Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session W25: Gels and Networks |
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Sponsoring Units: DPOLY Chair: Vijay Tirumala, National Institute of Standards and Technology Room: Baltimore Convention Center 322 |
Thursday, March 16, 2006 2:30PM - 2:42PM |
W25.00001: Tunable and Reversible Swelling of a p(tBA)-$b$-p(HEMA-\textit{co}-DMAEMA) Block Copolymer Kyle Guice, Yueh-Lin Loo Hydroxyethyl methacrylate (HEMA) and dimethylaminoethyl methacrylate (DMAEMA) have been investigated as precursor materials for pH-responsive hydrogels. DMAEMA in these hydrogel systems allows for pH-tunability, as it is reversibly protonated below its pKa (7.5). In this work, we present the design of a nano-structured hydrogel diblock copolymer whose major block consists of a statistical copolymer of p(HEMA-\textit{co}-DMAEMA) (30.5 kg/mol) polymerized at the azeotropic composition (71 mol{\%} HEMA), with a poly(tert-butyl acrylate), p(tBA), (12.1 kg/mol) minor block. The resulting diblock copolymer is narrow in molecular weight distribution (PDI = 1.24) and spontaneously self-assembles to form hexagonally-packed p(tBA) cylinders (R = 9.5 nm) within a p(HEMA-\textit{co}-DMAEMA) matrix in the solid state. When swollen in an aqueous medium, hydrophobic p(tBA) cylinders serve as physical cross-links. We monitor the extents of swelling by quantifying changes in the characteristic (10) spacing of the hexagonal lattice by SAXS. Swelling is tunable and reversible with changes in pH; we observe 35{\%} and 21{\%} swelling relative to the dry state at pH 5 and 8.5, respectively. [Preview Abstract] |
Thursday, March 16, 2006 2:42PM - 2:54PM |
W25.00002: Highly Responsive Self-Assembled Gels from Triblock Copolymers in Liquid Crystal Solvent Neal Scruggs, Rafael Verduzco, Julia Kornfield Triblock copolymers having random coil endblocks and a side-group liquid crystalline polymer (SGLCP) midblock self-assemble in small-molecule liquid crystal (LC) solvent to form highly responsive gels. In these block copolymers, the LC solvent switches from being strongly selective toward the SGLCP block below its isotropic-nematic transition (T$<$T$_{NI})$ to being a good solvent for both blocks in the isotropic phase. In the nematic phase, the LC solvent is a poor solvent for the polystyrene (PS) endblocks, driving them to physically associate to form the network crosslinks. In the isotropic phase, at dilute polymer concentrations, the PS endblocks are solubilized to yield a solution of free polymer chains. Synergistic coupling between polymer and solvent results in a gel with novel properties; the orientational order of the nematic LC solvent imparts electo-optic and mechano-optic properties that are forbidden by symmetry in isotropic gels, and the polymer network provides memory via long-time relaxation processes that do not exist in the bulk LC. Gels can be aligned into a single-crystal monodomain by applied shear, electric fields, magnetic fields, or surface effects, and the alignment state is preserved by the network's elastic restoring force. Insights into the structure and dynamics of the gels are gained by rheometry, small-angle neutron scattering, and electro-optical switching experiments. [Preview Abstract] |
Thursday, March 16, 2006 2:54PM - 3:06PM |
W25.00003: Nanoparticle-reinforced associative PLA-PEO-PLA hydrogels Sarvesh Agrawal, Naomi Sanabria-Delong, Surita Bhatia, Gregory Tew Hydrogels of poly(lactide)-poly (ethylene glycol)-poly (lactide) have potential applications in drug delivery and tissue engineering. Control over the structure and rheology of the gels is of fundamental importance for the use of this polymer in medical applications. We have performed a complete rheological and structural characterization of these hydrogels using dynamic mechanical rheology, SANS, and USAXS. These polymers form very stiff hydrogels and the structure and properties of these materials can be substantially modified by varying the crystallinity or degree of polymerization (DP) of the hydrophobic PLA block. We have also created reinforced hydrogels with enhanced mechanical properties by addition of laponite nanoparticles. Our recent studies show that the elasticity of the PLA-PEO-PLA hydrogels can be enhanced by orders of magnitude by addition of small amounts of laponite particles to the hydrogels. It is expected that the triblock copolymer micelles adsorb on the surface of the laponite particles to form additional junctions in the hydrogels leading to enhancement in their elasticity. We verify this hypothesis using DLS and SANS techniques. [Preview Abstract] |
Thursday, March 16, 2006 3:06PM - 3:18PM |
W25.00004: Controlling the Self-Assembly of ABCBA Pentablock Copolymer Gels in Water Solution by the Hydrophobic Effect Joshua Anderson, Alex Travesset We characterize the phases of a system of non-ionic pentablock copolymers with an ABCBA structure in water solution, where the A and C blocks are hydrophobic and the B blocks are hydrophilic. Coarse-grained simulations are performed using molecular dynamics with the solvent modeled implicitly, and the interaction potential includes a parameter that controls the quality of the solvent. In a good solvent, spherical micelles form and assemble into a swollen gel. We examine the aggregation number, gyration radii, micelle superstructures and percolation at various concentrations for this phase. As the B blocks become less hydrophilic, which occurs for increasing temperature, the micelles move close to one another and expel water. There is a gradual phase transition from spherical micelles to cylindrical worm-like micelles. We model the further expelling of water by increasing the concentration of polymers in the simulation and find that a lamellar phase forms. We compare our simulations with experimental results on recently synthesized modified Pluronic systems. [Preview Abstract] |
Thursday, March 16, 2006 3:18PM - 3:30PM |
W25.00005: PEO Hydrogels Prepared by End-linking with PAMAM Dendrimers Burcu Unal, Ronald C. Hedden End-linking is a preferred synthetic technique for preparing polymer networks and gels for fundamental structure/property studies. End-functionalized telechelic linear polymers are joined to a multifunctional crosslinker to form a network in which the molar mass of the polymer chains between chemical crosslink points is known. Although end-linked elastomers prepared in bulk have been well-studied over the preceding decade, much remains to be learned about how the presence of a good solvent affects the equilibrium swelling and modulus of end-linked gels. We prepared well-defined hydrogels in a good solvent (water) by linking epoxide end-functionalized, linear poly(ethylene oxide) (PEO) to the amine endgroups of poly(amidoamine) (PAMAM) dendrimers of generations 0, 2, and 4. Dendrimers can serve as well-defined macromolecular crosslink junctions because they can have nearly monodisperse numbers of reactive endgroups. We have characterized how reaction conditions such as junction functionality, polymer concentration at preparation, ratio of crosslinker endgroups to precursor endgroups, and precursor molar mass affect gelation and equilibrium swelling. We will discuss the somewhat surprising observation of ``superabsorbent'' behavior in selected PAMAM- PEO gels. [Preview Abstract] |
Thursday, March 16, 2006 3:30PM - 3:42PM |
W25.00006: Probe diffusion in polymer solutions and hydrogels using fluorescence correlation spectroscopy Ariel Michelman-Ribeiro, Hacene Boukari, Ferenc Horkay, Ralph Nossal We apply fluorescence correlation spectroscopy (FCS) to measure the diffusion of small fluorescent probes (TAMRA, Mw = 430 Da; dextran, Mw = 10 kDa) in poly(vinyl alcohol) (PVA) solutions and hydrogels. PVA is a linear, neutral, biocompatible polymer, whose hydrogels have many biotechnology applications, such as drug-delivery devices and tissue scaffolds. The FCS measurements indicate that the probe diffusion decreases when the polymer solution is cross-linked. Further, the more the polymer chains are cross-linked, the slower the particles diffuse. 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. Remarkably, we find that the apparent diffusion time and the elastic modulus of the gels show a linear correlation. [Preview Abstract] |
Thursday, March 16, 2006 3:42PM - 3:54PM |
W25.00007: Large-strain deformation and fracture of tough hydrogels Rebecca Webber, Guillaume Miquelard, Costantino Creton, Jian Ping Gong Highly-swollen, chemically-crosslinked hydrogels generally behave in a very brittle manner, fracturing suddenly after a small amount of reversible deformation. Because of their importance as biomaterials, it is useful to control and augment the resistance to fracture of these materials. Tougher, stronger hydrogels are emerging, and it is important to understand the structural origins of strength in these relatively robust, highly-swollen, polymer systems. We have investigated the rheological, mechanical and fracture properties of tough hydrogels, using novel testing techniques and focusing on the high-strain compression and tension behavior. Results from large-strain and fracture experiments were correlated to the chemical structure of the hydrogels. Because we believe that the mechanical properties of these tough hydrogels are due to the presence of dissipative mechanisms at the molecular level, we have explored several methods of synthesis to create these materials. [Preview Abstract] |
Thursday, March 16, 2006 3:54PM - 4:06PM |
W25.00008: Rheological behavior of Slide Ring Gels. Vivek Sharma, Jong Seung Park, Jung O. Park, Mohan Srinivasarao Slide ring gels were synthesized by chemically crosslinking, sparsely populated $\alpha $-cyclodextrin ($\alpha $-CD) present on the polyrotaxanes consisting of $\alpha $-CD and polyethylene glycol (PEG). [1] Unlike physically or chemically crosslinked gels, slide ring gels are topological gels where crosslinks can slide along the chain. [2] We investigate the rheological behavior of these gels swollen in water and compare their viscoelastic properties to those of physical and chemical gels. We also study the equilibrium swelling behavior of these gels. [1] Okumura and Ito, Adv. Mater. 2001, 13, 485 [2] C. Zhao et al, J. Phys. Cond. Mat. 2005, 17, S2841 [Preview Abstract] |
Thursday, March 16, 2006 4:06PM - 4:18PM |
W25.00009: Mechanical and swelling properties of PDMS interpenetrating polymer networks Claude Cohen, Seong Hyun Yoo Poly(dimethylsiloxane) (PDMS) interpenetrating networks (IPNs) of a large and a small molar mass PDMS were prepared. Six series of IPNs were obtained by first tetra-functionally end-linking long vinyl-terminated PDMS neat or in a 50 per cent solution with unreactive PDMS chains. These networks were then dried and swollen with short reactive telechelic PDMS that were subsequently end-linked. We found that the correlation between modulus (E) and equilibrium swelling (Q) in toluene of the PDMS IPNs obeys a scaling relation identical to that of normal uni-modal PDMS networks. The results of the toughness of the networks represented by the energy required to rupture them were analyzed in terms of a recent model by Okumura (Europhysics Letters 67(3), 470, 2004). A modified version of this model that assumes each component of the double network to be subjected to an equal stress gives a good representation of the data. [Preview Abstract] |
Thursday, March 16, 2006 4:18PM - 4:30PM |
W25.00010: Generation of Oriented Buckling Patterns by Modulation of Local Elastic Moduli Edwin Chan, Alfred Crosby Wrinkling patterns based on elastic instabilities are interesting due to the spontaneous formation of relief structures that consists of a dominant periodicity. While a wide variety of soft materials has been utilized to generate surface buckling patterns, alignment of these structures has only been demonstrated previously through pre-defined topographic patterns. In this contribution, we introduce a new methodology to producing aligned, or patterned, surface wrinkles through the manipulation of the local stress distributions. We define the specific regions of local differences in the elastic moduli of a poly(dimethyl siloxane) (PDMS) elastomer by selective oxidation of the PDMS surface into a silicate thin film. Subsequent swelling with a photopolymerizable monomer provides the buckling stress necessary for the formation of aligned surface wrinkles. We show that geometric confinement of the oxidized regions coupled with an osmotic stress controls the formation and orientation of the wrinkling structures. [Preview Abstract] |
Thursday, March 16, 2006 4:30PM - 4:42PM |
W25.00011: Melting Point Depression of Small Molecules in Cross-linked and Uncross-linked Polyisoprene: Deviations from Flory-Huggins Theory Qian Qin, Gregory McKenna Thermoporosimetry (TPY) is becoming increasingly used to study nano-scale heterogeneity in polymers. The starting point for TPY is the Gibbs-Thomson (GT) relation between melting point and inverse crystal size. In the case of polymers, the Flory-Huggins (FH) model predicts that there is a depression of melting point due to the mixing of the polymer and the solvent molecules, and this needs to be taken into account. The first step in analysis of heterogeneity size using TPY and the GT equation requires that there be quantitative agreement between FH and the melting points in the uncross-linked rubber. We find that both benzene and hexadecane exhibit excessive melting point depressions in uncross-linked polyisoprene. This may imply that the uncross-linked polymer is divided into `nanoheterogeneities.' We further find that the heat of fusion decreases as polymer concentration increases for the benzene, but not for the hexadecane. To our knowledge this is the first systematic investigation of the validity of melting of small organics in un-cross-linked polymers using the FH expressions. [Preview Abstract] |
Thursday, March 16, 2006 4:42PM - 4:54PM |
W25.00012: Elastic Fluctuations and Rubber Elasticity Xiangjun Xing, Paul Goldbart, Leo Rradzihovsky A coarse-grained phenomenological model is constructed to describe both phonon fluctuations and elastic heterogeneities in rubbery materials. It is a nonlocal, spatially heterogeneous generalization of the classical model of rubber elasticity, and with a tunable repulsion interaction. This model can also be derived from the Vulcanization theory. The residual stress and the non-affine deformation field, as well as their correlations, are calculated perturbatively, to the leading order of quenched randomness. It is explicitly shown that the interplay between the repulsive interaction and quenched randomness induces non- affine deformation. The spatial correlations of the non- affine deformation field and residual stress exhibit power-law scaling, with no characteristic length scale. We also calculate the contributions to the elastic free energy from both thermal and quenched fluctuations for arbitrary deformation. We find that they naturally explain the universal features in the Mooney-Rivlin plot of the stress-strain curve for rubbery materials. The (disorder averaged) thermal fluctuation of monomers is shown to depend on deformation, and becomes anisotropic upon shear deformation, as long as the repulsive interaction is finite. [Preview Abstract] |
Thursday, March 16, 2006 4:54PM - 5:06PM |
W25.00013: Developing a lattice spring model to simulate the behavior of polymer gels Victor Yashin, Anna Balazs A basic feature of responsive polymer gels is an inherent coupling of multiple physicochemical processes with a finite deformation of the material. We have developed a new, computationally efficient approach - the gel lattice spring model (gLSM) - which allows us to model responsive gels that undergo relatively large deformations in 2D. We start by writing an equation for the energy of the deformed gel in terms of the invariants of the strain tensor. We introduce the representative, rectangular-shaped unit element of the system, obtain an approximation of the total gel energy as a function of the coordinates (nodes) of this element, and derive the equations for the forces acting on the nodes. In accordance with the two-fluid model of gel dynamics, we assume a purely relaxational dynamics by taking the velocity of a node to be proportional to the force acting on that site. Using this gLSM, we simulate the structural evolution of a swelling gel in 2D, and the propagation of the swelling-deswelling waves through a rectangular chemo-responsive gel undergoing the Belousov-Zhabotinsky reaction. [Preview Abstract] |
Thursday, March 16, 2006 5:06PM - 5:18PM |
W25.00014: MD simulations of chemically reacting networks: analysis of permanent set Dana Rottach, John Curro, Joanne Budzien, Gary Grest, Aidan Thompson The Independent Network Model (INM) has proven to be a useful tool for understanding the development of permanent set in strained elastomers. Our previous work showed the applicability of the INM to our simulations of polymer systems crosslinking in strained states. This study looks at the INM applied to theoretical models incorporating entanglement effects, including Flory's constrained junction model and more recent tube models. The effect of entanglements has been treated as a separate network formed at gelation, with additional curing treated as traditional phantom contributions. Theoretical predictions are compared with large-scale molecular dynamics simulations. [Preview Abstract] |
Thursday, March 16, 2006 5:18PM - 5:30PM |
W25.00015: A Gaussian Slip-Link Model for Polymer Single and Double Networks Jay D. Schieber, Mahnaz Eskandari, Hamid Arastoopour In this study, we developed Schieber's slip-link model for lightly cross-linked polymers assuming the equilibration of deformed Gaussian chains. Our simulation consists of two steps: preparation and deformation. In the preparation step, cross-links and slip-links are assumed to be distributed randomly along the chain, but with independent statistical parameters: the average number of Kuhn steps between entanglements, $N_{e}$, and the average number of Kuhn steps between cross-links, $N_{c}$. In the second step, the cross-links and slip-links are deformed affinely, but since the chain can slide through the slip-links, its deformation is non-affine. The stress tensor can be determined as a function of deformation using Brownian dynamics as a sort of Monte Carlo algorithm. The Mooney plot of our simulation result has good agreement with most experimental data for uniaxial elongation deformation for cross-linked NR, PDMS, and PBd. The model is used to predict values for the Mooney plot parameters ($C_{1}$ and $C_{2}$) as a function of $N_{e}$ and the $N_{c}/N_{e}$ ratio. The $C_{2}/C_{1}$ ratio is found to be strongly dependent on $ N_{c}/N_{e}$, but weakly dependent on $N_{e}$. This observation provides a new way of predicting the cross-link density and separating it from the entanglement density and for systems of known $N_{e}$ and $N_{c}$, the model requires no adjustable parameters. We are also developing our model for double network polymers in order to demonstrate different applications for the model. [Preview Abstract] |
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