Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session P17: Focus Session: Glass Transition in Thin Films I |
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Sponsoring Units: DPOLY Chair: Rodney Priestley, Princeton University Room: B116 |
Wednesday, March 17, 2010 8:00AM - 8:36AM |
P17.00001: Mechanics of Polymer Thin Films: What Can We Learn from the Glass Transition? Invited Speaker: Mechanical properties of polymeric materials are critical to their utility in many applications. However, little is known regarding the mechanical properties of polymers when confined to dimensions approaching their intrinsic molecular size (R$_{g})$. Thermal properties, in particular the glass transition temperature (T$_{g})$, of nanoconfined polymers have been studied extensively over the past two decades due to the relative ease of measurements. Correlations between T$_{g}$ and modulus are well established for bulk polymers, but it is unclear if these hold at the nanoscale. Here, I will present direct comparisons of the thickness dependent T$_{g}$ and elastic moduli behavior to address this question. Additionally, correlations between bulk T$_{g}$ and the thickness dependent behavior will be explored by using a homologous series of poly(n-alkyl methacrylate)s and a series of widely varying molecular mass polystyrene films. [Preview Abstract] |
Wednesday, March 17, 2010 8:36AM - 8:48AM |
P17.00002: Glassy dynamics and glass transition in thin polymer layers of PMMA deposited on different substrates Friedrich Kremer, Michael Erber, Anatoli Serghei, Martin Tress, Emmanuel Mapesa, Klaus-Jochen Eichhorn, Brigitte Voit Broadband Dielectric Spectroscopy (BDS) and spectroscopic Vis-Ellipsometry are combined to study the glassy dynamics and the glass-transition of thin layers of atactic poly(methylmethacrylate) (PMMA) prepared under identical conditions. The interfacial interactions are systematically modified ranging from strong attractive interactions for covalently bonded PMMA brushes with high grafting densities to weak and strong repulsive interactions as realized by Au-coated and silanized SiO$_{x}$--surfaces, respectively. Down to layer thicknesses of $\sim $ 10 nm and independently from the substrate used, both methods deliver -- within the experimental accuracy ($\pm $1.0 K for BDS and $\pm $2 K for Ellipsometry) -- the coinciding result that the glassy dynamics and the glass transition are not altered due to the geometrical confinement in thin polymer layers. [Preview Abstract] |
Wednesday, March 17, 2010 8:48AM - 9:00AM |
P17.00003: Glass Transition Temperature of Polystyrene Thin Film on Polystyrene Brush Andrew Clough, Yoshihisa Fujii, Jessica Leach, Ophelia Tsui Numerous experiments have shown that the $T_{g}$ of thin polymer films can differ noticeably from the bulk value, exhibiting a dependence on both the film thickness and conditions at the film interfaces. Similar behavior has been observed in the $T_{g}$ of silica-nanoparticle-polystyrene composites where the average inter-particle separation assumes the role of the film thickness. However, the similarity ceases when the nanoparticles are altered by grafting a layer of polystyrene brush to the particle surface. We study the $T_{g}$ of polystyrene thin films supported by silicon grafted with a layer of polystyrene brush using ellipsometry. Preliminary result indicates that the $T_{g}$ depression observed on a low-molecular-weight brush is the same as that for polystyrene films on silicon. We will present our observations and discuss our interpretation for the data. [Preview Abstract] |
Wednesday, March 17, 2010 9:00AM - 9:12AM |
P17.00004: Confinement Effect on Glass Transition of Thin Polymer on Nanoparticles Wei Chen, Qi Xue A better understanding of the glass transition might come from the observation of key changes in the behavior of polymer when confined at the nanometer scale. We find that confinement has influences on both the affinity of the adsorbed polymer and its glass transition temperature (Tg) on nanoparticles. The dipolar filter proton solid state NMR signals of PMMA adsorbed on SiO2 nanoparticles indicated the multilayer of PMMA was more strongly confined, while the monolayer was less confined and had a higher mobility. The differential AC chip calorimeter studies on thin film of PMMA on flat surface shows little dependence of thickness of film on Tg, while PMMA thin layer on SiO2 nanoparticle was more mobile and has a lower Tg compared with the thick PMMA. These results indicated that the confinement plays an important role in the glass transition temperature of thin polymer film. This result established a link between the mobility detected by high resolution proton solid state NMR spectroscopic method and the glass transition measured by differential AC chip calorimeter. [Preview Abstract] |
Wednesday, March 17, 2010 9:12AM - 9:24AM |
P17.00005: Modeling glass transition and aging processes in nanocomposites and polymer thin films Victor Pryamitsyn, Venkat Ganesan We use a lattice kinetic model of glass transition to study the role of confinement and the presence of nano-inclusions. We have studied freely suspended films of glass-formers and its nanocomposites with ``plastifying'' and ``hardening'' nanoparticles. Using our model we determine the thickness and nanoparticle load dependencies of the Kauzmann temperature $T_0$ and the fragility parameter. We found the glass transition temperature increases with the thickness of the film and the volume fraction of ``hardening'' nanoparticles , while $T_g$ decreases with increase in the loading of ``plastifying'' nanoparticles. We found that the isothermal free volume relaxation rate of the nanocomposite thin film, usually referred as an aging, correlates with the glass transition temperature shift. We also studied the relations between our lattice model and Curro's, Kovacs and Struik's phenomenological models of free volume reduction to deduce physical insights into the mechanisms governing aging processes in thin films and nanocomposites. [Preview Abstract] |
Wednesday, March 17, 2010 9:24AM - 9:36AM |
P17.00006: A NanoThermal Analysis Method for Mapping Glass Transitions in Heterogeneous Soft Matter Films M.P. Nikiforov, S. Gam, S. Jesse, R.J. Composto, S.V. Kalinin Polymer thin films are utilized in many present day technologies because they exhibit attractive physico-chemical properties. By combing mixtures of polymers, new combinations of properties can be achieved that impart the coating with functionality, improved processibility and lower cost. Because these coatings are usually subjected to a range of environmental conditions (heat, moisture, pressure), the initial, desired properties can be perturbed because of phase separation, phase coarsening, and interfacial segregation. ~In this work we introduce a new method for local thermal analysis of polymeric materials (BE-NanoTA), which allows the measurement of glass transition temperature with 50 nm point-to-point resolution, provides new insight into domain growth of a model polymer blend system. Virtually non-destructive nature of BE-NanoTA allows for in-situ phase evolution studies for many soft matter systems containing multiple components such as polymer nanocomposites where the filler can perturb the local dynamics and corresponding processibility, mechanical properties, optical properties etc. [Preview Abstract] |
Wednesday, March 17, 2010 9:36AM - 9:48AM |
P17.00007: Interplay between glass transition and thermal expansivity in absorbed and spincoated polymer films Simone Napolitano, Michael Wubbenhorst We investigated the kinetics of formation, the glass transition dynamics and the thermal expansivity of absorbed layers of polystyrene and other amorphous polymers on aluminum oxide. Extremely thin films (2 - 10 nm) were prepared following the experiment of Guiselin: polymers were either spincoated or casted on metallic surfaces and annealed at constant temperature immediately after film formation; non-absorded chains were washed away by a good solvent. Different molecular weights and solvent conditions were explored. We analyzed the shape of the observed kinetics in terms of density of active absorption sites and compared with recent experimental results. The combination of a tremendous reduction of the thermal expansion coefficients, TEC, together with non-universal changes in Tg is discussed. Finally, we add more evidence on the unusual confinement effects of poly(tert-butylstyrene). Below 50 nm, both Tg and TEC decreased. Such a mixed behavior implies an enhancement of the molecular mobility, without the presence of any free surface, but dead layers. The effect of density-conformation coupling in proximity of a non-attractive interface allows coexistence of an immobilized fraction in contact with the metal and an excess of thermal expansivity, arising from the long range effects of packing frustration penetrating inside the bulk-like core of the film. [Preview Abstract] |
Wednesday, March 17, 2010 9:48AM - 10:00AM |
P17.00008: Observation of glassy physics within surface-bound organic molecular monolayers L.I. Clarke, D.R. Stevens, M.C. Scott, J.R. Bochinski Monolayer systems are an alternative approach to study glass transitions in confined geometries. In a sub-monolayer, where uniformly distributed molecules are covalently bound to the substrate, intermolecular interactions can be controlled by altering the areal density of the film. Thus the transition from simple rotations within an isolated molecule to more complex cooperative motion in a glassy system can be experimentally observed. We studied sub-monolayer collections of surface-bound alkyl chains (substituted alkylsilanes) with highly sensitive narrow-band dielectric spectroscopy, and observed a transition from independent dynamics to glassy motion as the density was increased. We identified the glassy relaxation [1] as similar to the poly-ethylene-like glass transitions previously observed in alkyl-side-chain polymers. Furthermore, we studied the fragility as a function of density, and molecular conformation (packing efficiency). At high densities, we observed the emergence of a sub-T$_{g}$ relaxation.\\[4pt] [1] \textit{ACS Nano} \textbf{2}, 2392 (2008). [Preview Abstract] |
Wednesday, March 17, 2010 10:00AM - 10:12AM |
P17.00009: Interfacial Rheology of Glassy and Non -- Glassy Polymer Monolayer at the air - water interface S. Srivastava, D. Leiske, J.K. Basu, G. Fuller Interfacial rheology is the study of the relationship between interfacial stress and the resultant deformation of the interface. We have used this technique to study the 2D rheology of two different polymers, Poly methylmetha acrylate, PMMA and Polyvinylacetate, PVAc, as a function of concentration and temperature. The measured polymers differs widely in their bulk Tg : PMMA (117C) and PVAc (30C). Our results suggest that there is transition from viscous to soft glassy like dynamics for PMMA monolayer above the cross - over concentration from semi - dilute to concentrated regime. In the glassy regime both the storage and the loss modulus of the monolayer becomes equal in magnitude and almost independent of the measured frequency. The PVAc monolayer remains viscous deep into the concentrated phase and at the lowest measured temperature. The transition from viscous to soft glassy dynamics for PMMA monolayer occurs at $\sim $ 43$^{\circ}$C. The cross over temperature is 75$^{\circ}$C less than the bulk Tg of PMMA. Our results are consistent to the earlier observation of the reduction in the Tg of confined polymeric systems with respect to bulk value due to enhanced mobility of the polymer segments at the interfaces. [Preview Abstract] |
Wednesday, March 17, 2010 10:12AM - 10:24AM |
P17.00010: Glass Transition in Thin Supported Polymer Films Probed by Temperature-Modulated Ellipsometry in Vacuum Mikhail Efremov, Paul Nealey Glass transition in model glass-forming polymer coatings is probed by ellipsometry in vacuum. Novel temperature-modulated modification of the technique is used alongside with traditional linear temperature program [1]. Spin-cast 2 - 200 nm thick polystyrene (PS) and 10 - 200 nm thick poly(methyl methacrylate) (PMMA) films on silicon are studied. Measurements are performed at $10^{-6} - 10^{-8}$ torr residual gas pressure. Temperature modulation allows effective separation of reversible glass transition from accompanying irreversible processes. It is found that glass transition in both polymers demonstrates no appreciable dependence on film thickness for more than 20 nm thick coatings. The temperature of the transition ($T_g$) in thinnest PS films does depend on film thickness, but does not follow often accepted $T_g(h)=T_g(\infty)[1-(\frac{A}{h})^{\delta}]$ function (where $h$ is film thickness, $A$ and $\delta$ are constants). Effects of polymer molecular weight and substrate surface pre-treatment on glass transition will be discussed also.\\[4pt] [1]. M. Yu. Efremov, A. V. Kiyanova, and P. F. Nealey, Macromolecules, 41, 5978 (2008). [Preview Abstract] |
Wednesday, March 17, 2010 10:24AM - 10:36AM |
P17.00011: Calorimetric investigations of ultrathin film of poly-vinylacetate under controlled humidity Heiko Huth, Christoph 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. AC-chip calorimetry is used as a very sensitive tool for calorimetric investigations of such thin films as demonstrated for thin polymeric films in a wide frequency range [1]. In several cases a direct comparison with results from other dynamic methods like dielectric spectroscopy is possible giving further insights. There is also an increasing interest in thin films from a technological point of view. As these applications often include the presence of water the controlled humidity is used as a new parameter in addition to temperature for calorimetry. As a first example thin films of polyvinylacetate are measured where a large influence of humidity on the glass transition is known from literature. \\[4pt] [1] Huth, H., Minakov, A. A., Schick, C., J. Polym. Sci. B Polym. Phys. 2006 44: 2996. [Preview Abstract] |
Wednesday, March 17, 2010 10:36AM - 10:48AM |
P17.00012: Measuring the relaxation dynamics and thermo-mechanical properties of ultrathin polymer glasses Christopher M. Stafford, Jun Young Chung, Doyoung Moon, Jack F. Douglas The measurement of the mechanical properties and relaxation dynamics in ultrathin films is experimentally difficult. In this talk, we present a new wrinkling-based method to measure the temperature dependence of the Young's modulus in thin and ultrathin polymer films. This method can also capture the relaxation dynamics of these films at temperatures near and below Tg, as the wrinkling patterns decays back to being flat. We find that the temperature dependence of the modulus conforms to a pattern of behavior found in diverse amorphous solids. Curiously, the apparent activation energy for the rate of wrinkling relaxation progressively decreases as the films becomes thinner, approaching a value comparable to the high temperature activation energy observed in bulk fluids. This trend, along with a broadening of Tg, is consistent with simulations and independent previous measurements, suggesting that film confinement progressively suppresses collective motion in ultrathin glassy polymer films. [Preview Abstract] |
Wednesday, March 17, 2010 10:48AM - 11:00AM |
P17.00013: Structural signal of a dynamic glass transition Sudeshna Chattopadhyay (Bandyopadhyay), Ahmet Uysal, Benjamin Stripe, Guennadi Evmenenko, Pulak Dutta, Steven Ehrlich, Evguenia A. Karapetrova Conventional wisdom states that there is no significant difference between the static structures of the glass and liquid states of a given material. Using x-ray reflectivity, we have studied pentaphenyl trimethyl trisiloxane, an isotropic liquid at room temperature with a dynamic glass transition at 224K. Surface density oscillations (surface layers) develop below 285K, similar to those seen in other metallic and dielectric liquids and in computer simulations [1]. Upon cooling further, there is a sharp increase in the penetration of the surface layers into the bulk material, i.e. an apparently discontinuous change in the static structure, exactly at the glass transition (224K) [2]. \\[4pt] [1]. e.g. O. M. Magnussen et al., PRL 74, 4444 (1995); H. Mo et al. PRL 96, 096107 (2006); E. Chac'on et al., PRL 87, 166101 (2001) \\[0pt] [2] S. Chattopadhyay et al, PRL 103, 175701 (2009) [Preview Abstract] |
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