Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session W17: Dynamics and Glass Transition Phenomena in Thin Polymer Films |
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Sponsoring Units: DPOLY Chair: Christopher Stafford, National Institute of Standards and Technology Room: Colorado Convention Center 102 |
Thursday, March 8, 2007 2:30PM - 2:42PM |
W17.00001: Effect of Confinement on the Relaxation Dynamics in an Antiplasticized Polymer Melt Robert Riggleman, Juan de Pablo We have developed a coarse grained model which exhibits antiplasticization in a polymer melt. Using molecular simulations, we have characterized our model and investigated the effect of antiplasticization on the relaxation dynamics of the polymer in both bulk and free-standing thin film geometries. In the bulk, we show that antiplasticization reduces the size of the cooperatively rearranging regions (CRRs) and leads to a weaker temperature dependence of the relaxation times of the system. We also show that it decreases the fragility of the material. Upon confinement, we find that the CRRs in the pure polymer film are strongly heterogeneous, and more cooperative near the free surfaces, leading to a large decrease in the glass-transition temperature (Tg). In contrast, the antiplasticized film shows a homogeneous distribution of the CRRs, which eliminates the effects of the free surface, and causes little change in the material properties upon confinement, including Tg, offering an explanation of recent experimental results. [Preview Abstract] |
Thursday, March 8, 2007 2:42PM - 2:54PM |
W17.00002: ABSTRACT WITHDRAWN |
Thursday, March 8, 2007 2:54PM - 3:06PM |
W17.00003: A New Look at Polymer Films and the Glass Transition Jane Lipson, Scott Milner The effect on the glass transition (Tg) of a polymer in going from bulk to film is still not well understood. Among the outstanding issues is the influence of a free surface relative to that of a hydrogen bonding substrate. We have developed a model able to capture the shift in Tg as a function of distance from two interfaces, accounting for the potentially different impacts of air and of a silicon oxide surface. While most of the literature reports Tg as a function of total film thickness, in ongoing work the Torkelson group has used labeling methods to probe the Tg of slices of controlled thickness within a polystyrene (PS) film.[1] In separate studies they have changed the thickness of a labeled surface layer with the underlayer held constant, the thickness of the underlayer with that of the surface layer held constant, and the thickness of the whole (labeled) film. Polystyrene is unusual in having relatively weak interactions with the substrate. Here we both apply our model to the PS film data and investigate the impact of changing from PS to a polymer able to hydrogen bond to the silicon oxide surface. [1] Ellison, CJ; Torkelson, JM Nature Mat. 2, 695 (2003). [Preview Abstract] |
Thursday, March 8, 2007 3:06PM - 3:18PM |
W17.00004: The Glass Transition of Miscible Binary Polymer-Polymer Thin Films Peter Green, Brian Besancon, Christopher Soles Studies of the glass transition temperatures, T$_{g}$, of completely miscible thin film blends of tetramethyl bisphenol-A polycarbonate (TMPC) and deuterated polystyrene (dPS), supported by SiO$_{x}$/Si, were examined using spectroscopic ellipsometry (SE) and incoherent elastic neutron scattering (INS). While both sets of measurements independently reveal that T$_{g}$ exhibits qualitatively similar trends with film thickness, h, there were important quantitative differences, which depended on composition. The T$_{g}$s measured by INS were consistently larger than those determined by SE for PS weight fractions $\phi$$>$0.1. These observations are rationalized in terms of theory based on the notion of a self- concentration and reveal evidence of heterogeneous component behavior in these miscible polymer-polymer systems. [Preview Abstract] |
Thursday, March 8, 2007 3:18PM - 3:30PM |
W17.00005: Molecular-dynamics simulations of thin films with a free surface Simone Peter, Hendrik Meyer, Joerg Baschnagel We present results [1,2] from molecular-dynamics simulations for a model of non-entangled short polymer chains in a free standing and a supported film geometry. We investigate the influence of confinement on static and dynamic properties of the melt. We find that the relaxation at the surfaces is faster in comparison to the bulk. We perform a layer-resolved analysis of the dynamics and show that it is possible to associate a gradient in critical temperatures Tc(y) with the gradient in the relaxation dynamics. This finding is in qualitative agreement with experimental results on supported polystyrene (PS) films [Ellison et al, Nat. Mater. 2, 695 (2003)]. Furthermore we show that the y-dependence of Tc(y) can be expressed in terms of the depression of Tc(h), the global Tc for a film of thickness h, if we assume that Tc(h) is the arithmetic mean of Tc(y) and parameterize the depression of Tc(h) by Tc(h)=Tc/(1+h0/h), a formula suggested by Herminghaus et al [Eur. Phys. J E 5, 531 (2001)] for the reduction of the glass transition temperature in supported PS films. We demonstrate the validity of this formula by comparing our simulation results to results from other simulations and experiments. \newline [1] S. Peter, H. Meyer and J. Baschnagel, J. Polym. Sci. B, 44, 2951 (2006) \newline [2] S. Peter, H. Meyer, J. Baschnagel and R, Seemann, J. Phys: Condens. Matter (2007) [Preview Abstract] |
Thursday, March 8, 2007 3:30PM - 3:42PM |
W17.00006: Dewetting of Thin Polymer Films Elie Raphael, Thomas Vilmin We study the dewetting of thin polymer films deposited on slippery substrate. Recent experiments on these systems have revealed many unexpected features. We develop here a model that takes into account the rheological properties of polymer melts, focussing on two dewetting geometries (the receding of a straight edge, and the opening of a hole). We show that the friction law associated with the slippage between the film and the substrate has a direct influence on the dewetting dynamic. In addition, we demonstrate that residual stresses, which can be stored in the films due to their viscoelasticity, are a source of destabilization for polymer films, and accelerate the dewetting process. [Preview Abstract] |
Thursday, March 8, 2007 3:42PM - 3:54PM |
W17.00007: Hole growth in free-standing block copolymer films: does lamellar structure imitate a support? Matthew J. Farrar, Andrew B. Croll, Kari Dalnoki-Veress We will discuss how the lamellar structure of a symmetric polystyrene-poly (methyl methacrylate) diblock co-polymer can affect the hole formation of free-standing films. It is found that ordered films (with lamellae aligned parallel to the film surface) exhibit a dramatically enhanced stability over disordered films. This stability is shown to be directly related to the lamellar structure through atomic force microscopy and optical microscopy. Secondly we note how the rim structure of the holes in these two experiments is extremely different. In particular, the steep rims observed in the ordered samples show a striking similarity to holes grown in supported films, which is difficult to reconcile with current theory. [Preview Abstract] |
Thursday, March 8, 2007 3:54PM - 4:06PM |
W17.00008: Confinement and interfacial effects on the alpha relaxation dynamics of thin polymer films Rodney Priestley, Linda Broadbelt, Koji Fukao, John Torkelson It is now known that both the glass transition temperature ($T_{g})$ and the alpha relaxation dynamics can deviate substantially from the bulk with confinement of polymer films. Using a novel fluorescence multilayer method, it has been established that the deviations in $T_{g}$ observed with decreasing film thickness for polystyrene and poly(methyl methacrylate) films supported on silica is a result of interfacial effects. Since the glass transition is related to the cooperative segmental (alpha) relaxation dynamics, it must be that these dynamics at the interfaces are different from the bulk. However, because of experimental limitations no direct measurement of the complete alpha relaxation time distribution at the interfaces has been conducted. Here we present results of a novel dielectric multilayer method that allows for such measurements. Our results indicate that both the alpha relaxation time distribution and the average alpha relaxation time are affected by the interfaces for polystyrene films. [Preview Abstract] |
Thursday, March 8, 2007 4:06PM - 4:18PM |
W17.00009: Free Volume behavior and Structure of Polymer Thin Film Seisuke Ata, Toshiaki Ougizawa, Makoto Muramatsu, Toshiyuki Ohdaira, Ryoichi Suzuki, Toshitaka Oka, Kenji Ito, Yoshinori Kobayashi Free volume hole size (Fv) of polystyrene thin films were measured by positron annihilation lifetime spectroscopy (PALS) by using slow positron beam. Though glass transition temperature was decreased in very thin film(less than 2Rg), decrease of Fv was observed concurrently. But these were inconsistent from the general recognition of relationship between Fv and glass transition temperature. In addition, increase of thermal expansion coefficient of Fv and decrease of generation temperature of positronium bubble which indicated decline of cohesive force of polymer chains, were observed. From these results, it was inferred that the physical properties change by thinning of polymer film was induced by change of polymer stracture due to the reduce of polymer chain entanglement. [Preview Abstract] |
Thursday, March 8, 2007 4:18PM - 4:30PM |
W17.00010: Effect of Confinement in Ultrathin Films on Translational Diffusion in Polymers near the Glass Transition Manish K. Mundra, John M. Torkelson A novel, extremely simple fluorescence-based multilayer method, based on breakthrough time assuming Fickian diffusion, has been developed to measure the impact of nanoscale confinement near free surfaces or polymer-substrate interfaces on translational diffusion in amorphous polymers near the glass transition temperature, Tg. With decreasing polystyrene film thickness, two dye molecules, Disperse Red 1 and decacyclene, exhibit reductions in their diffusion coefficients associated with translational diffusion away from the silica substrate interface and toward the free surface of the film. The effect of confinement on translational diffusion is much greater for the smaller dye, Disperse Red 1, with as much as an order of magnitude reduction in its translational diffusion coefficient being observed in the most confined film relative to its value in bulk polymer. [Preview Abstract] |
Thursday, March 8, 2007 4:30PM - 4:42PM |
W17.00011: Hindering Cooperative Segmental Dynamics at the Free Surface of Polystyrene: The Impact of Narrow Immiscible Interfaces in Polymer Multilayer Films Connie B. Roth, John M. Torkelson The glass transition temperature (Tg) of individual layers within multilayer films of different polymers are measured to investigate how competing free surface and substrate interactions are mediated by the narrow interfaces between the immiscible polymer layers. We demonstrate that the enhanced mobility at the free surface of polystyrene (PS), as characterized by the -32 K Tg reduction observed in a 14-nm thick PS surface layer on bulk PS, can be virtually eliminated by placing a 14-nm thick PS surface layer on bulk poly(methyl methacrylate) (PMMA) and bulk poly(2-vinyl pyridine) (P2VP). We explore the extent to which the interfacial widths and layer thicknesses affect the propagation of the enhanced cooperative segmental dynamics at the free surface of PS into the film. [Preview Abstract] |
Thursday, March 8, 2007 4:42PM - 4:54PM |
W17.00012: Dynamics of Polymer Melts Confined by Smooth Walls: Crossover from Non-entangled to Entangled Regime Qi Liao, Yijie Li, Dongshan Wei, Xigao Jin, Charles Han We present the results of molecular dynamics simulations of polymer films confined by smooth walls. Simulations were performed for a wide range of chain lengths covering both non-entangled and entangled polymer dynamics, as well as film thickness crossing over the order of chain size to the bulk state. The simulation results for chain size dependence on the film thickness are compared with the prediction of the scaling model. By measuring the correlation function of the end-to-end vectors, we have determined that relaxation time of polymer chains in the different entanglement state confined in the film. It is shown that there is a minimum of relaxation time with the decrease of the film thickness in the entangled regime, which may be caused by the confinement-induced disentanglement. [Preview Abstract] |
Thursday, March 8, 2007 4:54PM - 5:06PM |
W17.00013: Surface Dynamics of Polymer Brushes in the Melt State: An XPCS Study Mark D. Foster, Bulent Akgun, Gokce Ugur, William J. Brittain, Suresh Narayanan, Heeju Lee, Sanghoon Song, Hyunjung Kim, Zhang Jiang, Sunil K. Sinha The suppression of long-wavelength modes of surface fluctuations on a molten polymer brush has been demonstrated using direct measurements of dynamics for the first time. The surface dynamics of densely grafted polystyrene brushes of reasonably monodisperse chains were investigated by X-ray photon correlation spectroscopy. Within the range of time and length scale investigated, 0.2 s to 1000 s, and 200 nm to 5 um, there were no detectable dynamics on the brush surfaces, even 130 C above the polymer bulk glass transition temperature. A comparably thick film of untethered chains has a q-dependent surface relaxation time of the order of 30 s, indicating that the tethering of the chains alters the surface relaxation rate by at least 3 orders of magnitude. Such a suppression of long wavelength fluctuations on the surface of a molten brush was predicted by Frederickson and co-workers. [Preview Abstract] |
Thursday, March 8, 2007 5:06PM - 5:18PM |
W17.00014: Dynamics from Buried Polymer-Polymer Interfaces in Thin Films Jyotsana Lal, Xuesong Hu, Zhang Jiang, Sunil K. Sinha, Suresh Narayanan, Alec R. Sandy, Xuesong Jiao, Laurence B. Lurio We report a further development of X-ray photon correlation spectrocopy (XPCS) in order to probe capillary wave dynamics at a buried polymer interface of a bilayer. The bi-layer was chosen so that the critical angle for total external reflection for the top layer is smaller than that for the bottom layer. When x-rays are incident below the critical angle of the top layer only the structure and dynamics of the top layer are probed. When x-rays are incident above the critical angle of the top layer but below that of the bottom layer, a standing wave is set up. The phase of this standing wave can be adjusted to have a maxima at the polymer-polymer interface and simultaneously a node at the polymer-air interface. Consequently, one can isolate the static scattering and XPCS from the buried layer. Results on a system consisting of a 100 nm polystyrene (PS) film on top of an 100 nm polybromostyrene (PBrS) film, supported on a Si substrate will be reported. The dynamics are consistent with a low-viscosity mixed layer between the PS and PBrS. [Preview Abstract] |
Thursday, March 8, 2007 5:18PM - 5:30PM |
W17.00015: Dynamics of complicated phase behavior in ultrathin film of polymer blend by in situ AFM Tongfei Shi, Yonggui Liao, Lijia An The stability of polymer films is important for many applications. There are many reports on dewetting of polymer films, but most of them focused on dewetting behavior of single-component polymer films. In the phase-separated regions, reports suggest the formation of bilayer via phase separation, followed by the dewetting of the upper layer, which we call phase separation/bilayer-dewetting mechanism. Recently, it was theoretically predicted that a film of a binary miscible polymer blend can dewet on a solid substrate. In this work, we present first direct experimental observations of this dewetting dynamics, driven by a composition fluctuation, of ultrathin films of PMMA/SAN blend in miscible region by in situ AFM, and then the dynamics of the complicated dewetting and phase behavior of this ultrathin film in two-phase region, which is not the phase separation/bilayer-dewetting mechanism, but the dewetting of the film is followed by the phase separation in the droplets, coupling with the wetting of the substrate by the PMMA extracted by the strong attractive interaction between PMMA and the substrate, which we call dewetting-phase separation/wetting mechanism. [Preview Abstract] |
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