Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session V20: Thin Films and Adhesion II |
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Sponsoring Units: DPOLY Chair: Connie Roth, Emory University Room: 321 |
Thursday, March 19, 2009 8:00AM - 8:12AM |
V20.00001: ABSTRACT WITHDRAWN |
Thursday, March 19, 2009 8:12AM - 8:24AM |
V20.00002: Temperature-Modulated Ellipsometry in Vacuum: A New Tool for Probing Glass Transition in Thin Supported Polymer Films Mikhail Efremov, Shauheen Soofi, Anna Kiyanova, Franco Cerrina, Paul Nealey Observation of glass transition in thin polymer films is a good example of experimental challenges that measurements at the nanoscale may present. The standard technique in the field, ellipsometry, has been advanced by state-of-the-art approaches: measurements in vacuum and temperature modulation. Glass transition in 5 -- 200 nm thick spin-cast polystyrene (PS) and poly(methyl methacrylate) (PMMA) films on silicon has been studied at 10$^{-6}$ -- 10$^{-8}$ torr residual gas pressure, using both linear and modulated temperature scans [1, 2]. A well-defined glass transition in 5 nm thick PS and 10 nm thick PMMA films is observed. Factors that can alter glass transition temperature assignment will be discussed. Residual gas can affect data even at the pressures mentioned above [1]. An ionization gauge, the standard vacuum equipment, causes sample deterioration [1]. Temperature modulation effectively separates reversible glass transition from accompanying irreversible processes [2]. [1]. M. Yu. Efremov, S. S. Soofi, A. V. Kiyanova, C. J. Munoz, P. Burgardt, F. Cerrina, and P. F. Nealey, Rev. Sci. Instr., \textbf{79}, 043903 (2008). [2]. M. Yu. Efremov, A. V. Kiyanova, and P. F. Nealey, Macromolecules, \textbf{41}, 5978 (2008). [Preview Abstract] |
Thursday, March 19, 2009 8:24AM - 8:36AM |
V20.00003: Physical Aging in Nanoconfined Polymer Films: Importance of 3D vs. 1D Thermal Contraction in the Resulting Physical Aging Response Connie B. Roth, Elizabeth A. Baker, Perla Rittigstein, John M. Torkelson Studies of physical aging in confined geometries have reported conflicting observations of changes in physical aging rates with decreasing polymer film thickness. Accelerated physical aging with decreasing film thickness has been observed with gas permeation and ellipsometry in stiff backbone, so-called ``high free volume,'' polymers traditionally used in gas separation membranes. In contrast, no change or suppressed physical aging has been observed with fluorescence in flexible carbon-carbon backbone polymers. We have developed a new streamlined ellipsometry procedure that allows us to relatively quickly ($\sim$6 hrs) evaluate the physical aging characteristics of both stiff and flexible backbone polymers in a thin film geometry. We present measurements of physical aging rates using our new approach and compare these to existing results in the research literature. In addition, we also address the importance of 3D vs. 1D thermal contraction in the resulting physical aging response, which we believe is one of the key factors accounting for the observed qualitative differences in physical aging rate upon confinement between the existing studies. [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 8:48AM |
V20.00004: Segmental dynamics of supported and freestanding polystyrene thin films probed by dye reorientation. Keewook Paeng, Hau-Nan Lee, Stephen Swallen, Mark Ediger The dynamics of both freestanding and supported polystyrene thin films (down to 15 nm) were studied by measuring the reorientation of dilute dye molecules. Well below T$_{g, }$dye molecules were photobleached using intense linearly polarized light creating an anisotropic distribution. The anisotropy decay was measured using circularly polarized light and probing fluorescence parallel and perpendicular to the bleaching beam during linear temperature ramping. Temperature ramping anisotropy measurements allow us to compare both dynamics and the distribution of relaxation times in thin and thick films. Both freestanding and supported thin films show faster and more broadly distributed dynamics than thick films. For 17.5 nm supported films, temperature ramping experiments show up to 14K shift in dynamics. The corresponding shift for 16.5nm freestanding films was 22K. [Preview Abstract] |
Thursday, March 19, 2009 8:48AM - 9:00AM |
V20.00005: Modeling Dielectric Relaxation in Simulations of Polymer Glasses and Thin Films Hendrik Meyer, Simone Peter, J. Baschnagel We perform molecular dynamics simulations to study the dielectric relaxation of a bead-spring model for a polymer melt in the bulk and in supported films [1]. By assigning dipole moments parallel and perpendicular to the backbone of all chains in the completed simulation trajectories we calculate the dielectric spectra of so-called type-A polymers which exhibit relaxation processes due to the local motion of chain segments (``segmental mode'') and due to fluctuations of the end-to-end vector (``normal mode''). We find that the relaxation of both modes is enhanced in the films relative to the bulk. For the segmental mode this difference between film and bulk dynamics increases on cooling toward the glass transition. By a layer-resolved analysis of the segmental relaxation we show that the acceleration of the average film dynamics is a consequence of a smooth gradient in relaxation, originating from both interfaces where the segmental dipoles relax much faster than in the bulk. \\[3pt] [1] S. Peter et al Macromolecules 41 (2008) 7729. [Preview Abstract] |
Thursday, March 19, 2009 9:00AM - 9:12AM |
V20.00006: Neighboring Domains Perturb Glass Transition Temperature on Multilayer Films and Nanostructured Polymer Blend Systems Soyoung Kim, Connie Roth, Rodney Priestley, John Torkelson The impact of free surface and polymer-substrate interfaces on the glass transition temperature (Tg) in nanoconfined geometries has been studied for over a decade. Free surfaces reduce the requirement for cooperative dynamics and tend to decrease Tg; attractive interactions with a substrate interface reduce mobility and tend to increase Tg. Employing a multilayer fluorescence technique, we show how the Tg dynamics of PS layers are perturbed by immiscible polymer-polymer interfaces. We determine the length scale over which adjoining layers can perturb the PS layer. Finally, we demonstrate the tunability of the Tg of ultrathin PS layers atop different types of polymers. Our results indicate that the cooperative segmental dynamics of an ultrathin PS layer are strongly coupled to the neighboring domains through the narrow polymer-polymer interface. These results suggest a novel route to create new material properties controlled by the type and thickness of polymers in a multilayer film geometry. Studies with nanostructured blends to monitor Tg perturbation by neighboring domain are also underway. [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:24AM |
V20.00007: Near Surface Dynamics of Polymers Probed with Nanoparticle Embedding Dongping Qi, James A. Forrest We use nanoparticle embedding to probe the dynamics of the near surface layer of glassy polymer films. We observe evidence for heterogeneous dynamics in the first 5-10 nm near the free surface of glassy polymers. We observe that the relaxation into the polymer immediately below the free surface is irreversible, even after a period of 1 year. On the other hand, further embedding (5 -10 nm) appears to be reversible. The results are discussed in terms of posible models of near surface mechanical properties. [Preview Abstract] |
Thursday, March 19, 2009 9:24AM - 9:36AM |
V20.00008: Glass Transition of Thin Star Polymer Films Emmanouil Glynos, Peter Green The thickness dependence of the glass transition, $T_{g}$, of thin film polystyrene (PS) star molecules, supported by SiO$_{x}$ substrates, has been examined using spectroscopic ellipsometry and compared to the behavior of linear PS chains. Linear PS chains exhibit a film thickness dependence on SiO$_{x}$ substrates, decreasing with decreasing film thickness, for thicknesses $h$ less than approximately 45 nm. This thickness dependence, when normalized by the bulk $T_{g}$, is observed for chains with a wide range of degrees of polymerization $N$, from $N < \quad N_{e}$ (molecular weight between entanglements) to very large values of $N$. The $T_{g}$s of long chain star molecules, of low functionalities, $f$, exhibit the same thickness dependence. However, as the degree of polymerization of an arm length, \textit{Na}, decreases the thickness dependence undergoes a transition, wherein $T_{g}$ increases with decreasing $h$. These effects are discussed in terms of the role of architecture and entropic effects on the structure of the system. Implications on the chain dynamics will also be discussed. [Preview Abstract] |
Thursday, March 19, 2009 9:36AM - 9:48AM |
V20.00009: The Glass Transition in Ultra-Thin Polymer Filrms Confined between Structured Surfaces Vikram Kuppa, Gregory Rutledge Molecular Dynamics simulations are used to probe the structure and dynamics of polymers in ultra-thin slit pores. The simulation setup follows the structure of polymer nanocomposites, depicting chains intercalated between layered inorganic silicates. The structure and dynamics of bead-spring oligomers are studied for different film thicknesses, surface-segment interactions and temperature. In particular, we focus on the glass transition of the confined films as a function of layer thickness, interaction strength and density profiles: Tg is demonstrated to increase with confinement and attraction of the polymer with adjacent surfaces. The fragility of the polymer glasses is drastically different from the corresponding bulk system, and is shown to be dependent on the effective co-ordination number. [Preview Abstract] |
Thursday, March 19, 2009 9:48AM - 10:00AM |
V20.00010: Apparent changes in the molecular dynamics of thin polymer layers due to the impact of interfacial layers Anatoli Serghei, Martin Tress, Friedrich Kremer Possible mechanisms leading to an apparent faster glassy dynamics in thin polymer layers, as investigated by means of Broadband Dielectric Spectroscopy, are analyzed in detail. It is shown that manifold experimental findings can be traced back to the influence of interfacial sub-layers, where -- due to the proximity to solid interfaces -- the dielectric function of the polymer is altered and modifies, by that, the overall dielectric response of the polymer films. A large amount of experimental data is analyzed to evidence how the contribution of the interfacial dynamics combines with that of the bulk in order to give the total response of a thin polymer film. It is shown that the non-linear character of this combination could lead to apparently discrepant experimental results. [Preview Abstract] |
Thursday, March 19, 2009 10:00AM - 10:12AM |
V20.00011: Post-confinement Relaxation Behavior of Nanostructures on Polymer Surface Hua-Gen Peng, Yen Peng Kong, Albert Yee Dimensional relaxation of nanostructures on polymer film was studied to understand dynamics at surfaces and \textit{post-}confinement relaxation. Line gratings from 33 nm and up on PS surface were formed by nanoimprint lithography, and AFM was used to monitor their relaxation with time and temperature. When annealed at temperatures in the vicinity of the bulk Tg, the grating height slumps -- or shrinks -- as surface tension and other driving forces overcome the viscosity. The temperature for rapid slumping decreases at smaller and smaller gratings of all molecular weights, but a simple explanation based on enhanced surface mobility due to increased surface to volume ratio fails to explain the results. Analysis of viscosity shows that the stress from surface tension may cause shear thinning and thus contribute to the reduced nanostructure stability. More importantly, confinement of polymer chains to spatial dimensions comparable to or even smaller than the radius of gyration seems to enhance molecular relaxation, which may be the major factor for the surprisingly low slumping temperature. [Preview Abstract] |
Thursday, March 19, 2009 10:12AM - 10:24AM |
V20.00012: Molecular weight and chain architecture dependence of glassy compliance of ultrathin freely standing polymer films Shanhong Xu, Gregory B. McKenna Glassy compliance of ultrathin linear polystyrene films of different molecular weights was investigated with the novel nanobubble inflation techniques developed in our lab. Previous work by O'Connell, Hutcheson and McKenna [1] indicates that the glassy compliance decreases as the film thickness decreases for a polystyrene sample with molecular weight of 1M g/mol. However the glassy stiffening is not nearly as dramatic as that observed in the rubbery plateau regime [2]. Preliminary results in the present work show that the glassy compliance decreases as the molecular weight increases. We are now examining a three-arm star polystyrene with branch molecular weight the same as that of the lower molecular weight linear material and eight-arm stars will also be investigated with the purpose of determining chain architecture effects on the glassy and rubbery behaviors of ultrathin polymer films. \\[4pt] [1] P.A. O'Connell, S.A. Hutcheson and G.B. McKenna, J. Polym. Sci. Part B. Polym. Phys. Ed., \textbf{46}, 1952 (2008). \\[0pt] [2] P.A. O'Connell and G.B. McKenna, Eur. Phys. J. E, \textbf{20}, 143 (2006). [Preview Abstract] |
Thursday, March 19, 2009 10:24AM - 10:36AM |
V20.00013: Surface Chemistry Effects on the Reactivity and Properties of Nanoconfined Bisphenol M Dicyanate Ester in Controlled Pore Glass Sindee Simon, Qingxiu Li The influence of nanoconfinement on the cure kinetics and glass transition temperature of a bisphenol M dicyanate ester/polycyanurate material is investigated as a function of surface chemistry and nanoconfinement size in controlled pore glass (CPG). The glass transition temperature and conversion as a function of cure time is investigated using differential scanning calorimetry. The native CPG surface accelerates the cure of bisphenol M dicyanate to a larger extent compared to the silanized hydrophobic CPG presumably because of the catalytic nature of surface hydroxyl groups of the native pores. Two T$_{g}$s are observed for both monomer and polycyanurates confined in the native CPGs. For the fully cured polycyanurate, the primary T$_{g}$ is depressed by 60 K at 11.5 nm, and the secondary T$_{g}$ is 10 to 33 K above the primary T$_{g}$. The length scale associated with the secondary T$_{g}$ is approximately 0.8 nm. Based on the measurements of both T$_{g}$ and sol content as a function of conversion, the network structure does not change upon nanoconfinement. [Preview Abstract] |
Thursday, March 19, 2009 10:36AM - 10:48AM |
V20.00014: Relation Between Glass Transition Temperatures in Polymer Nanocomposites and Polymer Thin Films Jamie Kropka, Victor Pryamitsyn, Venkat Ganesan Motivated by recent experiments, we examine within a percolation model whether there is a quantitative equivalence in the glass transition temperatures of polymer thin films and polymer nanocomposites (PNCs). Our results indicate that while the qualitative behaviors of these systems are similar, a quantitative equivalence cannot be established in general. However, we propose a phenomenological scaling collapse of our results which suggests a simple framework by which the results of the thin films may be used to quantitatively predict the properties of PNCs. Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000. [Preview Abstract] |
Thursday, March 19, 2009 10:48AM - 11:00AM |
V20.00015: Finite Element Analysis of Nanoparticle embedding into glassy polymers Mark Ilton, James A. Forrest The embedding of rigid nanospheres into the surface of thin film glassy polymers was modelled using a Finite Element Analysis. This method of analysis readily adapts to various material models, film thicknesses, and substrates. Moreover, it allows for modelling of the related hole relaxation process when embedded nanospheres are removed from the film. Calculations using a viscoelastic constitutive model with depth dependent material properties are compared to recent experimental results. The results are discussed in the context of the proposed near surface enhanced dynamics in thin film glassy polymers. [Preview Abstract] |
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