Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session E33: Polymer Glass Formation and Stability |
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Sponsoring Units: DPOLY Chair: Connie Roth, Emory University Room: 336 |
Tuesday, March 15, 2016 8:00AM - 8:36AM |
E33.00001: DPOLY SESSION BREAK
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Tuesday, March 15, 2016 8:36AM - 8:48AM |
E33.00002: Theoretical Insights from Facile Microsecond Simulation of the Glass Transition Jui-Hsiang Hung, Tarak Patra, David Simmons Despite more than half a century of research, the fundamental nature of the glass transition remains one of the major open questions in polymer science and condensed matter physics. Molecular dynamics simulations have provided key insights into this problem, but their ability to firmly establish the underlying nature of glass formation have been limited by the extreme computational difficulty of directly probing the deeply supercooled regime most relevant to this process. Here we describe a new protocol for simulation of the glass transition enabling facile access to in-equilibrium segmental relaxation times approaching and exceeding one microsecond -- well into the deeply supercooled regime of most glass-forming liquids. Coupled with a well-validated strategy for extrapolation to experimental timescales, this approach provides vastly improved prediction of experimental glass transition temperatures. Here we combine data acquired through this protocol for the deeply supercooled regime of polymeric, inorganic, organic, and metallic glass formers to robustly test several theories of glass formation and identify microscopic phenomenological features shared across all classes of glass-forming liquid in the deeply supercooled regime. [Preview Abstract] |
Tuesday, March 15, 2016 8:48AM - 9:00AM |
E33.00003: Will it form a stable glass? How the stability of vapor deposited glasses depends on molecular structure Michael Tylinski, Madeleine Beasley, Yeong Zen Chua, Christoph Schick, Mark Ediger Over the past nine years physical vapor deposition has been used to prepare molecular glasses with exceptional properties. When heated, transformation of these highly stable glasses takes orders of magnitude longer than the transformation of liquid-cooled glasses. Until recently, it appeared that most organic molecules could form stable glasses when vapor deposited. We test the generality of stable glass formation by vapor-depositing a wide range of small organic molecules, including hydroxyl, carbonyl, phosphate, aromatic, and aliphatic functional groups. When prepared under conditions expected to yield highly stable glasses, we observe glasses with a wide range of kinetic stabilities, depending on the functional groups in the molecule. In general, alcohols and molecules with long aliphatic chains do not form highly stable glasses while aromatic molecules do. We also test the hypothesis that the surface mobility during deposition determines if a molecule is able to create highly stable glasses. [Preview Abstract] |
Tuesday, March 15, 2016 9:00AM - 9:12AM |
E33.00004: Kinetics of Dewetting of Ultra-Thin Films of Organic Glasses Zahra Fakhraai, Yue Zhang, Robert Riggleman Physical vapor deposition (PVD) is widely used in manufacturing ultra-thin layers of amorphous organic solids. It is generally assumed that the properties of these ultra-thin films are the same as bulk dynamics. In this work, we demonstrate that these films exhibit a sharp transition from glassy solid to liquid-like behavior with thickness below 30 nm. This liquid-like behavior persists even at temperatures well below the glass transition temperature, Tg, where bulk properties suggest that the film should be vitrified. The enhanced dynamics in these films can produce large scale morphological features during PVD and lead to dewetting instability in films held at temperatures as low as Tg-35 K. We measure an effective viscosity of organic glass films by monitoring the dewetting kinetics. These measurements combined with cooling rate-dependent Tg measurements show that the apparent activation barrier for rearrangement decreases sharply in films thinner than 30 nm. These observations suggest long-range facilitation of dynamics induced by the free surface. These observations can help understand correlated dynamics in glassy systems and elucidate the processes that lead to the formation of exceptionally stable glasses. [Preview Abstract] |
Tuesday, March 15, 2016 9:12AM - 9:24AM |
E33.00005: Dynamics of Vapor-Deposited Polymer Glasses from Simulation Wengang Zhang, Francis Starr, Jack Douglas We use molecular dynamics simulations to mimic the physical vapor deposition of glassy polymer films. Like experiments, the deposition results in "ultrastable glasses" that have lower energies, and greater kinetic stability than ordinary glasses. It has been suggested that these ultrastable glasses may be equivalent to very highly aged ordinary glasses. To explore this possibility, we contrast both the structure and dynamics of deposited and ordinary glasses. Our modeling indicates that the deposited polymer glass is structurally distinct from the ordinary glass due to anisotropy of chain packing. If the deposited glasses correspond to highly aged ordinary glasses, we would expect vastly larger relaxation times for the deposited glass. Instead, we find that relaxation times of the vapor-deposited glass are nearly the same as that of the ordinary glass. These findings do not support the view that vapor-deposited glassy polymer films are equivalent to highly-aged ordinary glassy polymer films. We further study the dynamical heterogeneity of highly out-of-equilibrium polymer films. [Preview Abstract] |
Tuesday, March 15, 2016 9:24AM - 9:36AM |
E33.00006: Molecular Orientation in Two Component Vapor-Deposited Glasses: Effect of Substrate Temperature and Molecular Shape Charles Powell, Jing Jiang, Diane Walters, Mark Ediger Vapor-deposited glasses are widely investigated for use in organic electronics including the emitting layers of OLED devices. These materials, while macroscopically homogenous, have anisotropic packing and molecular orientation. By controlling this orientation, outcoupling efficiency can be increased by aligning the transition dipole moment of the light-emitting molecules parallel to the substrate. Light-emitting molecules are typically dispersed in a host matrix, as such, it is imperative to understand molecular orientation in two-component systems. In this study we examine two-component vapor-deposited films and the orientations of the constituent molecules using spectroscopic ellipsometry, UV-vis and IR spectroscopy. The role of temperature, composition and molecular shape as it effects molecular orientation is examined for mixtures of DSA-Ph in Alq$_{3}$ and in TPD. Deposition temperature relative to the glass transition temperature of the two-component mixture is the primary controlling factor for molecular orientation. In mixtures of DSA-Ph in Alq$_{3}$, the linear DSA-Ph has a horizontal orientation at low temperatures and slight vertical orientation maximized at 0.96T$_{g,mixture}$, analogous to one-component films. [Preview Abstract] |
Tuesday, March 15, 2016 9:36AM - 9:48AM |
E33.00007: Measuring Surface Diffusion of Organic Glasses Using Tobacco Mosaic Virus as Probe Nanoparticles Yue Zhang, Richard Potter, Zahra Fakhraai Recent studies have shown that diffusion on the surface of organic glasses can be many orders of magnitude faster than bulk diffusion, with lower activation barrier. Developing new probes that can readily measure the diffusion at the surface of an organic glass can help study the effect of chemical structure and molecule's size on the enhanced surface diffusion. In this study, surface diffusion coefficient of molecular glass (TPD) is measured using tobacco mosaic virus (TMV) as probe particles. TMV is placed on the surface of bulk TPD films. The evolution of the meniscus formed around TMV, driven by curvature gradient, is probed at various temperatures. TMV has a well-defined cylindrical shape, with a large aspect ratio (18 nm wide, 300 nm long). As such, the shape of the meniscus around the center of TMV is semi-one dimensional. Based on the self-similarity nature of surface diffusion flow in one dimension, the surface diffusion coefficient and its temperature dependence are measured. It is found that the surface diffusion is greatly enhanced and has weak temperature dependence compared to bulk counterpart, consistent with previous studies, showing that TMV probes serve as an efficient method of measuring surface diffusion. [Preview Abstract] |
Tuesday, March 15, 2016 9:48AM - 10:00AM |
E33.00008: Relationship between Fragility and Tg Changes on Confinement for Three Cyanurates Evelyn Lopez, Sindee L. Simon The glass transition temperature (Tg) is known to deviate from the bulk when subjected to both thin film and nanopore confinement. Previous work from our laboratory has analyzed the effect of nanopore confinement on the Tg of three materials: a cyanurate trimer, an uncrosslinked polycyanurate, and a polycyanurate network. The results showed that the Tgs of the three materials decreased under confinement and that with increasing molecular weight and molecular stiffness, the Tg depression increased. However, recent studies have pointed to fragility, and not stiffness, as a key factor in determining how the material’s Tg will be affected by confinement, with fragile polymers showing greater confinement effects. In this work, we analyze the effect of both nanopore and thin film confinement on Tg and calculate the fragilities of the three materials to determine the relationship between the two properties. Fragility is calculated from the dependence of Tg on the cooling rate, with a fast-scanning calorimeter used to extend the range of cooling rates. [Preview Abstract] |
Tuesday, March 15, 2016 10:00AM - 10:12AM |
E33.00009: Liquid and Glassy Specific Volume Variations in Thin Supported Polystyrene Films Xinru Huang, Connie Roth Studies of density or specific volume in thin films have been previously studied as a possible means of understanding changes in the glass transition temperature Tg(h) with decreasing film thickness. In the late 1990s, studies reported no change in the glassy, room temperature density of thin films outside of the plus or minus 1 percent experimental error, while more recent studies have claimed large 25-30 percent increases in film density below about 40 nm.In addition, recent theoretical efforts have suggested that Tg(h) decreases may be associated with small less than 1 percent increases in specific volume of the equilibrium liquid-line. We use ellipsometry to investigate variations in the liquid and glassy specific volume lines for polystyrene films supported on silicon.We observe small, reproducible increases in specific volume of 0.4 plus or minus 0.2 percent for both the liquid and glassy regimes that are uncorrelated with the observed Tg(h) decrease.Below 20-30 nm, we also observe what appears to be large increases in density that we attribute to breakdown of the assumptions used to derive the Lorentz-Lorenz formula. [Preview Abstract] |
Tuesday, March 15, 2016 10:12AM - 10:24AM |
E33.00010: ABSTRACT WITHDRAWN |
Tuesday, March 15, 2016 10:24AM - 10:36AM |
E33.00011: Are polymers glassier upon confinement? Simone Napolitano, Jean Spiece, Daniel E. Martinez-Tong, Michele Sferrazza, Aurora Nogales Glass forming systems are characterized by a stability against crystallization upon heating and by the easiness with which their liquid phase can be transformed into a solid lacking of long-range order upon cooling (glass forming ability). Here, we discuss on the the thickness dependence of the thermal phase transition temperatures of poly(L-lactide acid) thin films supported onto solid substrates [1]. The determination of the glass transition (T$_{g}$), cold crystallization (T$_{CC}$) and melting (T$_{m}$) temperatures down to a thickness of 6 nm via ellipsometry, permitted us to build up parameters describing glass stability and glass forming ability. We observed a strong influence of the film thickness on the latter, while the former is not affected by 1D confinement. Remarkably, the increase in T$_{g}$/T$_{m}$ ratio, a parameter related to glass forming ability, is not accompanied by an increase in T$_{CC}$-T$_{g}$, as observed on the contrary, in bulk metallic glasses. We explained this peculiar behavior of soft matter in confinement considering the impact of irreversible adsorption on local free volume content [2]. [1] J. Spiece et al. Soft Matter, 2015,11, 6179-6186 [2] Non-equiibrium Phenomena in Confined Soft Matter, S Napolitano (ed.) Springer, 2015 [Preview Abstract] |
Tuesday, March 15, 2016 10:36AM - 10:48AM |
E33.00012: ENGINEERING THE CRYSTALLINE MORPHOLOGY OF POLYMER THIN FILMS VIA PHYSICAL VAPOR DEPOSITION Hyuncheol Jeong, Craig Arnold, Rodney Priestley Thin-film growth via physical vapor deposition (PVD) has been successfully exploited for the delicate control of film structure for molecular and atomic systems. The application of such a high-energetic process to polymeric film growth has been challenged by chemical degradation. However, recent development of Matrix Assisted Pulsed Laser Evaporation (MAPLE) technique opened up a way to deposit a variety of macromolecules in a PVD manner. Here, employing MAPLE technique to the growth of semicrystalline polymer thin films, we show the engineering of crystalline film morphology can be achieved via manipulation of substrate temperature. This is accomplished by exploiting temperature effect on crystallization kinetics of polymers. During the slow film growth crystallization can either be permitted or suppressed, and crystal thickness can be tuned via temperature modulation. In addition, we report that the crystallinity of polymer thin films may be significantly altered with deposition temperature in MAPLE processing. We expect that this ability to manipulate crystallization kinetics during polymeric film growth will open the possibility to engineer structure in thin film polymeric-based devices in ways that are difficult by other means. [Preview Abstract] |
Tuesday, March 15, 2016 10:48AM - 11:00AM |
E33.00013: Comparing the Bending Stiffness Measurements of Brittle Paper Andrea Hall, Molly McGath, Patricia McGuiggan It has been estimated that one third of the paper materials in libraries are too brittle to handle. A typical paper sheet is comprised of semi-rigid cellulose fibers that are more than ten times longer than the sheet thickness and can be considered a two dimensional random fiber network. The main pathways of degradation, acid-catalyzed hydrolysis and oxidation, cause depolymerization of the cellulose chains and breaking of the intrafiber bonds. Conventional mechanical measurements of aged paper are destructive and often too severe to understand the true extent of deterioration. By comparing the roll test, folding endurance tests, tensile tests and stiffness tests of naturally aged papers with varying amounts of brittleness, we intend to show the limits of each test and relate the state of the paper degradation to the mechanical test results. [Preview Abstract] |
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