Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session V4: Dynamics of Polymers: Phenomena Due to Confinement |
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Sponsoring Units: DPOLY Chair: Zahra Fakhraai, University of Wisconsin Room: Ballroom A4 |
Thursday, March 24, 2011 8:00AM - 8:36AM |
V4.00001: Direct measurement of molecular motion in freestanding polystyrene thin films Invited Speaker: An optical photobleaching technique has been used to measure the reorientation of dilute probes in freestanding polystyrene films as thin as 14 nm. Temperature-ramping and isothermal anisotropy measurements reveal the existence of two subsets of probe molecules with differing dynamics. While the slow subset shows bulk-like dynamics, the more mobile subset reorients within a few hundred seconds even at Tg- 25 K (Tg is the glass transition temperature of bulk polystyrene). At Tg -5 K, the mobility of these two subsets differs by 4 orders of magnitude. These data are consistent with the presence of a high mobility layer at the film surface whose thickness is independent of polymer molecular weight and total film thickness. The thickness of the mobile surface layer increases with temperature and equals 7 nm at Tg. [Preview Abstract] |
Thursday, March 24, 2011 8:36AM - 9:12AM |
V4.00002: Molecular dynamics at nanometric length-scales Invited Speaker: Broadband Dielectric Spectroscopy, Spectroscopic vis-Ellipsometry, X-Ray Reflectometry, Alternating and Differential Scanning Calorimetry are combined to study glassy dynamics and the glass transition in nanometric thin ($\ge $5 nm) layers of polystyrene (PS) having widely varying molecular weights and Polymethylmethacrylate (PMMA) deposited on different substrates. For the dielectric measurements two sample geometries are employed, the common technique using evaporated electrodes and a recently developed approach taking advantage of nanostructures as spacers. \textit{All} applied methods deliver the concurring result that deviations from glassy dynamics and from the glass transition of the bulk never exceed margins of $\pm $3 K \textit{independent} of the layer thickness, the molecular weight of the polymer under study and the underlying substrate. Our findings are discussed in the context of the highly controversial literature and prove that an appropriate sample preparation is of paramount importance in order to avoid artefacts. \\[4pt] [1] Erber et al., \textit{Macromolecules} \textbf{43}, 7729 (2010).\\[0pt] [2] Mapesa et al., \textit{Europ. Phys. J. - Special Topics} \textbf{189}, 173-180 (2010).\\[0pt] [3] Tre{\ss} et al., \textit{Macromolecules} (2010). DOI:10.1021/ma 102031k. [Preview Abstract] |
Thursday, March 24, 2011 9:12AM - 9:48AM |
V4.00003: Gas Permeation in Thin Glassy Polymer Films Invited Speaker: The development of asymmetric and composite membranes with very thin dense ``skins'' needed to achieve high gas fluxes enabled the commercial use of membranes for molecular level separations. It has been generally assumed that these thin skins, with thicknesses of the order of 100 nm, have the same permeation characteristics as films with thicknesses of 25 microns or more. Thick films are easily made in the laboratory and have been used extensively for measuring permeation characteristics to evaluate the potential of new polymers for membrane applications. There is now evidence that this assumption can be in very significant error, and use of thick film data to select membrane materials or predict performance should be done with caution. This presentation will summarize our work on preparing films of glassy polymers as thin as 20 nm and characterizing their behavior by gas permeation, ellipsometry and positron annihilation lifetime spectroscopy. Some of the most important polymers used commercially as gas separation membranes, i.e., Matrimid$^{\mbox{{\textregistered}}}$ polyimide, polysulfone (PSF) and poly(2,6-dimethyl-1,4-phenylene oxide) (PPO), have been made into well-defined thin films in our laboratories by spin casting techniques and their properties studied using the techniques we have developed. These thin films densify (or physically age) much faster than thicker films, and, as result, the permeability decreases, sometimes by several-fold over weeks or months for thin films. This means that the properties of these thin films can be very different from bulk films. The techniques, interpretations and implications of these observations will be discussed. In a broader sense, gas permeation measurements can be a powerful way of developing a better understanding of the effects of polymer chain confinement and/or surface mobility on the behavior of thin films. [Preview Abstract] |
Thursday, March 24, 2011 9:48AM - 10:24AM |
V4.00004: Surface Dynamics in Glass forming Materials Invited Speaker: There is mounting evidence that the surface of glassy polymers exhibits enhanced dynamics compared to the bulk material at the same temperature. Using nanoparticle embedding and relaxation of nanodeformations on the surface, we have developed a detailed characterization of the dynamics of glassy polymers (polystyrene (PS), isotactic-poly(methyl methacrylate) ). This includes the effects of temperature, molecular weight and film thickness on the surface dynamics. We have extended the studies to the molecular glass former TNB, which display striking similarities to PS. The results of these studies allow us to begin to develop an understanding of the surface properties of glassy material, and how these properties may lead to observed changes in thin film polymer properties.\\[4pt] Work done in collaboration with Chad Daley and Dongping Qi, University of Waterloo; Zahra Fakhraaim, University of Pennsylvania; and Mark Ilton, University of Waterloo. [Preview Abstract] |
Thursday, March 24, 2011 10:24AM - 11:00AM |
V4.00005: Confinement effects on polymer structure and glassy dynamics Invited Speaker: We have performed molecular-dynamics simulations to explore the influence of confinement on the glass-transition temperature $T_{g}$ for supported atactic-polystyrene thin films of different thickness (1 nm $\div $ 10 nm) and different strengths of attraction to the substrate (0.1 kcal/mol $\div $ 3.0 kcal/mol). The films have been equilibrated in a melt at 540 K and further cooled down with a constant cooling velocity of 0.01 K/ps below $T_{g}$ to room temperature, 300 K. Based on the density measurements we have defined three different (substrate, middle and surface) layers for each film. We found that the monomers close to the surface and in the substrate layer are partially oriented, which leads to more effective monomer packing. For the whole film the average $T_{g}$ value remains almost constant for films down to 2 nm thickness, where middle layer vanishes. For the middle layer itself $T_{g}$ does not depend on the total film thickness, while an increase up to 70 K is measured for the substrate layer depending on the strength of attraction to the actual substrate. The surface layer remains liquid-like in the whole temperature range (300 K $\div $ 540 K). We claim that the redistribution of mass in the three film layers may explain the change with film thickness of the average $T_{g}$, if the latter is determined from linear fits of the average glass and melt densities. First results on the shear cycling and the rejuvenation phenomena will be discussed as well. [Preview Abstract] |
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