Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Q20: Thin Films and Adhesion I |
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Sponsoring Units: DPOLY Chair: Chris Stafford, National Institute of Standards and Technology Room: 321 |
Wednesday, March 18, 2009 11:15AM - 11:27AM |
Q20.00001: Capillary wrinkling of a floating sheet under differential surface tension Jiangshui Huang, Wim H. de Jeu, Narayanan Menon, Thomas P. Russell We have previously studied the radial wrinkling of a thin polymer film floating on the surface of water under the capillary force exerted by a drop of water placed on its surface. Here, the same surface tension both sets the radial stress in the unperturbed film as well as the source of the perturbation that leads to the wrinkling instability. We now report the effect on the wrinkling instability of a differential surface tension by using fluids with different surface tensions for the liquid the film is floating on and the drop put on the film. We use both surfactants and a variety of pure liquids to control the surface tension of water. When the base radial stress of the the films floating was decreased, the length of the wrinkles increased, but the number of wrinkles decreased. \\[3pt] [1] Full reference here. Science 317, 650(2007) [Preview Abstract] |
Wednesday, March 18, 2009 11:27AM - 11:39AM |
Q20.00002: Effect of Long-chain Branching on Surface Dynamics of Polymer Films Mark D. Foster, Shih-fan Wang, Jae Sik Lee, Sewoo Yang, Zhang Jiang, Suresh Narayanan, David Wu Thermally stimulated fluctuations of the surface of films of branched polystyrene chains have been studied using x-ray photon correlation spectroscopy (XPCS), a recently-developed technique that has already been applied to study the surfaces of melts of linear polystyrene chains. Surface relaxations of films of branched chains are faster than are those of films of linear analogs. However, the Tg's of the branched molecules are also lower. The variation in surface relaxation time as a function of scattering vector can be described well by a continuum hydrodynamic theory of thermally stimulated capillary waves with a nonslip boundary condition. However, the film viscosities inferred from fits of the theory to the data differ markedly from viscosities from bulk measurements. Acknowledgements: NSF support (CBET 0730692) [Preview Abstract] |
Wednesday, March 18, 2009 11:39AM - 11:51AM |
Q20.00003: Entanglement swelling in polymer glasses Joshua D. McGraw, Kari Dalnoki-Veress A polymer system in which the chains are much longer than the entanglement molecular weight, $M_{+} \gg M_{e}$, is well entangled. When a thin, glassy polymer film is uniaxially strained, deformations which are almost visible to the naked eye called crazes may be formed. Measuring volume fractions of deformed to undeformed regions provides a method by which entanglement densities of similar systems can be compared\footnote{ACM Yang, EJ Kramer, CC Kuo, SL Phoenix, \emph{Macromolecules} \textbf{19} 2020 (1986)}. We present results of deformation experiments, probed using atomic force microscopy, in which well entangled polystyrene networks have been diluted with various weight fractions of polystyrene with molecular weight in the vicinity of $M_{e}$. Upon dilution the system assumes an effective reduction in the entanglement density. The effective entanglement density is predicted by a conceptually simple model with $M_e$ as the only free parameter. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q20.00004: Microstructure Evolution during Solvent Evaporation from Thin Film Polymer Mixtures Nigel Clarke, Mireille Souche, Gavin Buxton We present simulations of the phase separation dynamics in a thin film polymer blend solution subject to solvent evaporation [1]. If the upper and lower surfaces are neutral with respect to the different components, we find that as the solvent diffuses through the film, and evaporates from the surface, phase separation becomes energetically favourable progressively throughout the film. This produces an ordering front which propagates through the film and leaves an ordered lateral morphology in its wake. In order to understand microstructure evolution if the surface interactions are strong enough that the film initially separates into a two layers, we have perfomed a linear analysis of the Marangoni instability of a deformable interface between two fluid layers of finite depths, submitted to a gradient of solvent concentration induced by the evaporation [2]. Qualitative comparison with experimental observations of spin-coating processes of solution of two immiscible polymers are then performed, yielding satisfactory agreement.\\[0pt] [1] G. A. Buxton and N.Clarke, Europhysics Letters, 78, 56006, 2007.\\[0pt] [2] M. Souche and N. Clarke, European Physical Journal E, in press. [Preview Abstract] |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q20.00005: Dimension-Dependent Mechanical Properties of Pure and Antiplasticized Polymer Nanostructures Sean Delcambre, Robert Riggleman, Juan de Pablo, Paul Nealey Dense arrays of poly(methyl methacrylate) (PMMA) grating nanostructures 80 nm to 120 nm in pitch were fabricated by electron-beam and extreme ultraviolet interferometric lithography. During development and rinse drying, the nanostructures are subjected to capillary forces that are defined by the rinse fluid properties and spacing between adjacent structures. The applied capillary forces and structure aspect ratios were varied experimentally to induce structure collapse. By coupling nanostructure collapse data with continuum cantilever beam bending models, mechanical properties such as the elastic modulus and yield stress are determined. The elastic moduli of PMMA structures at this scale are observed to decrease with structure linewidth. This behavior is counteracted by the addition of a low molecular weight diluent, tris(2-chloropropyl) phosphate (TCPP). At concentrations up to 5 wt{\%}, TCPP acts as an antiplasticizing agent, decreasing the glass transition temperature while simultaneously increasing the elastic modulus. For a given applied capillary force, nanostructures containing 5 wt{\%} TCPP are observed to remain stable at aspect ratios up to 20{\%} higher than the pure material. [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q20.00006: The stiffening of ultrathin polymer films in the rubbery regime -- the relative contributions of bending, membrane stress and surface tension Paul O'Connell, Gregory McKenna A novel nano-bubble inflation technique has been developed which allows the determination of the absolute creep compliance of ultrathin polymer films as thin as 9 nm. Previous results have shown that the degree of reduction in Tg with film thickness is not universal, with PVAc showing no change in Tg down to 23nm while PS shows a significant reduction at thicknesses below approximately 80nm. Interestingly the rubbery plateau region for both materials shows a similar stiffening as the thickness is reduced. At low inflation pressures the film is dominated by the bending stiffness of the film while at higher pressure the film is under membrane conditions and the response is dominated by the biaxial stiffness of the film. In addition the film is subject to surface tension effects. Compliance data have been measured as a function of applied pressure and have been analyzed to determine the relative contribution to the response from these three modes. The results suggest that at sufficiently large deformations the bending contribution is small while the surface tension contribution varies depending on loading conditions, film thickness etc. However neither contribution is sufficient to account for the observed stiffening in the rubbery regime [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q20.00007: Molecular confinement and residual stress in ultrathin polymer films Arnold Yang, Gunter Reiter, Yi-Hsin Chang, Yi Chien The residual stress operative in thin films of a polymer (polystyrene) prepared by spin coating was determined from local elastic stress release induced by pinhole nucleation during dewetting instability. The measured stress was orders of magnitude greater than the capillary force and attributed to chain recoiling of the confined macromolecules. The entropy-driven stress was found to be small for thicker films but increase dramatically as film thickness became less than the unperturbed molecular dimensions. The chain conformations in these films can only be described by the Langevin, rather than Gaussian, statistic and the draw ratio was determined to be around 5, comparable to that in craze fibrils, for film thickness of 4 nm. The effects of spin speed, aging-induced relaxation, and molecular packing were investigated. The molecular processes during spin coating were proposed. In addition, conjugated polymers when squeezed into the molecular thicknesses were found to emit light with much enhanced efficiencies due to the large molecular deformation. [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q20.00008: Semifluorinated Polymers Confined at the Solid-Air Interface Umesh Shrestha, Stephen Clarson, Dvora Perahia Effective responsive layers should exhibit stability while retaining a dynamic mode that will allow reaction of the interface to external stimuli. Semifluorinated polymers have a potential for forming energy controlled responsive interfaces. Because of the high segregation between the fluorinated and protonated segments, well defined structures are induced at relatively short chains, retaining the capability to rearrange on short time scales. Fluorinated segments affect the interfacial energies as well as enhance thermal stability and controls the refractive index and dielectric properties. The present study investigates the interfacial response of poly trifluoro propylmethyl siloxane-polystyrene diblock copolymer (PTFPMS-PS) at volume fractions varying from 0.003 to 0.5 of fluorinated block, at the interface of oxidized silicon wafers. In all volume fractions we found that the air interface is fluorine rich and the solid surface in proton rich. Layering is detected across the films for all volume fractions. Upon annealing the layering is retained, however the interfacial compositions changes. [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q20.00009: Crosslink Density Variations with Process Conditions in Plasma Polymerized Photonic Films Somesh Peri, Mark Foster, Jesse Enlow, Hao Jiang, Timothy Bunning, Bulent Akgun, Sushil Satija, Charles Majkrzak The structures of plasma polymerized homopolymer octafluorocyclobutane (PP-OFCB) films made under different processing conditions were studied using x-ray reflectivity (XR) and neutron reflectivity (NR). The processing parameters varied were monomer feed location, plasma power, and pressure. Each dry film had a surface layer of thickness $\sim $20{\AA} and a thin layer of $\sim $10{\AA} thickness at the substrate in which the crosslink density was lower than in the bulk polymer film. The region of lower cross-link density at the film-air interface reflects the extent of a reaction zone that moves with the deposition and is responsible for dictating the width of interfaces that are formed when a layer of different precursor is deposited atop the first layer. Results from bilayer films support this view. Such a reaction zone is also seen for benzene and iron-containing plasma polymerized films. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q20.00010: Spatially Organized Polymer Films Prepared by Oblique Angle Polymerization Niranjan Malvadkar, Melik Demirel We developed a novel, bottom-up method to prepare nanostructured poly($p$-xylylene) (PPX) films called oblique angle polymerization (OAP). In OAP, the monomer vapor flux is directed at an angle ($<$ 10$^{o})$ to the substrate. The morphology of the film is influenced by the combination of nucleation, surface diffusion and geometrical self-shadowing. The final nanostructure consists of 40 $\times $ 10$^{6}$ /mm$^{2}$ obliquely aligned, quasi-periodic PPX nanowires on the substrate[1]. The nanostructure can be controlled by tuning the physical deposition parameters and/or the monomer chemistry. Functional materials prepared by depositing conformal metal layer on these nanostructured PPX films have opened new avenues of application in the areas of biodetection[2] and catalysis[3]. [1] Cetinkaya, M., Malvadkar, N., Demirel, M. J. Poly. Sci. B, 46, 640 (2008). [2] Kao, P., Malvadkar N., Wang, H., Allara, D., Demirel, M. Adv. Mat., 20, 3562 (2008). [3] Malvadkar, N., Park, S., MacDonald, M., Wang, H., Demirel, M. J. Power Sources, 182, 323 (2008). {\copyright} 2008 Elsevier [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q20.00011: Rubber Friction -A Molecular Picture Anish Kurian, Kumar Nanjundiah, Ali Dhinojwala Understanding the relationship between adhesion, friction and the interfacial structure has been of significant interest for many years. Most tribological experiments involve measuring friction and adhesion values to develop a molecular model to explain the macroscopic results. Here, we have used surface sensitive infrared-visible sum frequency generation spectroscopy (SFG) to study the interface between elastomer and solid surfaces. SFG is a second order nonlinear optical technique that provides information on the chemical identity; orientation and concentration of the molecules at the interface. We have designed a friction cell to probe the changes in the interfacial structure during sliding using a femto-second laser spectroscopy. These results will be presented. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q20.00012: In-situ Adhesion Measurements Utilizing Layer-by-layer Functionalized Surfaces Christopher M. Stafford, Adam J. Nolte, Jun Young Chung, Marlon L. Walker The adhesion between poly(dimethylsiloxane) (PDMS) hemispheres coated with layer-by-layer (LbL) assemblies of polyelectrolytes and rigid, planar substrates was investigated using Johnson, Kendall, and Roberts (JKR) contact mechanics. Measurements were performed against amine-functionalized glass slides both in air and in aqueous solutions of controlled pH. Despite the increased density of negatively charged carboxylate groups, LbL-functionalized PDMS exhibited lower adhesion due to the combined effects of increased surface roughness and the high Young's modulus of the coating. Measurements of coated PDMS in aqueous solutions revealed tunable adhesion behavior dominated by pH-mediated changes in the mechanical properties of the coating. Smoothing the surface of the LbL coatings by aqueous salt annealing led to a significant increase in adhesion. Our results suggest that LbL assembly can be an effective means of surface functionalization for in-situ adhesion measurements, but understanding and predicting the adhesion behavior requires comprehensive knowledge of the chemical, mechanical, and topological properties of the coating and how such properties change in response to the ambient environment. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q20.00013: Tethered Lubricant Films Based On Cross-linked Polydimethylsiloxane Lucas Landherr, Claude Cohen, Lynden Archer We report on the interfacial friction and wear properties of surface-tethered cross-linked polymer thin films. We show that thin, two-tiered films produced by covalently tethering polydimethylsiloxane (PDMS) networks to self-assembled monolayers manifest the lowest friction coefficient (mu = 0.0039) recorded for a dry lubricant film. Using a combination of lateral force microscopy, equilibrium swelling, and adhesion measurements we determine the physical processes responsible for these low friction coefficients. We also investigate the effect of free (unattached) and pendent polymer chains dispersed in thin PDMS network films on transient mechanical properties, interfacial friction, and wear characteristics. [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q20.00014: Effect of Adsorbed Films on Nanoscale Mechanical Contacts Shengfeng Cheng, Binquan Luan, Mark Robbins For surfaces exposed to ambient air, the presence of adsorbed molecules cannot generally be avoided. Molecular simulations are presented which show that the compliance of these adsorbed films can have a profound effect on the mechanical behavior of contacts. An adsorbed film of short chain molecules is equilibrated on a flat, elastic substrate. The film is then contacted by a non-adhesive spherical tip. The atomic scale structure of the tip is varied from amorphous to crystalline, since this has a substantial effect on contacts with clean substrates. Including adsorbed molecules reduces sensitivity to tip geometry, but introduces new effects. One is that the contact region is broadened dramatically, with measured contact radii increased from predictions of continuum theory by a constant shift. The variation of tip displacement and substrate deformation with normal load show a crossover between two regimes. At small loads, the effective elastic modulus is set by the soft adsorbed film, while at large loads the modulus is that of the substrate. Variations in friction with tip geometry are much smaller than for bare substrates and the friction rises linearly with load in almost all cases. [Preview Abstract] |
Wednesday, March 18, 2009 2:03PM - 2:15PM |
Q20.00015: The Promotion of Au Adhesion on Polymer Surfaces Using Polyhedral Oligomeric Silsequioxane. Chris Hughes, Brian Augustine, Alan Mo, Jonathan Wyrick, Bruno Caputo, Ethan Rosenthal The adhesion of Au on polymer surfaces is weak because of the inert nature of Au and the non-polarity of the hydrocarbon surface of the polymer. We seek to fabricate microfluidic devices in which vacuum deposited gold thin films will be used in electrical contacts and optical reflectors. Various fabrication steps involve the use of solvents which easily wash away the gold. To overcome this, we have explored the use of a thin layer of polyhedral oligomeric silsequioxane-methacrylate (POSS-MA) which is a nanocomposite having both polymer and inorganic silica glass characteristics. The POSS-MA is spun cast onto the surface of PMMA creating a film which is on the order of 100 nm thick. Au dots that are 1 mm in diameter were deposited onto both the virgin PMMA surface and the POSS-MA coated surface and the samples were covered with acetone, a known solvent for PMMA. Optical microscope video images of the dots revealed their delamination from the surface and image analysis was used to determine the time that it took for the dots to be undercut -- typically in the range of seconds to minutes. An obvious increase in the time required to undercut the Au was observed for the POSS-MA treated surface. A model explaining the improved adhesion will be discussed as will future plans for device fabrication. [Preview Abstract] |
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