Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session U30: Polymer Thin Films: General |
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Sponsoring Units: DPOLY Chair: Azar Alizadeh, GE Room: LACC 505 |
Thursday, March 24, 2005 8:00AM - 8:12AM |
U30.00001: Rheological Response of Ultrathin Polymer Films Gregory McKenna, Paul O'Connell A novel microbubble inflation experiment has been developed for the purpose of measuring the biaxial creep compliance response of ultrathin polymer films. Here we have performed experiments on two polymeric films, poly(vinyl acetate) (PVAC) and polystyrene (PS) having thicknesses ranging from 25 to 100 nm. Three findings come from these studies. 1) PVAc films show a segmental dynamics that is quantitatively the same as in the bulk with no reduction in the glass temperature T$_{g}$ for even the thinnest films; 2) early results suggest that for the thinnest PS films the T$_{g}$ decreases by up to 30 $^{o}$C which demonstrates dramatically and quantitatively that the segmental dynamics of nanometer thick polymer films do not follow a universal behavior, i.e., the results seem to depend on chemical structure; 3) for both polymers we find that the creep compliance increases from the glassy value to a rubbery plateau that is greatly reduced from the bulk behavior, i.e., the stiffness is increased. This result suggests that entanglements in ultra thin films are much more effective than they are in the bulk---perhaps because of the two-dimensional aspect of the film. [Preview Abstract] |
Thursday, March 24, 2005 8:12AM - 8:24AM |
U30.00002: Direct measurement of the counterion distribution within swollen polyelectrolyte films Vivek Prabhu, Bryan Vogt, Wen-li Wu, Jack Douglas, Eric Lin, Sushil Satija, Dario Goldfarb, Hiroshi Ito A depth profile of the counterion concentration within thin polyelectrolyte films was measured \textit{in-situ} using contrast variant specular neutron reflectivity to characterize the initial swelling stage of the film dissolution. We find a substantial counterion depletion near the substrate and an enrichment near the periphery of the film extending into the solution. These observations challenge our understanding of the charge distribution in polyelectrolyte films and are important for understanding film dissolution in medical and technological applications. [Preview Abstract] |
Thursday, March 24, 2005 8:24AM - 8:36AM |
U30.00003: Crumpling and uncrumpling of thin polymer films Megan Juszkiewicz, Narayanan Menon, Yao Lin, T.P. Russell With a view to study the forced crumpling of thin elastic sheets, we have developed a procedure for creating thin copolymer films of thickness, $h \quad \sim $ 50nm and lateral dimensions $X \quad \sim $1cm x 1cm. This geometry yields an aspect ratio, $X/h \quad \sim $ 2x10$^{5}$, much closer to the theoretical idealization of infinitely thin sheets than can be obtained with macroscopic sheets. Even though the films are exceedingly thin, they are robust enough that they can be mechanically manipulated without tearing. The films are crumpled by forcing them into small volumes. The copolymer is tagged with a fluorescent dye that allows us to study the three-dimensional crumpled structure of the film by confocal microscopy. The film can then be uncrumpled at an oil-water interface to study the statistics of the plastic creases created in the course of the crumpling process. We will also show data on the kinetics of uncrumpling - a little-studied process with many promising applications. We acknowledge support from NSF-DMR 0305936 and the REU program at the polymer science MRSEC at UMass-Amherst [Preview Abstract] |
Thursday, March 24, 2005 8:36AM - 8:48AM |
U30.00004: Grain Structure in Block Copolymer Thin Films Studied by Guided Wave Depolarized Light Scattering Jeffrey Wilbur, Amish Patel, David Durkee, Rachel Segalman, Nitash Balsara, Bruce Garetz, Maurice Newstein, Alexander Liddle A new optical technique for characterizing the grain structure of ordered block copolymer thin films has been developed. The technique is an adaptation of previous work wherein polarized light was used to characterize the grain structure in bulk block copolymer samples. Thin films of a cylindrically ordered poly(alpha methyl styrene-\textit{block}-isoprene) copolymer were prepared on flat fused silica substrates. A plane-polarized laser beam was coupled into and out of a transverse magnetic (TM) mode of each film, which acts as planar waveguide. The polarization of some of the incident light changes due to encounters with randomly oriented optically anisotropic grains. This results in the coupling of light into transverse electric (TE) modes in the sample. We show that the TE intensity of samples with well-developed grain structure is significantly larger than that obtained from samples with poorly developed grain structure. [Preview Abstract] |
Thursday, March 24, 2005 8:48AM - 9:00AM |
U30.00005: Swelling and surface modification of ultrathin chitosan films Chris Murray, Oleg Stukalov, Amy Jacina, John Dutcher Chitosan is a biodegradable polysaccharide derived from seashell waste products. The high water absorbency and biocompatibility of chitosan have enabled its use as a hydrogel in specialty biomedical applications. We present the results of several experiments focused on characterizing properties of ultrathin films of chitosan critical to their use in techniques such as wound dressings, medical implants and drug delivery systems. Uniform thin films with thicknesses of 15 to 600 nm and rms roughness of the order of 1 nm were prepared using techniques previously developed in our research group. The swelling of these films in the presence of high humidity has been characterized using reflection ellipsometry, atomic force microscopy and quartz crystal microbalance techniques. The effects of exposure to elevated temperature and UV/ozone (a common surface modification technique) on the surface properties such as hydrophobicity are described. [Preview Abstract] |
Thursday, March 24, 2005 9:00AM - 9:12AM |
U30.00006: Surface Tension Driven Laser Lithography of Thin Polymer Films John M. Hudson, Michael V. Massa, Kari Dalnoki-Veress, John S. Preston, An-Chang Shi We have developed a technique for the non-destructive laser lithography of supported polymer films. Using a focused laser beam we induce a sharp thermal gradient within the film, which leads to a variation in surface tension across the free surface. As a result of the gradient in the surface tension, material flows away from the center of the beam via the thermo- capillary effect. This non-destructive process can be used to rapidly write trenches and patterns in polymer films, with fine control over the spatial dimensions and features (width, depth, etc.). The experimental results will be discussed in the context of a linearised model which predicts the topography of the feature as a function of experimental parameters. [Preview Abstract] |
Thursday, March 24, 2005 9:12AM - 9:24AM |
U30.00007: Photoinduced Trans-Cis Isomerization of Azobenzene Probes Tagged to Polystyrene in Thin and Ultrathin Films Yohei Tateishi, Keiji Tanaka, Toshihiko Nagamura For the last decade, molecular motion in confined polymer systems have been extensively studied. In the most studied case, polystyrene (PS), a conclusion obtained is that glass transition temperature in the PS thin and ultrathin films is lower than the corresponding bulk value. Although why the enhanced mobility must be manifested in the thin and ultrathin states is far from clear on molecular level for the moment, it is no wonder the presence of the free surface is one of responsible factors in the active molecular motion. In this study, photoinduced transcis isomerization of azobenzene chromophores in PS films was examined as a function of film thickness. The photoisomerization was composed of fast and slow modes. The fractional amount of the fast mode increased with decreasing thickness, implying that an effect of the surface becomes remarkable with decreasing thickness. Finally, local free volume and molecular motion with a relatively small scale in the surface region were discussed. Such enables us to gain access to information about hierarchical molecular motion in the surface region of polymer films. [Preview Abstract] |
Thursday, March 24, 2005 9:24AM - 9:36AM |
U30.00008: Single chain structure in thin polymer films: corrections to Flory's and Silverberg's hypotheses A. Cavallo, M. M\"uller, J.P. Wittmer, K. Binder Conformational properties of polymer melts confined between two hard structureless walls are investigated by the bond-fluctuation model.Chain extension,bond-bond correlation function and structure factor are computed and compared with recent theoretical approaches attempting to go beyond the Flory's and Silverberg's hypotheses. We demonstrate that for ultrathin films the chain size parallel to the walls diverges logarithmically, $R^{2}/2N \approx b^{2}+d\log N$ with $c\sim 1/H$.The corresponding bond-bond correlation function decreases like a power law $C(s)=d/s^{\omega}$,being $s$ the curvilinear distance between bonds and $\omega=1$.Upon increasing $H$ we find--at variance with Flory--the bulk exponent $\omega=3/2$ and,more importantly,a strongly decreasing amplitude $d(H)$ that gives direct evidence of an {\em enhanced} self-interaction of chain segments reflected at the walls.Systematic deviations from the Kratky plateau as a function of $H$ are found for the single chain form factor parallel to the walls in agreement with the {\em non-monotoneous} behavior predicted by theory. For large $H$ the deviations are linear with the wave vector $q$ but very weak.In contrast,for thin films, very strong corrections are found (albeit logarithmic in $q$) suggesting a possible experimental verification of our results. [Preview Abstract] |
Thursday, March 24, 2005 9:36AM - 9:48AM |
U30.00009: Photoemission studies of the photo-degraded polyethylene and polystyrene ultrathin films Orhan Kizilkaya, Masaki Ono, Eizi Morikawa The structural degradation induced by photon radiation in polyethylene and polystyrene ultrathin films has been investigated by ultraviolet photoemission spectroscopy (UPS) and molecular orbital (MO) calculations. The UPS results of pristine and degraded films show very good agreement with the calculated density of states by model MO calculations. The UPS and MO calculations reveal double bond conjugation and cross linking structure. Even though a considerable difference exists in the pristine chemical structure of both polymers, the photodegraded films exhibit very similar UPS spectrum, which indicates that the chemical structure of polyethylene and polystyrene films becomes indistinguishable after the prolong degradation. [Preview Abstract] |
Thursday, March 24, 2005 9:48AM - 10:00AM |
U30.00010: Crystalline polymer thin films characterized with NEXAFS dichroism microscopy H. Ade, T. Araki, Y. Zou, Y. Wang, M. Rafailovich, J. Sokolov The sensitivity of Near Edge X-ray Absorption Spectroscopy (NEXAFS) to bond orientation holds the promise that it can be used in a conjunction with an x-ray microscope to the study the organization of thin films of semi-crystalline polymers. Linear Medium Density Polyethylene (LMDPE) ($\rho $=0.95 g/cm$^{3})$ has been processed into thin films 20-60 nm thick, which were subsequently recrystallized, and characterized with the 5.3.2 x-ray microscope at the Advanced Light Source. Films thicker than 35 nm show spherulitic crystals with primarily edge-on lamellar orientation. Films 25 nm thick, show feather-like structures with significantly more flat-on lamellar character. The results show that improved sample handling should be implemented to allow for in-situ sample rotation. This would significantly improve the sensitivity to small title angles of the carbon-carbon backbone relative to the surface normal. [Preview Abstract] |
Thursday, March 24, 2005 10:00AM - 10:12AM |
U30.00011: Dynamics of Thin Film Mixtures from Incoherent Neutron Scattering Brian Besancon, Christopher Soles, Peter Green We examined the influence of film thickness on the segmental dynamics of thin film mixtures of polystyrene (PS) and tetramethylbisphenol-A polycarbonate (TMPC) on Si/SiOx substrates using incoherent elastic neutron scattering. By fitting the elastic scattering intensities to the Debye-Waller factor, a mean square atomic displacement (MSD) was calculated. The MSD was found to decrease with decreasing film thickness. Dissipative motions, such as those associated with the glass transition, are manifested as ``kinks'' in the curve of elastic scattered intensity (or MSD) versus temperature. The glass transition temperature was determined to decrease with decreasing film thickness despite the decrease in the segmental mobility with decreasing film thickness. The values of Tg extracted from the neutron scattering are in quantitative agreement with prior Tg measurements made using ellipsometry. These results are examined in light of existing models on the thin film glass transition. [Preview Abstract] |
Thursday, March 24, 2005 10:12AM - 10:24AM |
U30.00012: Diffuse X-ray Scattering from Polystyrene Films Xuesong Jiao, Jarrett Stark, Laurence Lurio, Suresh Narayanan, Alec Sandy, Zhang Jiang, Sinha Sunil The diffuse x-ray scattering from a series of thin polystyrene (PS) films spun cast onto Si substrates has been measured. A standing wave technique was used to decompose the measured x-ray scattering into the contribution from the surfaces of the film and the contribution from density fluctuations in the films interior. The scattering from the interior yields the compressibility of the film. For thick films (100 nm) the compressibility is found to equal the bulk value. For thinner films the compressibility is increases with decreasing thickness up to 20{\%} for the thinnest film measured (35 nm). Additional diffuse scattering was found to originate from both the top and bottom interfaces. When the contribution from surface capillary waves is taken into account there is a residual scattering, which is of similar magnitude at each interface. We attribute this scattering to a near surface region in the polymer where there is incomplete chain interpenetration. [Preview Abstract] |
Thursday, March 24, 2005 10:24AM - 10:36AM |
U30.00013: Qualitative Discrepancy Between Motion on Different Length Scales in Thin Polymer Films Zahra Fakhraai, Girjesh Dubey, James A. Forrest Ellipsometry is used to measure the interface healing in two- layer polystyrene films at different annealing temperatures. Since the interface healing involves center of mass motion, it serves as a probe of chain diffusion in thin PS films. Using an appropriate model, the time constant of the interface healing can be obtained. The results indicate that at temperatures above the glass transition temperature, as the thickness of layers is decreased, the time constant of interface healing increases, showing slower chain motion. Ellipsometry is also used to measure the glass transition temperature of the same films. Although the chain motion is slower in these films, the Tg reduction indicates enhanced dynamics. This study shows than not all measures of dynamics can be used to determine the Tg. [Preview Abstract] |
Thursday, March 24, 2005 10:36AM - 10:48AM |
U30.00014: In-Situ Hot Stage Atomic Force Microscopy Study of Poly(E-Caprolactone) Crystal Growth in Ultrathin Films Robert E. Prud'homme, Vincent H. Mareau Morphologies, growth rates and melting of isothermally crystallized ultrathin (200 to 1 nm) poly(e-caprolactone) (PCL) films have been investigated in real-time by atomic force microscopy. The flat-on orientation of the lamellar crystals relative to the substrate was determined by electron diffraction. The truncated lozenge shape PCL crystals observed at low undercooling become distorted for films of thicknesses equal or thinner than the lamellar thickness, which depends on the crystallization temperature but not on the initial film thickness. The melting behavior of distorted crystals differs from that of undistorted ones, and their growth is slower and non-linear. The crystal growth rate decreases greatly with the film thickness. All these observations are discussed in terms of the diffusion of the polymer chains from the melt to the crystal growth front. [Preview Abstract] |
Thursday, March 24, 2005 10:48AM - 11:00AM |
U30.00015: Polymer Crystallization in Ultrathin Films Alan Esker, Suolong Ni, Bingbing Li, Melinda Ferguson-McPherson, John Morris Confinement of a polymer to a thin film can dramatically alter the morphology and crystallinity. In this study, Brewster Angle Microscopy (BAM) is used to follow the dendritic crystallization of poly ($\varepsilon $-caprolactone) in Langmuir monolayers at the air/water interface. In a separate study, atomic force microscopy (AFM) and reflection absorption infrared spectroscopy (RAIRS) on Langmuir-Blodgett (LB) films show poly($_{L}$-lactic acid) form nearly 100{\%} crystalline single chain helices. These studies identify two model systems for studying crystallization and enzymatic degradation in ultrathin systems. [Preview Abstract] |
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