Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session V4: Dynamics in Polymeric Systems |
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Sponsoring Units: DPOLY Chair: Ophelia Tsui, Boston University Room: Colorado Convention Center Korbel 2B-3B |
Thursday, March 8, 2007 11:15AM - 11:51AM |
V4.00001: Dynamics of polymer glasses under active deformation. Invited Speaker: Polymer glasses can often be deformed significantly without breaking. What microscopic mechanism allows this ``flow'' under conditions where mobility is otherwise absent? We utilize an optical photobleaching technique to measure the segmental mobility of polymer glasses and nanocomposites during active deformation. It has been previously established that the reorientation of dilute dye molecules (on the time scale of thousands of seconds) can monitor the segmental dynamics of a polymer melt. Here we utilize this method to measure mobility during tensile deformation of a free-standing poly(methyl methacrylate) glass. We have observed increases in mobility during deformation from Tg-10 K to Tg -30 K, with larger changes at lower temperatures. At Tg-18 K, with a strain rate of 0.00001/s, segmental mobility increases slowly at first and then dramatically, so that the increase in mobility during deformation reaches a factor of about 200. After removing the stress, we observe that the enhanced mobility disappears slowly. These measurements are compared to continuum and mesoscopic models of polymer glass dynamics and rheology. [Preview Abstract] |
Thursday, March 8, 2007 11:51AM - 12:27PM |
V4.00002: Dynamics of Polymer Blends: Beyond Self-Concentration. Invited Speaker: The self-concentration model has been shown to describe the component dynamics of a wide variety of miscible polymer blends remarkably well. However, there are some systems, such as PEO/PMMA and polymer/solvent systems, where the model is clearly inadequate. We will present new experimental results on such systems, and explore the possible origins of the discrepancies. [Preview Abstract] |
Thursday, March 8, 2007 12:27PM - 1:03PM |
V4.00003: Looking inside the tube: what molecular dynamics simulations are revealing about polymer entanglements Invited Speaker: Using concepts developed over the years by de Gennes, Doi, Edwards, Marrucci, Rubinstein, McLeish, Milner, and others, a kind of ``standard model'' for entangled polymer relaxation and rheology has been developed, which, like the ``standard model'' of high-energy physics, has a number of ad hoc assumptions and fitting parameters. The ``standard model'' of polymer relaxation is based on a phenomenological ``tube'' surrounding each polymer chain that represents the effect on that chain of non-crossability constraints imposed by surrounding chains. As a result of its confinement to the tube, the chain relaxes by reptation -- or sliding along the tube, accordion-like fluctuations of the chain within the tube, and movement of, or dilation of, the tube due to motion of the surrounding chains creating the tube-like region. Increasing computer speed and advanced simulation methods are now making possible the direct molecular dynamics simulations of entangled polymers resolved at the monomer scale, over time scales sufficient to test the underlying assumptions of the tube model and allow direct calculation of some of the phenomenological parameters. Here we illustrate how these simulations allow us to estimate the distribution of tube lengths, the average diameter of the tube, and the mobility of the branch point in a simple ``star'' branched polymer. These findings confirm the validity of the tube ansatz, but suggest that some corrections to the ``standard model'' may be needed. [Preview Abstract] |
Thursday, March 8, 2007 1:03PM - 1:39PM |
V4.00004: Glass Transition Temperature Reductions in Freely-Standing Films of Different Polymers Invited Speaker: The effects of confinement and free surfaces on the dynamics of polymers in thin films have been studied extensively since the original observation of reductions in $T_{g}$ with decreasing film thickness $h$ in thin polystyrene (PS) films [1]. One particularly striking result, which is yet to be understood in detail, is the observation of very large, molecular weight (\textit{MW}) dependent reductions of $T_{g}$ in very thin, freely-standing PS films using Brillouin light scattering and ellipsometry [2]. We have recently measured $T_{g}$ ($h$, \textit{MW}) for freely-standing PMMA films [3] and we find that the results are in qualitative agreement with those obtained for freely-standing PS films. However, the overall magnitude of the $T_{g}$ reduction is much less (by roughly a factor of three) for the high-\textit{MW} freely-standing PMMA films than for freely-standing PS films of comparable \textit{MW} and $h$. The observed differences between the freely-standing PMMA and PS film data suggest that differences in chemical structure determine the magnitude of the $T_{g}$ reduction and we discuss the possible origins of these differences. Our analysis of the \textit{MW}-dependence of the $T_{g}$ reductions suggests that the mechanism responsible for the \textit{MW}-dependent $T_{g}$ reductions observed in the high-\textit{MW} freely-standing films is different than that responsible for the \textit{MW}-independent $T_{g}$ reductions observed in low-\textit{MW} freely-standing and supported films. [1] Keddie et al., Europhys. Lett. \textbf{27}, 59 (1994); [2] Dalnoki-Veress et al., Phys. Rev. E \textbf{63}, 031801 (2001); [3] Roth and Dutcher, Eur. Phys. J. E \textbf{20}, 441 (2006). [Preview Abstract] |
Thursday, March 8, 2007 1:39PM - 2:15PM |
V4.00005: Dynamics in Confined Systems:~ Polymer Thin Films and Surfaces Invited Speaker: Here we visit the problem of dynamics of polymers confined in ultrathin films and at surfaces. We present results obtained on ultrathin polymer films using a novel nanobubble inflation device to measure the biaxial creep of films as thin as 13 nm. The results show that the viscoelastic response of PVAc in the glass transition regime is similar to that of the bulk material, while polystyrene shows considerable acceleration of the molecular dynamics that corresponds to a 40 K or more reduction in the glass transition. We will discuss the breadth of the transition, as this is related to the presence of a liquid layer at the surface as well as to the thickness of such a layer. We also discuss the effects of film thickness on the rubbery plateau response of these films. An observed stiffening of the material is found that gives an apparent plateau compliance that is over 100 times smaller than that of the bulk material. In the thinnest films a significant fraction of the deformation energy comes from surface tensions. However, in films above approximately 30 nm, this contribution is less than 30{\%} of the total energy of deformation and suggests that much of the stiffening is due to a thin film effect. This is discussed in detail. Finally, we show results for nanoparticle embedment experiments and argue that the mobility increases observed on the polymer surface are insufficient to explain a reduction of 40 K or more of the glass transition in ultrathin polymer films. [Preview Abstract] |
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