Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session G4: Polymer Physics Prize |
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Sponsoring Units: DPOLY Chair: Murugappan Muthukumar, University of Massachusetts Room: Baltimore Convention Center 308 |
Tuesday, March 14, 2006 8:00AM - 8:36AM |
G4.00001: On nanostructured dispersions and toughness of semi-crystalline polymers. Invited Speaker: In the past 50 years, great research efforts have been devoted to increase toughness of plastics. Today, impact strength of millions of tons of plastics used in all sorts of applications relies upon dispersing rubber particles in semi-crystalline polymers (polyolefins, polyamides or polyesters). Yet, the underlying mechanisms controlling such toughening are controversial. Based on a large body of literature and some simple observations, we propose a theory of toughening that explains the existence of an often evoked critical distance between rubber particles which controls the brittle-to-tough transition and predicts how it depends on size and concentration of particles. Our model predicts the brittle-to- tough transition temperature and emphasizes the role of crystal organization and orientation of matrix. Adding rubber particles induces a substantial loss in stiffness. We report how to obtain super-tough polymers and avoid such softening by using nanostructured dispersions of block- copolymers. We show that block copolymers having a three-dimensional co-continuous structure are particularly attractive as they disperse in a polymer matrix very differently from a lamellar one. In particular, droplet coalescence is much less pronounced for the co-continuous structure. Our observations arise interesting general questions about break-up and coalescence of structured fluid droplets. With Laurent Cort\'{e}. [Preview Abstract] |
Tuesday, March 14, 2006 8:36AM - 9:12AM |
G4.00002: Function in block copolymer assembly Invited Speaker: Spontaneous mesophase and colloidal organization of linear blockcopolymers in bulk and in thin films yields a wealth of well defined metastable and equilibrium structures with intriguing perspectives of new materials functionalities. Besides length, composition, and the type and number of the constituent blocks, branching and chemical transformation provides a further parameter for controlling the molecular conformation and structure. Here we will report on thermal and photo induced single molecule collapse and micellar assembly switching.for comb, palmtree blockcoplymers, as well as for amphiphilic polymer complexes as a means to introduce novel and responsive functionalities, such as motility and hydrophilic/hydrophobic switching. [Preview Abstract] |
Tuesday, March 14, 2006 9:12AM - 9:48AM |
G4.00003: Field-Theoretic Polymer Simulations: At the Frontier Invited Speaker: I will review our recent work on the development of a theoretical framework and numerical strategies for conducting computer simulations of polymer field theory models. We invoke Hubbard-Stratonovich and related auxiliary field transforms to convert ``particle-based'' models of polymeric fluids into statistical field theories. This transformation has the effect of replacing polymer-polymer interactions with polymer-field interactions, and renders the action complex. We have developed efficient numerical techniques for computing saddle point field configurations (mean-field or SCFT solutions), as well as for sampling field fluctuations near or away from saddle points (``field-theoretic simulations''). This talk will focus on a number of contemporary topics including: high resolution and variable cell shape SCFT, geometrically complex domains, hybrid particle-field simulations, and strategies for avoiding the sign problem. Examples of applications to polymer alloys, block copolymers, and copolymer nanocomposites will be presented. [Preview Abstract] |
Tuesday, March 14, 2006 9:48AM - 10:24AM |
G4.00004: Structural an Dynamical Properties of Some Equilibrium Polymers. Invited Speaker: The most detailed studies on equilibrium polymers have been performed on surfactant wormlike micellar solutions. Recently the formation of reversible supramolecular polymers through H bond mediated self assembly of two homofitopic heterocomplementary monomers were reported. In apolar solvents, highly viscoelastic solutions were observed at concentrations as low as 2 mM, thus indicating a network-like structure. We have studied a supramolecular polymeric system formed through a sextuple hydrogen bonding of a Janus-type wedge and a corresponding receptor. Small Angle Neutron Scattering experiments showed that the monomers self assemble into long unidimensionnal aggregates whose local structure is independent of temperature and concentration. From the values of the cross section and the mass per unit length of the fibers it can be inferred that these fibers contain several monomolecular wires. The concentration and temperature dependences of the average polymer length were found to be similar to those of worm-like micelles. Non linear rheological experiments showed a shear-banding instability as in micellar systems. However the concentration dependence of the terminal time of the stress relaxation suggests the formation of additional transient crosslinks in the supramolecular polymeric systems resulting in a slowing down of the relaxation as in associating polymers. [Preview Abstract] |
Tuesday, March 14, 2006 10:24AM - 11:00AM |
G4.00005: A Lattice Model for Segmental Dynamics of Miscible Polymer Blends Invited Speaker: Thermally-driven concentration fluctuations make local regions (at the scale of monomers) have a wide range of local compositions for weakly interacting miscible blends of long chain polymers. These fluctuations remain important hundreds of degrees from the critical temperature because the entropy (and hence free energy) of mixing is small in polymer mixtures. The connected nature of the chain biases the local composition distribution, making the range of effective compositions surrounding a given monomer extend from the self-composition to environments very rich in that type of monomer. These two polymer physics issues make blends of polymers vastly more interesting than mixtures of small molecules. Time-temperature superposition can fail and motions can persist far below the glass transition temperature of the blend; both of these results are enhanced as the glass transition contrast between the two components increases. A simple lattice model is used to describe the segmental dynamics of miscible polymer blends. Concentration fluctuations and chain connectivity effects are calculated at the scale of the Kuhn length, by considering a central monomer to be surrounded, out to the second shell of monomers, by 24 lattice sites. Including the central monomer, fraction 5/25 = 0.2 of the lattice sites are part of the central monomer's chain (the self-composition) and the other 20 sites are occupied stochastically, while preserving connectivity of all chains. The resulting concentration distributions are mapped onto segmental relaxation time distributions for each blend component using the composition dependence of the glass transition and dynamic scaling. The predicted distributions are compared with experimental dielectric data on miscible polymer blends using three methods: (1) A Debye (single exponential) relaxation of each composition predicts dielectric loss peaks for each blend component which are too narrow because the lattice model ignores density fluctuations. (2) The empirical Havriliak-Negami distribution can be fit to the dielectric loss of each pure component and then assigned to each composition in that component's distribution in the blend. (3) The pure component data can be modeled with a Gaussian distribution of density fluctuations with times related to free volume using the Doolittle equation, and subsequently this distribution is assigned to each composition in that component's blend distribution. The relative merits of these three approaches will be discussed in detail. [Preview Abstract] |
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