Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session B25: Polymer Blends |
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Sponsoring Units: DPOLY Chair: Rick Register, Princeton University Room: Morial Convention Center 217 |
Monday, March 10, 2008 11:15AM - 11:27AM |
B25.00001: Effective Coordination Number and Interaction Parameter In Simple Models of Polymer Blends David Morse One challenge faced when trying to quantity corrections to the RPA in either simulations or experimental measurements of correlations in polymer blends is the need to estimate the temperature or parameter dependence of a self-consistent field interaction parameter $\chi(T)$. In simulations of simple models, a useful independent definition of $\chi(T)$ may be obtained from a thermodynamic perturbation theory in which the difference between AB and AA interactions is treated as a perturbation of a one-component melt. For lattice models, this yields a value that, in the limit of long chains, is related to the Flory-Huggins value by replacing the coordination number $z$ by an effective coordination number given by the average number of inter-molecular contacts per monomer, as proposed previously by M. Mueller. Generalizations of the idea are provided for continuum models, and for shorter chains. An analytic theory is presented that quantitatively predicts the N-dependence of the effective coordination number found in lattice simulations. The approach provides a clean to way to separate RPA from non-RPA effects in the analysis of simulations. [Preview Abstract] |
Monday, March 10, 2008 11:27AM - 11:39AM |
B25.00002: Nucleation in Polymer Blends Edward Feng, Nitash Balsara Balsara and co-workers used small angle neutron scattering experiments on binary homopolymers blends to determine the size of the critical nucleus during phase separation. This suggests measuring the size of a single critical nucleus through a measurement of total density fluctuations. We carefully analyze this idea through kinetic Monte Carlo simulations of the Ising model, performing simulations of phase separation that conserve and do not conserve the magnetization. Calculations of the structure factor and spin-spin correlation function reveal clear differences in the nucleation mechanism for these two dynamics. Simulations that conserve the magnetization qualitatively agree with the experimental results on binary blends. Moreover, we calculate the cluster distribution during nucleation to determine the critical nucleus size of the Ising model. Comparing this result with the value determined from the structure factor shows qualitative agreement with increasing supersaturation. [Preview Abstract] |
Monday, March 10, 2008 11:39AM - 11:51AM |
B25.00003: Molecular dynamics simulations of constraint release effects in entangled binary blends of linear polymers Zuowei Wang, Ronald G. Larson We present extensive molecular dynamics simulations of the dynamics of entangled binary blends consisting of long test chains diluted in shorter chain matrix. The ratio between the long and short chain lengths is varied by a factor of ten covering the crossover from the chain reptation regime to the tube Rouse relaxation regime. Consistent with Neutron Spin Echo experiments, the dynamic structure factor of the long chains is found to decay faster in the matrix with shorter chain lengths, owing to the stronger constraint release effect. Correspondingly the monomers and centers of mass of the long chains show a faster time-dependent diffusivity than that expected from pure reptation. The simulation results for the diffusion properties agree qualitatively with the predictions based on constraint release Rouse motion model at long time scales, but show deviations from the theoretical predictions in the intermediate time regime. Our preliminary analysis of diffusion of the matrix chains in the tube-region of the long chains indicates that this discrepancy results from neglect of the broad distribution of the lifetimes of constraint release events in the theoretical treatment. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B25.00004: Flory Theorem for Structurally Asymmetric Mixtures Andrey Dobrynin, Frank Sun, David Shirvanyants, Gregory Rubinstein, Michael Rubinstein, Sergei Sheiko, Hyung-Il Lee, Krzysztof Matyjaszewski The generalization of the Flory theorem for structurally asymmetric mixtures was derived and tested by direct visualization of conformational transformations of brushlike macromolecules embedded in a melt of linear chains. Swelling of a brush molecule was shown to be controlled not only by the degree of polymerization of the surrounding linear chains, \textit{NB}, but also by the degree of polymerization of the brush's side chains, $N$, which determines the structural asymmetry of the mixed species. The boundaries of the swelling region were established by scaling analysis as $N^2$ $<$\textit{NB} $<$\textit{NA/N}, where \textit{NA} is the degree of polymerization of the brush backbone. Experiment and theory demonstrated good agreement. [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B25.00005: The Molecular Weight and Composition Dependence of Measured Flory-Huggins Interaction Parameters for Blends of Model Polyolefins Alisyn Nedoma, Megan Robertson, Nisita Wanakule, Nitash Balsara The thermodynamics of binary blends of model polyolefins (deuterated polybutadiene and polyisobutylene) was studied using small angle neutron scattering (SANS) and small angle light scattering (SALS). SANS experiments on single phase blends reveal that the Flory-Huggins interaction parameter, chi, for this system depends on both blend composition and component molecular weights. This is in contrast to numerous previous studies on thermodynamics of polymer blends where chi is reported to be independent of molecular weight. The phase separation temperatures of our blends, determined by SALS and SANS, will be compared to predictions based on the measured chi parameters. [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B25.00006: Porod SAXS Studies of Shear-Induced Droplet Deformation in a Concentrated Immiscible Polymer Blend Wesley Burghardt, Kristin Brinker Droplet deformation, break-up and coalescence of immiscible polymer blends under flow is fundamental to understanding the effect of processing on ultimate blend properties, as well as the excess rheological properties associated with deformation of the multiphase structure. Rheo-optical methods have frequently been employed to gain insight into these processes, but in most cases, optical methods are restricted to quite dilute concentrations, owing to concerns of high turbidity and multiple scattering. Here we explore used of time-resolved synchrotron small-angle x-ray scattering as an alternative method to study multiphase materials under shear. Typical blend droplet sizes of $\sim $ 1 micron are large relative to length scales typically probed by SAXS; however, the wide-angle limit of small-angle scattering (i.e. the Porod regime) is directly sensitive to interfacial structure of multiphase materials and, when rendered anisotropic by shear flow, provides direct insights into the deformation and orientation of interface. We report scattering studies in the flow-gradient plane of a polystyrene/poly(methyl methacrylate) blend, which is approximately viscosity matched, to step-strain deformations. Postulating that each droplet is deformed to a geometrically similar shape, data are analyzed in the context of a model of Porod scattering from ellipsoids. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B25.00007: Measurements of the Onsager coefficient in a phase-separating polymer blend Amish Patel, Nitash Balsara Phase separation in a polymer blend of critical concentration was studied using time-resolved Small Angle Neutron Scattering (SANS). To start off, the blend was homogenized by taking it well into the 1-phase region of the phase diagram. It was then quenched into the 2-phase region by using a rapid change in pressure. Since, the sample enters the two-phase region at the critical point, it is expected to phase separate by spinodal decomposition. Hence, the time dependent SANS intensity was fit to the Cahn-Hilliard-Cook equation for systems undergoing spinodal decomposition. The fit parameters were then used to calculate the non-local Onsager coefficient at several different pressures. The functional form of the Onsager coefficient, as a function of the wave-vector, was compared to that predicted by theory. Finally, the molecular parameters that go into the theory were extracted. [Preview Abstract] |
Monday, March 10, 2008 12:39PM - 12:51PM |
B25.00008: Structure And Dynamics Of Semi-crystalline Polyethylene Oxide / Polyvinyl Acetate Blends James Runt, Daniel Fragiadakis The structure and dynamics of semi-crystalline, melt-miscible polyethylene oxide / polyvinyl acetate (PEO/PVAc) blends were investigated using small-angle X-ray scattering and dielectric relaxation spectroscopy. PEO/PVAc blends with selected compositions were crystallized at various temperatures. Small-angle X-ray scattering was used to quantitatively determine the semi-crystalline microstructure, including the location(s) of the non-crystallizable PVAc in the structure. Segmental and local dynamics were studied using broadband dielectric relaxation spectroscopy. We attempt to clarify the origin of an additional relaxation, located at intermediate frequencies between the segmental and local processes, which has been proposed to be related to initial stages of crystallization even in blends lacking macroscopic crystallinity. [Preview Abstract] |
Monday, March 10, 2008 12:51PM - 1:03PM |
B25.00009: The Structure and Thermodynamics of Cellulose Acetates. Mark Dadmun, Rujul Mehta, Gary Lynn Cellulose acetate (CA) polymers having different degrees of substitution (DS) vary widely in their properties, such as glass transition, miscibility, chemical resistance, and gas permeation. These variations hint at differences in their molecular structure. We have systematically studied the variation in the molecular structure of CA with DS, utilizing small angle neutron scattering (SANS). SANS measurements were carried out in 1{\%} (w/v) solutions in dDMSO (which is sufficiently dilute to avoid intermolecular scattering). The polymer chains display rigidity along their main chain and have persistence lengths in the range of 12 to 14 nm. The effect of DS on the extent of miscibility is explained on the basis of available intra-molecular hydrogen bonding sites along the polymer chain. SANS measurements were also carried out on 50-50(w/w) miscible blends of two sets of CA, where the difference in the DS between the polymers varied from 0.05 to 0.25. Random phase approximation analysis provides an estimate of the Flory-Huggins interaction parameter between the two polymers in the blend. The influence of both the amount of substitution and hydrogen bonding on the polymer miscibility will be discussed. [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B25.00010: Effect of solvent evaporation and coagulation on morphology development of asymmetric membranes Neelakandan Chandrasekaran, Thein Kyu Miscibility behavior of blends of amorphous polyamide (PA) and polyvinylpyrrolidone (PVP) was studied in relation to membrane formation. Dimethylsulfoxide (DMSO) and water were used as solvent and non-solvent, respectively. Differential scanning calorimetry and cloud point measurements revealed that the binary PA/PVP blends as well as the ternary PA/PVP/DMSO system were completely miscible at all compositions. However, the addition of non-solvent (water) to this ternary system has led to phase separation. Visual turbidity study was used to establish a ternary liquid-liquid phase diagram of the PA-PVP/DMSO/water system. Scanning Electron Microscopy (SEM) showed the development of finger-like and sponge-like cross sectional morphologies during coagulation. Effects of polymer concentration, PA/PVP blend ratio, solvent/non-solvent quality, and evaporation time on the resulting membrane morphology will be discussed. [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B25.00011: The Glass Transition and Dynamics in Athermal Poly(a-Methyl Styrene)/Oligomer Blends Wei Zheng, Sindee Simon The glass transition and dynamics in athermal blends of poly(a-methyl styrene) (PaMS) and its short chain oligomers are investigated using differential scanning calorimetry (DSC). A methodology is described to partition the calorimetric transition in order to obtain effective Tgs for each component of the blend. The dependences of these effective Tgs on overall blend composition are described by the Lodge-McLeish model although the self-concentration effect is less than expected based on the Kuhn length. The kinetics associated with the glass temperature, Tg, are examined by studying the cooling rate dependence of Tg for the pure components and the blends, as well as by examining the enthalpy overshoots in the heating DSC scans. Extension of Colmenero's model to describe the dynamics in these materials will be discussed. [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B25.00012: Effect of compositional heterogeneity on the phase structure and crystallization behavior of polypropylene in-reactor alloys. Dujin Wang, Haijin Zhu, Benjamin Monrabal, Charles C. Han Although the compositional heterogeneity and chain structure of PP/EPR in-situ blends have been extensively investigated, little is known about the conclusive relationship among the molecular/phase structure and the ultimate mechanical properties due to its complex compositions in such system. A systematic study was conducted on the compositional heterogeneity, phase structure, the crystallization and subsequent melting behavior of two in-reactor alloys EB-P and EP-P. The composition of the alloys and the chain structure of each component were characterized by preparative TREF and 13C-NMR technique. The results showed that the excellent balance between toughness and rigidity of EB-P primarily benefits from the polyethylene homopolymer (HPE) phase and the ethylene-$\alpha $-olefin copolymer (EC) component, which is enriched at the interface between the dispersed phase (HPP) and the matrix (HPE). As for EP-P, the amorphous EC and the interpenetrating phase are mainly responsible for the outstanding low temperature impact toughness. [Preview Abstract] |
Monday, March 10, 2008 1:39PM - 1:51PM |
B25.00013: Mesoscopic drop dynamics and rheological modeling for polymer blends Yuanze Xu, Wei Yu, Jianmao Yang Fundamental challenge to rheology for polyblends originates from the dynamic coupling between interfacial morphology and viscoelastic flow. This work will outline our approaches. The framework of irreversible thermodynamics was employed to model immiscible viscoelastic blends considering the chain conformation, the drop deformation, drop break-up and collapse and the hydrodynamic interaction of drops in concentrated blends. The theoretical predictions were proved by measuring the drop dynamics in a four-roll mill rheometer with success. Great challenge to the model exists in systems where the drop dynamics severely deviating from emulsions of Newtonian droplets. A new type of drop merging by a string pulling two beads together (BSB) was observed and analyzed. The relevant material and process conditions causing BSB or beads-on-string are discussed elucidating its origin of non-linear viscoelasticity of polymers. [Preview Abstract] |
Monday, March 10, 2008 1:51PM - 2:03PM |
B25.00014: Self Similar Growth of Polyolefin Blends On a Micro-Nano Granule Reactor Charles Han, Jiang Du, Kun Meng, Xia Dong, Jin-yong Dong, Dujin Wang A Ziegler-Natta/metallocene hybrid catalyst was used in this MRGT (Multi-catalyst Reactor Granule Technology) synthesis. Isotactic polypropylene/polyethylene-co-octene (iPP/PEOc) polymer blends were prepared on the micro-nano granule reactors. A self-similar growth mechanism has been observed and deciphered. The self-similar structure is extended and observed at least for 5 decades in a combined real and reciprocal spatial range. With thermal treatment, structure growth and crystallization kinetics has been studied on these single reactor granules. [Preview Abstract] |
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