Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session Y41: Elastomers and Gels |
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Sponsoring Units: DPOLY Chair: Kevin Cavicchi, University of Akron Room: A115/117 |
Friday, March 25, 2011 8:00AM - 8:12AM |
Y41.00001: Swelling Kinetics of a Microgel Shell Joshua Wahrmund, Jin-Woong Kim, Liang-Yin Chu, Chanjie Wang, Yong Li, Alberto Fernandez-Nieves, David A. Weitz, Arkadii Krokhin, Zhibing Hu Tanaka's approach to swelling kinetics of a solid gel sphere is extended to a spherical microgel shell. The boundary condition at the inner surface is obtained from the minimization of shear elastic energy. Temporal evolution of a shell is represented in a form of expansion over eigenfunctions of the corresponding diffusion equation. The swelling of Tanaka's solid spherical gel is recovered as a special case of our general solution if the inner radius approaches zero. To test our theoretical model, we prepared monodisperse poly-N-isopropylacrylamide (PNIPAM) hydrogel shells using a microfluidic device. The temporal dependence of the inner and outer radii of the shell was measured and the data was fitted to our theoretical model. As a result, we obtained the collective diffusion constants for shrinking and for swelling processes. The obtained values for microgel shells are in excellent agreement with the previous results obtained for sub-millimeter PNIPAM solid spheres in the same temperature interval. Our model shows that the characteristic swelling time of a gel shell should be proportional to the square of its outer radius---just as with Tanaka's model. [Preview Abstract] |
Friday, March 25, 2011 8:12AM - 8:24AM |
Y41.00002: Indentation of polydimethylsiloxane submerged in organic solvents Yuhang Hu, Xin Chen, George Whitesides, Joost Vlassak, Zhigang Suo This study uses a method based on indentation to characterize a polydimethylsiloxane (PDMS) elastomer submerged in an organic solvent (decane, heptane, pentane, or cyclohexane). An indenter is pressed into a disk of a swollen elastomer to a fixed depth, and the force on the indenter is recorded as a function of time. By examining how the relaxation time scales with the radius of contact, one can differentiate the poroelastic behavior from the viscoelastic behavior. By matching the relaxation curve measured experimentally to that derived from the theory of poroelasticity, one can identify elastic constants and permeability. The measured elastic constants are interpreted within the Flory-Huggins theory. The measured permeabilities indicate that the solvents migrate in PDMS by diffusion, rather than by convection. This work confirms that indentation is a reliable and convenient method to characterize swollen elastomers. [Preview Abstract] |
Friday, March 25, 2011 8:24AM - 8:36AM |
Y41.00003: Synthesis and mechanical properties of resilin-like hydrogels Jun Cui, Melissa Lackey, Gregory Tew, Alfred Crosby Resilience measures a material's efficiency for mechanical energy storage. Many materials exhibit high resilience at low strains, but relatively few can maintain this performance at high strain levels. One of the most notable examples of a resilient material is resilin, a protein used strategically when Nature requires elasticity with minimal loss over large deformations. Similar to resilin in many aspects, we present a novel hydrogel network with well-defined architecture by introducing hydrophobic polydimethylsiloxane (PDMS) into hydrophilic polyethylene glycol (PEG)-based network. As a function of the PDMS to PEG ratio, we demonstrate that maximum water content can range from 97{\%} to 80{\%} and Young's modulus from 5kPa to 75kPa. Across this full range of network compositions and water content, the resiliency is nearly 100{\%} for uniaxial strains exceeding 80{\%}. This unique balance of properties is associated with two network attributes: uniformity in network connectivity and negligible secondary structures. [Preview Abstract] |
Friday, March 25, 2011 8:36AM - 8:48AM |
Y41.00004: Pre-Stressed Double Network Elastomers And Hydrogels Naveen Singh, Alan Lesser A new approach to prepare and characterize pre-stressed double network elastomers and hydrogel systems is investigated. In one example, a styrene-butadiene-styrene (SBS) tri-block copolymer system containing physical cross-links is used to achieve a pre-stressed double network by additional chemical crosslinking in a strained state using ultra-violet (UV) light. Unusual physical and mechanical properties that result from the interactions between each network are presented. These double network elastomers show a transition between competitive and collaborative behavior in their mechanical properties, as well as lower permanent set in both low and high strain regimes along with lower hysteresis. These networks exhibit lower modulus, along with lower coefficient of thermal expansion, still showing lower swelling ratios, which results from a competition of the networks. In another example, a new two-step curing schedule is utilized for Polyacrylamide based hydrogels, where a strain is induced in the middle of curing reaction. The final mechanical properties of these double network hydrogels are studied and compared to both first network and the single network formed without any step strain. [Preview Abstract] |
Friday, March 25, 2011 8:48AM - 9:00AM |
Y41.00005: Topological effects on viscoelasticity of polyacrylamide hydrogels Jan Kalfus, Alan Lesser Viscoelastic behavior of long linear chains in a concentrated solution is governed by the topology of the molecules and interchain excluded volume interaction. As a consequence, chain diffusive motion is significantly retarded and such an assembly of chains exhibits highly pronounced entropy elastic behavior. In this contribution, two types of additional chain confinements imposed on a concentrated solution of linear polyacrylamide (L-PAA) will be discussed. The confinement was realized either by adding silica nano-filler into the concentrated solution of L-PAA or by cross-linking of acrylamide in the concentrated solution of L-PAA. While in the first case the trapped entanglement interaction is caused by interaction of chains with large nano-filler surface, in the second case the L-PAA chains are trapped among the cross-links of the PAA network. Viscoelastic response of both types of composite systems exhibited generic characteristics. In both cases, the trapped entanglement interaction significantly changed the relaxation spectrum of the matrix polymer solution and considerably enhanced the linear elastic modulus. [Preview Abstract] |
Friday, March 25, 2011 9:00AM - 9:12AM |
Y41.00006: Memory Effects in Strained Polymer Networks Caused By Multiple Stages of Crosslinking Joanne Budzien Polymer networks crosslinked in multiple strain states usually are analyzed with the independent network model. For networks that undergo scission in addition to crosslinking, however, the networks have been shown not to be completely independent. Even with complete removal of all crosslinks from a given network reacted in a particular strain state, the system still responds as though a portion of the original network remains. This talk will present simulation results of a coarse-grained model that has multiple networks with crosslinking and scission occurring in stages. [Preview Abstract] |
Friday, March 25, 2011 9:12AM - 9:24AM |
Y41.00007: Viscoelastic Properties and Ionic Conductivity of Block Copolymer-Based Ion Gel Electrolytes Sipei Zhang, Keun Hyung Lee, C. Daniel Frisbie, Timothy P. Lodge The viscoelastic properties and ionic conductivity of block copolymer-based ion gels were investigated with polymer concentrations of 10 -- 50 wt{\%} over a temperature range of 25 -- 200 $^{\circ}$C. Ion gels were prepared through the self-assembly of poly(styrene-$b$-ethylene oxide-$b$-styrene) (SOS) and poly(styrene-$b$-methyl methacrylate-$b$-styrene) (SMS) triblock copolymers in a room-temperature ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsufonyl)imide ([EMI][TFSI]). The S end-blocks associate into micelles, whereas the O and M midblocks are well-solvated by this ionic liquid. Under oscillatory mechanical shear, two relaxation modes have been observed in the SMS ion gels. The faster mode corresponds to the relaxation of the M midblocks in the ionic liquid, while the slow mode reflects motion of the S end blocks within their micellar cores. Comparison of the solid gels and the liquid homopolymer solutions showed that the reduction of ionic conductivity of the gels with respect to that of the solutions is relatively small, and depends primarily on the volume fraction of S micelles. [Preview Abstract] |
Friday, March 25, 2011 9:24AM - 9:36AM |
Y41.00008: Fracture Behavior of High-Toughness, Ionically Cross-linked Triblock Copolymer Hydrogels Kevin Henderson, Kathryn Otim, Kenneth Shull Mechanisms for enhancing energy dissipation and hence toughness are important for the generation of robust synthetic soft materials for biomedical applications. Ionic cross-linking in particular has been explored in triblock copolymer hydrogels and affords a remarkable change in mechanical performance comparable to non-cross-linked analogs. Here we employ a physically associated base triblock copolymer network composed of hydrophobic poly(methyl methacrylate) endblocks and a hydrophilic poly(methacrylic acid) midblock capable of complexing with divalent cations. Increases in stiffness and strength have previously been reported, with the extent dependent upon the identity, concentration, and pH of a cross-linking cation solution. We delineate the measured toughness in such systems using tensile tear tests and relate the mechanical performance to a damage zone model reminiscent of loading behavior observed in double network hydrogels. [Preview Abstract] |
Friday, March 25, 2011 9:36AM - 9:48AM |
Y41.00009: Origin of the Toughness and the Elastomeric Properties of Gels from Block Copolymers with Semicrystalline Syndiotactic Polypropylene Blocks Fanny Deplace, Zhigang Wang, Glenn H. Fredrickson, Edward J. Kramer, Jeffrey M. Rose, Geoffrey W. Coates, Fumihiko Shimizu, Lixia Rong, Benjamin S. Hsiao The exceptional toughness and elastomeric properties of gels from block copolymers with semicrystalline syndiotactic polypropylene blocks have been reported. From results obtained from small angle and wide angle X-ray scattering experiments simultaneously performed with step cycle tensile stretching, the toughness can be attributed to the formation, orientation and elongation of crystalline fibrils along the tensile direction. The evolution of the crystalline structure during deformation is confirmed by FTIR measurements and the crystalline morphology is characterized by polarized microscopy imaging. Both polypropylene crystals and the rubbery phase play a role in the elasticity of the gels. Due to the viscoelasticity of the rubbery phase, an increase in the elastic recovery is observed when the gels are allowed to relax at zero load before starting the next cycle. [Preview Abstract] |
Friday, March 25, 2011 9:48AM - 10:00AM |
Y41.00010: Chemomechanical Characterization of Autonomic Polyacrylamide Gels Matthew Smith, Kevin Heitfeld, Ryan Kramb, Maxim Tchoul, Daniel Gallagher, Richard Vaia Autonomic behavior is a distinctive attribute of complex biological systems. Like biological tissue, self-oscillating hydrogels driven by the Belousov-Zhabotinsky (BZ) reaction can convert chemical signals into a mechanical response. Under appropriate conditions BZ gels exhibit sustained mechanical swell-deswell oscillations; and arrays of these gels have the potential to form networks of coupled oscillators. One of the key challenges to developing criteria for device design and assessing practical performance limits of these materials is the need for detailed knowledge of the chemomechanical characteristics of the BZ gels at various states of autonomic behavior. Recently we developed an easily synthesized BZ gel system based on polyacrylamide. Here in, the swell-deswell amplitude, mechanical forces produced during uniform oscillations, and the chemical response to external loads are discussed in context with current poly(N-isopropylacrylamide)-based systems. These studies establish the parameter space leading to robust chemomechanical oscillations and provide an experimental foundation to refine currently available theoretical models to guide the design of autonomic materials and devices. [Preview Abstract] |
Friday, March 25, 2011 10:00AM - 10:12AM |
Y41.00011: Block copolymer photonic crystal gels for mechanochromic sensing Edwin Chan, Joseph Walish, Edwin Thomas, Christopher Stafford Block copolymer based photonic crystal gels (BCPG) have been previously demonstrated for chemical sensing by taking advantage of dynamic changes in structural color upon interactions with their environment. With their high degree of tunability in structural color and mechanical properties, these materials can function as mechanochromic sensors with the potential application for measuring local mechanical deformation such as cell adhesion and mechanics. In this work, we demonstrate the application of a BCPG for local mechanical sensing by investigating the changes in structural color in response to mechanical deformation. The BCPG consists of a hydrophobic block (polystyrene) -- hydrophilic polyelectrolyte (poly(2-vinyl pyridine)) block copolymer that self-assembles into a one- dimensional periodic lamellar structure and functions as a one dimensional Bragg reflector. Contact adhesion testing is used to measure and relate the changes in structural color of the BCPG films as a function of mechanical deformation. We explore the effects of solvent conditions and applied mechanical deformation in determining the relationships between structural color changes and mechanical strain. [Preview Abstract] |
Friday, March 25, 2011 10:12AM - 10:24AM |
Y41.00012: Theoretically Informed Coarse-Grained Simulations of Polymer Nanogels Prateek Jha, Jos Zwanikken, Francois Detcheverry, Juan de Pablo, Monica Olvera de la Cruz Nanoscale finite-sized polymer networks (nanogels) are smart responsive materials that undergo large reversible volume changes with moderate changes in environmental conditions such as temperature, pH, light, and electric field. We develop a coarse-grained model of nanogels in terms of experimentally measurable physical quantities, and perform a theoretically informed Monte Carlo simulation that combines ideas from both the particle and continuum approaches of polymer physics. The elastic interactions are treated through beads connected by harmonic springs (``particles''), and the van der Waals and electrostatic interactions are treated by weighted densities (``fields''). Our simulations predict high degrees of swelling and a discontinuous volume phase transition in ionic nanogels, in contrast to moderate swelling and a continuous volume phase transition for the non-ionic case. We analyze the effects of mesh-size, polymer charge fraction, ionic strength, and solvent quality, on the swelling behavior of nanogels. A comparison is made with the results of a simplified continuum model, where the electrostatic interactions are treated using the Poisson-Boltzmann approximation. [Preview Abstract] |
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