Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session X29: Wetting & Dewetting-Stability of Polymer Thin Films |
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Sponsoring Units: DPOLY Chair: John Dutcher, University of Guelph Room: LACC 504 |
Friday, March 25, 2005 8:00AM - 8:12AM |
X29.00001: Drop-on-Demand Based Inkjet Printing for Making Patterned Surfaces with Controlled Surface Wetting. Amit Sankhe, Michael Kilbey Combining inkjet printing and atom-transfer radical polymerization (ATRP) provides a facile and versatile method for producing patterned surfaces that may serve as platforms for a variety of applications. We report the use of drop-on-demand technology to print chemically-graded interfacial layers or simple patterns that allow surface wetting characteristics to be tailored. Inkjet printing can be coupled with surface-confined ATRP to amplify the printed patterns and gradients. Chemically graded monolayers prepared by inkjet printing dodecanethiol and backfilling with 11-mercaptoundecanol showed continuous changes in water contact angle along the gradient. These samples also exhibited a distinct change in the intensity of methyl group and C-O stretching modes along the gradient. Graded or patterned polymer layers were produced by growing, with ATRP, tethered poly(methyl methacrylate) (PMMA) layers from gradient or patterned printed monolayers that contained an ATRP initiator. Atomic force microscopy and optical microscopy confirmed that the PMMA layers amplified the underlying printed layer with fidelity that agrees with the printer resolution. [Preview Abstract] |
Friday, March 25, 2005 8:12AM - 8:24AM |
X29.00002: Role of molecular motion in the Dewetting of Diblock Copolymer Thin Films Gang Cheng, Dvora Perahia The dewetting kinetics of polystyrene-block-polyisoprene (PS-PI) thin films on silicon has been studied as a function of temperature using Atomic Force Microscopy (AFM). Thin films of $\sim $100{\AA} were annealed above the glass transition temperature for the separate blocks and the surface morphology was followed as a function of annealing time. It has been found that the surface morphology of dewetted samples resembles a bicontinuous spinodal-like pattern. The dominant length scale, characterized by the inverse of the wave vector q* shows apparent power law dependence on annealing time. Several distinctive stages with of power law dependencies are observed at each temperature. Current theory such as Cahn-Hilliard equation that describes spinodal kinetics of binary alloys and thin film equation that has been adapted from liquid theory cannot predict the observed various stages of power law dependence with increasing time. The differences observed as a function of temperature show that the molecular motions play an important role in the dewetting kinetics. [Preview Abstract] |
Friday, March 25, 2005 8:24AM - 8:36AM |
X29.00003: Probing molecular mobility in freely-standing polystyrene films using hole growth Connie Roth, John Dutcher Hole growth provides an elegant technique for measuring the viscoelastic response of a thin film of material to the constant stress applied at the edge of the growing hole due to surface tension. The time-dependent deformation of the material has been studied in detail for thin freely-standing polystyrene (PS) films using both optical microscopy and a unique differential pressure experiment (DPE).~We present shear strain rate dependent viscosity results that span reductions in viscosity by 8 orders of magnitude, and we discuss our findings in the context of the large glass transition temperature reductions observed for very thin freely-standing PS films. [Preview Abstract] |
Friday, March 25, 2005 8:36AM - 8:48AM |
X29.00004: Instability of Polymer Films on a Polymer Substrate with Interfacial Heterogeneities Bin Wei, Peter G. Lam, Jan Genzer, Richard J. Spontak Incorporation of an AB diblock copolymer into a thin polymer film is found to alter the rate by which the film dewets from an immiscible polymer substrate. Films with little or no copolymer dewet from the substrate by nucleation and growth of circular holes. Increasing the AB content, however, induces dewetting by a different mechanism. The copolymer molecules migrate to the polymer/polymer interface to form a chemically and structurally heterogeneous interface, which promotes spinodal-like dewetting. In this case, the surface of the top layer uniformly perforates and subsequently roughens. At higher AB concentration, dewetting is initially inhibited ($>$10 h) but eventually proceeds via the formation of flower-shaped holes due presumably to copolymer pinning of the top layer. AFM of the polymer/polymer interface reveals the existence of laterally organized copolymer nanostructural features. On the basis of these observations, we have constructed a dewetting rate/mechanism diagram. [Preview Abstract] |
Friday, March 25, 2005 8:48AM - 9:00AM |
X29.00005: Suppression of Dewetting in Polystyrene Thin Films by Polymer Nanoparticles Hongxia Feng, R. M. Briber, Victor Y. Lee, Robert D. Miller, Ho-Cheol Kim The addition of a small amount of polymer nanoparticles (based on star polymers) to linear polystyrene (PS) can lead to inhibition of dewetting in thin (30 nm) spun-cast films. The star polymers have polystyrene-benzocyclobutene random copolymer arms which can undergo \textit{intra}-molecular crosslinking to form compact nanoparticles. Differences in the dewetting behavior between films containing the uncrosslinked and crosslinked star polymers suggests that the system has tunable dewetting behavior, depending on the specifics of the added star molecules. The suppression of dewetting may be related to a segregation layer of nanoparticles at the polymer/silicon interface as observed by neutron reflectivity (NR) in as-cast nanoparticle-containing films. Small angle neutron scattering data shows the nanoparticles are miscible with the PS, implying that the segregation is not due to bulk phase separation. Characterization of the hole morphology in the films by atomic force microscopy indicates there is a layer of nanoparticles left behind on the Si substrate inside the hole, which should be related to the segregation layer observed by NR. [Preview Abstract] |
Friday, March 25, 2005 9:00AM - 9:12AM |
X29.00006: `Finger-like' instabilities of the moving rim during the dewetting of thin polymers films Sylvain Gabriele, Pascal Damman Due to its wide applications field, understanding and controlling the mechanisms that lead to thin films instability is of outstanding interest. In this communication, we focus on a physical process arising at the moving rim during the dewetting of viscoelastic fluids. In the purely viscous regime, the three-phase contact line undulates and finally leads to droplets formation. This transversal instability of the rim is clearly reminiscent of the Rayleigh-Plateau instability. This study gives a special attention to the role of the width of the rim and of the dewetting velocity to understand the early stages of the instability and its characteristic wavelength. In fact, the droplets formation induces a self-regulation of the width of the rim which provides a faster way for the fluid to dewet the substrate. [Preview Abstract] |
Friday, March 25, 2005 9:12AM - 9:24AM |
X29.00007: Molecular Visualization of the Spreading Process Hui Xu, David Shirvanyants, Kathryn Beers, Krzysztof Matyjaszewski, Michael Rubinstein, Sergei Sheiko We carried out atomic force microscopy studies of a polymer melt spontaneously spreading on a solid substrate with molecular resolution. Along with the position of the contact line, AFM enabled clear visualization of the molecules at every stage of the spreading process. Thus, one obtained direct information about (i) the position of the center of mass, (ii) orientation, and (iii) the local curvature for every individual molecule in its unique environment. Three characteristic rates, i.e. the spreading rate of the precursor film $D_{spread}$=(3.9$\pm $0.2)$\times $10$^ {3}$ nm$^{2}$/s, the flow-induced diffusion rate of molecules within the film $D_{induced}$=1.3$\pm $0.1 nm$^{2}$/s, and the thermal diffusion coefficient of single molecules $D_{therm}\le $0.10$\pm $0.03 nm$^{2}$/s, were independently measured. Since $D_{spread}>>D_{induced}$, plug flow of polymer chains was identified as the main mass-transport mechanism of spreading with insignificant contribution from the molecular diffusion. [Preview Abstract] |
Friday, March 25, 2005 9:24AM - 9:36AM |
X29.00008: Morphological stability of thin film PS/TMPC mixtures on SiOx-Si substrates Jamie Kropka, Peter Green Thin homopolymer liquid films may exist in a morphologically unstable or metastable state. The morphological stability is determined by the nature of long-and short-range intermolecular forces. For the case of PS on SiO$_{x}$-Si substrates ($\sim $2 nm oxide layer), the potential is predicted to exhibit a minimum at thicknesses, h*$\sim $2 nm, and a maximum at larger h. The maximum represents a barrier to nucleation of holes. The addition of TMPC, with which PS is miscible, stabilizes the mixture at sufficiently large TMPC concentrations. This stabilization is associated with an increase of the energy barrier to nucleation and a decrease in the magnitude of the potential minimum. These effects are implied by the film morphology through measurements of thickness uniformity, wetting layer thickness (h*), and contact angle. The changes in the interface potential described by the morphology are dictated by the change in long- range (VdW) forces alone. These results are discussed in light of morphological instabilities observed in other thin film polymer-polymer mixtures. [Preview Abstract] |
Friday, March 25, 2005 9:36AM - 9:48AM |
X29.00009: Dynamics of thin liquid polystyrene films Hyunjung Kim, Young Joo Lee, Heeju Lee, Zhang Jiang, S. K. Sinha, Xuesong Jiao, L. B. Lurio, A. Ruehm, K. Shin, C. Li, M. Rafailovich We have applied x-ray photon correlation spectroscopy (XPCS) to study the dynamics of surface fluctuations in thin supported polystyrene films. Film thicknesses, ranging from four times the radius of gyration (Rg) to two times Rg, were used in this study. We found different behaviors in the relaxation times as a function of wave vector with different molecular weight. The observed behavior shows a deviation from the conventional capillary wave predictions. The analysis will be discussed in terms of surface tension, viscosity and effective interactions with the substrate. [Preview Abstract] |
Friday, March 25, 2005 9:48AM - 10:00AM |
X29.00010: Time Evolution Study on the Spinodal Dewetting of Polymer Films Yong Jian Wang, Fengchao Xie, Ophelia K.C. Tsui Liquid films deposited on non-wettable substrate surfaces are unstable and may rupture spontaneously into liquid droplets. It has been suggested that the initial rupturing process is similar to that of spinodal decomposition. We used atomic force microscopy to investigate the time evolution of the spinodal dewetting of polystyrene (PS) thin films on oxide- coated silicon, which enables us to measure $\Gamma (\lambda )$, the spectroscopic initial rupturing rate of the films. We found that $\Gamma (\lambda )$ demonstrates a peak at a wavelength, $\lambda _{max}$, and falls to zero either when $\lambda $ increases towards $\infty$ or decreases towards $\surd $2/2 $\lambda _{max}$. The data indeed fit well to $\Gamma (\lambda ) =\Gamma (\lambda _{\max } )(2\lambda ^{-2}_{max}\lambda ^{-2} -\lambda ^{-4})\lambda _{\max }^4 $, expected from the result of Cahn for the spinodal decomposition rate of phase separating binary mixtures. [Preview Abstract] |
Friday, March 25, 2005 10:00AM - 10:12AM |
X29.00011: A Generalized Approach to Surface Modification using Random Copolymers Du Yeol Ryu, Kyusoon Shin, Eric Drockenmuller, Craig Hawker, Thomas Russell Interfacial interactions underpin phenomena ranging from adhesion to surface wetting, and are key for controlling the orientation of structures in thin films. However, modifying interfacial interactions has required either surface specific chemistries, the deposition of monolayers, or the chemical attachment of materials to a surface. While these processes are effective, they are not general and must be optimized for each surface. Here, an approach to surface modification is described that is independent of the nature of the underlying substrate and replaces these traditional approaches with an ultra-thin crosslinkable film of a random copolymer. Specifically, thin films of benzocyclobutene (BCB)- functionalized random copolymers of styrene and methyl methacrylate were thermally-crosslinked on Au, Al, Si, SiOx, and even polymer surfaces to produce robust coatings with controlled thickness where the composition of the random copolymer precisely defines the surface energy. This very simple, rapid procedure represents a substantial improvement in controlling interfacial interactions in a robust manner that can be used on any surface leading to a process that can easily be implemented and adapted to other systems. [Preview Abstract] |
Friday, March 25, 2005 10:12AM - 10:24AM |
X29.00012: Structural evolution of thiol-capped gold nanoparticle monolayers undergoing controlled nanowetting Diego Pontoni, Kyle Alvine, Antonio Checco, Oleg Gang, Ben Ocko, Peter Pershan, Francesco Stellacci Monolayers of thiol-capped gold nanoparticles characterized by a poydisperse/bimodal size distribution were prepared on silicon substrates and were subsequently wet by nano-thin liquid layers of solvents of varying quality. The structural changes were probed in-situ by synchrotron X-ray surface scattering. It is found that thin solvent films of thickness comparable to the nanoparticle size tend to improve the monolayer uniformity regardless of the quality of the solvent. On the other hand, thick films dissolve the monolayer if the liquid is a good solvent, while induce nanoparticle aggregation in the case of a bad solvent. The interplay of size-selective segregation and nanoparticle aggregation overcomes the intrinsically large polydispersity and leads to the formation of oriented colloidal crystallites. [Preview Abstract] |
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