Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session W34: Thin Films, Surfaces and Interfaces II |
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Sponsoring Units: DPOLY Chair: Dvora Perahia, Clemson University Room: 342 |
Thursday, March 21, 2013 2:30PM - 2:42PM |
W34.00001: Photo-crosslinkable polymers for fabrication of photonic multilayer sensors Maria Chiappelli, Ryan C. Hayward We have used photo-crosslinkable polymers to fabricate photonic multilayer sensors. Benzophenone is utilized as a covalently incorporated pendent photo-crosslinker, providing a convenient means of fabricating multilayer films by sequential spin-coating and crosslinking processes. Colorimetric temperature sensors were designed from thermally-responsive, low-refractive index poly($N$-isopropylacrylamide) (PNIPAM) and high-refractive index poly(para-methyl styrene) (P$p$MS). Copolymer chemistries and layer thicknesses were selected to provide robust multilayer sensors which show color changes across nearly the full visible spectrum due to changes in temperature of the hydrated film stack. We have characterized the uniformity and interfacial broadening within the multilayers, the kinetics of swelling and de-swelling, and the reversibility over multiple hydration/dehydration cycles. We also describe how the approach can be extended to alternative sensor designs through the ability to tailor each layer independently, as well as to additional stimuli by selecting alternative copolymer chemistries. [Preview Abstract] |
Thursday, March 21, 2013 2:42PM - 2:54PM |
W34.00002: Diffusion of single molecules on surface tethered polymer brushes Zhenyu Zhang, Matt Mears, Mark Moxey, Nicolas Warren, Jeppe Madsen, Steven Armes, Andrew Lewis, Mark Geoghegan The interaction between polymer molecules and brush surfaces in aqueous media is a multi-dimensional problem; the polymer competes with the solvent for surface sites, and the resultant molecular conformation controls its diffusion properties. The diffusion coefficients of single fluorescence-labeled poly(ethylene glycol) (PEG) molecules on surface-immobilized PEG brushes are measured by fluorescence correlation spectroscopy, and are shown to slow down by nearly 10 times when grafting density increased from 0.11 to 0.42 chain per nm\textasciicircum 2. This diffusion dynamics can be explained by Stokes-Einstein treatment of the surface-adsorbed polymer. Subsequently, we prepared a series of surface-grown poly(oligo(ethylene glycol) methacrylate) (POEGMA) brushes with varying grafting density. Diffusion coefficients of three types of fluorescence-labeled polymer (PEG, POEGMA, PGMA) on the POEGMA brushes were quantitatively measured. It was found that diffusion coefficient of PEG changed substantially over those POEGMA samples, with POEGMA to a small degree, and PGMA not affected. The data indicates that not only grafting density of polymer brushes, but also intermolecular interaction could affect the transport of macromolecules on polymer brushes. [Preview Abstract] |
Thursday, March 21, 2013 2:54PM - 3:06PM |
W34.00003: Molecular Dynamics Simulations of Tension Amplification in Tethered Bottle-brushes Gary M. Leuty, Mesfin Tsige, Michael Rubinstein, Gary S. Grest Bottle-brush polymers are grafted comb polymers in which the density of side chains grafted to the polymer backbone is large enough that steric repulsions between the side chains force the backbone to stretch and preclude it from forming random-coil configurations. Tethering one end of the bottle-brush backbone to a solid substrate restricts the conformations of the side chains near the surface and leads to side-chain repulsion that induces significant amplification of the tension along the polymer backbone. Depending on the grafting density on the substrate, the density of side chains and the length of the side chains, this tension amplification may be large enough break bonds along the bottle-brush backbone, especially at the site of the link with the substrate, where the tension is maximized. We have performed coarse-grained molecular dynamics simulations to understand the interplay between the factors affecting backbone tension amplification and whether the amplification effects can be controlled in such a way as to predict, for a given architecture and surface coverage, the maximum allowable packing of chains on the substrate surface prior to tethering failure. [Preview Abstract] |
Thursday, March 21, 2013 3:06PM - 3:18PM |
W34.00004: Fast Lattice Monte Carlo Simulations of Grafted Homopolymers under Compression Pengfei Zhang, Qiang Wang Fast lattice Monte Carlo (FLMC) simulations [Q. Wang, Soft Matter 5, 4564 (2009); 6, 6206 (2010)] with multiple occupancy of lattice sites and Kronecker $\delta$-function interactions give orders of magnitude faster/better sampling of configuration space for many-chain systems than conventional lattice MC simulations with the self- and mutual- avoiding walk and nearest-neighbor interactions. Using FLMC simulations with the novel Wang-Landau-Optimized-Ensemble sampling, we have studied homopolymers end-grafted on an impenetrable and flat substrate under the compression by another impenetrable and flat surface. Comparing various quantities (including chain dimensions, internal energy, Helmholtz free energy, and pressure) obtained from FLMC simulations with predictions from the corresponding lattice self-consistent field (LSCF) calculations, both using the same model system (Hamiltonian) and thus without any parameter-fitting, we unambiguously quantify the effects of system fluctuations and correlations neglected in LSCF theory. In particular, we find LSCF theory underestimates the pressure for compression of mushrooms in the athermal and $\theta$-solvents and for compression of brushes in the $\theta$-solvent, but overestimates it for compression of brushes in the athermal solvent. [Preview Abstract] |
Thursday, March 21, 2013 3:18PM - 3:30PM |
W34.00005: ABSTRACT WITHDRAWN |
Thursday, March 21, 2013 3:30PM - 3:42PM |
W34.00006: Low voltage switching of crease patterns on gel surfaces with topographically patterned microelectrodes Bin Xu, Ryan Hayward Exercising precise control over surface instability patterns on soft hydrogels is of significant interest for applications in biological and biomedical contexts. Here, we show that patterns of surface creases can be successfully programmed on thin hydrogel layers by applying a direct current electric voltage to underlying micro-patterned electrodes. We characterize the dependence of the critical switching voltage on the swelling of the gel layer and the geometry of the electrode array, as well as the depth of creases as a function of applied voltage and the switching kinetics. We also show that introducing topographically structured electrodes lowers the critical voltage slightly, and provides better control over crease shape. To better understand the mechanism of electrically-triggered creasing, we have developed an \textit{in situ} strain mapping technique based on bleaching of markers within the gel layer. [Preview Abstract] |
Thursday, March 21, 2013 3:42PM - 3:54PM |
W34.00007: Tunable Surface Properties from Bioinspired Comb Copolymers Wendy van Zoelen, Hilda Buss, Nathan Ellebracht, Ronald Zuckermann, Rachel Segalman A modular polymer system which incorporates multiple functionalities simultaneously while keeping an identical backbone chemistry is a useful tool in determining necessary functionalities for marine antifouling properties. We have investigated the surface properties and antifouling behavior of polypeptoids, a class of non-natural biomimetic polymers based on an N-substituted glycine backbone, that combine many of the advantageous properties of bulk polymers with those of synthetically produced proteins, including controllable chain shape, sequence, and self-assembled structure. Using thiol-ene click chemistry, thiol functionalized amphiphilic peptoid sequences consisting of hydrophilic methoxyethyl and hydrophobic heptafluorobutyl side chains were attached to polystyrene-block-poly(ethylene oxide-stat-allyl glycidyl ether), creating comb-shaped molecules. Near edge X-ray absorption fine structure spectroscopy (NEXAFS) was used to study the surface characteristics as a function of peptoid length and composition. Only 20{\%} of fluorinated groups in the peptoid were sufficient for promoting surface display of the otherwise hydrophilic PEO/peptoid comb block. Antifouling experiments with spores of the green algae Ulva indicated an influence of sequence. [Preview Abstract] |
Thursday, March 21, 2013 3:54PM - 4:06PM |
W34.00008: Microwave-Assisted Surface-Initiated Free Radical Polymerization Erich Bain, Xinfang Hu, Christopher Gorman, Jan Genzer We investigate microwave ($\mu $w) irradiation as a heat source for surface-initiated (SI) free-radical polymerization (FRP). First, we consider the possibility of SI controlled radical polymerization (CRP) without chemical additives, based on local heating due to microwave absorption by the substrate. A simple model is developed to predict the temperature gradient at the interface between a microwave absorbing substrate and a nonabsorbing medium. Stochastic simulations are then applied to predict the molecular weight distribution for polymerizations with decoupled kinetics of initiation, propagation, and termination due to the temperature gradient. The simulations shed light on experimental requirements for $\mu $w-induced SI-CRP, as well as general conditions for any successful CRP. Secondly, we consider whether $\mu $w irradiation may increase throughput of SI-FRP, affording either faster brush growth, thicker brushes, or both, compared with conventional heating (CH) (e.g. by an oil bath). Experimental results of $\mu $w SI-FRP are compared against CH on silicon wafers, quartz slides, particles, and in bulk. Reproducibility of heating for silicon wafers is found to depend on orientation relative to the incident irradiation. [Preview Abstract] |
Thursday, March 21, 2013 4:06PM - 4:18PM |
W34.00009: Free Volume Model of Enhanced Mobility at a Free Surface Nicholas B. Tito, Jane E. G. Lipson, Scott T. Milner Experiments on polymer thin films during the past two decades have revealed a number of intriguing properties as they approach the glass transition. In addition to dynamic heterogeneity, which is also characteristic of the bulk, there is a substantial body of evidence for enhanced mobility at and near a free surface, leading to local suppression of the glass transition temperature. We have developed a simple kinetic lattice model of free volume and mobility transport in a near-glassy fluid. The model qualitatively exhibits hallmarks of the glass transition in bulk fluids, e.g. power-law growth of the cooperative length scale of glassified material, and slowing global dynamics on approach to a ``kinetic arrest'' transition. In this talk we discuss how introducing a free surface into the model yields a gradient of mobility, the depth of which depends on proximity to the bulk glass transition. [Preview Abstract] |
Thursday, March 21, 2013 4:18PM - 4:30PM |
W34.00010: Quantification of tip-sample forces on and below resonance in tapping mode atomic force microscopy Orsolya Karacsony, Tomasz Kowalewski, Brian Cusick There has been a recent resurgence of interest in multi-frequency tapping mode AFM techniques, in which quantifying the tip-sample force is crucial. In particular, knowledge of the magnitude of tip-sample force may be essential in understanding the nature of contrast in imaging soft materials such as block copolymers or novel complex macromolecular architectures. This presentation will focus on the quantitative understanding of the dependence of average tip-sample forces on imaging conditions such as set-point ratio and operating frequency. First, the derivation of an analytical expression for the average tip-sample force will be presented. Its predictions will be then shown to be in excellent agreement with the results of numerical simulations using a single degree of freedom, driven damped harmonic oscillator model of tapping mode AFM. [Preview Abstract] |
Thursday, March 21, 2013 4:30PM - 4:42PM |
W34.00011: Grazing Resonant Soft X-ray Scattering: A New Way to See Inside Mesoscale Thin Films Eliot Gann, Anne Watson, Cheng Wang, Justin Cochran, Joshua Carpenter, Terry McAfee, Hongping Yan, Christopher McNeill, Michael Chabinyc, Harald Ade Thin film structures are becoming increasingly important in energy and engineering applications as functional films and specifically as thin film electronics. Often the most important structures in these thin films are the interfaces between different materials. The internal structure of thin film complex systems, particularly interfacial structure, has been difficult and often impossible to characterize with traditional characterization techniques. Existing methods either lack materials contrast necessary to distinguish different components, lack penetrating power to see structure beneath the film surface, require special sample preparation which may change important features, or are too local a probe to get statistically meaningful information. This talk highlights a new technique, Grazing Resonant Soft X-ray Scattering (GR-SOXS), capable of probing buried structures in thin film systems. GR-SOXS uses varying energy x-rays near the 1S core electron absorption peak of carbon to scatter from thin polymer films at a grazing angle. Using simulations of the electric field propagation and scattering contrast of different features in model systems as a guide, Scattered X-rays from different structures within the film can be disentangled, elucidating their internal structure. [Preview Abstract] |
Thursday, March 21, 2013 4:42PM - 4:54PM |
W34.00012: Marangoni-Driven Topographic Patterning of Polymer Thin Films Christopher Ellison, Joshua Katzenstein, Dustin Janes, Julia Cushen, Nathan Prisco, Nikhil Hira, Dana McGuffin When exposed to UV light polystyrene (PS) undergoes partial dehydrogenation of its polymer backbone, raising its surface energy. By exposing a PS thin film to UV light through a photomask, a surface energy pattern can be programmed in to the polymer film. Upon heating the film to a liquid state without the mask present, the polymer flows from the unexposed (relatively low surface energy) to exposed (relatively high surface energy) regions of the film. The driving force for this phenomenon is the Marangoni Effect, familiar to most in the `tears' or `legs' in a glass of wine, which describes convective mass transfer due to surface energy gradients. This flow results in three-dimensional topography reflective of the photomask used in the patterning step, which can be preserved indefinitely by quenching the film below its glass transition temperature. In this talk, this process, a preliminary model, and its kinetics will be described. [Preview Abstract] |
Thursday, March 21, 2013 4:54PM - 5:06PM |
W34.00013: Polymer Thin Films and Interfaces; a Layer-by-Layer Approach Ronald White, Jane Lipson In this talk we discuss new ways to model polymer films and interfaces, including properties such as density and concentration gradients, interfacial tension, and surface enrichment. We build on recent work where we developed a very simple equation of state approach for polymer thin films, and successfully applied it to determine thermodynamic properties and even to make predictions for the thickness-dependent depression of the thin film glass transition temperature. In that very simplified mean field model, the film properties across the entire interface region were treated as a ``whole sample'' average. Here, we take the next step, and develop a layer-by-layer equation of state model wherein details of the interface region are captured by allowing properties to vary from one discretized layer (within which properties are uniform) to the next. The model can be solved by imposing hydrostatic equilibrium in each layer, which then leads to predictions for the corresponding density gradient and other key interface properties. [Preview Abstract] |
Thursday, March 21, 2013 5:06PM - 5:18PM |
W34.00014: Melting of Linear Alkanes between Swollen Elastomers and Solid Substrates Ali Dhinojwala, Kumar Nanjundiah We have measured the melting and freezing behavior of linear alkanes confined between a poly(dimethylsiloxane) (PDMS) elastomer and a solid sapphire substrate. For shorter alkanes (15 and 17 carbons) the interfacial layer has a higher melting temperature (T$_m$) than the majority of the alkane crystals inside the swollen PDMS elastomer. For longer alkanes (19, 21, and 22 carbons), a large depression in T$_m$ was observed and the crystallization takes place outside the contact region first and then proceeds to the PDMS-sapphire boundary. In heating, the sapphire/alkane interface shows a pre-melting layer (or melts first) before the melting of a thicker alkane layer next to the sapphire surface. The observation of this unusual depression of T$_m$ of the interfacial layers was unexpected and these findings have important implications in understanding friction and adhesion of soft elastomeric materials. [Preview Abstract] |
Thursday, March 21, 2013 5:18PM - 5:30PM |
W34.00015: Tacticity Effects on the Local Conformation and Interfacial Properties of poly (methyl methacrylate) at the Liquid-Vapor Interface Kshitij C. Jha, He Zhu, Ali Dhinojwala, Mesfin Tsige The orientation of functional groups of poly (methyl methacrylate) (PMMA) play a key role in understanding functionalities like wettability, aggregation and solvent interaction. We have studied the orientation of different functional groups such as the $\alpha $-methyl, ester methyl, methylene and carbonyl groups of the PMMA chain through all atom Molecular Dynamics (MD) simulations for different chain lengths of the polymer. Through orientational correlation, and number density computations we are able to establish the identity and extent of groups coming to the surface. Surface tensions are computed to validate our PMMA model. Analysis has been carried out for all three tacticities-atactic, syndiotactic, and isotactic. Sum Frequency Generation (SFG) spectroscopy also provides insight into the orientation of various groups at the liquidvapor interface. Characterization of the SFG peaks is the point of some debate and MD simulations aim to aid in the understanding of local ordering. [Preview Abstract] |
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