Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session N24: Structure and Dynamics in Polymer Thin Films |
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Sponsoring Units: DPOLY Chair: Steve Granick, University of Illinois at Urbana Champaign Room: Baltimore Convention Center 321 |
Wednesday, March 15, 2006 8:00AM - 8:12AM |
N24.00001: Unique refractive index and thickness values for polymer films via ellipsometry Alan R. Esker, Ufuk Karabiyik, Min Mao, Sushil K. Satija In this study, elliposometry using multiple ambient media at Brewster's angle is used to determine unique values of refractive index and film thickness for polymer thin films. Measurements were made for polymer thin films on HF etched silicon wafers against air and water. Results obtained for poly(t-butyl acrylate), polystyrene, and trimethylsilyl cellulose confirm that the refractive indices agree well with the literature values and that the film thicknesses agree with values obtained from X-ray reflectivity. This technique provides a rapid unambiguous method for determining a film's thickness and refractive index for polymers. [Preview Abstract] |
Wednesday, March 15, 2006 8:12AM - 8:24AM |
N24.00002: Watching How Molecules Orient in a Surface Forces Apparatus, Using Confocal Raman Spectroscopy Shan Jiang, Minsu Kim, Sung Chul Bae, Steve Granick Much is known about surface forces, less about where they come from. This laboratory is engaged in direct vibrational spectroscopic measurements of how molecules orient in confined geometries. Regarding force measurements, PDMS (polydimethylsiloxane) was a model system for many years. In this study, we describe direct experiments using a novel version of confocal Raman spectroscopy. This experiment allows direct measurement of how the PDMS molecules orient under confinment as well as under subsequent shear. When the thickness of the fluid film is less than the unperturbed radius of gyration of the polymer, we obtain two novel findings: (a) linewidth analysis of peaks reveals that vibrational relaxation times are perturbed in this confined geometry; (b) orientation of the chain backbone is not everywhere the same within the molecularly-thin film; domains of various orientation are observed instead. [Preview Abstract] |
Wednesday, March 15, 2006 8:24AM - 8:36AM |
N24.00003: Polymer dynamics at local scales: origin of ripples formation Robert Szoszkiewicz, Takashi Okada, Enrico Gnecco, William King, Seth Marder, Elisa Riedo A plethora of nanowear patterns in polymers has been obtained by heating the polymers and scanning their surfaces with an atomic force microscope (AFM) tip (1, 2). These morphologies represent the nanoscale realization of aeolian ripples in sandy deserts and are similar to patterns obtained during evolution of surfaces during ion sputtering (3). By means of locally heated AFM probes we studied ripples on various polymer films. While the theory of aeolian ripples formation is very complicated (3), we show that the key morphological features in our results can be explained in terms of the elastic and diffusive properties of the polymer. From measurements of the ripple spacing we study the local dynamics of polymers in the vicinity to the glass transition. (1) B. Gotsmann and U. Durig, Langmuir 20, 1495 (2004) (2) R. H. Schmidt, G. Haugstad and W. L. Gladfelter, Langmuir 15, 317 (1999) (3) T. Aste and U. Valbusa, New Journal of Physics 7, 122 (2005). [Preview Abstract] |
Wednesday, March 15, 2006 8:36AM - 8:48AM |
N24.00004: Probing surface relaxation of polystyrene films using gold nano-particles Zahra Fakhraai, James A Forrest Polymer thin films are usually used to probe the confinement effects on the dynamics of glass former materials. Many studies show that the glass transition temperature of thin polymer films is decreased below bulk Tg for very thin films. There is evidence that this Tg reduction is due to the existence of a more mobile region near the surface of the film. However, it is very hard to directly measure the existence of this layer, let alone more quantitative aspects such as thickness or viscoelastic properties. In this work we probe the near surface behaviour of thin polystyrene films using a nanorheological technique. To do this, we uniformly distribute gold nanoparticles on the surface. A particle's motion on the surface is then driven by surface capillary waves. The motion of nanoparticles is detected by dynamic light scattering. Since the particles only interact with the surface region (determined independently using AFM), these experiments tell us about the viscoelastic properties of the near surface region. [Preview Abstract] |
Wednesday, March 15, 2006 8:48AM - 9:00AM |
N24.00005: Grazing-incidence x-ray scattering studies on surface melting in ultrathin polymer films Tadanori Koga, Y. Wang, M. Rafailovich, J. Sokolov, A. Tikhonov, D. Schultz, M. Lee, M. Tolan The aim of this study is to investigate surface crystal structures formed in ultrathin (thickness below 100 nm) polymer films by using surface-sensitive x-ray scattering techniques. This study was motivated by our current experimental finding that showed a drastic suppression (the decrease of $\sim $50\r{ }C) in the surface melting temperature (T$_{m})$ of ultrathin polymer films, which was determined as the onset of surface softening by using the shear modulation force microscopy (SMFM) method[1]. In order to clarify the relationship between the melting behavior and surface crystal structures, we integrated a variety of grazing-angle x-ray scattering techniques including reflectivity, diffuse scattering, grazing-incidence diffraction, and grazing-incidence small-angle scattering. As a result, we found that diffuse scattering, which is sensitive to surface roughness, drastically changed at T$_{m}$ determined from SMFM, while the surface crystallinity decreased with increasing temperature, but remained up to the bulk melting temperature. A model to explain the mechanism of the surface melting will be discussed. [1]Wang, Y. et al. Macromolecules, \textbf{37}, 3319 (2004). [Preview Abstract] |
Wednesday, March 15, 2006 9:00AM - 9:12AM |
N24.00006: The Tg-Nanoconfinement Effect and the Relaxation of Residual Stresses in Spin-Coated Films of Polystyrene and Styrene-Containing Copolymers: Characterization by Intrinsic Fluorescence. Manish K. Mundra, Christopher J. Ellison, Ross Behling, John M. Torkelson The glass transition temperatures (Tgs) of films of polystyrene (PS) and styrene (S)-methyl methacrylate (MMA) copolymers have been determined using intrinsic fluorescence from styrene units. The Tgs are measured by a break in the temperature dependence of fluorescence intensity measured upon cooling from the equilibrium liquid state. As the film thickness decreases below 50 nm, there is a substantial deviation in Tg from bulk Tg, with PS and high S-content copolymers exhibiting a reduction in Tg and high MMA-content copolymers exhibiting an increase in Tg. This is explained by a competition of free surface effects and the effects of attractive polymer-substrate interactions. As the intrinsic fluorescence is a combination of monomer and excimer fluorescence, it reflects the local conformational population. This is used to determine the conditions at which residual stresses induced by spin coating are relaxed away, leading to a steady-state conformational population and fluorescence signal independent of annealing time. Films must be heated to temperatures well above Tg (Tg + 30 K) for several minutes to achieve constant fluorescence independent of further annealing. Annealing for short times close to Tg is insufficient to achieve an equilibrium conformational population. [Preview Abstract] |
Wednesday, March 15, 2006 9:12AM - 9:24AM |
N24.00007: Dynamics of very thin polymer films on supported surface Chunhua Li, Hyunjung Kim, Jun Jiang, Clive Li, Tadanori Koga, Laurence Lurio, Sunil Sinha, Jonathan Sokolov, Miriam Rafailovich In this work, we study the effect of solid substrate surface on the viscosity of polymer thin films. We found that the viscosity of polymer thin film increased about two orders of magnitude near the solid substrate. Measurements performed on split layer substrates indicated that this layer was responsible for trapping the subsequent layers, and propagating the effect of the surface interactions to the chains without direct contacts to the surface. If this layer was applied prior to the rest of the film, it in fact screened the surface interactions and even caused auto-dewetting of the other layers in the film. These results will be discussed in terms of the ``two fluid'' hypothesis. This surface interaction was also found to induce the ``melt fracture'' of the polymer thin film during the dewetting process where we see the cracks in the less viscous bottom layer. [Preview Abstract] |
Wednesday, March 15, 2006 9:24AM - 9:36AM |
N24.00008: The Distribution of Tgs in Thin and Ultrathin Methacrylate-Based Polymer Films: Percolation of Free Surface and Interface Effects over Tens and Hundreds of Nanometers. Rodney D. Priestley, Manish K. Mundra, Perla Rittigstein, Linda J. Broadbelt, John M. Torkelson A multilayer/fluorescence method is used to measure the distribution of Tgs in poly(methyl methacrylate) (PMMA) films. The average Tg increases with decreasing total film thickness (h) below 90 nm. In bulk, bilayer films, the free surface layer exhibits a reduced Tg at a thickness of 30 nm or less. The Tg reduction at the free surface is a fraction of that in polystyrene (PS) films; a 14-nm-thick free surface layer exhibits a Tg reduction of $\sim $ 6 K in PMMA and $\sim $ 32 K in PS. The Tg increase observed in the substrate layer of a PMMA bilayer, bulk film exceeds the Tg reduction observed at the free surface and occurs over longer length scales. An amazing effect of confinement of the total multilayer film on the free surface layer Tg of PMMA has been observed. When h $<$ 250 nm, a 12-nm-thick free surface layer exhibits an increase in Tg with decreasing h, and the free surface layer Tg exceeds that of bulk Tg when h $<$ 90 nm. This is the first demonstration that a free surface layer can exhibit Tg $>$ bulk Tg and means that the perturbation of Tg dynamics at the substrate can percolate over hundreds of nanometers in PMMA films. These results will be contrasted with those of poly(isobutyl methacrylate) films which exhibit no average Tg-confinement effect. [Preview Abstract] |
Wednesday, March 15, 2006 9:36AM - 9:48AM |
N24.00009: Dynamics of Polymer Thin Film Mixtures Brian M. Besancon, Peter F. Green, Christopher L. Soles We examined the influence of film thickness and composition on the glass transition temperature ($T_{g})$ and mean square atomic displacements (MSD) of thin film mixtures of deuterated polystyrene (dPS) and tetramethyl bisphenol-A polycarbonate (TMPC) on Si/SiO$_{x}$ substrates using incoherent elastic neutron scattering (ICNS). The onset of dissipative motions, such as those associated with the glass transition and sub-Tg relaxations, are manifested as ``kinks'' in the curve of elastic intensity (or MSD) versus temperature. From the relevant kinks, the $T_{g}$ was determined as a function of composition and of film thickness. The dependence of the $T_{g}$ on film thickness exhibited qualitatively similar trends, at a given composition, as determined by the ICNS and ellipsometry measurements. However, with increasing PS content, the values of $T_{g}$ measured by INS were consistently larger then those measured by ellipsometry. These results are examined in light of existing models on the thin film glass transition and component blend dynamics. [Preview Abstract] |
Wednesday, March 15, 2006 9:48AM - 10:00AM |
N24.00010: Dynamics of Block copolymer films Hyunjung Kim, Heeju Lee, Young Joo Lee, Zhang Jiang, Sunil Sinha, Xuesong Jiao, Adrian Ruehm, S. G. J. Mochrie We have investigated the dynamics of thin block copolymer films of ploy(styrene)-b-poly(dimethylsiloxane) using X-ray Photon Correlation Spectroscopy (XPCS). The films were supported on Si substrates and measured at melt. The results are compared with the theory of overdamped thermal capillary waves on thin films. The lateral length scales examined were between 600 and 6000 nm. We selectively measured the dynamics from the surface and from the micelles by changing incident angles and found the different behaviors between them. [Preview Abstract] |
Wednesday, March 15, 2006 10:00AM - 10:12AM |
N24.00011: Conformational Anisotropy and the Glass Transition in Polymer Thin Films Folusho Oyerokun, Anna Cavallo, Marcus Mueller, Kenneth Schweizer A segmental level theory of the ideal kinetic glass transition, or dynamic crossover, temperature $\left( {T_c } \right)$ in confined polymer films has been developed. The theory is based on an anisotropic generalization of a coarse grained polymer mode coupling theory which utilizes conformational and thermodynamic information from anisotropic PRISM theory and computer simulations. Confinement is found to enhance the bulk compressibility and induce anisotropic segmental dynamics. For non-capped films (free standing or supported films on neutral substrates) the theory predicts suppression of $T_c $, with confinement. The underlying mechanism is reduction of the degree of coil interpenetration and intermolecular repulsive contacts due to segmental scale alignment and deformation. The predicted suppression of $T_c $ is nonuniversal and follows an inverse power law dependence on film thickness in reasonable agreement with experiments. For capped films simulations find a weak variation of the dimensionless compressibility with confinement suggesting little or no shift of $T_g $. [Preview Abstract] |
Wednesday, March 15, 2006 10:12AM - 10:24AM |
N24.00012: Radial Thickness Profiles of Spincoated Thickness Gradient Films Monika Michalek, Bernie Nickel, John Dutcher By dropping a polymer solution onto a spinning substrate at a position displaced from the axis of rotation, one can produce a film in which the thickness increases with increasing radial distance (thickness gradient film). Since each film contains a continuous range of thickness values, one can track subtle changes in the physical properties with film thickness by using a local probe of the film properties. We have used two such local probes, focused ellipsometry and atomic force microscopy, to measure the radial thickness profiles. We have also developed a simple, fluid flow model that describes the dependence of the polymer film thickness on radial distance from the axis of rotation. A detailed comparison between the measured and calculated radial thickness profiles will be discussed. [Preview Abstract] |
Wednesday, March 15, 2006 10:24AM - 10:36AM |
N24.00013: Internal Phase Separation Induces Dewetting in Multicomponent Polymer Films Hyun-joong Chung, Russell J. Composto, Kohji Ohno, Takeshi Fukuda Thin liquid films that dewet from their substrate are ubiquitous as demonstrated by the beading of paint on oily surface. Although most coatings contain more than one component, the dewetting mechanisms in multicomponent films are not understood. Using dPMMA:SAN (50:50) films (550 nm) with or without nanoparticles (NP), we demonstrate, for the first time, that the Laplace pressure induced by internal phase-separated structure is the driving force for roughening and rupture in polymer blend films. Three NP were investigated, namely NP$_{A}$, NP$_{B}$, and NP$_{C}$ which either partition into dPMMA or weakly and strongly segregate to the dPMMA/SAN interface, respectively. NP$_{B}$ are more effective than NP$_{A}$ at stabilizing the film, whereas NP$_{C}$ are able to prevent film rupture. Upon annealing, roughened films display a periodic, lacey structure, resembling patterns from spinodal dewetting. The fluctuation periodicity scales with roughness evolution as \textit{$\lambda $}$_{s} \quad \propto \quad R_{q}^{1/4}$ for neat blends and blends with NP$_{A}$, whereas the scaling breaks down for blends containing NP$_{B}$ and NP$_{C}$. These studies show that phase separation is responsible for film roughening. [Preview Abstract] |
Wednesday, March 15, 2006 10:36AM - 10:48AM |
N24.00014: Dewetting Morphology and Dynamics of Ordered Symmetric Block Copolymer Films: Stability of Nanoscopic Liquid Bilayers Matthew J. Farrar, Andrew B. Croll, Kari Dalnoki-Veress Symmetric diblock copolymers, which form lamella upon micro-phase separation, can have unique dewetting properties. In this experimental study we explore the effects of the microphase separation on the dewetting of three different systems. We begin with the dewetting of disordered thin poly(styrene)-b-poly(methyl methacrylate) films on poly(dymethyl siloxane) coated Silicon. In this case, the film is not allowed to relax to its lamellar state before dewetting begins. The complex interplay between dewetting and microphase separation leads to hole growth that appears dendritic and deviates dramatically from the conventional circular hole growth. In a second experiment, the thin films are arrange into their lamellar equilibrium before being transferred onto an unfavourable substrate, which initiates dewetting. On an unfavourable substrate, these films show remarkable stability. Holes that do form are cylindrical but grow at a much-reduced rate when compared to a homopolymer system. Finally, hole growth in free-standing ordered lamellar films is explored. Here we again see significant stability and extremely slow dynamics -- an ordered free-standing film is stable, or nearly stable, even though the liquid film is well above the glass transition temperature and only of order 1 lamella ($\sim $30nm) thick! [Preview Abstract] |
Wednesday, March 15, 2006 10:48AM - 11:00AM |
N24.00015: Interfacial segregation and micellization of hydrogen bonding copolymers Michelle Lefebvre, Murat Guvendiren, Monica Olvera de la Cruz, Kenneth Shull An AB diblock copolymer in which A and B have unfavorable interactions will segregate to an interface between A and B homopolymers. The driving force for segregation is increased if the B homopolymer is replaced by a C homopolymer and B and C have favorable interactions. When copolymer accumulates at the interface, the preferred interfacial curvature changes as a function of the copolymer composition. This change in curvature leads to a variety of possible morphologies, including micelles, swollen micelles, or inverted micelles. To examine this effect we use a model system where A is polystyrene (PS), B is poly(4-hydroxystyrene) (PHS), and C is poly(2-vinylpyridine) (PVP), and the PHS and PVP can undergo hydrogen bonding. We measure the interfacial segregation of PS- PHS copolymers at an interface between PS and PVP using dynamic secondary ion mass spectrometry. [Preview Abstract] |
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