Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session J31: Frank J. Padden Award Symposium |
Hide Abstracts |
Sponsoring Units: DPOLY Chair: Steve Granick, UIUC Room: LACC 503 |
Tuesday, March 22, 2005 11:15AM - 11:27AM |
J31.00001: Local Dynamic Mechanical Properties of Model Free-Standing Polymer Thin Films Kenji Yoshimoto, Tushar Jain, Juan de Pablo We present results that strongly support a heterogeneous distribution of dynamic mechanical properties (DMP) in model free-standing polymer thin films. While a number of groups are investigating the mechanical properties of glassy polymer thin films on a substrate, enhanced molecular-level understanding will be required to further rationalize their observations. This study highlights the free-standing geometry; we eliminate any convolution of effects arising from the presence of a free surface and a substrate. The local DMP are calculated from nonequilibrium MD simulations with a coarse-grained polymer model. Our simulation results suggest that the soft surface layers coexist with the bulk-like rigid regions in the thin films below the glass transition temperature (Tg). By increasing the temperature, we show that the thickness of the soft surface layers substantially increases. The overall stiffness of the free-standing films is thereby shown to be much smaller than that of the bulk. The molecular mechanism of surface softening is revealed by a normal mode analysis of the glassy thin films; large, cooperative motions of polymer segments are enhanced near the free surfaces in the vicinity of Tg. [Preview Abstract] |
Tuesday, March 22, 2005 11:27AM - 11:39AM |
J31.00002: Self-assembly and cross-linking of nanoparticles at liquid-liquid interfaces Yao Lin, Alexander Boeker, Habib Skaff, Jinbo He, Kevin Sill, Todd Emrick, Anthony Dinsmore, Thomas Russell, Su Long, Qian Wang The fabrication of functional nanostructured materials for sensing, encapsulation and delivery requires practical approaches to self-assembly on multiple length scales and the synthesis of tough yet permeable structures. Here, ligand-stabilized nanoparticles assembled into three-dimensional constructs at fluid-fluid interfaces driven by the reduction in interfacial energy were investigated. Studies on the dynamics of the nanoparticles and the self-assembled structures formed at the interface, using fluorescence photobleaching methods and in-situ grazing incidence small angle x-ray scattering, suggest a liquid-like behavior and ordering at the interfaces. Cross-linking of the nanoparticle assembly using functional ligands, affords robust membranes that maintain their integrity even when they are removed from the interface. These composite membranes, nanometers in thickness, are elastic yet permeable. The assembly of virus and other biological complexes at fluid interfaces was also investigated where interfacial assembly rendered an easy route to direct and assemble the bioparticles into 2-D and 3-D constructs with hierarchical ordering. These assemblies enable the potential use of the bioparticles as a natural supramolecular building block to obtain materials with well-defined bio-functionalities. [Preview Abstract] |
Tuesday, March 22, 2005 11:39AM - 11:51AM |
J31.00003: Predicting the Viscosity of a Miscible Polymer Blend Jeffrey Haley, Timothy Lodge The composition-dependent rheological properties of polymer mixtures have been a topic of longstanding interest. We present a roadmap for viscosity predictions for the simplest binary polymer mixture, a miscible polymer blend. Drawing on the Lodge-McLeish model to predict the composition dependences of segmental dynamics, and the double reptation model to account for the chain dynamics, we present new comparisons of predictions with model blend viscosity data. The results suggest that some outstanding issues regarding the prediction of component chain dynamics remain. A modified constraint release model improves the prediction of component longest relaxation times and is able to resolve some of the observed discrepancies between experiment and theory. [Preview Abstract] |
Tuesday, March 22, 2005 11:51AM - 12:03PM |
J31.00004: Host polymer influence on dilute polystyrene segmental dynamics T.R. Lutz, Y.Y. He, M.D. Ediger We have utilized deuterium NMR to investigate the segmental dynamics of dilute (2{\%}) d$_{3}$-polystyrene (PS) chains in miscible polymer blends with polybutadiene, poly(vinyl ethylene), polyisoprene, poly(vinyl methylether) and poly(methyl methacrylate). In the dilute limit, we find qualitative differences depending upon whether the host polymer has dynamics that are faster or slower than that of pure PS. In blends where PS is the fast (low T$_{g})$ component, segmental dynamics are slowed upon blending and can be fit by the Lodge-McLeish model. When PS is the slow (high T$_{g})$ component, PS segmental dynamics speed up upon blending, but cannot be fit by the Lodge-McLeish model unless a temperature dependent self-concentration is employed. These results are qualitatively consistent with a recent suggestion by Kant, Kumar and Colby (Macromolecules, 2003, 10087), based upon data at higher concentrations. Furthermore, as the slow component, we find the segmental dynamics of PS has a temperature dependence similar to that of its host. This suggests viewing the high T$_{g}$ component dynamics in a miscible blend as similar to a polymer in a low molecular weight solvent. [Preview Abstract] |
Tuesday, March 22, 2005 12:03PM - 12:15PM |
J31.00005: Tailoring Protein and Cell Adsorption Using Surface-grafted Polymer Gradients Rajendra Bhat, Jan Genzer, Bryce Chaney, Andrea Liebmann-Vinson We report on tuning the adsorption of proteins and cells on surfaces using grafted polymer gradients. Specifically, we manipulated the amount of adsorbed fibronectin (FN) by adsorbing it onto surface-anchored poly (2-hydroxyethyl methacrylate) (PHEMA) with orthogonal variation of PHEMA molecular weight and grafting density. The amount of FN adsorbed on the surface decreased as molecular weight and/or grafting density of the PHEMA increased. Incubating cells that recognize FN resulted in cell density gradient on PHEMA gradient surface. The number of adsorbed cells decreased as FN concentration decreased along PHEMA gradient. [Preview Abstract] |
Tuesday, March 22, 2005 12:15PM - 12:27PM |
J31.00006: Experimental Investigation of Entangled Polymer Flow Behavior Prashant Tapadia, Shi-Qing Wang In this presentation, we report additional features of entangled polymer solutions and melts undergoing either continuous shear at high stresses comparable to the dynamic plateau modulus or step shear at large strain amplitudes, following the initial observations of a yield-like bulk flow transition in entangled polybutadiene solutions, made of ultra-high molecular weight PBD and lower molecular weight PBD (as the ``solvent'') [1]. Specifically, the dynamics of the flow transition have been first examined as a function of the chain length of the solvent. In a separate investigation, the stress responses after large step-strain deformation have been studied in detail for several PBD melts and solutions. In both cases, flow birefringence observations using white light were made to delineate the spatial distribution of the optical retardation and to provide further information on the microscopic origins of the observed flow behavior, with the polarized light incident onto either 1,2 plane or 1,3 plane. [1] Tapadia, P.; Wang, S. Q. \textit{Phys Rev. Lett}, \textbf{91}, 198301 (2003); Tapadia, P.; Wang, S. Q. \textit{Macromolecules }\textbf{37}, 9083 (2004). [Preview Abstract] |
Tuesday, March 22, 2005 12:27PM - 12:39PM |
J31.00007: The distribution of Tgs in bulk and nanoconfined polymer films measured by a novel fluorescence method Christopher J. Ellison, John M. Torkelson We have recently developed (Nature Mater., 2, p695, 2003) fluorescence approaches that have allowed the direct measurement of Tg in specific layers 10-15 nm in thickness near interfaces and in between. This novel approach has made possible the unique characterization of the distribution of Tgs in bulk and ultrathin polymer films where it has been observed that the distribution in Tgs is highly dependent on the type of interfacial interaction (as at the free surface or substrate) and the degree of nanoconfinement. These measurements have revealed that interfacial effects almost entirely explain Tg deviations observed in ultrathin polymer films and that these interfacial effects have the ability to impact the properties (Tg and associated cooperative segmental dynamics) of material that is even tens of nanometers away from an interface. For example, these measurements have indicated that the 14 nm nearest the free surface of a relatively thick polystyrene (PS) film has a Tg that is reduced by more than 30 K and that this Tg reduction at the free surface influences material even 30 nm away from that interface. [Preview Abstract] |
Tuesday, March 22, 2005 12:39PM - 12:51PM |
J31.00008: Growth of the Cooperative Length Scale Below the Caging Temperature of Glass-forming Liquids Brian Erwin, Ralph Colby, Sudesh Kamath, Sanat Kumar A cooperative mechanism is invoked to explain the acute property changes observed in glass-forming liquids near the glass transition temperature $T_g$. This theory implies the existence of cooperatively rearranging regions (CRR) as characterized by a dynamic length scale $\xi$, which is present in both experiments and simulation. Armed with the temperature dependence of this length scale and the fractal dimension of the CRR (from simulations) a simple scaling model for glassy behavior can be constructed. This scaling model has been applied to measurements of viscosity, the $\alpha$-relaxation and probe dynamics to estimate the $\xi(T)$ for numerous glass forming liquids. [Preview Abstract] |
Tuesday, March 22, 2005 12:51PM - 1:03PM |
J31.00009: Conjugated Polymer Nanowires: Preparation, Morphology, Optical Properties and Field-Effect Transistors Amit Babel, Yan Zhu, Dan Li, Younan Xia, Samson A. Jenekhe Nanowires of conjugated polymers are ideal system for studying 1-D confinement effects on optical and electrical properties and hold promise as building blocks for nanoelectronics. We have prepared high quality nanowires of conjugated polymer semiconductors, such as poly(3-hexylthiophene) (PHT), poly(4-hexylquinoline) (PHQ), poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEHPPV), poly(9,9-dioctylfluorene) (PFO) and their blends, using self-assembly and electrospinning techniques and investigated their morphological, optical and electrical properties. Self-assembled crystalline nanowires of binary blends of regioregular PHT and PHQ showed good ambipolar charge transport with hole and electron mobilities of 0.012 and 0.004 cm$^{2}$/Vs, respectively. Electrospun fibers of MEH-PPV and its blends with PHT and PFO had diameters of 30-500 nm and tunable optical and charge transport properties. $p$-Channel field-effect transistors based on the MEH-PPV/PHT blend nanofibers had hole mobility of up to 1$\times $10$^{-4}$ cm$^{2}$/Vs. [Preview Abstract] |
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