Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session A31: Molecular Motion in Miscible Blends |
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Sponsoring Units: DPOLY Chair: Sanat Kumar, Rensselaer Polytechnic Institute Room: LACC 503 |
Monday, March 21, 2005 8:00AM - 8:36AM |
A31.00001: Dynamics in Miscible Blends: Recent Results and Open Questions Invited Speaker: The study of polymer blends has been a very active field in polymer physics during the past 20 years. However, many questions still remain open. From the point of view of the polymer blend dynamics, the so-called dynamic miscibility, i.e., the question concerning how the dynamics of each component is modified in the blend, has been deeply investigated. In an ideal two-component miscible polymer blend one could expect a completely homogeneous dynamic behavior, in the meaning that the time scale as well as the relaxation function of each component becomes similar in the blend. However, miscible polymer blends are in general dynamically heterogeneous. This heterogeneity manifests in two ways. On the one hand, when the segmental dynamics (alpha-relaxation) of a single component of a blend is selectively investigated, it is found that the response extends over a very broad time/frequency range, in particular in the vicinity of the glass transition temperature. On the other hand, distinctly different local segmental mobilities for the two blend components are observed even at temperatures well above the glass transition. Whether these two manifestations have the same microscopic origin or they put in evidence two different aspects of the polymer blend dynamics is still controversial. Two different but perhaps complementary concepts have been used to describe the above-described phenomenology: thermal concentration fluctuations and the so-called self-concentration. However, a complete theoretical description still remains elusive. The main goal of this talk is to present an overview of the current status on this topic. New results - not only from relaxation techniques but also from neutron scattering and molecular dynamics simulation as well - will be also presented and discussed. [Preview Abstract] |
Monday, March 21, 2005 8:36AM - 9:12AM |
A31.00002: Component Dynamics in Miscible Blends of PEO and PMMA Invited Speaker: We investigate component dynamics in miscible blends of PEO and PMMA as determined from quasielastic neutron scattering combined with deuterium labeling. This blend has many unusual features. The glass transition temperatures of the two components are widely separated leading to extreme differences in the timescales of component mobility. This, in addition to the weak interactions in this system, makes it more likely than other blends to be influenced by concentration fluctuations. The chain connectivity picture does not appear to provide an adequate description of this blend for those cases in which it has been tested, and the small relaxation times of PEO have interesting consequences in the coupling model. We present results for the segmental dynamics of both components, in blends of 10-30 percent PEO by weight, the composition range where PEO remains amorphous. Incoherent measurements, which provide an estimate of self mobility, are discussed within the framework of each model, using molecular simulation to provide additional information where needed. [Preview Abstract] |
Monday, March 21, 2005 9:12AM - 9:24AM |
A31.00003: Modelling the Segmental Relaxation Time Distribution of Miscible Polymer Blends Jane Lipson, Ralph Colby In considering the relaxation of segments in a blend whose components have reasonably disparate glass transition temperatures, local concentration fluctuations and density fluctuations each play a role. The result is a distribution of environments around a given segment in the blend, which translates into a distribution of segmental relaxation times. In this work we focus on concentration fluctuations, making use of a simple lattice model to generate a distribution of environments which we then translate into a dielectric relaxation spectrum. We analyze experimental data for several polymer blends and show that, by accounting for the relatively strong composition dependence of the blend glass transition temperature, it is possible to model the dielectric relaxation spectrum using a Kuhn length which is both composition and temperature independent. [Preview Abstract] |
Monday, March 21, 2005 9:24AM - 9:36AM |
A31.00004: Miscible Polyisoprene/Polystyrene Blends: An Unusual Combination of Heterogeneous Segmental Dynamics and Homogeneous Diffusion Yiyong He, Tom Lutz, Mark Ediger, Marinos Pitsikalis, Nikos Hadjichristidis, Ernst von Meerwall The segmental and terminal dynamics of each component in miscible blends of polyisoprene (PI) and polystyrene (d$_{3}$PS) were characterized over a wide temperature and composition range. Though the system has a large positive interaction parameter \textit{$\chi $} up to 0.15, it is miscible in the temperature range of study due to selected low molecular weight. C-13 and H-2 NMR relaxation measurements were performed to extract the segmental dynamics. Pulse-gradient spin-echo NMR was used to determine the diffusion coefficients. Though the segmental dynamics of PI and PS components differ by more than 2 decades at $T_{g}$+50 K, their terminal dynamics (monomeric friction) are identical. We know of no other system with zero to positive \textit{$\chi $ }showing this feature. The distinct component segmental dynamics can be reasonably interpreted by the Lodge/McLeish model. The unusual homogeneous terminal dynamics are most likely due to a large thermodynamic barrier to diffusion. [Preview Abstract] |
Monday, March 21, 2005 9:36AM - 9:48AM |
A31.00005: A molecular picture: How composition influences the dynamic and static properties in a polyolefin blend, as observed with molecular simulation Andrew May, Janna Maranas We use molecular dynamics simulation to investigate dynamic and static properties in a blend of poly(ethylene-propylene) [PEP] and poly(ethylene-butene) [PEB]: this is a simple model for blend dynamics because the mixture behaves athermally and each component has similar pure packing characteristics and glass transition temperatures. The use of simulation allows us to examine a full spectrum of compositions, ranging from the dilute (single chain) to concentrated limits (all but one chain). As composition is varied, mobility is observed through the self-intermediate scattering function, while the pair distribution function and local concentrations are used to examine static features. Attention is given to both average values and the distribution within the average. Despite the simplicity of this system, the influence of composition varies between the two components, most noticeable in the dilute region. Molecular packing and concentrations on a local length scale are investigated as a possible source for this variation. [Preview Abstract] |
Monday, March 21, 2005 9:48AM - 10:00AM |
A31.00006: Interdiffusion in a Polydisperse Polymer Blend Anna C. Balazs, Victor V. Yashin We present a theoretical description of interdiffusion in a binary blend of polymers that exhibit polydispersity in length. The diffusion equations are formulated in terms of the volume fractions and the chain concentrations of the components. This choice of variables is equivalent to the assumption that the local molecular weigh distributions of the components are described by the Flory distribution. The Onsager kinetic coefficients are obtained based on the Green-Kubo equation and correspond to the fast-mode interdiffusion theory. As demonstrated by numerical simulations, the resulting equations describe the simultaneous processes of the evolution of blend composition and the relaxation of the local molecular weight distributions of the components. The developed approach can be used to study polymer systems in which the degree of polymerization changes due to interfacial or bulk chemical reactions. [Preview Abstract] |
Monday, March 21, 2005 10:00AM - 10:12AM |
A31.00007: A Molecular Dynamics Simulation Study of the Alpha- and Beta-Relaxation Processes in a Realistic Model Polymer Dmitry Bedrov, Grant D. Smith Molecular dynamics simulations of a melt of freely rotating chains of 1,4-polybutadiene (FRC-PBD) have been performed over a wide range of temperature. Removal of the dihedral barriers in FRC-PBD allows for complete resolution of the Johari-Goldstein $\beta $-process from the primary $\alpha $-process in the simulation time window. We find that relaxation in the $\beta $-regime occurs as the result of large-angle excursions of all backbone dihedrals that are largely decoupled from the dynamics of the polymer matrix, while the $\alpha $-relaxation exhibits strong coupling between matrix motion and polymer dihedral relaxation. [Preview Abstract] |
Monday, March 21, 2005 10:12AM - 10:24AM |
A31.00008: Entropy Theory of Polymer Glass-Formation Revisted Jack Douglas, Jacek Dudowicz, Karl Freed Considerable physical evidence supports the idea of Gibbs and DiMarzio that glass-formation arises when the configurational entropy of a liquid becomes critically small and the subsequent arguments by Adam and Gibbs (AG) that quantitatively relate the configurational entropy to the rate of long wavelength structural relaxation. We revisit this classical `entropy theory' of glass-formation, based on a minimal lattice model that incorporates monomer structure and the different rigidities of the polymer chain backbone and side-groups into a thermodynamic description of compressible polymer melts with van der Waals interactions. Previous observations of an apparent breakdown of the AG theory at elevated temperatures (20- 30 K above the glass-transition temperature Tg) led us to identify the `configurational entropy' of the AG model with the site rather than mass normalized configurational entropy. This reinterpretation of the entropy theory has little effect near the glass transition, but it completely revises the theory at more elevated temperatures. In particular, we predict a series of characteristic temperatures describing respectively, the onset of a drop in s upon cooling, an inflection point in sT and the extrapolation of s to 0. The T-dependence of s is quite distinct above and below the inflection point `crossover temperature'. We complete our specification of the characteristic temperatures of glass-formation by defining kinetic transition temperature Tg through a Lindemann criterion. [Preview Abstract] |
Monday, March 21, 2005 10:24AM - 10:36AM |
A31.00009: Correlation between static and dynamic heterogenities in polymer mixtures Roland Faller, Florence Pon, Qi Sun Computer simulations cannot only address average properties of the system under study but also the distribution over any observable of interest. Here we are using this advantage to study mixtures of polystyrene and polyisoprene by atomistic molecular dynamics and calculate correlation times for all segments in the system. We then identify fast and slow segments. We are able to correlate the segment speed with the local neighborhood and obtain that fast segments have a surplus of the faster component in their neighborhood and vice versa [1]. We are additionally studying other influences on the dynamics such as end effects. As these studies are performed on a mixture with strongly different glass transition temperatures, we are able to study the behavior in a a temperature range where one constituent would be a glass whereas the other one a melt. [1] R. Faller Macromolecules 37 (2004) 1095 [Preview Abstract] |
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