Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session Y22: Interfaces and Adhesion II |
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Sponsoring Units: DPOLY Chair: Alexei Sokolov, University of Akron Room: Morial Convention Center 214 |
Friday, March 14, 2008 11:15AM - 11:27AM |
Y22.00001: Polymer monolayer -- substrate adhesion strength Moshe Gottlieb, Haim Dvir Polymer monolayers have been deposited on several chemically different solid substrates. The substrates ranged from hydrophobic to hydrophilic and from chemically inert to highly reactive. In addition few of the surfaces were also exposed to ionizing irradiation. The extent of surface coverage and surface topology were experimentally determined for the different surfaces and polymers. The adsorbed layer thickness was determined optically. The strength of polymer interaction with the substrate was investigated using contact-mode Atomic Force Microscopy. Typically, for each polymer a characteristic layer thickness was measured irrespective of the nature of the surface or strength of adhesion. Adhesion strength was attributed mainly to van der Waals interactions with no indications of large scale covalent bonding between the polymer and the surface even for highly reactive surfaces. Hydrophobic interactions, surface topology, and initial conditions existing during film deposition seem to dominate the interaction between the polymer and the substrate. [Preview Abstract] |
Friday, March 14, 2008 11:27AM - 11:39AM |
Y22.00002: Role of Interfacially Active Diblock Copolymers toward Controlling the Glass Transition of Thin Polymer Films Hyunjoon Oh, Peter Green We show that small concentrations of polystyrene-b-polymethylmethacrylate (PS-b-PMMA) diblock copolymers significantly alter the thickness, h, dependence of the glass transition of polystyrene (PS) films supported by silicon substrates. The T$_{g}$ can be induced to increase, or decrease, with decreasing h or to be independent of h. T$_{g}$-shifts of as much of 35\r{ }C are obtained for films h $<$ 30 nm. The copolymers form micelles and the critical micelle concentration ($\phi _{cmc})$ of the copolymer in the thin films is considerably larger than for the bulk, larger than 15{\%}. In fact micelles form only beyond a critical film thickness, determined by the size of the chains and by the number of chains required to saturate the interfaces. [Preview Abstract] |
Friday, March 14, 2008 11:39AM - 11:51AM |
Y22.00003: Weak interfaces for UV cure nanoimprint lithography Frances Houle, Ann Fornof, Eva Simonyi, Dolores Miller, Hoa Truong Nanoimprint lithography using a photocurable organic resist provides a means of patterning substrates with a spatial resolution in the few nm range. The usefulness of the technique is limited by defect generation during template removal, which involves fracture at the interface between the template and the newly cured polymer. Although it is critical to have the lowest possible interfacial fracture toughness (Gc less than 0.1 Jm-2) to avoid cohesive failure in the polymer, there is little understanding on how to achieve this using reacting low viscosity resist fluids. Studies of debonding of a series of free-radical cured polyhedral silsesquioxane crosslinker formulations containing selected reactive diluents from fluorosilane-coated quartz template materials will be described. At constant diluent fraction the storage modulus of cured resists follows trends in initial reaction rate, not diluent Tg. Adhesion is uncorrelated with both Tg and storage modulus. XPS studies of near-interface compositions indicate that component segregation within the resist fluid on contact with the template, prior to cure, plays a significant role in controlling the fracture process. [Preview Abstract] |
Friday, March 14, 2008 11:51AM - 12:03PM |
Y22.00004: Spincoating of ultrathin chitosan films Chris Murray, John Dutcher We have studied the spincoating of ultrathin chitosan films onto silicon wafer substrates from dilute solutions of chitosan dissolved in acetic acid solutions. This particular example of spincoating presents unique difficulties due to the non-volatility of the solvent, but also provides unique information since the spincoating process is slow enough to allow detailed measurements of the drying of the film. The resulting film thickness, as measured using ellipsometry, is a strong function of the relative humidity (RH) in the surrounding atmosphere, which can be easily controlled. By using a simple model for the dependence of film thickness on spin speed and RH, we obtain a measure of water uptake in chitosan films that can be compared with that estimated from sorption isotherms measured using ellipsometry. [Preview Abstract] |
Friday, March 14, 2008 12:03PM - 12:15PM |
Y22.00005: Protein Diffusion at the Interface of Responsive Polymer Thin Films Shengqin Wang, Yingxi Elaine Zhu Protein adhesion at polymer interfaces has been much explored, yet the interfacial friction is not. We employ fluorescence correlation spectroscopy (FCS) and single-molecule imaging to examine the translational dynamic processes of protein at the responsive polymer interfaces, whose surface hydrophobicity and interfacial viscoelasticity are tunable experimentally. We focus on the dynamics of human serum albumin (HSA) and lubrin, a nutritious protein in synovial fluids, at the interface of responsive poly (N-isopropylacrylamide) (PNIPAM) brush layers. The effects of PNIPAM brush thickness, grafting density, and surface hydrophobicity on protein interfacial diffusivity are investigated. We observe the coupling of the local protein dynamics at the protein-PNIPAM interfaces with the interfacial viscoelasicity of PNIPAM brush thin films. [Preview Abstract] |
Friday, March 14, 2008 12:15PM - 12:27PM |
Y22.00006: Structure and dynamics of molecules undergoing lubricated sliding Kumar Nanjundiah, Anish Kurian, Ping Hsu, Ali Dhinojwala The presence of a thin fluid layer is crucial in reducing wear and energy dissipation and is important in many areas such as tribology, adhesion, micro-fluidics, study of earthquakes and biolubrication. It has been shown using force measurements that the molecules undergo abrupt liquid-to-solid transition upon confinement and shear melting on sliding. Experiments and simulations have provided important clues but no definite answers. We have designed a friction cell that allows us to simultaneously probe the structure of the confined molecules using infrared-visible sum frequency generation spectroscopy in conjunction with friction and adhesion. Changes in the structure of liquid molecules upon confinement and during sliding will be presented. [Preview Abstract] |
Friday, March 14, 2008 12:27PM - 12:39PM |
Y22.00007: Measurement advances to follow polymer thin film reaction-diffusion processes. Vivek Prabhu, Shuhui Kang, Kristopher Lavery, Kwang-Woo Choi, Wen-li Wu, Eric K. Lin Polymer thin films are used as imaging layers for photolithography to define high spatial resolution features for the semiconductor industry. These chemically amplified photoresist materials, however, may be reaching their intrinsic limits as desired feature sizes approach macromolecular dimensions. A photoacid-catalyzed reaction defines a chemical image which is subsequently resolved by dissolution in an aqueous base solution. A method to characterize the reaction-diffusion process was developed using infrared spectroscopy and tested by neutron reflectivity. We determine the thin film reaction kinetics, photoacid trapping behavior, and photoacid diffusivity by measuring the reaction kinetics. The temperature-dependence and mechanism for observed pinning of the reaction-diffusion front will be discussed. These results permit an analysis of the latent image formation which is a crucial for photolithography resolution and fidelity. [Preview Abstract] |
Friday, March 14, 2008 12:39PM - 12:51PM |
Y22.00008: Methanol Diffusion into Thin Ionomer Films: An \textit{in situ} Study Using Neutron Reflectometry . Lilin He THUSITHA, N. ETAMPAWALA DVORA, PERAHIA $^{ }$Department of Chemistry, Clemson University, Clemson, SC 29634 JAROSLAW MAJEWSKI, Lujan Neutron Scattering Center, Los Alamos National Laboratory, Los Alamos, NM 87545 CHRISTOPHER J. CORNELIUS$^{ }$Sandia National Laboratories, MS 0886, Albuquerque, New Mexico 87185-0886 The penetration of solvent into a polymer that consists of incompatable groups is determined by the specific interactions with the guest molecule, where interfacial structure and dynamics of the polymer affect the onset of the process. The current work presents a neutron reflectometry study of the penetration of methanol into sulfonated polyphenlylene thin films. The ionomer films were exposed to saturated deuterated methanol vapor and reflectometry patterns were recorded until equilibrium was reached. The process incorporates two stages where the vapors first wet the surface and then penetrate into the film. Significant swelling takes place as soon as the film is exposed to the vapors. Similar to previous studied in water, the onset diffusion is Fickian followed by an anomalous diffusion process. The entire process however is faster than that observed for water. [Preview Abstract] |
Friday, March 14, 2008 12:51PM - 1:03PM |
Y22.00009: Pattern Formation in Dewetting Nanoparticle/Polymer Bilayers Alan Esker, Rituparna Paul, Ufuk Karabiyik, Michael Swift, John Hottle Comprised of inorganic cores and flexible organic coronae with 1 -- 2 nm diameter monodisperse sizes, polyhedral oligomeric silsesquioxanes (POSS) are ideal model nanofillers. Our discovery that one POSS derivative, trisilanolphenyl-POSS (TPP), can form Langmuir-Blodgett (LB) films on hydrophobic substrates, allows us to create thin film bilayers of precisely controlled thickness and architecture. Work with poly(t-butylacrylate) (PtBA)/TPP bilayers reveals a two-step dewetting mechanism in which the upper TPP layer dewets first, followed by the formation of isolated holes with intricate, fractal, nanofiller aggregates. Like the PtBA/TPP bilayers, polystyrene (PS)/TPP bilayers also undergo a two-step dewetting mechanism. However, the upper TPP layer initially forms cracks that may arise from mismatches in thermal expansion coefficients. These cracks then serve as nucleation sites for complete dewetting of the entire bilayer. Understanding the rich diversity of surface patterns that can be formed from relatively simple processes is a key feature of this work. [Preview Abstract] |
Friday, March 14, 2008 1:03PM - 1:15PM |
Y22.00010: Case II diffusion and solvent-polymer films drying: a meso-scale model Didier Long, Mireille Souche This model is based on the fact that dynamics in liquids close to the glass transition is spatially heterogeneous [Ediger2000,Souche2007], with characteristic size 3 to 4nm in van der Waals liquids. Before considering large scale diffusion experiments, we consider first the evolution of the dynamics of a layer of thickness 3 to 4nm, submitted to an arbitrary time varying activity a(t). This procedure allows in principle to calculate a constitutive relation for the dynamics of solvent- polymer mixtures, that can then be used for calculating the evolution of macroscopic samples in contact with a reservoir of solvent. We show how these constitutive relations allow for explaining case-II diffusion in glassy polymers and provide a physical interpretation for the parameters of the Thomas-Windle model. Regarding the process of film drying, we show that films up to 1 micrometer thick can be almost completely dried in an accessible experimental time, even at temperatures well below the polymer glass transition temperature. This is a consequence: 1- of the presence of the fast path 2- of the film being out equilibrium, and in a dynamical state which is must faster than the one it would have at equilibrium. When drying a thicker film, we show that a glassy crust may appear on the free surface, as has been shown experimentally. Ediger M.D., Annu. Rev. Chem., 51 (2000) 99; Souche M. and Long D., Europhys. Lett, 77 (2007) 48002 [Preview Abstract] |
Friday, March 14, 2008 1:15PM - 1:27PM |
Y22.00011: Nonsolvent-induced dewetting of thin polymer films Tong-Fei Shi, Lin Xu, Li-Jia An The dewetting of thin liquid films is important to various technological processes. Most of the studies on dewetting are through thermal dewetting, whereas solvent-induced dewetting has received very little attention. Generally the main difference between thermal dewetting and solvent-induced dewetting is that the cause of instability is the long-range force of van der Waals interactions in the thermal dewetting whereas it is the short-range force of polar interactions in the solvent-induced dewetting. In these reports on solvent-induced dewetting, nearly all solvents, which are chosen, can dissolve the polymers. However, few reports focus on the dewetting induced by nonsolvent, which cannot dissolve polymer. In this work, the process of nonsolvent-induced dewetting of thin polystyrene (PS) films on hydrophilic surfaces at room temperature has been studied by using water as a nonsolvent. It is observed that the process of nonsolvent-induced dewetting is greatly different from other previous dewetting processes. The PS film is in non-viscous state. A mechanism of nonsolvent-induced dewetting, different from other previous dewetting mechanisms, is deduced: penetration, replacement and coalescent. [Preview Abstract] |
Friday, March 14, 2008 1:27PM - 1:39PM |
Y22.00012: Mechanical Properties of Thin Polymer Films Studied by Atomic Force Microscopy Blandine Jerome, Christian Vialleton, Laurent Chazeau, Elisabeth Charlaix Introducing inorganic nanoparticles into a polymer is known to modify the macroscopic mechanical properties of the material. This is often interpreted by assuming the presence of a polymer layer with different properties at the interface with the particles. There is however little direct information available on the mechanical properties of such an interfacial layer. We have used an Atomic Force Microscope (AFM) as a nano-indenter to probe the mechanical response of thin poly(styrene butadiene) random copolymers deposited on oxidized silicon wafers (model silica surface). Indentations were performed at different approach and retraction speeds at room temperature (polymer in the rubbery state) on films with thicknesses ranging from 40nm to 500nm. Approach and retraction curves obtained at high speeds are characteristic of the indentation of an elastic material with an adhesive tip/polymer contact. At low speeds, the adhesion forces dominate for low applied forces, while the elasticity of the polymer dominates the behaviour at high applied load. This allows us to separate the mechanical response of the polymer film from the tip-polymer adhesion that involves some dissipation taking place close to the contact line between the polymer free surface and the tip. [Preview Abstract] |
Friday, March 14, 2008 1:39PM - 1:51PM |
Y22.00013: Molecular Dynamics Simulations of Adhesion at Epoxy Interfaces Sarah-Jane Frankland, Thomas Clancy, Thomas Gates With composite materials becoming more prevalent as metal parts are being replaced on aircraft, adhesives are being developed for composite bonds which are suitable for the various thermal, mechanical and environmental changes that take place over the lifetime of the aircraft. The key molecular structure-property relationships that enable the chemical compatibility of the adhesive with the adherend can be identified with molecular dynamics simulation (MD). MD can assess the role of different chemical moieties in the adhesive, and their behavior in the presence or absence of solvents under different thermo-mechanical conditions. In the present work, MD simulations are used to calculate factors that affect the work of adhesion with and without solvent present. MD simulations are carried out at the interface between components of epoxy-based adhesives and composite adherends. The simulations utilize molecular models of networks which are representative of specific chemistries of the epoxy system. The simulations include both bulk and interface models of the components. The paper will present the simulation methodology, and the results for the work of adhesion.. [Preview Abstract] |
Friday, March 14, 2008 1:51PM - 2:03PM |
Y22.00014: Interfacial Properties of Polydimethylsiloxane-Water Systems Ahmed E. Ismail, Gary S. Grest, Mark J. Stevens, Mesfin Tsige, David R. Heine Polydimethylsiloxane (PDMS) is a main constituent of silicone adhesives, which have a wide use as adhesives. Often these adhesives are used as sealants. The interaction between water and PDMS is of fundamental importance. To improve our understanding at the molecular level, we have performed molecular dynamics (MD) simulations of PDMS in the presence of water, with the long-term goal of studying how water molecules effect debonding at the surface. Knowledge of the basic interfacial properties of a multicomponent system, such as the surface tension, contact angle, and diffusion constant, are essential to obtain the proper dynamic behavior in a molecular simulation of adhesion and wetting processes. Explicit-atom simulations of 10$^5$ or more atoms were used to determine liquid-vapor surface tension and the contact angle for water on the surface of PDMS. We present results for the dependence of the surface tension on chain length and end-group functionality. [Preview Abstract] |
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