the shear dependence and mechanism of the Shih-kebab formation in the isotactic polypropylene (iPP) and isotactic polypropylene/polyethylene-co-octene blends have been studied. The network strands deformation and the primary nucleation mechanism have been studied by the time resolved small angle light scattering and AFM. New mechanism has been proposed. The above studies are aimed to understand the enhanced primary nucleation mechanisms in crystallizable polymers and polymer blends which have not been emphasized in most of the traditional nucleation and crystallization research in polymers. The detailed experimental evidences and proposed physical model will be presented. [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 8:48AM |
U24.00002: Formation of micelles in homopolymer-copolymer mixtures Marcus M{\"u}ller, Anna Cavallo, Kurt Binder Using Monte Carlo (MC) simulations of the bond fluctuation model and self-consistent field (SCF) calculations, we study the formation of micelles in a mixture of homopolymers and asymmetric AB-diblock copolymers with composition, $f_{A}=1/8$. We work in the semi--grandcanonical ensemble, {\it i.e.}, we fix the monomer density and incompatibility, $\chi N\simeq 100$, and control the composition of the mixture via the exchange chemical potential, $\delta\mu$ between the copolymer and homopolymer solvent. The MC simulation comprises moves that allow homopolymers to mutate into AB-diblock copolymers and {\it vice versa}. These moves are very efficient in equilibrating the configurations. We accurately locate the critical micelle concentration, study the micellar size distribution and characterize the shape of the micelles by the tensor of gyration and radial density profiles. The simulation results are {\it quantitatively} compared to predictions of the SCF theory in the grandcanonical ensemble without adjustable parameter. Only in the limit of high molecular weight the simulation results gradually approach the theoretical predictions. The structure and phase behavior of mixed micelles is investigated by SCF calculations. [Preview Abstract] |
Thursday, March 8, 2007 8:48AM - 9:00AM |
U24.00003: Enhancing the segregation strength of amphiphilic block copolymer melts using selectively associating homopolymers: Well ordered systems from inexpensive components. James Watkins, Vijay Tirumala, Alvin Romang, Eric Lin Amphiphilic block copolymers based on poly (oxyethylene-oxypropylene-oxyethylene) are commercially available (Pluronic{\texttrademark}, BASF), inexpensive and used in a variety of solution-based applications. But their use as thin film templates in applications is limited by their low molecular mass, presence of impurities, and weak segregation. Here, we show that the segregation strength of such copolymer melts can be dramatically enhanced by blending with a homopolymer that selectively associates with one of the segments via hydrogen bonding. Examples include poly (acrylic acid), poly (4-vinyl phenol) and poly (styrene sulfonate). SANS measurements indicate that the order-disorder transition temperature of a deuterated copolymer analogous to Pluronic F68 increases by at least 180 $^{o}$C when blended with poly (acrylic acid) suggesting a significantly higher effective interaction parameter. This approach has many implications including the preparation of highly ordered templates from inexpensive components. [Preview Abstract] |
Thursday, March 8, 2007 9:00AM - 9:12AM |
U24.00004: Hexagonal Phases in Rod Coil Block Copolymers Rachel Segalman, Bradley Olsen The thermodynamics of rod containing block copolymers is distinct from classical block copolymers due to the conformational asymmetry between the blocks and liquid crystalline interactions between the rods. Understanding the self-assembly of rod containing block copolymers is potentially useful for organic electronics and biopolymers. In the weak segregation limit, a model rod-coil block copolymer shows only lamellar ordered structures, with nematic and isotropic phases observed on heating above the order-disorder transition. As both the strength of segregation and the asymmetry between the rod and coil are increased, new hexagonal phases are observed. Self-assembly of these hexagonal phases requires both high asymmetry in volume fraction of the blocks and high asymmetry between the interfacial area occupied by coil and rod such that there is a strong driving force to break lamellae. Heating of hexagonal polymers with a comparatively low coil fraction can result in an order-order transition to the lamellar phase. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U24.00005: Morphological Characteristics and Phase Behavior of Nanoparticle-Modified Block Copolymers Michelle Bowman, Michael Bockstaller, Kim Rasmussen, Jon Samseth, Steven Smith, Russell Thompson, Richard Spontak Block copolymers exhibit a wealth of nanoscale morphologies that continue to find use in a diverse variety of emergent (nano)technologies. While numerous studies have explored the effects of molecular confinement on such copolymers, few have examined the use of such objects to modify the morphological characteristics and phase behavior of microphase-ordered block copolymers. In this work, a poly(styrene-$b$-methyl methacrylate) (SM) diblock copolymer has been modified with surface-functionalized fumed silica (FS) and colloidal silica (CS). Dynamic rheological measurements have been conducted on the neat and nanoparticle-modified copolymer to generate a quantitative comparison with SM/FS and SM/CS nanocomposites. Transmission electron microscopy (TEM) and self-consistent field theory (SCFT) calculations have also been performed to further elucidate results obtained via dynamic rheology by establishing the morphological characteristics of the copolymer and the dispersion of the functionalized nanoparticles within the resultant nanocomposites. [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U24.00006: Structure and Phase Transition in Sulfonated Block Copolymer Moon Jeong Park, Nitash Balsara Ion conducting polymers containing sulfonic acid are of interest for the fuel cell applications. This study is based on sulfonated poly(styrene-b-methylbutylene) block copolymers. Small-angle X-ray scattering experiment was performed on the dry state and these materials show remarkably rich sequence of phase transitions, i.e., lamellae, gyroid, hexagonally perforated lamellae, hexagonal cylinder, and disorder, depending on temperature, molecular weight, and fraction of sulfonated monomers. Small-angle Neutron scattering measurements under controlled temperature and humidity provided us powerful insight into the structure of hydrated materials. We will present data on the relationship between water uptake and morphology. In future work we plan to measure the proton conductivity of these materials. [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U24.00007: Symmetry Breaking in Block Copolymer Thin Films Eric Cochran, Gila Stein, Kirill Katsov, Ed Kramer, Glenn Fredrickson This contribution is concerned with the packing of spherical domain block copolymer mesophases in the thin film geometry as a function of the number of layers $n$. In a single layer, $n = 1$, the spheres pack on a hexagonal lattice; in the bulk, $n = \infty$, the $bcc$ ($Im\overline{3}m$) packing is preferred. These symmetries minimize packing frustration in 2- and 3- dimensions, respectively. We find that intermediately segregated films of sphere-forming poly(styrene-\emph{b}-vinylpyridine) pack on a hexagonal lattice ($P6_3/mmc$) for $n$ = 1--3, and then abrupty transform to a $Fmmm$ orthorhombic packing for $n \ge 4$. Beyond the $P6_3/mmc$-$Fmmm$ transition, the unit cell deforms to asymptotically approach the bulk $bcc$ packing. These results are interpreted in terms of the competition between the excess surface energy imposed by the interfaces and the free energy of the bulk system. We construct a simple theory, parameterized using high-resolution SCFT calculations, that successfully accounts for the experimentally observed symmetry breaking. From these calculations we conclude that character of the transition from thin-film to bulk behavior may be either continuous or discontinuous, depending on the degree of the block copolymer segregation. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U24.00008: Dewetting and Phase Separation in Thin Film Polymer Blends Nigel Clarke We present a dynamic theory for simultaneous phase separation and dewetting in ultra thin-film binary mixtures with free surfaces. The model assumes that phase separation only occurs in the plane of the film, thus reducing a three dimensional problem to two dimensions. If the height of the film is orders of magnitude less than the typical lateral dimensions of phase separation and dewetting then such an approximation is highly desirable from a computational viewpoint. We show some preliminary computational results, illustrating how coupling between the two processes can significantly affect pattern formation. [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U24.00009: Shear-Induced Phase Transitions in Ternary Polymer Blends Venkat Ganesan, Bharad Narayanan We present a theoretical study of flow-induced phase transitions in the microemulsion phases of ternary polymer blends. The results match qualitatively with recent experimental observations, and suggests that flow transforms the microemulsion phases into distinct lamellar phases at strong shears followed by a macrophase separation of the homopolymers at even stronger shear flows. These transitions are also accompanied by a strong shear-thinning behavior in the rheological response. The results suggest significant differences between ternary polymeric systems and oil-water- surfactant systems, which we rationalize from a molecular viewpoint. [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U24.00010: Formation of a superlattice in mixtures of block copolymer micelles Sayeed Abbas, Timothy P. Lodge The formation of a superlattice was observed in a binary solution mixture of diblock copolymer micelles. The larger micelles formed from polystyrene-$b$-polyisoprene (SI) diblocks had an overall radius of 29 nm, and the smaller micelles formed from polystyrene-$b$-polydimethylsiloxane (SD) diblocks had an overall radius of 16 nm. The radius ratio of the micelles was therefore 0.55. By indexing the peaks observed in the scattering pattern from the sample, the structure was assigned to the $Fm\bar {3}c$ space group. The structure is more commonly known in the literature as the binary AB$_{13}$ superlattice. The same lattice was again observed in a similar SI + SD system where the hydrodynamic radii of the bigger and the smaller micelles were 38 and 22 nm, respectively. In this system the size ratio of the micelles was 0.57. Similar to sterically stabilized colloidal hard spheres, the micelles were uncharged and hence the formation of the superlattice was driven solely by free volume entropy. [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U24.00011: Phase transitions in block copolymers induced by external fields Marco Pinna, Andrei Zvelindovsky By means of computer simulation we investigate structures formed by polymers consisting of several chemically different blocks, block copolymers. Aim of this contribution is to describe phase transitions between different structures induced by applied electric field and shear flow. We study behaviour of bicontinuous gyroid structure under electric field and compare it with the transition under shear flow. The transition demonstrates non-trivial topological transformations under various fields. We also study spherical, cylindrical, and lamellae phases. Depending on temperature the sphere morphology shows different behaviour under applied shear flow or electric field. We describe kinetics of sphere-to-cylinder transition. We found that about the critical value of electric field the spherical phase transforms into cylinders. While applying shear flow we find sphere-to-cylinders coexistence at one temperature and arrays of hexagonal ordered spheres at another temperature. The results are compared with the experiments and suggest ways of block copolymer structure tailoring. [Preview Abstract] |
Thursday, March 8, 2007 10:36AM - 10:48AM |
U24.00012: Phase behavior and morphology of high hard block content polyurethanes. Alberto Saiani, Julia S. Higgins We have investigated the phase behavior and morphology of a set of high hard block content polyurethanes: 50 to 100{\%} (wt.) hard segments. Using mainly calorimetry, scattering and microscopy techniques we were able to elucidate the origins of all the thermal events observed through differential scanning calorimetry. Correlating our thermodynamic work with our structural work we were able to propose a new morphological model of the structure and the phase behavior of high hard block content polyurethanes. We have shown that above 65{\%} hard segment content the melt-quenched samples present a two-phase morphology one pure hard segment phase co-existing with a mixed phase with the same hard segment content of 65{\%} for all samples. When annealed at high temperature the mixed phase undergoes phase separation resulting in the same phase-separated mesophase. This model allowed us to account for our small angle X-ray experiments that showed a single scattering peak at the same position for all the samples. [Preview Abstract] |
Thursday, March 8, 2007 10:48AM - 11:00AM |
U24.00013: Monte Carlo simulation of self-assembled polymer chains with inter-chain attractions Xinjiang L{\"u}, James Kindt We have used Monte Carlo simulations to study the phase behavior of self-assembled semiflexible chain polymers with inter-chain attractions. Approximate phase diagrams are obtained with varied physical parameters, such as the chain flexibility, intra- and inter-chain bonding energies. The attraction induced phase separation results in an equilibrium between a bundle and isotropic short chains. The chain length distribution of the phase separated system, as well as the bundle's shape and aspect ratio are presented and discussed. Simulation results are analyzed and compared with related experimental and theoretical work. We also present some other interesting results of observed toroids and branched bundles. [Preview Abstract] |
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