Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session L40: Dillon Medal Symposium |
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Sponsoring Units: DPOLY Chair: Russel Composto, Univ. of Penn Room: LACC 408A |
Tuesday, March 22, 2005 2:30PM - 3:06PM |
L40.00001: Surface engineering with soft matter Invited Speaker: In my presentation, I will outline several novel strategies facilitating the generation of functional polymeric surfaces. In particular, I will present and discuss simple methodologies leading to the formation of complex surface assemblies of surface-tethered polymers with continuous variation of physico- chemical properties (e.g., wettability, molecular weight, grafting density, composition). I will illustrate how these grafted “gradient” surfaces can be utilized to control the spatial distribution of adsorbates, such as nanoparticles and proteins, and administer the proliferation of living cells on the surfaces. Furthermore, I will illustrate how flexible elastomeric networks can be utilized to tailor the grafting density of oligomers or polymers, create responsive (``smart'') surfaces, and generate topographically corrugated surfaces comprising multidimensional cascades of wrinkles. Application of these wrinkled surfaces for material assembly will also be demonstrated. [Preview Abstract] |
Tuesday, March 22, 2005 3:06PM - 3:18PM |
L40.00002: Grafting reactions between end-functional polymers at polymer interfaces E.J. Kramer, B.J. Kim, K. Katsov, G.H. Fredrickson, H. Kang, K. Char Reactions to produce graft copolymers at polymer interfaces during extruder mixing are important for controlling dispersed phase size by retarding droplet coalescence and reducing interfacial tension while providing interface reinforcement. We investigate such reactions at various temperatures in a model bilayer film system consisting of amine end-functional deuterated polystyrene (dPS-NH$_{2})$ in PS and anhydride end-functional poly(2-vinylpyridine) (P2VP-anh) in P2VP as a function of molecular weight M and initial volume fraction $\phi _{0}$ of the end functional chains. After various times of reaction the interfacial excess z* of block copolymer formed at the interface is determined by detecting the $^{2}$H$^{- }$ion using dynamic SIMS depth profiling. At low $\phi _{0}$ ($\sim $ 0.01) of dPS-NH$_{2}$ and P2VP-anh, such that the normalized interface excess z*/R$_{g} \quad <$ 1 and the blocks are unstretched, the forward reaction rate constant k$^{+}$ decreases as M$^{-0.68}$ in rough agreement with theoretical predictions (k$^{+ }\sim $ M$^{-0.55})$ for this regime. The rate constant is thermally activated with an activation enthalpy 165 kJ/mol that is independent of M. [Preview Abstract] |
Tuesday, March 22, 2005 3:18PM - 3:30PM |
L40.00003: Design and Realization of Temperature-Responsive Polymers with Tunable Onset of Response Evangelos Manias Temperature responsive polymers offer high potential for applications involving chemical sensing and/or stimuli-driven actuation, but their proliferation has been hampered by the inability to tailor by design the onset-point of their response. A systematic series of temperature-responsive polymers were designed, synthesized, and studied, and the onset of their T-response was tailored by design of their monomer. Their T-response was studied both for their water solutions, and when they were end-tethered on a surface. Thermodynamic considerations for the monomer design, afford the possibility to fine-tune the lower critical solution temperature (LCST) point, at values ranging from 5 to 70$^{\circ}$C, in water. Solubility studies and phase diagrams will be presented for their aqueous solutions, whereas water contact angle, ellipsometry, and atomic force microscopy will be shown for end- grafted polymers. [Preview Abstract] |
Tuesday, March 22, 2005 3:30PM - 3:42PM |
L40.00004: Structure and Phase Behavior of End-Tethered Weak Polyelectrolytes Igal Szleifer, Peng Gong The behavior of weak polyelectrolytes tethered at one of their ends to a surface and/or interface is studied using a molecular theory that explicitly accounts for the inhomogeneous acid-base equilibrium of the polymer segments. The predictions for the thickness of polyacrylic acid as a function of salt concentration are in excellent quantitative agreement with the experimental observations from the Genzer group. The local degree of dissociation of the polymer segments varies with bulk salt concentration and the local pH can change by two units in the interior of the polymer layer. In general, water is a relatively poor solvent for the polymer segments, which are soluble due to the presence of the charged groups. Therefore, one expects phase separation as the quality of the solvent decreases, depending on the degree of charging of the polymer. We will show how the phase diagrams of mobile weak polyelectrolytes tethered layers depend upon the salt concentration and bulk pH, together with the structure of the polymer layers along the coexistence curves. The two coexisting phases show very different degree of charge and as a result different structural properties. The possibility of microphase separation in the case of polymers end-grafted to the surface will be discussed. The implications of the results for practical application of weak polyelectrolyte layers will be presented. [Preview Abstract] |
Tuesday, March 22, 2005 3:42PM - 3:54PM |
L40.00005: How do grafting points influence the structure formation in binary and one-component polymer brushes? Marcus M{\"u}ller, Ludger Wenning Grafting of incompatible polymers onto a substrate one prevents macroscopic phase separation. Theory predicts a rich phase diagram of laterally periodic morphologies of brushes consisting of incompatible polymers or one-component brushes in a bad solvent, however, structures observed in experiments lack long-range periodic order.\\ We employ MC simulations of a coarse-grained model to investigate the influence of spatial correlations of the grafting points (patterns) onto the morphology of one component brushes in a bad solvent and binary brushes. Comparing morphologies on identical sets of grafting point we observe a pronounced correlation between the average morphology of the brush and density fluctuations of the grafting points. These fluctuations prevent long-range ordering. Rather than a sharp thermodynamic transition, we observe a gradual building up of local structure upon increasing the incompatibility. The structure formation occurs at smaller incompatibility and the length scale is slightly larger than in case of grafting on a regular lattice. Different morphologies as a function of composition give rise to very similar structure factors but can be well distinguished by their Euler-characteristics. [Preview Abstract] |
Tuesday, March 22, 2005 3:54PM - 4:06PM |
L40.00006: Dendronized polymer is a Single Molecule Glass Jayajit Das, Yoshida Masaru, Zachary Fresco, Tae-Lim Choi, Jean Frechet, Arup Chakraborty The molecular architecture of dendronized polymers can be tuned to obtain nanoscale objects with desired properties. In this work, we bring together experiments and computer simulations to study the thermodynamic and dynamic properties of a single dendronized polymer chain. We find that, upon changing certain architectural features, dynamic correlations characterizing backbone conformational fluctuations of a dendronized polymer exhibit dynamical arrest akin to glass-forming bulk liquids. Thus, a dendronized polymer chain is a novel macromolecule that is a single molecule glass. The range of conditions that leads to dynamic arrest does not, however, correspond to any thermodynamic singularities. Therefore, a dendronized polymer provides the first example of an experimental system that can directly test theories of constrained dynamics. We also show that defect densities characteristic of typical synthesis conditions do not alter the material properties of dendronized polymers. The self-assembly of the chains studied using the results of the single chain yields different phases from lamellar to gyroid phases and nematic phases depending the relative volume fractions of the backbone and the dendron units and the flexibility of the backbone. [Preview Abstract] |
Tuesday, March 22, 2005 4:06PM - 4:18PM |
L40.00007: Understanding the Assembly of Pi-Conjugated Dithiol Molecules on Metal and Semiconductor Surfaces Yueh-Lin Loo, Dmitry Krapchetov, Hong Ma, Alex Jen, Daniel Fischer We examined the assembly of terphenyl- (TPDT) and quaterphenyl-dithiol (QPDT) molecules on gold and gallium arsenide (GaAs) surfaces from ethanol (EtOH), tetrahydrofuran (THF), and mixtures of the two solvents using a combination of x-ray photoelectron spectroscopy, synchrotron-based near-edge x-ray absorption fine structure spectroscopy, and Fourier transform infra-red spectroscopy. While the molecular assembly on gold is solvent independent, our experimental results suggest that the assembly of both TPDT and QPDT on GaAs is extremely solvent dependent. Specifically, TPDT and QPDT form highly oriented monolayers with excellent surface coverage on gold substrates regardless of the solvent from which assembly occurred. When the molecules are assembled on GaAs, however, the surface coverage degrades with increasing THF fraction. Correspondingly, the molecules also become progressively less ordered. When assembled from pure THF, the molecules on GaAs are completely disordered and exhibit poor surface coverage. The origin of this dramatic solvent effect is currently under investigation. [Preview Abstract] |
Tuesday, March 22, 2005 4:18PM - 4:30PM |
L40.00008: Adhesive Transfer of Thin Viscoelastic Films Kenneth Shull, Rachel McSwain Micellar suspensions of acrylic diblock copolymers are excellent model materials for studying the adhesive transfer of viscoelastic solids. The micellar structure is maintained in films with a variety of thicknesses, giving films with a well-defined structure and viscoelastic character. Thin films were cast onto elastomeric silicone substrates from micellar suspensions in butanol, and the adhesive interactions between these coated elastomeric substrates and a rigid indenter were quantified. By controlling the adhesive properties of the film/indenter and film/substrate interfaces we were able to obtain very clean transfer of the film from the substrate to the portion of the glass indenter with which the film was in contact. Adhesive failure at the film/substrate begins with the nucleation of a cavity at the film/substrate interface, followed by complete delamination of this interface. The final stage in the transfer process involves the failure of the film that bridges the indenter and the elastomeric substrate at the periphery of the contact area. This film is remarkably robust, and is extended to three times its original length prior to failure. Failure of this film occurs at the periphery of the indenter, giving a transferred film that conforms to the original contact area between the indenter and the coated substrate. [Preview Abstract] |
Tuesday, March 22, 2005 4:30PM - 4:42PM |
L40.00009: Adhesion Induced Instability in Thin Polymer Films Manoj Chaudhury Geometric confinement strongly influences the adhesion and fracture of soft elastic films. Aided by external force, confined elastic films develop undulations almost instantaneously and instability patterns appear in the form of cavitations and fingers at the interface. The characteristic wavelength of the instability is remarkably insensitive to the materials properties of the system except the thickness of the film. Geometric confinement and its relief via pattern formation shed light on the mechanism of adhesion of various thin film adhesives. In particular, when discontinuities are generated on the films via incisions, the length scale of the instability dictate to which the incised segments communicate with each other via shear field. This information is useful in the design of the adhesives that mimic the behavior of those found in biological world. . [Preview Abstract] |
Tuesday, March 22, 2005 4:42PM - 4:54PM |
L40.00010: Symmetry, Equivalence and Molecular Self-Or\-ganization Jack Douglas, Kevin Van Workum Molecular self-organization at equilibrium is central to the formation of many biological structures and the emulation of this process through the creation of synthetic counterparts offers great promise for nanofabrication. The central problems in this field area are an understanding of how the symmetry of the interacting particles encodes the geometry of the organized structure and the nature of the thermodynamic transitions involved. Our approach is inspired by the self-organization of actin, tubulin and the icosahedral self-organization of clathrin and spherical viruses and proceeds from the general observation that biological macromolecules and synthetic molecules exhibiting supermolecular self-organization often exhibit large dipolar or other highly directional interactions, in addition to short range interactions responsible for phase separation. Correspondingly, we find chain-like, membrane-like, tubular and icosahedral particle self-organization using `equivalent' particles exhibiting an interplay between directional (dipolar and multipolar) interactions and short-range (van der Waals) interactions. Specifically, a dipolar potential having a continuous rotational symmetry gives rise to chain formation while potentials having discrete rotational symmetries [e.g., square quadrupole or a triangular ring of dipoles (`hexapole')] led to the self-organization of sheets, hollow tubes and icosahedral structures with resemblance to biological and synthetic structures. [Preview Abstract] |
Tuesday, March 22, 2005 4:54PM - 5:06PM |
L40.00011: Direct Comparison of Surface and Bulk Relaxation of PS - A Temperature Dependent Study Wen-li Wu, Sharadha Sambasivan, Chia-Ying Wang, William E. Wallace, Jan Genzer, Daniel A. Fischer Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy was used to measure simultaneously the relaxation rates of polystyrene (PS) molecules at the free surface and in the bulk. The samples were uniaxially oriented at room temperature via a modified cold rolling process. The density of the oriented samples as determined by liquid immersion technique is identical to that of bulk PS. At temperatures below its bulk glass transition temperature the rate of surface and bulk chain relaxation was monitored by measuring the partial-electron yield (PEY) and the fluorescence NEXAFS yields (FS), respectively, both parallel and perpendicular to the stretching direction. The decay rate of the dichroic ratios from both PEY and FY at various temperatures was taken as a measure of the relaxation rate of surface and bulk molecules respectively. In addition, the decay rate of the optical birefringence was also measured to provide an independent measure of the bulk relaxation. Relaxation of PS chains was found to occur faster on the surface relative to the bulk. The magnitude of the surface glass transition temperature suppression over the bulk was estimated to be 18 \r { }C based on the measured temperature dependence of the relaxation rates. [Preview Abstract] |
Tuesday, March 22, 2005 5:06PM - 5:18PM |
L40.00012: Does Coarsening Begin During the Initial Stages of Spinodal Decomposition? Nitash Balsara, Timothy Rappl We have studied the early stages of spinodal decomposition for a critical 50/50 binary blend of high molecular weight rubbery polymers by time-resolved small angle neutron scattering. Many aspects of the data are described by the well-established linearized theory of Cahn, Hilliard and Cook. In this theory, the time-dependent scattering profiles are given by three time-independent but wave vector ($q)$ dependent functions: the initial structure factor [$S_{0}(q)$], the terminal structure [$S_{t}(q)$], and a kinetic parameter [R(q)]. Phase separation leads to a periodic bicontinuous structure with a well-defined lower cut-off. This lower cut-off is characterized by a pole in $S_{t}(q)$ and $R(q)$=0. The linearized theory also predicts a wave vector ($q_{peak})$ corresponding to a maximum in $R(q)$. Our experiments do not support this prediction. Instead, the scattering peak decreases linearly with timer indicating that coarsening occurs throughout the initial stages of spinodal decomposition. [Preview Abstract] |
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L40.00013: Temperature-Dependent Conformational Changes of PNIPAM Grafted Chains in Water: Effects of Molecular Weight and Grafting Density Michael Kent, Hyun Yim, Sergio Mendez, S.S. Balamurugan, S. Balamurugan, Gabriel Lopez, Sushil Satija, Young-Soo Seo Poly(N-isopropyl acrylamide) (PNIPAM) is perhaps the most well known member of the class of responsive polymers. Free PNIPAM chains have a lower critical solution temperature (LCST) in water at about 31$^{\circ}$ C. This very sharp transition (about 5$^{\circ}$ C) is attributed to alterations in the hydrogen bonding interactions of the amide groups. Grafted chains of PNIPAM have shown promise for creating responsive surfaces. Conformational changes of the polymer are likely to play a role in some of these applications, in addition to changes in local interactions. In this work we investigated the temperature-dependent conformational changes of grafted PNIPAM chains in D2O over a range of surface density and molecular weight using neutron reflection and AFM. The molecular weight and surface density of the PNIPAM brushes were controlled using atom transfer radical polymerization (ATRP). We discovered a strong effect of surface density and molecular weight. Large conformational changes were observed for intermediate grafting densities and high molecular weights. [Preview Abstract] |
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