Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session Y18: Dynamics of Thin Polymer Films |
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Sponsoring Units: DPOLY Chair: Peter Green, University of Michigan Room: Morial Convention Center 210 |
Friday, March 14, 2008 11:15AM - 11:27AM |
Y18.00001: The effect of confinement on the structure of polystyrene melt films Mrinmay K. Mukhopadhyay, Sunil K. Sinha, Laurence B. Lurio, Curt DeCaro, Zhang Jiang, Michael Sprung The structure factor of thin, Si supported, polystyrene films has been measured using grazing incidence wide angle diffuse x-ray scattering. Measurements were made as function of thickness and molecular weight from bulk-like films down to films of thickness of the polymer radius of gyration. A standing wave technique was employed to isolate the scattering component from the film interior. We observe a diffuse background and a liquid scattering ring whose intensity, for thick films, depends only on the magnitude of the scattering vector. In thinner films the intensity in the scattering ring is strongly concentrated along the surface normal direction. We interpret this peak as due to the chain-chain correlations and the concentration of scattering along the surface normal is indicating preferential stacking of the polymer chains parallel to the surface. [Preview Abstract] |
Friday, March 14, 2008 11:27AM - 11:39AM |
Y18.00002: Tuning the Glass Transition Temperature over 100 K using Polymer-Polymer Interfaces Connie B. Roth, Rodney D. Priestley, Soyoung Kim, John M. Torkelson During the past decade considerable research has focused on the impact of the free surface and substrate interactions on the glass transition temperature (Tg) in nanoconfined geometries. For example, the large (up to 80 K) Tg reductions that have been observed in free-standing films indicate that we still have much to learn about the nature of the glass transition. Here we focus on an entirely different kind of interface, a polymer-polymer interface, which we show can have an even stronger impact on the Tg dynamics than a free surface. We demonstrate that the interactions across a narrow polymer-polymer interface are sufficient to tune the Tg of a single polymer material by over a 100 K simply by changing the type of polymer in the adjoining layer. The cooperative segmental dynamics of the two immiscible polymers are strongly coupled over length scales of several tens of nanometers. These results have significant impact on our understanding of the glass transition in multilayer films and nanostructured polymer blends with large amounts of polymer-polymer interface. These findings also suggest new methods for controlling polymer properties in nanoconfined geometries. [Preview Abstract] |
Friday, March 14, 2008 11:39AM - 11:51AM |
Y18.00003: The viscoelastic properties of ultrathin polymer films as measured with a novel nanobubble inflation technique. Paul OConnell, Gregory McKenna Using a nano-bubble inflation technique developed within our laboratory, we have measured the absolute biaxial compliance of polymer films as thin as 11.3 nm. Previous results have shown that the degree of reduction in Tg with film thickness is not universal viz., PVAc shows no reduction even for the thinnest films while the PS shows a significant reduction at a thickness below approximately 80nm. In addition the rubbery plateau region for both materials shows dramatic stiffening as the thickness is reduced ($>$300 times) and scales as approximately the square of film thickness. We have extended the analysis of the data to directly determine the creep compliance function from the measured data rather than the minimization routine used previously. Creep compliance master curves constructed from data at varying thicknesses show that time-temperature superposition is valid even at the thinnest film thickness. The time-temperature shift factors are consistent with a WLF-type dependence and indicate a reduction in Tg for PS at 11.3nm of 53K while no significant reduction ($<$ 3K) is seen for PVAc. [Preview Abstract] |
Friday, March 14, 2008 11:51AM - 12:03PM |
Y18.00004: Fabricating Nanoscale Gratings with Gradient Pattern Height by Annealing Imprinted Polymer Patterns Yifu Ding, Hyunwook Ro, Jirun Sun, Jing Zhou, Sheng Lin-Gibson, Christopher Soles The evolution of nanoimprinted polymer patterns during isothermal annealing is driven by the interplay of the pattern features, material properties of the polymer, and the polymer/substrate interactions. With proper control of these factors, a range of hierarchical nanostructures can be fabricated through thermal annealing of the imprinted polymer patterns. Here we demonstrate an example of creating polystyrene (PS) gratings with gradient pattern height. This is achieved by annealing the imprinted PS gratings under a temperature gradient. In the simplest case, the pattern decay rate is determined by the viscosity and surface tension of the PS. Consequently, the degree of the gradient pattern height can be well controlled through the ``fragility'' of the PS, i.e. its temperature dependence of the viscosity. Such a gradient grating is extremely useful in the combinatorial studies of the effect of the surface topology on the cell behaviors and controlled wettability. [Preview Abstract] |
Friday, March 14, 2008 12:03PM - 12:15PM |
Y18.00005: Substrate and chain size dependence of near surface dynamics of glassy polymers Dongping Qi, Zahra Fakhraai, James Forrest We report on the application of nanohole relaxation technique to study the surface relaxation of i-PMMA thin films. This allows us to obtain the time dependent relaxation function at a number of different sample temperatures for the first 2-3 nm of the free surface. By studying the film thickness dependence of the near-free surface relaxation for films on different substrates we are able to determine the range over which the substrate directly affects the free surface relaxation. This also allows us to determine a limiting thickness where the free surface relaxation is not affected by the substrate. For such films we determine the Mw dependence of the near free surface relaxation time and find a surprising linear Mw dependence. The Mw dependence is discussed in terms of possible motions as well as polymer configurations near the free surface. [Preview Abstract] |
Friday, March 14, 2008 12:15PM - 12:27PM |
Y18.00006: Molecular Simulation of Confined Polymer Films : Structure, Dynamics and the Glass Transition Vikram Kuppa, Gregory Rutledge Molecular Dynamics simulations are used to probe the structure and dynamics of polymers in extreme confinements. The simulations mimic intercalated nanocomposites in which polymer chains are trapped in nanometer sized slit pores between layered inorganic surfaces : our system consists of thin films of bead-spring chains spatially restricted in one dimension by surfaces comprised of monomer beads arranged in an FCC configuration. The responses of the system are studied as a function of slit spacing, polymer-wall interaction strength and temperature. The glass transition temperatures, as well as the fragility of the confined films are seen to increase with increasing confinement and with increasing attraction of the polymer with the confining wall. [Preview Abstract] |
Friday, March 14, 2008 12:27PM - 12:39PM |
Y18.00007: Probing Relaxation in Glassy Freestanding Diblock Copolymer Films Adam N. Raegen, Andrew B. Croll, Kari Dalnoki-Veress We employ an axi-symmetric deformation and modulus test (ADAM) to measure the response of a thin freestanding diblock copolymer film to an external load. The method measures the deformation of a spincast film when an axi-symmetric load is applied by a flat circular punch. The flat punch minimises uncertainties in the experiment, while the use of spincast films provides a very smooth contact surface. The use of diblock copolymers allows us to change the internal structure of the film from disordered to ordered (lamellar) and surface topography (flat if there are an integer number of lamellae, and islands, bicontinuous or holes for non-integer) by changing the annealing history and thickness of the sample. We discuss our results in terms of the elastic modulus and creep compliance of the films. [Preview Abstract] |
Friday, March 14, 2008 12:39PM - 12:51PM |
Y18.00008: Ellipsometric Investigation of the Surface Dynamics of a Polymer Film near the Glass Transition Temperature Ashis Mukhopadhyay, Christopher Grabowski We have investigated the surface dynamics of poly (butyl methacrylate) films by using a phase-modulated ellipsometer, which can measure thickness at angstrom-level resolutions. Experiments were performed for a range of temperatures, both above and below the glass transition temperature (Tg) of this system. Thickness-thickness correlation functions were calculated at each temperature using ellipsometry data collected at 200 Hz frequency. Our results indicate that, above Tg, the relaxation time stays relatively constant ($\sim $1 sec) and the correlation functions obey a simple exponential decay. As Tg is approached, a dramatic increase in the relaxation time is observed and the correlation functions are best fitted with a stretched exponential Kohlrausch-Williams-Watts (KWW) relation. [Preview Abstract] |
Friday, March 14, 2008 12:51PM - 1:03PM |
Y18.00009: Confinement effects on the dynamics of polymers Hugues Bodiguel, Guang Yin Jing, Christian Fretigny Dewetting experiments of thin polystyrene films on a liquid substrate are performed in the vicinity of the glass transition. It is shown that this technique reveals the extensional creep compliance function of the polymer. The viscosity in the flow regime is very much reduced when the film thickness becomes comparable to the gyration radius of the polymer. This long time behaviour may be associated with the large length scales involved in the viscous flow which should be modified by confinement. On the contrary, the rubbery plateau remains unaffected by the confinement up to a fraction of the coil size. This can be related to the short length scales involved in the rubber elasticity. In the viscoelastic region, physical aging of the sample is clearly evidenced: Structural recovery complicates the short times dewetting response. Preliminary results are presented of the confinement effects on aging properties of ultra thin films as revealed in this original way. [Preview Abstract] |
Friday, March 14, 2008 1:03PM - 1:15PM |
Y18.00010: Suppression of the Tg-Confinement Effect in Thin Polymer Films by the Presence of an Anti-Plasticizer Soyoung Kim, Manish Mundra, Connie Roth, John Torkelson The effect of film thickness on the glass transition temperature (Tg) of poly(vinyl acetate) supported on silica was studied via ellipsometry for films that were in the bone-dry state and also with several weight percent water sorbed into the film. The presence of water resulted in a decrease of the Tg of bulk poly(vinyl acetate) films but an increase in the density of the films. This combination of effects indicates that water acts as an anti-plasticizer in poly(vinyl acetate). We demonstrate that the bone-dry poly(vinyl acetate) films exhibit a significant reduction in Tg with decreasing film thickness below about 50 nm. In contrast, the poly(vinyl acetate films) containing several weight percent water exhibited no reduction in Tg relative to bulk Tg with decreasing film thickness down to a thickness of about 10 nm. These results are in accord with recent theoretical analysis (Riggelman et al., PRL 97, 045502 (2006)) indicating that the presence of anti-plasticizers leads to a suppression of confinement effects on the behavior of polymer films and indicate that caution should be employed when studying confinement effects in polymer systems that may sorb significant levels of water from the atmosphere. [Preview Abstract] |
Friday, March 14, 2008 1:15PM - 1:27PM |
Y18.00011: New Measurements of the Effects of Confinement on the Glass Transition Temperature of Freely Standing Polymer Films John Torkelson, Soyoung Kim, Connie Roth Pioneering work by the Dutcher group (PRL 77, 2002 (1996)) a dozen years ago led to the first measurement of the Tg-confinement effect in freely standing polymer films. Their studies were especially intriguing because of the observations of very large Tg reductions relative to bulk Tg and a strong molecular weight dependence of the Tg-confinement effect. Such a molecular weight dependence is absent in the Tg-confinement effect of supported polymer films. Because of experimental difficulties associated with freely standing films, especially when the films are less than 100 nm thick, only a few related experiments have been reported by other research groups. Here we describe new results involving the measurement of Tg via the temperature dependence of fluorescence intensity of dyes labeled at trace levels to the polymer chains. Present measurements on freely standing films of poly(methyl methacrylate) (PMMA) have demonstrated reductions in Tg relative to bulk values of 20 to 25 K in films of 25-40 nm thickness. Tg reductions of at least 5 K are observed when PMMA films are 80 nm thick. Studies are also underway with polystyrene films and with polymers of different molecular weight. [Preview Abstract] |
Friday, March 14, 2008 1:27PM - 1:39PM |
Y18.00012: Glass transition in ultra thin polymeric films measured by differential AC-Chip calorimetry H. Huth, A. Minakov, C. Schick The film thickness dependency of glass transition in polymer films is still controversially discussed. For different experimental probes different dependencies are observed and a generally accepted link to molecular mobility is not yet established. Calorimetry has proven to provide useful information about glass transition, because it establishes a direct link to energetic characterization. In several cases a direct comparison with results from other dynamic methods like dielectric spectroscopy is possible giving further insights. For thin films in the $\mu $m{\ldots}nm range standard calorimetric methods are mostly not applicable. In the recent years there are new developments in the field of calorimetry which overcome these limitations. We set up a differential AC-chip calorimeter capable to measure the glass transition in nanometer thin films with pJ/K sensitivity. Changes in heat capacity can be measured for sample masses below one nanogram even above room temperature as needed for the study of the glass transition in nanometer thin polymeric films. The glass transition in thin films was determined at well defined experimental time scales. No thickness dependency of the glass transition temperature was observed within the error limits - neither at constant frequency nor for the traces in the activation diagrams. [Preview Abstract] |
Friday, March 14, 2008 1:39PM - 1:51PM |
Y18.00013: Relaxation Kinetics of Nanostructures on Polymer Surface: Effect of Orientation, Spatial Confinement, and Chain Mobility H.G. Peng, Y.P. Kong, A.F. Yee Nanostructures provide an opportunity for studying relaxation and chain dynamics of polymers when the radius of gyration is not small compared with the dimension of the structure. PS (PDI=1.03-1.05, Mw=6.4 to 1571 kg/mol) gratings of varying line-widths (600 nm, 270 nm, and 30 nm) were fabricated by nanoimprint lithography. When annealed at T $\sim $ bulk Tg, the grating height monitored with an AFM relaxes as surface tension and other driving forces overcome the polymer viscosity. The temperature for rapid relaxation decreases as the feature size diminishes for all molecular weights (MWs), but a simple explanation based on surface enhanced mobility fails to explain the results. The residual molecular orientation effect is identified as the main relaxation driving force for gratings of MWs much larger than the entanglement MW. Comparison between the various nanostructure sizes allows to observe the spatial confinement effect and to determine whether a thin mobile surface layer exists. [Preview Abstract] |
Friday, March 14, 2008 1:51PM - 2:03PM |
Y18.00014: Direct Measurements of Heterogeneous Viscosity Distributions in Ultrathin Polymer Films Tadanori Koga, C. Li, J. Koo, J. Jiang, M. Rafailovich, S. Narayanan, D. Lee, L. Lurio, S. Sinha We will present direct measurements of depth dependence of the polymer chain dynamics in single ultrathin films via the diffusive motion of gold nanoparticles dispersed in the films. The technique used was x-ray photon correlation spectroscopy (XPCS) which serves a fingerprint of the Brownian motion of the gold nanoparticles. By intensifying the probing electrical field in the region of interest, we could measure the viscosity at the surface and at the position close to the center of the film separately. The XPCS results showed that most chains had at least some direct contacts with an immobile layer (absorbed with a substrate) up to 3$R_{g}$ thick from the substrate, where R$_{g}$ is the radius of polymer gyration, and hence the lateral dynamics of the polymer chains was significantly reduced. [Preview Abstract] |
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