Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session H4: Polymer Physics Prize |
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Sponsoring Units: DPOLY Chair: Philip Pincus, University of California, Santa Barbara Room: Ballroom A4 |
Tuesday, March 22, 2011 8:00AM - 8:36AM |
H4.00001: Polymer Physics Prize Talk: Polymer Brushes: Why do we still care? Invited Speaker: Polymer molecules have been widely used to modify the properties of surfaces including its adhesion. Among the most studied have been polymer brushes, in which polymer chains are grafted at one end to a surface and immersed in a small molecule solvents. Experiments and simulations have shown that the conformation of the chains grafted onto a flat surface depends on the grafting density and the interaction of the polymer with the solvent. As the molecular weight of the solvent increases, the structure of the brush changes. Consequently the brush chains are expelled from the solvent due to entropic loss that originate from the fact that the melt chains penetrating the brush cannot overcome the translational, or mixing, entropy. This crossover from wetted to non-wetted brushes, has important implications for polymer adhesion, where the phase separation of melt and brush chains reduces entanglements at the interface. As polymers are grafted to nanoparticles, the curvature of the surface offers the polymer brush chains a significantly larger space to explore compared to a flat surface, reducing the tendency for autophobic dewetting. Using large scale molecular dynamics simulations we have studied the interface between brush coated nanoparticles and a polymer melt. Effects of chain length of the brush, and that of the polymer melt, the coverage of the nanoparticle and its curvature on the brush/melt interface will be discussed. The role of individual entanglements, between the brush chains and the melt, as identified by primitive path analysis will be introduced. These simulations provide insight into the structure of the brush/polymer interface which is not accessible through other theoretical or experimental means. [Preview Abstract] |
Tuesday, March 22, 2011 8:36AM - 9:12AM |
H4.00002: Polymer Physics Prize Talk: Topological Constraints Matter -- or Back to the Origin Invited Speaker: Topological constraints, being permanent or temporal, influence many properties of soft matter, especially polymers. While at a first glance the simple Rouse models describes the motion of short chains surprisingly well, the fact that chains cannot cut through each other dominates the dynamics of long chain melts, ring polymers and the relaxation in networks and gels. Furthermore new phenomena in special melts and mixtures even make this more obvious. The talk will review some developments and will also address new problems linked to material science as well as biology. To illustrate the importance of topological constraints, numerical simulations for a melt on non concatenated ring polymers with and without linear contaminants will be presented. While the static properties of long rings can be rationalized by the concept of a crumpled globule, dynamic properties are much less understood. Our simulations clearly show that diffusion and stress relaxation in such a system of globules decouple. In addition the first results for non concatenated rings added to a melt of linear polymers and for a few linear polymers added to a melt of rings will be discussed. [Preview Abstract] |
Tuesday, March 22, 2011 9:12AM - 9:48AM |
H4.00003: Bottle-brush polymers versus worm-like chains: Do we understand the stiffness of macromolecules? Invited Speaker: Bottle-brush polymers contain a long flexible macromolecule as a backbone to which flexible side-chains are grafted. Through the choice of the grafting density of the length of the side chains to the local stiffness of this cylindrical molecular brush can be controlled. However, understanding mesoscopic length scales (cross-sectional radius, persistence length, contour length) of these semiflexible cylindrical brushes poses a challenging problem. While self-avoiding walks of variable stiffness show a crossover to the Kratky-Porod worm-like chain model, and hence a (pre-asymptotic) regime of Gaussian behavior, bottle-brushes under good solvent conditions are not compatible with this model. Consequences for the description of chain stiffness in terms of the concept of the persistence length are discussed, as welll as pertinent experiments. [Preview Abstract] |
Tuesday, March 22, 2011 9:48AM - 10:24AM |
H4.00004: Entanglements and the Mechanical Properties of Glassy Polymers Invited Speaker: The response of glassy polymers to shear or tensile strain is strongly influenced by the entanglement network that is inherited from the melt. Molecular dynamics simulations are used to probe the microscopic origins of stress-strain curves and their connection to entanglements. The latter are identified in real space by examining topological constraints along the primitive path. The first part of the talk will consider the process of craze formation, where the entanglement density is correlated to the volume increase during crazing. Simulations show that entanglements are preserved during crazing, but the craze density does not correspond to pulling chains taut between entanglements. The second part of the talk will examine the effect of entanglements on strain hardening under uniaxial strain. The stress is directly associated with the degree of orientational order along the strain axis, and nearly independent of order along perpendicular directions [1]. Studies with mixtures of short and long chains show that the degree of order is independent of the surrounding chains [2]. The final part of the talk will examine the strength of welds formed by diffusion across polymer interfaces. The shear stress follows the bulk response until chains are pulled taut on the scale of the length of segments that have diffused across the interface. When this length is several times the entanglement length, the maximum shear stress saturates at the bulk value and chains fail through scission. Similar trends are found for the fracture energy in tensile loading.\\[4pt] [1] T. Ge and M. O. Robbins, J. Polymer Sci. B: Polymer Physics {\bf 48}, 1473-1482 (2010).\\[0pt] [2] R. S. Hoy and M. O. Robbins, J. Chem. Phys. {\bf 131}, 244901 (2009) [Preview Abstract] |
Tuesday, March 22, 2011 10:24AM - 11:00AM |
H4.00005: Conjugated Polymer Nanoparticle Hybrids: Structure, Dynamics and Forces Invited Speaker: While nanoparticles (NPs) have unique tunable elctro-optical properties and exceptional mechanical strength, it remains a challenge to integrate them into devices while retaining the advantages of the nanoscale. Tethering polymeric materials to the NPs surfaces has the potential to stabilize single NPs and direct their assembly. The polymers may serve in several capacities from a simple tether to a matrix to directed assembly tool taking advantage of the inherent structure of the polymers and as an active component in a complex material. However confining a large molecule to a highly curved surface affects the inherent configuration of the polymer. These effects are of particular interests in conjugated polymer-nanoparticle hybrids, where the conformation of the polymers affects not only the assembly of the nanoparticles but also the optical and electronic communication between the NPs. Using molecular dynamic simulations we have studied the structure of a single hybrid of \textit{para} dialkyl phenylene ethynelyne (PPE) grafted nanoparticles. PPEs are polymers whose conformation determines their degree of conjugation and therefore their electro-optical response. Using simulations coupled with neutron scattering studies we have shown that PPE is a rigid polymer that is fully extended in dilute solutions in good and theta solvents but can be forced into a collapsed configuration in a poor a solvent. When confined to a nanoparticle surface, the PPE chains are fully extended but cluster as the solvent quality is reduced. Results for the conformation of grafted PPE molecules on a single nanoparticle and the forces between two nanoparticles as a function of chain length and solvent quality will be presented. These simulations provide insight to the interactions that result in formation of tunable hybrids. [Preview Abstract] |
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