Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session J30: Elastomers and Gels |
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Sponsoring Units: DPOLY Chair: Claude Cohen, Cornell University Room: LACC 505 |
Tuesday, March 22, 2005 11:15AM - 11:27AM |
J30.00001: Mechanical and swelling properties of end-linked polydimethylsiloxane networks with hydrogen bonding or ionic interactions Claude Cohen, Ashish Batra We have synthesized a series of endlinked polydimethylsiloxane networks in which the polydimethylsiloxane chain has carboxyl groups at regular intervals along the polymer backbone. The spacing between these carboxyl groups and the number of these carboxyl groups/chain has been varied. Modulus studies at room temperature when carboxyl groups have very weak interactions show moduli similar to conventional endlinked PDMS networks. At high temperatures such as 150 C, carboxyl groups undergo intermolecular interactions and act as additional cross-link points thereby strengthening the networks. On bringing the networks back back to room temperature a number of these intermolecular bonds are retained. Swelling data on networks annealed at 150 C for several days as well as those not annealed will be presented. Carboxyl based networks have also been converted to Gallium and Cobalt networks by treatment with appropriate salts. Swelling and moduli studies at room temperature and on thermally annealed samples of these ionomer networks with both covalent endlinks and physical ionic domains will be presented. [Preview Abstract] |
Tuesday, March 22, 2005 11:27AM - 11:39AM |
J30.00002: Nonaffinity and nonlinearity in random elastic networks Brian DiDonna, Tom Lubensky, Paul Jamney We develop a general framework for the elasticity of networks with spatially varying elastic constants. We consider spring networks with randomness in either the spring constants, node position and connectivity, or internal stresses (through frustrated bond lengths). The non-affine component of the elastic response and corrections to bulk elasticity are calculated as a function of the magnitude and spatial correlations of the effective local elastic constants. Our calculations are verified through extensive numerical simulations. We believe this framework will apply to stiff polymer gels, foams, and random bead packings. [Preview Abstract] |
Tuesday, March 22, 2005 11:39AM - 11:51AM |
J30.00003: MD simulations of chemically reacting networks Dana Rottach, John Curro, Aidan Thompson, Gary Grest The aging of polymeric networks is difficult to investigate in the laboratory. Chemical control is at the mercy of nature, and resulting network structures aren't easily observable. Molecular dynamics simulations, without these limitations, are used to perform virtual stress relaxation and permanent set measurements. Tobolsky's Independent Network Model (INM) and Flory's modifications are tested. The INM states that crosslinks added to an existing network can be treated as forming a second, parallel network. Simulations of these two-stage systems support this model. Flory hypothesized that the effect of removing first stage crosslinks is reduced by an effective switching of second-stage crosslinks. This is supported by simulations in which all first-stage crosslinks are removed. Tobolsky and Flory assumed Gaussian network segments and, more problematically, affine deformation in the derivation of quantitative relationships for the INM. The simulation results match their predictions only qualitatively. More advanced models of elasticity are discussed. [Preview Abstract] |
Tuesday, March 22, 2005 11:51AM - 12:03PM |
J30.00004: Thiol-Vinyl Photopolymerizations: Controlled Network Evolution Sirish Reddy, Amber Rydholm, Kristi Anseth, Christopher Bowman Presently, the photopolymerization field is dominated by acrylic systems. Thiol-vinyl photopolymerizations have many advantages over acrylic polymerizations, like reduced oxygen inhibition, initiatorless polymerization, delayed gelation, and greater control over network structure. However, currently there are no theories for predicting and thereby controlling network evolution of these kinetically controlled mixed step-chain thiol-vinyl polymerizations. Here, a combined kinetic and statistical modeling framework is developed to predict network properties including molecular weight, gel point, and crosslinking density. Further, non-mean field kinetic modeling is used to include network non-ideality, cyclization, and ensuing network predictions are successfully contrasted with experiments. Then, network control aspect of these polymerizations is employed to design degradable materials with tunable degradation kinetics that are suitable for controlled drug delivery and tissue engineering. Both the network properties and concomitant degradation kinetics are adjusted by changing thiol functionality or stoichiometric ratio. [Preview Abstract] |
Tuesday, March 22, 2005 12:03PM - 12:15PM |
J30.00005: Traveling Waves in a Reactive Polymer Gel Victor V. Yashin, Anna C. Balazs We consider a theoretical model of a polymer gel, which exhibits a swelling-deswelling behavior in response to the Belousov-Zhabotinsky (BZ) reaction. The BZ reaction generates periodic redox changes of a metal catalyst, and a wide variety of spatiotemporal structures have been observed in the course of the BZ reaction in solution. If the catalyst is covalently bonded to a responsive hydrogel soaked in a solution containing the rest of the BZ reagents, then the metal redox changes may cause variations in the gel volume. The self-oscillation of the gel volume and the traveling chemical waves accompanied by the local swelling have been experimentally observed by Yoshida and co-workers. Here, we present a simplified theoretical description of a hydrogel with the BZ reaction. The description is based on the Oregonator model of the BZ reaction, and on the two-fluid model of the gel dynamics. The formulated model is applied to studying one-dimensional wave trains in the reactive gel. We focus on the dispersion law as it reflects the inherent coupling between the chemical and mechanical degrees of freedom. [Preview Abstract] |
Tuesday, March 22, 2005 12:15PM - 12:27PM |
J30.00006: Scaling of entropic shear rigidity Swagatam Mukhopadhyay, Xiangjun Xing, Paul Goldbart The scaling of shear modulus near the gelation/vulcanization transition is explored heuristically and analytically [1]. It is found that in a dense melt the effective chains of the infinite cluster have sizes that scale{\it sub-linearly\/} with their contour length. Consequently, each chain contributes $k_{\rm B} T$ to the rigidity, which leads to a shear modulus exponent $d\nu$. In contrast, in phantom elastic networks the scaling is {\it linear\/} in the contour length, yielding an exponent identical to that of the random resistor network conductivity, as predicted by de Gennes. For non-dense systems, the exponent should cross over to $d\nu$ when the percolation correlation length is much larger than the density-fluctuation length. [1] X. Xing, S. Mukhopadhyay and P. M. Goldbart, Phys. Rev. Lett. 93, 225701 (2004). [Preview Abstract] |
Tuesday, March 22, 2005 12:27PM - 12:39PM |
J30.00007: A cavity approach to the heterogeneity of the random solid state Xiaoming Mao, Paul Goldbart, Marc Mezard, Jean-Philippe Bouchaud Replica statistical mechanics has been invoked to explore a wide collection of properties of the random solid state of matter, as formed, e.g., by vulcanized macromolecular systems. Even at the mean-field level, this approach yields a rich and illuminating description of the state in terms of a fraction of localized particles and the statistical distribution of their localization lengths. An application of the cavity method -- successfully used to analyze a wide range of spin glass models and other statistical-mechanical systems with quenched disorder -- allows replica-based results for random solids to be recovered in a way that sheds light both on their physical origin and their limitations. An extension of this cavity approach, involving an assembly of Ornstein-Uhlenbeck processes, points towards a strategy for addressing certain aspects of the dynamics of the random solid state, at least at the mean-field level. [Preview Abstract] |
Tuesday, March 22, 2005 12:39PM - 12:51PM |
J30.00008: Structural changes in polymer gels probed by Fluorescence Correlation Spectroscopy Ariel Michelman-Ribeiro, Hacene Boukari, Ralph Nossal, Ferenc Horkay We apply fluorescence correlation spectroscopy (FCS), a non-invasive optical technique, to measure the diffusion of small fluorescent probes (TAMRA, Mw = 430 Da; TAMRA-labeled dextran, Mw = 10 kDa) in semidilute, non-fluorescent --hence invisible-- poly(vinyl alcohol) (PVA, Mw = 85 kDa) solutions and cross-linked PVA gels. The measurements indicate that for the same polymer concentration, the diffusion of the particles slows down when the polymer solution is cross-linked. Further, the more the polymer chains are cross-linked, the slower the particles diffuse. We attribute this effect to the formation of large-scale structural changes caused by cross-linking of the PVA chains. These results suggest that the cross-link density, which is often ignored in the analysis of probe diffusion data in gels, must be taken into account. Measurements of the elastic modulus support this conclusion, as indicated by the linear correlation between the diffusion time of the particles and the elastic modulus of the gels. [Preview Abstract] |
Tuesday, March 22, 2005 12:51PM - 1:03PM |
J30.00009: The elasticity of smectic liquid crystal elastomers James Adams, Mark Warner The elasticity of smectic elastomers is remarkable: response is solid-like along the layer normal and rubbery in the plane. These 2-D elastomers have extreme Poisson ratios and a reversible threshold above which the elastic modulus is drastically reduced. This behavior can be understood from a microscopic model based on the anisotropic chain shape of liquid crystalline polymers that couple to the smectic layers through the crosslinks. This coupling constrains the layer normal to deform affinely with the rubber matrix. Additionally chain shape distribution anisotropy colors the complex shears occurring after the mechanical instability occurs. Results fit well with experimental elastic and X-ray scattering data. \newline Smectic C elastomers are predicted to display soft elasticity, accompanied by an intricate microstructure to accommodate the strains required for softness. They are anticipated to be of great technological importance because their symmetry permits piezoelectricity, ferroelectricity and second harmonic generation. [Preview Abstract] |
Tuesday, March 22, 2005 1:03PM - 1:15PM |
J30.00010: Self-Adhesion of uncrosslinked elastomers using a probe method Regis Schach, Costantino Creton Relatively few studies have been carried out on the adhesion between uncrosslinked elastomers. A key experimental obstacle for the understanding of this problem is the separation of the surface and the bulk contributions to adhesion for such highly deformable and viscoelastic materials. A modification of the probe tack experiment used in the Pressure Sensitive Adhesive industry allows us to study the self-adhesion of elastomers with a very good control of the experimental parameters (contact time down to 1s, pressure of contact, debonding velocity). A video acquisition also allows the detailed analysis of the debonding mechanisms. We present here results on the self adhesion of three SBR Rubbers with the same microstructure (20{\%} styrene, 42{\%} vinyl, 19{\%} cis and 19{\%} trans, Mw/Mn lower than 1.1) but with different molecular weights (80~000, 160~000 and 240~000 g/mol). We observed different debonding mechanisms depending on the time of contact, the debonding velocity and the polymer used. We found that these different fracture behaviours are directly related to the bulk rheology of the polymers, especially their reptation time. Finally, we propose a map of the mechanisms as a function of two reduced parameters, the ratio of contact time to reptation time and the Deborah number. [Preview Abstract] |
Tuesday, March 22, 2005 1:15PM - 1:27PM |
J30.00011: Effects of Substitutes on the Self-Assembling of Rigid Polymers Yunfei Jiang, Dvora Perahia, Uwe H.F. Bunz Beyond the basic understanding of the assembly process of highly conjugated rigid polymers, these macromolecules are inherently semiconductors and may be used as molecular wires with the appropriate doping. In device applications, the molecules have to be in contact with either an interface or other molecules. In an effort to correlate the elctro-optical properties of substituted poly(para phenyleneethynylene) (PPE), we have recently shown that PPE substituted by short alkyl chains, associate to form flat aggregates in solutions of toluene as well as at interfaces. With increasing concentration in solution, these aggregates form fragile gels. The shape and size of the PPE aggregates as well as the overall phase diagrams are strongly dependent on molecular parameters and also on the environment in which the association takes place. AFM and small angle neutron scattering studies have shown that a bulky substitutent triisopropylsilyloxy changes the structure of the assembly, the nature of the gels and the morphology at interfaces. [Preview Abstract] |
Tuesday, March 22, 2005 1:27PM - 1:39PM |
J30.00012: Ultrasound Devulcanization of Natural Rubber, Studied by NMR Relaxation and Diffusion E. von Meerwall, J.L. Massey, C.-K. Hong, A.I. Isayev In support of recycling of industrial rubbers, we have studied the effect of intense ultrasound on unfilled natural rubber networks using proton and carbon transverse NMR relaxation and diffusion, sol extraction, and bulk characterization. At 70.5$^{\circ}$ C the proton echo decay exhibits three components, due to entangled sol and crosslinked network; unentangled polymeric sol plus dangling chain ends; and oligomer remnants. Contrary to the 13C results which indicate backbone mobilities decreasing with sonication (hence with increasing sol fraction), all 1H component T2 values, hence intermolecular mobilities, increase by similar modest factors; the network component amplitude decreases strongly. Diffusion of unentangled sol is sharply bimodal, arising from intermediate fractions and unreactive oligomers. Both diffusion rates decrease slightly with sonication in spite of plasticization by sol, because degradation adds sol primarily of intermediate molecular weights. We compare these results with our earlier work in SBR. Although ultrasound devulcanization does not recover many properties of the virgin melt, recyclability is not compromised. [Preview Abstract] |
Tuesday, March 22, 2005 1:39PM - 1:51PM |
J30.00013: Local Conversion model for Phase Diagrams and Calorimetric properties of gel-forming LCST-type polymers. Fraancisco Solis, Brent Vernon Assuming a sharp internal conversion of the properties of monomers in a polymer system, we derive the phase diagrams and calorimetric properties of gel-forming LCST-type (lower critical solution temperature) polymers. The transition models the release of water molecules associated with the polymer backbone. After the release, monomers are free to associate and form gel structures. We compare these results with properties of experimentally studied systems. [Preview Abstract] |
Tuesday, March 22, 2005 1:51PM - 2:03PM |
J30.00014: Studies on Phase Separation in a-PMMA/PEG Gels Xiaoliang Wang, Liang Li, Dongshan Zhou, Gi Xue Stereo-irregular atactic poly(methyl methacrylate) ($a$-PMMA) is known incapable of forming gels in common solvents, irrespective of the solvent quality. However, we recently found a rigid opaque thermal-reversible $a$-PMMA gel in the solvent of the polyethyl glycol oligomer (PEG) (the PEG molecule mass differ from 400 to 4000 were used). FT-IR, dynamic mechanical temperature analysis and Solid state NMR measurements were used to study the gel properties and gelation mechanism. The \textit{in situ }IR studies in $a$-PMMA/PEG gel suggested that some $a$-PMMA segments were in the aggregated state in solution, which became a node in the solution. With decreasing temperature, the fraction of aggregated $a$-PMMA in solution increases, resulting in the formation of physical network finally. Spin diffusion was used to determine the size (\textit{$\xi $}) of domains in the gels. We found that, $a$-PMMA/PEG4000 was miscible (\textit{$\xi $ }$\sim $ 9nm), while $a$-PMMA/PEG1000 was micro phase separated (\textit{$\xi $ }$\sim $ 57nm) and $a$-PMMA/PEG400 was macro phase separated (\textit{$\xi $} $>$ 300nm). The $a$-PMMA self-aggregation was attributed to the depletion interaction that becomes important in the case of middle-sized solvents. [Preview Abstract] |
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J30.00015: Stretching Networks of Helical Polymers Gustavo A. Carri, Richard Batman We present a computational study of the stress-strain behavior of a network of helical polymers. For this study, we employ a combination of Monte Carlo simulations based on the Wang-Landau algorithm and the traditional three-chain model of polymer networks. The helical polymers are described with a recently developed model (V. Varshney \textit{et. al}., \textit{Macromolecules} \textbf{2004}, $37$, 8794) that has proven to capture the configurational, conformational and thermodynamic properties of single helical polymers correctly. In this talk, we will focus on the mechanical and thermodynamic properties of the network as a whole together with the conformational and configurational characteristics of a single chain in the network. We find that the mechanical response of the network is strongly dependent on temperature. For example, for the same amount of strain the network is stiffer at temperatures below the helix-coil transition temperature than at temperatures above it. In addition, we find that the stress-strain curve is \textit{non-monotonic}, indicating that the network \textit{softens} at high enough elongations due to the \textit{force-induced melting} of the helical structures. [Preview Abstract] |
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J30.00016: Normal stresses and elastic modulus in sol gels polyester blends Suresh Ahuja Visco-elasticity and normal stresses in gels can be analyzed using scaling arguments. In our study, polyester gels were made and melt mixed in an extruder at different volume concentrations. Normal stresses and elastic modulus as functions of volume concentration and fractals were analyzed and compared to existing models. [Preview Abstract] |
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