Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Y18: Focus Session: Surface Instabilities and Adsorbed or Grafted Layers |
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Sponsoring Units: DPOLY Room: 319 |
Friday, March 20, 2009 8:00AM - 8:36AM |
Y18.00001: Drying-Mediated Self-Assembly of Highly Ordered Complex Structures: From Polymers to Nanoparticles Invited Speaker: Drying of a sessile drop containing nonvolatile solutes readily self-assembles into a number of concentric ``coffee rings'' by repetitive ``stick-slip'' motion of the three-phase contact line. However, due mainly to lack of control over the evaporation process of the drop, the challenge remains to use evaporative self-assembly to rationally ``synthesize'' ``coffee rings'' of different shapes and sizes of high regularity and fidelity. Here, we report a facile, robust, and one-step evaporation method for producing in a precisely controllable manner versatile microstructures possessing high regularity, dispensing with the need for lithographic techniques and externally applied fields. Polymer or nanocrystal solutions are confined in a simple geometry comprised of a curved surface placed upon a flat substrate. By changing the shape of the upper surface of the imposed geometry, the controlled, evaporative self-assembly of polymer or nanocrystal solutions yields a variety of complex, intriguing, and well-ordered structures over large areas. As such, this method represents a significant advance in creating regularly organized, complex structures with potential applications in microelectronics, optoelectronics, and biotechnology, among other areas. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y18.00002: Morphology of Osmotically-Driven Surface Buckles Derek Breid, Alfred Crosby The ordering of osmotically-driven surface buckles on an elastically-supported stiff plate depends strongly upon the dominant stresses acting on the surface during formation. For example, conditions which lead to buckle initiation at the center of the plate yield hexagonal dimple arrays characteristic of an isotropic surface stress, while buckles initiated at the edges align radially, indicating a theta-dominated state of compressive stress. Here, we present experimentally-observed buckling morphologies resulting from a variety of surface geometries, as well as Finite Element Modeling results which provide insight into the specific evolution of stresses which led to the formation of these morphologies. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y18.00003: Lateral Instability of Nanoimprinted Polymer Patterns during Thermal Annealing Yifu Ding, Kyle Alvine, Hyunwook Ro, Manish Kulkarni, Jack Douglas, Christopher Soles Nanoimprint Lithography (NIL) is a promising candidate for next generation lithography. Dense polymer patterns with relatively high aspect ratios can be created using NIL. Upon thermal annealing, the as-imprinted polymer patterns will smooth out due to the surface energy and sometimes residual stress within the patterns. Both effects will predominately drive the pattern decay vertically. Here, we present that under certain patterning conditions, lateral instability of the surface patterns, driven by surface fluctuations, can also become an effective mechanism for reducing the total surface area. The characteristics of the lateral instability were successfully carried out by annealing the patterns under a temperature gradient. We demonstrate that the lateral instability can be controlled by the imprinting conditions, and can be greatly enhanced by the additions of surfactants. [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y18.00004: Mechanical instabilities in periodic porous elasto-plastic solids. Srikanth Singamaneni, Katia Bertoldi, Sehoon Chang, Ji-Hyun Jang, Seth Young, Edwin Thomas, Mary Boyce, Vladimir Tsukruk We describe the transformation of the periodic microporous structures fabricated by interference lithography followed by their freezing below glass transition. Periodic porous microstructures subjected to internal compressive stresses can undergo sudden structural transformation at a critical strain. The pattern transformation of collapsed pores is caused by the stresses originated during the polymerization of acrylic acid (rubbery component) inside of cylindrical pores and the subsequent solvent evaporation in the organized microporous structure. The results of a non-linear numerical investigation confirm the critical role of the bifurcation of the periodic solid under compressive stresses. In striking contrast to the earlier observations of elastic instabilities in porous elastomeric solids, the elastic-plastic nature of the crosslinked periodic microstructure studied here provides for the ability to lock in the transformed pattern with complete relaxation of the internal stresses. By confining the polymerization of acrylic acid to localized porous areas complex microscopic periodic structures are obtained. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:24AM |
Y18.00005: Peculiarities in liquid phase of Styrene Butadiene rubber surface induced by Atomic Force Microscopy-assisted electrostatic nanolithography Mindaugas Rackaitis, Sergei Lyuksyutov, Dmytro Kashyn, Pavel Paramonov, Robert Mallik Nanoscale surface changes are reported for styrene butadiene rubber (SBR) films (10-100 nm) using protocol derived from Atomic Force Microscopy (AFM) electrostatic nanolithography. Under appropriate tip bias conditions, the electric field magnitude induced in SBR films is of the order of 10$^{8}$-10$^{9}$ V m$^{-1}$, which is sufficiently large to initiate cross-linking in the rubber. Peaklike surface features, surrounded by a circular trough and a raised ring, are observed after completing AFMEN-based protocol. The nanostructure dimensions vary from 0.5-20-nm high and 50-200-nm in diameter. The topology of the nanostructures is attributed to the interplay between film thickness (10 nm and thinner) and the radial component electrostatic pressure. Modeling of the electric field based on the numerical solution of Laplace equation for cylindrical geometry suggests that non-uniformity of electric field plays an important role in nanostructure formation. The stability of the features which remain stable for days suggests cross-linking between macromolecules at the nanoscale. [Preview Abstract] |
Friday, March 20, 2009 9:24AM - 9:36AM |
Y18.00006: Diffusion-Controlled, Self-Organized Growth of Symmetric Wrinkling Patterns Christopher M. Stafford, Jun Young Chung, Adam J. Nolte The formation of self-organized wrinkling patterns is a potential route for generating such tunable ordered patterns on surfaces across many length scales. Here, we demonstrate that surface wrinkling of ultraviolet/ozone (UVO) treated polymer films through osmotically driven swelling by solvent vapor sorption leads to unique and intriguing patterns, some of which have not been previously reported. The type of pattern and speed of its growth is coupled to the degree of UVO crosslinking and the rate of solvent diffusion into the film from a localized defect. This simple yet novel approach could serve as a test-bed for studying topography-driven phenomena such as wettability and adhesion and diffusion related processes, as well as facilitate a better understanding of dynamic self-assembly. [Preview Abstract] |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y18.00007: Transition from Non-interacting to Interacting Regime of Tethered Polymer Chains Ryan Van Horn, Joseph X. Zheng, Ming-Siao Hsiao, Bernard Lotz, Edwin L. Thomas, Jutta Luettmer-Strathmann, Stephen Z.D. Cheng Tethered polymer chains have become an important area of research over the last few decades. Their unique properties make them appealing for various applications. The tethering density of the chains determines the state of the chains as well as the resulting properties, as shown by various theoretical and experimental work. Our group uses single crystals of crystalline-amorphous diblock copolymers to study tethered polymer chains. This system provides better control of tethering density and molecular weight as compared to previous methods. Previous work on PS-b-PEO and PS-b-PLLA shows, for the first time, the reduced tethering density value of the interacting regime transition regardless of molecular weight or solvent quality. Other factors that were not addressed include adsorption and chain stiffness effects. The work presented here will look at the role of chain adsorption using miscible polymers, namely PMMA-b-PLLA, and chain stiffness with PEO-b-PCL where PEO is the amorphous block. In addition to experimental results, Monte Carlo simulations were used to estimate the chain conformation of adsorbed PMMA. [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y18.00008: Internal Structure of Diblock Copolymer Brushes. Bulent Akgun, Charles F. Majkrzak, Sushil Satija, Gokce Ugur, William J. Brittain, Mark D. Foster, Xuefa Li, Jin Wang Although diblock copolymer brushes (DCBs) have been studied due to their stimuli-responsive behavior, their internal structure has been unknown. We have resolved the internal structure of DCBs using neutron reflectivity and grazing incidence small-angle X-ray scattering (GISAXS). The internal brush structure of DCBs of polystyrene-$b$-poly(methyl acrylate) (PS-b-PMA) and PMA-$b$-PS depends strongly on the block sequence and the value of $\chi $N. For the thinnest films a model of two layers with an interfacial region of finite width provides a good description of the data. For dPS-b-PMA films that are thicker and of sufficiently asymmetric composition, a third layer must be included. The necessity of including a third layer is consistent with lateral ordering of some type in the center of the brush, as evidenced by correlation peaks in GISAXS data. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y18.00009: Steric effects on the phase behavior of end-tethered temperature-responsive polymers Charles Hogshead, Evangelos Manias End-tethering polymers from a planar surface at high grafting density results in an apparent spatial confinement originating from interchain steric repulsion. This effective confinement can alter the phase behavior of temperature-responsive polymers in solution, relative to their bulk solution behavior. Here we report on experiments where we synthesized tethered polymer layers with a gradient in grafting density. The systems studied were temperature-responsive alternating copolymers in aqueous solutions. Under-water AFM was used to directly observe the tethered chain collapse upon heating through the bulk LCST, and the resulting reversible adhesion switch. The transition of the tethered layers occurs at temperatures that are similar to the binodal points of the respective solution, but over a much broader temperature range. This behavior is consistent with the collapse being a cooperative conformational transition, reflecting the effects of chain confinement, rather than a first-order thermodynamic LCST transition (as these same polymers exhibit in bulk aqueous solutions). The onset-temperature of the transition, of both the tethered layers and the LCST, was tailored by varying the copolymer composition, i.e., by tuning the hydrophilic/hydrophobic balance within the chain. [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y18.00010: Self-Assembling Pi-conjugated Monolayer on Silicon J.-C. Lin, J. Kellar, J.-H. Kim, S. Nguyen, M. Hersam, M. Bedzyk, K. Bevan Pi-conjugated molecules play an important role in molecular electronic applications. For conductivity, the intrinsic ordering of the structure strongly influences its efficiency but is difficult to characterize by conventional scanning probe and IR spectroscopy techniques. In the present study, we combine a compliment of techniques, including XSW(X-ray standing waves), AFM, XPS, XRR(X-ray reflectivity), XRF(X-ray fluorescence), and DFT(Density functional theory) to determine the atomic scale molecular configurations and packing densities of two self-assembled aromatic monolayers (SAMs) grown on H-passivated silicon. P-4-bromophenyl-ethynyl-phenyl-acetylene, which has two phenyl rings, is directly compared with p-4-bromophenyl-acetylene, which has only one phenyl ring. The results show a local dense packing in spite of the overall coverage being somewhat less than 0.5 monolayers. This packing of the p-4-bromophenyl-ethynyl-phenyl-acetylene SAM suggests the average spacing between molecules is within the pi-pi interaction range, which will contribute to the charge transport. The detailed atomic structure of SAMs are also constructed using our characterization package. The result suggests the possibility of the application of self-assembling method on the growth of molecular electronics. [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y18.00011: The Dynamic Reinforcement of Polyvinyl Alcohol (PVA) as a Result of Non-equilibrium State of Polymer Supermolecular Structures and their Confinement in Nanofibers Eyal Zussman, Emil Shaked, Arkadi Arinstein The results of mechanical testing of PVA -based electrospun nanofibers and bulk in static and dynamic modes are presented. An increase in the elastic moduli resulting from sample deformation was observed in both the bulk and as-spun fibers. This increase occurs when the deformation rate exceeds a critical value and can be attributed to the non-equilibrium dynamics of the supermolecular structures of the polymer matrix. That is, the evolution of these supermolecular structures results in an observably extended relaxation time. It is noted that the rate of the modulus increase of the nanofibers is nearly double that of the bulk fibers' rate. This difference can be explained by confinement influence on the polymer matrix of the nanofibers. In addition, the tests revealed that the, \textit{Tg}, of the nanofiber is noticeably higher than that of bulk specimen. Reinforcing the nanofibrs by cellulose whiskers showing that the dependence of the effective modulus on the whisker concentration has an initial increase that changes to a decrease when the whisker concentration exceeds 2 {\%}. Such behavior can be explained in the framework of an aggregation concept -- when the cluster size reaches that of the fiber diameter (cluster confinement), the whisker distribution becomes inhomogeneous and results in a measurable weakening of the composite. [Preview Abstract] |
Friday, March 20, 2009 10:36AM - 10:48AM |
Y18.00012: Phenomenology of Polymer Thin Film Dewetting during Vapor Deposition Polymerization Mitchell Anthamatten, Xichong Chen Initiated chemical vapor deposition (iCVD) is a solventless technique to grow polymer thin films directly from gas phase feeds. The free radical technique involves the dissociation of gaseous initiator followed by adsorption onto a surface and subsequent polymerization with monomer to produce linear or crosslinked polymer films. We have designed and built an axisymmetric hot-zone iCVD vacuum reactor. Using this reactor, smooth poly (methyl methacrylate) films are grown from methyl methacrylate and t-butyl peroxide gas feeds. When solvent vapors are added to the process, we observe dewetting of vapor deposited polymer films. The objective of the current study is to understand the phenomenology of the observed solvent-induced dewetting. White light interferometery was used to investigate the surface topography of dewetted structures. The observed length scale depends on several process parameters including the deposition rate, the type of solvent used, and the surface free energy. Higher deposition rates suppress dewetting, and higher solvent content leads to dewetted structures with larger length scales. A dynamic model is applied to explain how droplet size and droplet aerial density depend on time and process parameters. [Preview Abstract] |
Friday, March 20, 2009 10:48AM - 11:00AM |
Y18.00013: Nano-adsorbents control surface properties of polyurethane SuPing Lyu, Darrel Untereker, Jim Schley, Tom Grailer, Anna Belu, Chris Hobot, Terri Bartlett, Randy Sparer Additives are minor but critical components that polymers need for processing and applications. However, these additives may also have adverse effects, e.g. for polymeric biomaterials, leaching additives can change surface properties, and may lead to poor biocompatibility. How to use additives yet keep them from detrimental behaviors is challenging. Diffusion barriers may be used to slow down the additive migration but it is difficult to stop it. In this paper, we introduced the concept of ``nano-adsorbents'' in polymers. These nano-adsorbents confined the additives within the polymers via thermodynamically interacting with them. While the additives are still present in polymers to provide intended functions, they are thermodynamically constrained from freely migrating to the surface. Nano sized-fillers were selected due to their high surface-to-volume ratio. This new use of nano-fillers for polymers was demonstrated with a biomedical polyurethane and a surface coated nano-clay that thermodynamically attracts the additive in the polyurethane. [Preview Abstract] |
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