Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session X4: Dynamics of Nano-confined Polymer Films |
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Sponsoring Units: DPOLY Chair: James Forrest, University of Waterloo Room: Oregon Ballroom 204 |
Thursday, March 18, 2010 2:30PM - 3:06PM |
X4.00001: Consequences of Residual Stresses in Thin Polymer Films Invited Speaker: In our quest for making functional devices smaller, the thickness of polymer films has reached values even smaller than the diameter of the unperturbed molecule. However, despite enormous efforts over the last decade, our understanding of the origin of some puzzling properties of such thin films is still not satisfactory and several peculiar observations remain rather mysterious. In this context, we explore the consequences of the transition from a dilute polymer solution to the glassy state with respect to the properties of polymers in thin films. This transition is likely to result in residual stresses, arising from out-of-equilibrium chain conformations due to rapid solvent loss. Consequently, depending on thermal history and ageing time, such films exhibit significant changes even in the glassy state $^{ }$which we quantify by performing detailed studies of viscoelastic dewetting of thin polystyrene films on solid substrates. We explored relaxation times, residual stresses, and temporal changes of the stability of non-equilibrated thin films as they progress toward stable equilibrium behaviors. To do so, we have focused primarily on times shorter than the reptation time of the polymer. The number of spontaneously nucleated holes per unit area is seen to decrease as the films were aged below the glass transition, showing the meta-stability of the system. The ratio of stress over elastic modulus was found to increase strongly with decreasing film thickness and increasing chain length. Full equilibration of chain conformations required long times comparable to bulk reptation times. However, for chains longer than about 3000 monomers, the residual stress relaxed faster, at a rate independent of chain length. We present some tentative ideas on the relation between these observed atypical mechanical and relaxational behaviors and meta-stable states introduced by sample preparation. [Preview Abstract] |
Thursday, March 18, 2010 3:06PM - 3:42PM |
X4.00002: Distributions of Glass Transition Temperatures and Physical Aging and Diffusion Behaviors in Confined Polymer Films and Nanocomposites Invited Speaker: Thin polymer films and nanocomosites exhibit strong effects of nanoscale confinement: apparent Tg's can change by 50+ K; glassy-state relaxation can be strongly suppressed; and diffusion coefficients of small molecules in polymer can be altered by an order of magnitude or more. Here, we summarize key results from fluorescence studies that show how interfaces (polymer-air, polymer-substrate, polymer-nanofiller) perturb properties and the length scales, often $>$ 100 nm, over which interfacial perturbations can propagate into the polymer. We also show via multilayer film studies that ultrathin layers of one species can have their glass transition dynamics ``slaved'' to that of neighboring domains of other polymers. [Preview Abstract] |
Thursday, March 18, 2010 3:42PM - 4:18PM |
X4.00003: Substrate effects on the relaxation dynamics of nano-confined polymers: Insight from MD simulations Invited Speaker: In this talk, I survey recent progress in understanding the structure and dynamics of ultra thin polymer melts confined between solid walls. Here, ``ultra'' refers to distances between the substrates of the order of a few radius of gyration of the polymer chain. Using the example of a coarse-grained polymer model, we explore in detail the influence of the boundary condition, imposed by smooth or rough walls, on the structure and dynamics of the polymer melt. Depending on the atomic structure of the substrate, geometric confinement is found to have quite opposite effects on the properties of the melt. While polymers diffuse faster compared to the bulk in the vicinity of a perfectly smooth wall (no corrugation at all), their dynamics is strongly hampered close to a wall exhibiting corrugation on the length scale of the monomer diameter. The effect of corrugation is further enhanced if the wall atoms are not distributed randomly but are placed on a crystalline lattice. These effects are by no means negligible since they can change the structural relaxation time by many orders of magnitude. As a consequence, in nano-confined polymer systems, the glass transition temperature is expected to significantly differ from its bulk value. We compare our and other simulation results with experimental data and survey some theoretical ideas to explain these shifts. [Preview Abstract] |
Thursday, March 18, 2010 4:18PM - 4:54PM |
X4.00004: Flow dynamics of thin polymer films: Influence of the solid/liquid interface Invited Speaker: Nanoscale liquid polymer films are ideal candidates to probe the solid/liquid boundary condition: Prepared on a nonwettable surface like a hydrophobic Si wafer, the films are not stable, they dewet and bead off the substrate. That way, a flow is induced without applying an external force. Probing the dynamics of the dewetting process and the morphology of the liquid front, we can deduce the slip length, which is a characteristic for the solid/liquid boundary condition. A variation of the type of hydrophobic layer as top coating enables us to tune the boundary condition from a no-slip to a nearly full-slip condition. Changing the molecular weight of the polymer reveals that slippage is directly linked to chain entanglements. We find a reduced entanglement density at the solid/liquid interface (factor 3 to 4), which stresses the importance of considering non-bulk polymer properties in the vicinity of an interface.\footnote{O. B\"aumchen, R. Fetzer and K. Jacobs, PRL 103, 247801 (2009)} [Preview Abstract] |
Thursday, March 18, 2010 4:54PM - 5:30PM |
X4.00005: Dynamics in ultrathin films Invited Speaker: There is considerable interest in, yet large variability in results on the dynamics of ultrathin films. A question that arises in all cases is whether or not we are measuring dynamics and/or are different measurements probing the same dynamics. Hence, there are multiple experiments reporting changes in the glass transition temperature as based on a ``pseudo-thermodynamic'' measurement in which a parameter (film thickness, fluorescence intensity, index of refraction) vary as a function of temperature in a way that is reminiscent of the macroscopic volume-temperature in glass-forming liquids. Yet, these are not direct measurements of the dynamics. In the case of supported films, dynamics have been directly probed by dielectric measurements and mechanical measurements, the latter being complicated by contact mechanics problems or limited to extremely thin surface layers. Perhaps, supported films are the most practical problem, but not the most interesting because the effects of apparently increased mobility are small relative to what has been seen in freely standing polystyrene films and similar to the behavior seen in nanopore confined liquids. In the case of essentially freely standing polystyrene films, two types of measurement stand out. Hole growth and bubble inflation measurements. The former seems to give relatively little change of dynamics until the glass transition of the material in the bulk state is reached while the latter, and consistent with the behavior reported for Tg by ``pseudo-thermodynamic'' methods shows very large decreases in relaxation times corresponding to up to 50 K reductions in the Tg for 10 nm films of polystyrene. Some possible reasons for the differences in behaviors and other contradictory results will be discussed. [Preview Abstract] |
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