Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session L20: Focus Session: Dynamics of Glassy Polymers under Nanoscale Confinement I |
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Sponsoring Units: DPOLY Chair: Connie Roth, Emory University Room: 405 |
Wednesday, March 5, 2014 8:00AM - 8:12AM |
L20.00001: Elastic modulus and surface tension of a polyurethane rubber in nanometer thick films Meiyu Zhai, Gregory McKenna Estane is a kind of polyurethane with thermodynamically incompatible hard and soft segments. In this study the macro and micro properties of Estane have been characterized and compared. The viscoelastic properties of this material in bulk scale have been determined using dynamic rheometry. Time-temperature superposition was found to be applicable for this material, and a master curve was successfully constructed from the dynamic shear responses of G'($\omega )$ and G''($\omega )$. Also a novel nano bubble inflation method was used to obtain the creep compliance of the Estane ultrathin films and the results show stiffening in the rubbery region for the Estane over thicknesses ranging from 110nm to 22nm. The dependence of the rubbery stiffening on film thickness is studied and the relative influences of nano confinement and surface tension effect are analyzed using both a direct stress strain analysis and an energy balance method for the membrane. The contributions of surface tension and nano confinement are considered separately. [Preview Abstract] |
Wednesday, March 5, 2014 8:12AM - 8:24AM |
L20.00002: Effect of Microstructural Order on Plasticity in Thin PS-P2VP Films Bekele Gurmessa, Andrew B. Croll We report the results of an experimental investigation of the onset of plastic deformation in polystyrene-b-poly (2-vinylpyridine) (PS-P2VP) thin films. PS-P2VP is a glass-forming diblock copolymer which serves as a model material for the study of the effects of microstructure on mechanical response due to the similarities of the mechanical properties (glass transition temperature, entanglement molecular weight, and bulk elastic modulus) of each block. In particular, we measure the onset of plasticity using an elastic instability technique to locally bend and locally impart a tensile stress in a thin film that is subsequently examined for damage. Similar to our earlier results from experiments conducted on homopolymer polystyrene, we show that failure in PS-P2VP is initiated at extremely low strain and that the failure strain is influenced by thin film confinement effects. For the first time, we show that the onset of plasticity increases as a sample is annealed from a disorganized, as-cast state to that of a film with a well ordered internal microstructure. [Preview Abstract] |
Wednesday, March 5, 2014 8:24AM - 8:36AM |
L20.00003: Theory of Activated Relaxation in Nanoscale Confined Liquids Stephen Mirigian, Kenneth Schweizer We extend the recently developed Elastically Cooperative Nonlinear Langevin Equation(ECNLE) theory of activated relaxation in supercooled liquids to treat the case of geometrically confined liquids. Generically, confinement of supercooled liquids leads to a speeding up of the dynamics(with a consequent depression of the glass transition temperature) extending on the order of tens of molecular diameters away from a free surface. At present, this behavior is not theoretically well understood. Our theory interprets the speed up in dynamics in terms of two coupled effects. First, a direct surface effect, extending two to three molecular diameters from a free surface, and related to a local rearrangement of molecules with a single cage. The second is a longer ranged ``confinement'' effect, extending tens of molecular diameters from a free surface and related to the long range elastic penalty necessary for a local rearrangement. The theory allows for the calculation of relaxation time and T$_{g}$ profiles within a given geometry and first principles calculations of relevant length scales. Comparison to both dynamic and pseudo-thermodynamic measurements shows reasonable agreement to experiment with no adjustable parameters. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L20.00004: Dynamics and mechanical properties of glassy polymers under cylindrical confinement Amit Shavit, Robert Riggleman Even after two decades of active research surrounding glassy polymers under confinement, we still lack a complete understanding of the changes in mechanical properties as a bulk material is confined. Understanding the properties of glassy polymers in confinement is relevant not only for our fundamental understanding, but also for applications in semiconductor manufacturing and fabrication of novel materials. Here, we used molecular dynamics simulations to investigate dynamical and mechanical properties of glass-forming polymers in bulk and pillar geometries. We examine how the free surface influences the dynamics locally in the film, and we show that the dynamics in the surface are several orders of magnitude faster than in the bulk, which is similar to the enhancement we see in thin-films. Finally, we show that the mechanical properties of the bulk differ significantly from the pillar, and that the path to the glassy state has significant consequences for the overall material properties. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L20.00005: Role of free surface and interface effects on viscoelastic properties of ultrathin polystyrene films Heedong Yoon, Gregory McKenna The surface properties of 20 nm polystyrene (PS) films with different under layer substrates were investigated by employing a silica particle embedment method. The under layer substrates used were PS, poly(2-vinyl pyridine) (P2VP), and poly(methyl methacrylate) (PMMA) with thicknesses ranging from 17 nm to 350 nm. The apparent particle height change was monitored through the use of Atomic Force Microscopy at different experimental temperatures ranging from T$_{\mathrm{g}}$-10 to T$_{\mathrm{g}}+$10 $^{\mathrm{o}}$C. The Hutcheson and McKenna model [Phys. Rev. Lett. \textbf{94}, 076103 (2005)] was applied to the particle embedment depth to obtain the surface rheological temperatures. The results showed that the 20 nm top layer of PS films soften below T$_{\mathrm{g}}$ and stiffen above T$_{\mathrm{g}}$ for the different types of under layer substrates. The rheological temperatures of the 20 nm surface layer PS films were independent of under layer PS thickness in the range from 17 nm to 350 nm. Furthermore, the rheological temperature of the top layer of PS film also showed that different types of under layer substrates such as PS, P2VP, and PMMA can slightly alter the T$_{\mathrm{g}}$ of the top layer of the PS film, but the change was less than 10 K. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L20.00006: Molecular weight dependence of surface flow near the bulk glass transition temperature Yu Chai, Thomas Salez, Michael Benzaquen, Elie Raphael, James A. Forrest We present the study on molecular weight dependent sub-T$_{\mathrm{g}}$ surface dynamics of polymer thin films by using the Nano-step experiment [McGraw et al. Soft Matter 7, 7832 (2011)]. By varying the molecular weight, we are able to probe the surface dynamics of the free surface below T$_{\mathrm{g}}$ with the polymer size comparable to the surface depth. In particular, we define and use a correlation function to compare measured and calculated profiles to analyze the transition from the bulk flow to flow restricted to the surface region. Surprisingly, even for the polymers with M$_{\mathrm{w}}=$22,000 surface flow is still observed below the bulk T$_{\mathrm{g}}$ value. A numerical simulation of random walk is used to find the fraction of polymer of which all of the polymer segments are located in the free surface region. The simulation results indicate that there are still a significant fraction of polymer molecules where all segments are in the near free surface region. These molecules can undergo flow consistent with the experimental results. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:48AM |
L20.00007: Viscosity of Polymer Nanometer Films Invited Speaker: Ophelia Tsui Ample studies have shown that the $T_g$ of polymer films can change visibly when the film thickness ($h$) is decreased below 100 nm. It implies that the viscosity ($\eta$) of the films can differ notably from the bulk. In this talk, I shall discuss the viscosity measurement performed on two systems - polystyrene and poly(methal methacrylate) on silicon oxide (PS/SiO$_x$ and PMMA/SiO$_x$) where the $T_g$ decreases and increases with decreasing $h$, respectively. At low molecular weight ($M_w$), $\eta$ of both systems can be described by a layer model. For PS/SiO$_x$, a two-layer model assuming the films to comprise a mobile layer on top of a bulklike layer is sufficient. For PMMA/SiO$_x$, a three-layer model including an additional slow substrate layer is needed. For PS/SiO$_x$, the layer model is also able to describe the data at high $M_w$. However, the $M_w$-dependence displays a qualitative change when the polymer size exceeds the estimated thickness of the surface mobile layer. Beyond that, the surface chains are partially embedded in the bulklike layer so cannot facilitate enhanced transport as in the low-$M_w$ films. We hypothesize that the surface mobile layer operates in the low-$M_w$ films, but a confinement effect in the high-$M_w$ films in lowering $\eta$. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L20.00008: Surface Dynamics and Structures of Swollen Polymer Brushes Mark Foster, Liang Sun, Bulent Akgun, Suresh Narayanan, Jim Browning The surface height fluctuations of a film of untethered linear polystyrene (PS) chains can be well described by a continuum hydrodynamic theory of overdamped capillary waves. When the chains in a film are tethered to the substrate to form a ``dry'', densely grafted PS brush, the surface fluctuations are dramatically suppressed so that they are no longer observable in the time and scattering vector windows available for X-ray Photon Correlation Spectroscopy (XPCS). Here, surface fluctuations of PS brushes highly swollen in toluene vapor are investigated using XPCS and the structures of these swollen polymer brushes are investigated using Neutron Reflectivity (NR). Surface fluctuations of densely grafted PS brushes are still strongly hindered and not observable even if the brush is substantially swollen in a good solvent vapor. When there is a condensed liquid toluene layer on top of the brush, the surface fluctuations are observed with a relaxation time orders of magnitude larger than that of a thick film of pure toluene. [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L20.00009: Solvent-polymer thin films drying below Tg Didier Long, Gregoire Julien, Elian Masnada We propose a model for describing the dynamics of polymer-solvents systems close to the glass transition. We extend to these systems the facilitation mechanism due to free volume diffusion proposed by Merabia and Long (EPJ E 2002; J. Chem. Phys. 2006, Chen et al 2009) to the case of polymer-solvent systems. Our model is solved on a 2D lattice with a spatial resolution corresponding to the scale of dynamical heterogeneities. It allows to describe how a solvent penetrates a glassy film (case II diffusion, Kramer et al 1988). Regarding the process of film drying, we show that films up to a few hundred of nm thick can be almost completely dried in an accessible experimental time scale, even at temperatures 100 K below the glass transition temperature of the pure polymer as shown experimentally by Catherine Allain and co-workers (2002, 2003). The thinner the films, the shorter the drying time, allowing to obtain far from equilibrium polymer films a few tens of nanometer thick in a state very different from bulk glassy polymers. For thicker films, a glassy crust a few hundred of nanometers thick appear and the subsequent evaporation of solvent slows down progressively over very long time scales. [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L20.00010: Dynamics of bound polymer layers in CO2 Naisheng Jiang, Levent Sendogdular, Mani Sen, Maya K. Endoh, Tadanori Koga, Bulent Akgun, Michael Dimitriou, Sushil Satija Recently, there has been growing interest in bound polymer layers (BPLs) on planar solids due to their strong influence on the physical and mechanical properties of confined polymeric materials. It is known that BPLs are immobile (in air) even at temperature far above the bulk glass transition temperature. Here, we used CO$_{2}$ as a plasticizer for polystyrene (PS) bound layers ($\sim$10 nm in thickness) formed on planar silicon (Si) substrates. By using high pressure neutron reflectivity, we studied the swelling behavior of the BPL and the interdiffusion process for bilayers of the bottom BPL and an deuterated PS overlayer (about 50 nm-thick) in CO$_{2}$ at T = 36$^{\circ}$C. The results have clearly shown that CO$_{2}$ swells the BPL and induces the interdiffusion of polymer chains across the BPL/overlayer interface. We further studied the interdiffusion processes as a function of molecular weights of the overlayer, allowing us to highlight the unique interdiffusion process relative to the bulk. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L20.00011: Confinement for Thin Film on Substrates with Different Geometric Curvatures Jie Xu, Jiao Chen, Gi Xue Molecular chain conformation in thin polymer film on substrates with different geometric curvature was examined using fluorescence non-radiative energy transfer (NRET) spectroscopy. We find that thin film on concave substrate exihibits significant differences in vitrification behavior, in both magnitude and thickness dependence, from the planar film. NRET measured a more compact morphology, while dynamical scanning calorimetry detected an increased glass transition temperature (Tg) for the concave thin film, with respect to bulk film. In contrast to planar film where properties are thickness dependent, polymer concave film shows that its conformation and Tg are solely dependent on curvature radius. Surprisingly, these properties converted back to the bulk values when the substrate was removed, indicating the crucial importance of interaction imposed by the concave hard wall. These spectroscopic data matched perfectly the calorimetric results and provided a new implication to understanding geometric confinement on dynamics. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L20.00012: The effect of nanoconfinement on network topology and thermo-mechanical properties of glassy polymers dynamically reacted using MD simulation Katherine Sebeck, John Kieffer The interaction of polymers with an ordered surface affects the thermo-mechanical properties of polymer matrix composite and nanoconfined polymeric materials. Atomistic simulations of glassy polymers such as epoxides offer unique insights to the network topology of such structures. A series of epoxide network structures using a bifunctional epoxide and polyfunctional aliphatic amines have been generated using a dynamic polymerization simulation technique. This allows for natural evolution of the network topology in both bulk and confined structures. The nature of structural organization at the surface of confined structures will be analyzed as a function of distance from the interface using various metrics including spatial distribution functions. These structural characteristics are then correlated with dynamical properties such as Tg and modulus, comparing nanoconfined networks to bulk systems. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L20.00013: A Simple Model of Dynamic Heterogeneity: Connection with Experimental Results Jane Lipson, Nicholas Tito, Scott Milner We have developed the Limited Mobility (LM) model to study dynamic heterogeneity in a system that exhibits kinetic arrest, i.e. a glass transition. In recent work we have investigated the approach to the bulk transition from above and below, as well as the effects of perturbations on the transition. Results in the latter area have included looking at buried slabs of different mobility than the surround, as well as studies on a supported film. In this talk we will focus on characterizing sample mobility in the bulk, via measurement of the diffusion constant of mobile material, D, as well as in a film, via characterization of mobility fronts. We find that as the bulk glass transition is approached the LM model exhibits the same kind of deviation from Stokes-Einstein behaviour as is observed in experiment and other model studies.~ In the film the LM model shows a time-dependent growth of the mobility front that scales with the same D that characterizes mobility in analogous bulk samples; this has also been seen experimentally in glass-forming liquids. These results will be discussed, in addition to others that help connect the LM model with data on real systems. [Preview Abstract] |
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