Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session U17: Polymer Surfaces |
Hide Abstracts |
Sponsoring Units: DPOLY Chair: Vivek Prabhu, National Institute of Standards and Technology Room: Colorado Convention Center 102 |
Thursday, March 8, 2007 8:00AM - 8:12AM |
U17.00001: Forces between polyelectrolyte brushes in various ionic environments Matthew Tirrell End-tethered polyelectrolyte layers (``brushes'') shrink monotonically in response to addition of mono-valent salt, which also produces corresponding monotonic changes in the range of the repulsive normal forces exerted by such brushes. High swelling and very low frictional forces have been reported under low salt concentrations. A new pattern of behavior is demonstrated here via surface force measurement on polyelectrolyte brushes in the presence of multi-valent ionic interactions, introduced via tri-valent aluminum cations (Al$^{3+})$ or aggregates of cationic surfactants. Very low concentrations of added Al$^{3+ }$or surfactant produce much stronger shrinkage of the brush than does mono-valent salt. Normal forces become strongly attractive under these circumstances. Multi-valent interactions enable tuning of polyelectrolyte brush structure and properties over a wide range, from compact, stiff and sticky to swollen, soft and repulsive. [Preview Abstract] |
Thursday, March 8, 2007 8:12AM - 8:24AM |
U17.00002: Evolution of Polymer Brush Dynamics by X-ray Photon Correlation Spectroscopy. Pinar Akcora, Suresh Narayanan, Pappannan Thiyagarajan, Linda Schadler, Sanat Kumar We studied the dynamics of PS grafted silica particles dispersed in PS homopolymers of two molecular weights (44k and 150k) through x-ray photon correlation spectroscopy measurements. The particle motions were related to the traditional aging process and also to the wetting/dewetting behavior of polymer brushes controlled by changing the molecular weight of the matrix PS homopolymer. By increasing the particle concentration, relaxations slowed down in both wet and dry brushes. At 5{\%} particle concentration, relaxations were independent of the matrix molecular weight. We observed that particle dynamics represented the features of structural relaxation in wet brush case which was not related directly to the aging. Two modes of relaxations were seen at early times (in the first 2 hours) for 5{\%} particle loading. Slow relaxations were seen at low q's where tau dependency of q was 0.8. Particles that were arranged at closer distances (below average interparticle spacing) showed diffusive motions. In the dry brush, particle motions were found to be ballistic similarly observed in the pure brush case. [Preview Abstract] |
Thursday, March 8, 2007 8:24AM - 8:36AM |
U17.00003: Nanoparticle decoration overlayer for producing a surface enhanced Raman scattering spectrum of a pre-existing polymer surface. Bettina Roan, Thomas Furtak A new method was used for obtaining a surface enhanced Raman scattering (SERS) spectrum of a polymer surface. SERS reactive silver nanoparticles were deposited directly onto the surface of the polymer. This differs from conventional methods of producing SERS-active samples where the analyte polymer is deposited onto a SERS reactive metal nanoparticle substrate, an approach that can only give a SERS spectrum of the buried metal-polymer interface. The new technique makes it possible to study the outermost, pre-existing surface of the polymer. We obtained a SERS spectrum of the external surface of poly(methyl methacrylate) (PMMA) using this method. The vacuum evaporation parameters of the silver nanoparticles were adjusted to optimize the SERS intensity of PMMA. We demonstrate the utility of this technique by measurements on surface-modified polymers. [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 8:48AM |
U17.00004: Dewetting of a Polymer Melt on a Chemically Identical Brush Ophelia Tsui, Xueyun Zhang The use of an end-grafted polymer or oligomer layer has been a popular means of engineering surfaces with designable wetting properties. We investigated the factors governing the equilibrium states of the autophobic polystyrene/polystyrene brush/SiO$_{2}$/Si model system, namely the ratio of the molecular weight of the homopolymer to that of the brush, $r$, and the grafting density of the brush, $\sigma $. We found that the wettability of the polymer melt film on the brush -- as measured by the residual thickness, $d$, of the final dewetted film -- decreased monotonically with increasing $r$ or $\sigma $. The experimentally determined form of the $d(r$,$\sigma )$ curves and the polymer-brush interfacial tensions can be described reasonably well by the self-consistent field theory. [Preview Abstract] |
Thursday, March 8, 2007 8:48AM - 9:00AM |
U17.00005: Probing Molecular Mobility at the PNIPAM Brush Surface Jiang Zhao, Wei Wang, Shengqin Wang Diffusion of single polyelectrolyte molecules, poly 2- vinylpyrindine (P2VP), on N-isopropylacrylamide (PNIPAM) brush was studied by single molecule fluorescence techniques. The dependence of the surface diffusivity on varying temperature and brush thickness was studied. We observed a strong dependence of the surface diffusivity on the brush thickness while a minor dependence on the temperature. We attribute these effects to the surface mobility of the adsorption sites of the P2VP to the PNIPAM brush surface. [Preview Abstract] |
Thursday, March 8, 2007 9:00AM - 9:12AM |
U17.00006: Influence of entropic configurational effects on the surface tension of symmetric star polymers Zhenyu Qian, Venkatachala Minnikanti, Lynden Archer, Bryan Sauer Surface tension of symmetric 4 and 11 arm polystyrenes have been measured for a series of different molecular weights. We observe that entropic effects contribute significantly to the variation of surface tension as a function of molecular weight. Minnikanti and Archer have developed an expression that relates the surface tension of a symmetric star polymer with its molecular weight. To determine the dependence of surface tension on the reciprocal of molecular weight, the number of arms and dimensionless attraction of the ends and branched point towards the surface play a significant role. We estimated the entropic attraction of the ends and joints from independent self consistent field simulations of a polymer on a lattice. The predicted variations in surface tensions due to entropic reasons were found to constitute a large part of the experimentally observed surface tension variation with molecular weights for 4 and 11 arm stars. [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U17.00007: Contact Properties of Surface Modified Elastomeric Membranes and the Recognition of Specific Interactions David A. Brass, Kenneth R. Shull Specific interactions (i.e. Biotin/Avidin, etc.) between modified brushes on both a thin elastomeric membrane and a gold coated quartz crystal surface are investigated. To quantify the effect of different end groups, the interactions between opposing unfunctionalized brushes or between an unfunctionalized brush and a substrate were first analyzed. The quartz crystal resonator provides a measured distance between the membrane and quartz surfaces. The surface of the quartz is modified with poly(ethylene glycol) (PEG) molecules with thiol end groups that bond to the gold electrode surface. The membrane surface is modified through the use of Langmuir layers of amphiphilic molecules, producing PEG brushes that extend into the aqueous environment. Self consistent field theory and models of the acoustic wave propagation were used to establish the sensitivity of the technique to the specific interactions arriving from the end groups. [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U17.00008: Photoresponsive Polymer Surfaces Spiros H. Anastasiadis, M.I. Lygeraki, K. Lakiotaki, M. Varda, A. Athanassiou, M. Farsari, C. Fotakis Photochromic spiropyran molecules are utilized as additives for the development of polymer surfaces whose wetting characteristics can reversibly respond to irradiation with laser beams of properly chosen photon energy. The hydrophilicity is enhanced upon UV laser irradiation since the embedded non-polar spiropyran molecules convert to their polar merocyanine isomers, which is reversed upon green laser irradiation. Micropatterning of the photochromic-polymer films using soft lithography or photo-polymerization techniques affects their wettability towards a more hydrophobic or more hydrophilic behavior depending on the dimensions of the patterned features and on the hydrophilicity-hydrophobicity of the flat surface. The light-induced wettability variations of the structured surfaces are enhanced by up to a factor of three as compared to those on the flat surfaces. This enhancement is attributed to the photoinduced reversible volume changes to the imprinted gratings, which additionally contribute to the wettability changes due to the light-induced photochromic interconversions. [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U17.00009: Learning from the Venus Flytrap: A Biomimetic Responsive Interface Douglas P. Holmes, Alfred J. Crosby The ability to have controllable and fast property changes to a material surface over large length scales is desirable for a variety of functions including chemical sensors, antimicrobial devices, ``smart'' adhesives, and drug delivery coatings. Recently, many strategies, including shape memory and environment selectivity, have been developed for synthesizing responsive surfaces, but the response times are often too slow or non-sensitive. Here, we present a new strategy for responsive surfaces or interfaces that builds upon the response mechanism of the Venus flytrap, which exhibits one of the most rapid movements in the plant kingdom. This rapid, controllable movement is largely attributed to the geometry of the leaflets, which can undergo a snap-through elastic instability upon development of a critical pressure. Learning from nature, we have made a responsive interface with controlled, elastic instabilities capable of large geometric changes across very short timescales. We present the fabrication of a responsive surface of microlenses with a controlled, predictable geometry that undergoes a rapid snap-through transition when triggered by various stimuli. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U17.00010: ``Smart'' Surfaces of Diblock Copolymer Brushes Dong Meng, Qiang Wang We use a self-consistent field (SCF) theory to study the morphological response of diblock copolymers grafted to a planar substrate. One of the two blocks can carry weakly dissociating charges (e.g., poly(acrylic acid)), making the surface responsive to the solution pH, ionic strength and applied electric field, in addition to the solvent selectivity. Three-dimensional, parallel SCF calculations are performed in real space with high accuracy to identify the thermodynamically stable morphology under a given set of controlling parameters. The internal structure and surface switchability of the copolymer brushes are studied in detail as a function of chain length, volume fraction of the two blocks and chain-grafting density. [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U17.00011: Polymers containing azobenzene as photo-mechanical materials. Christopher Barrett Polymers containing Azobenzene have received much interest as photo-reversible materials for a variety of optical and photonic applications. Most recently however, Azo Polymers have also been shown to respond physically and mechanically to light, to act as all-optical patterning materials, and photo-mechanical devices. In particular, a photo-induced pressure in soft amorphous thin films of azo polymers can lead to the facile inscription of efficient surface relief gratings (SRGs) upon irradiation with an interference pattern. Irradiation with CW light is also shown to lead to a reversible photo-expansion of these films, allowing the materials to function as photo-mechanical switches or light-actuators. New azo polymers to optimize this effect with be presented, and some simple macroscopic devices will be demonstrated that take mechanical advantage of this effect for larger scale motion driven by light. The mechanism for this effect will be discussed from studies using ellipsometry, surface plasmon resonance spectroscopy, and neutron reflectometry. [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U17.00012: Light-directed Control of Macromolecule Organization on a Surface Gregory Carroll, Jeffrey Koberstein, Nicholas Turro This report describes a versatile method to photo-generate and control self-organized polymer patterns on a surface within a larger pattern. Thin polymer films are cross-linked by irradiation with UV light. Crosslinked thin polymer films resist dewetting when heated above the glass transition temperature. Combining pattern formation via instability with pattern formation via photolithography allows the dewetting patterns to be localized to specific areas of a surface, resulting in a self-organized pattern within a light-directed pattern. By confining the uncrosslinked polymer to an area that approaches the size of the equilibrium dewetting morphology, new mesoscopic features result. For thicker films, the polymer organizes into ribbons at the interface between crosslinked and uncrosslinked polymer. When the width of the uncrosslinked area is large enough, droplets form between the ribbons. As the width gets larger, droplet organization evolves from incomplete to complete polygons. In addition, the structure of the dewetting morphologies changes as the thickness of the film changes. [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U17.00013: High-speed, sub-15 nm feature size thermochemical nanolithography Elisa Riedo, Robert Szoszkiewicz, Takashi Okada, Simon Jones, Tai-De Li, William King, Seth Marder The past decade has witnessed an explosion of techniques used to pattern materials on the nano and submicrometer scale, driven by a diversity of applications, such as molecular electronics, data storage, optoelectronics, displays, and all forms of sensors. However, there are many challenges to conventional techniques as they are approaching their fundamental size limit. Here we report a nanolithography technique that allows simultaneous direct control of the local chemistry and topography of thin polymer films. Specifically, a heated atomic force microscope tip can write sub-15 nanometer hydrophilic features over a hydrophobic polymer at the rate of 1.4 millimeters per second. This method is simple, direct, extremely rapid, achievable in a range of environments, and easily adaptable to other materials systems. [Preview Abstract] |
Thursday, March 8, 2007 10:36AM - 10:48AM |
U17.00014: In-Plane Ordering in Diblock Copolymer Brushes. Bulent Akgun, Gokce Ugur, William J. Brittain, Mark D. Foster, Xuefa Li, Jin Wang Internal and surface structures of polystyrene-b-polyacrylate and polyacrylate-b-polystyrene diblock copolymer brushes have been studied using grazing-incidence small-angle X-ray scattering (GISAXS) and atomic force microscopy (AFM). Each asymmetric, as-deposited diblock brush that contains a poly(methyl acrylate) (PMA) block shows an in-plane structure with a spacing comparable to the PMA layer thickness. The correlation length of the in-plane ordering is about the nearest neighbor distance and grows with annealing at 180$^{\circ}$ C. After a brush is treated with a solvent selective for the bottom block, Bragg rods appear in the GISAXS pattern. The lateral spacing corresponding to the Bragg rods is on the order of the brush total thickness. This lateral correlation is also detected by power spectral density analysis of AFM images of the samples' surfaces. The Bragg rods disappear upon heating to 80$^{\circ}$ C. [Preview Abstract] |
Thursday, March 8, 2007 10:48AM - 11:00AM |
U17.00015: ABSTRACT WITHDRAWN |
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