Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session D21: Polymeric Elastomers and Gels |
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Sponsoring Units: DPOLY Chair: Bradley Olsen, Massachusetts Institute of Technology Room: 406 |
Monday, March 3, 2014 2:30PM - 2:42PM |
D21.00001: Double network physical gels from elastin-like polypeptide block copolymers: nanoscale control of thermoresponsive reinforcement Matthew Glassman, Bradley Olsen Triblock copolymers with associative protein midblocks and thermoresponsive endblocks form shear thinning hydrogels with a low yield stress at low temperatures, but can be reinforced by a self-assembled network of the endblock aggregates. Here, we compare the use of bioengineered elastin-like polypeptides (ELPs) to synthetic poly(N-isopropylacrylamide) (PNIPAM) as endblocks to control the self-assembly of the reinforcing network. The temperature dependence of the mechanics of these hydrogels is a strong function of the domain size and morphology in the endblock network. Despite the architectural similarities, triblock ELP fusions and PNIPAM bioconjugates exhibit distinct reinforcement maxima at fixed block composition and polymer concentration, and these differences can be attributed to the nanostructural features of the two systems. Furthermore, in ELP fusions, the amino acid sequence can be readily modified to manipulate the solvation kinetics of the endblock domains. Finally, various endblocks have been combined to form triblock terpolymer hydrogels, demonstrating how the choice of thermoresponsive blocks can be used to tune the reinforcement of shear thinning hydrogels. [Preview Abstract] |
Monday, March 3, 2014 2:42PM - 2:54PM |
D21.00002: Mechanical Characterization of Photo-crosslinked, Thermoresponsive Hydrogel Thin Films via AFM Nanoindentation Thao Le, Katherine Aidala, Ryan Hayward Thin hydrogel films with patterned swelling are known to buckle into programmed three-dimensional shapes, offering approaches to fabricate reversibly self-folding micro-devices for actuators and drug delivery devices. To precisely control the shapes adopted, it is important to quantitatively understand the relationship between swelling and mechanical properties. Furthermore, to understand the buckling pathways and the mechanical responses of the swelled materials, it is also important to identify how the gels undergo stress relaxation. However, the low moduli, high water contents, and micrometer-scale thicknesses of these materials have so far made mechanical characterization difficult. In this study, we use an AFM nanoindentation technique to characterize the mechanical properties of photo-crosslinked, thermoresponsive poly(N-isopropylacrylamide) hydrogel thin films. Simultaneously, we conduct stress relaxation experiments at microscopic indentation lengths to differentiate between the effects of viscoelastic and poroelastic response mechanisms. [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D21.00003: Mechanical properties of Tetra-PEG gels with supercoiled networks Takuya Katashima, Kenji Urayama, Ung-il Chung, Takamasa Sakai We investigated the effects of swelling and deswelling on the mechanical properties of polymer gels with variable polymer volume fractions of interest ($\varphi_{\mathrm{m}})$. We employed the Tetra-PEG gel as a model system. Tetra-PEG gels were prepared by the AB type crosslink-coupling between the two symmetrical tetra-arm prepolymers with precisely tuning the network strand length ($N_{\mathrm{c}})$ and polymer fractions at preparation ($\varphi_{\mathrm{0}})$. The drastic increase in the elastic modulus was observed in the high $\varphi_{\mathrm{m}}$ region due to the unusually contracted conformation of the network strands, called supercoiling. The Obukhov model can describe the $\varphi_{\mathrm{m}}$-dependence of the elastic modulus in all $\varphi_{\mathrm{m}}$ regions. We analyzed the stress-elongation relationships for the swollen and deswollen networks. We estimated the fractal dimensions based on the Pincus blob concept, and for the first time observed the $\varphi_{\mathrm{m}}$-, $N_{\mathrm{c}}$-, $\varphi_{\mathrm{0}}$-dependence of the fractal dimension. We found that the gyration radius exhibits the affine deformation in the supercoiling region. These findings will help to understand the structure and formation mechanism of supercoiling. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D21.00004: Rheology of Hyperbranched Poly(triglyceride)-Based Thermoplastic Elastomers via RAFT polymerization Mengguo Yan, Eric Cochran In this contribution we discuss how melt- and solid-state properties are influenced by the degree of branching and molecular weight in a family of hyperbranched thermoplastics derived from soybean oil. Acrylated epoxidized triglycerides from soybean oils have been polymerized to hyperbranched thermoplastic elastomers using reversible addition-fragmentation chain transfer (RAFT) polymerization. With the proper choice of chain transfer agent, both homopolymer and block copolymer can be synthesized. By changing the number of acrylic groups per triglycerides, the chain architectures can range from nearly linear to highly branched. We show how the fundamental viscoelastic properties (e.g. entanglement molecular weight, plateau modulus, etc.) are influenced by chain architecture and molecular weight. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D21.00005: Stability analyses of the model for photosensitive self-oscillating polymer gels Pratyush Dayal, Olga Kuksenok, Anna C. Balazs Via theory and simulations, we investigate the behavior of polymer gels undergoing Belousov-Zhabotinsky (BZ) reaction. Driven by periodic reduction and oxidation of the ruthenium catalyst, which is grafted to the polymer network, BZ gels undergo rhythmic mechanical oscillations and thereby exhibit chemo-mechanical transduction. The oscillations within the BZ gels, however, can be completely suppressed with light of a certain intensity and wavelength. We simulate the behavior of photosensitive BZ gels by our 3D gel lattice spring model. Using this model we have successfully demonstrated that it is possible to direct the movement of BZ gels, along complex paths, guiding them to bend, reorient and turn. The mechanism of chemo-mechanical transduction, however, works for a particular set of conditions. Through linear stability and normal form analyses, we isolate parameters for which the gel switches from oscillatory mode to stationary mode and vice versa. Specifically, we characterize the nature of Hopf bifurcations and identify regimes where this bifurcation is subcritical or supercritical. We also determine several other types of bifurcations within our system. These analyses allow us to establish optimal conditions required to guide the movement of BZ gels along complex paths. [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D21.00006: Connecting structural rigidity and dynamical heterogeneity to the rheology of colloidal gels Lilian Hsiao, Heekyoung Kang, Richmond Newman, Sharon Glotzer, Kyung Ahn, Michael Solomon Colloidal gels are known to exhibit complex structural and dynamical changes when sheared, particularly when the applied flow is strong enough to cause rupture. Such systems of colloids interacting through short range attractive forces are good models of associating species, such as associating polymers. Here, we investigate the effect of structural rigidity and dynamical heterogeneity on the nonlinear elasticity of colloidal gels that have undergone yielding. These gels are comprised of fluorescent, sterically stabilized poly(methyl methacrylate) colloids that are suspended at intermediate volume fractions. Non-adsorbing polystyrene is added to induce gelation with weak, short-ranged attraction. Our work shows that the nonlinear elasticity in sheared gels can be attributed to the stress-bearing capability imparted by rigid, slow-diffusing clusters that persist after the flow ceases (L.C. Hsiao et al. (2012). Proc. Natl. Acad. Sci USA 109, 16029-16034). In addition, we observe a decrease in the subdiffusive motion of the particles as the applied strain increases. This deformation introduces a bimodal distribution in the van Hove self-correlation function, suggesting the existence of a fast and slow subpopulation of colloids within sheared gels. We show that the predictive power of microscopic theories that connect elasticity to localization length can be improved by considering only this slow subpopulation. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D21.00007: Soft and Ultra-soft Elastomers William Daniel, Joanna Burdynska, Sam Kirby, Yang Zhou, Krzysztof Matyjaszewski, Michael Rubinstein, Sergei Sheiko Polymeric networks are attractive engineering materials utilized for various mechanically demanding applications. As such, much attention has been paid to reinforcement of polymer mechanical properties with little interest in how to make softer elastomers to address numerous biomedical applications including implants and cell differentiation. Without swelling in a solvent, it is challenging to obtain materials with a modulus below ca.105 Pa, which is dictated by chain entanglements. Here we present two methodologies for the creation of soft and ultra-soft dry elastomeric compounds. The first method utilizes polymer capsules as temperature responsive filler. Depending on volume fraction of microcapsules this method is capable of fine tuning modulus within an order of magnitude. The second technique uses the densely grafted molecular brush architecture to create solvent-free polymer melts and elastomers with plateau moduli in the range one hundred to ten hundred Pa. Such compounds may find uses in biomedical applications including reconstructive surgery and cell differentiation. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D21.00008: Deformation of Unentangled Swollen Gels Ozan Sariyer, Sergey Panyukov, Michael Rubinstein We study the deformation characteristics (Poisson's ratios and stress-strain relations) of unentangled gels swollen and uniaxially or biaxially deformed in excess solvent by considering the balance of osmotic pressure and elastic stress in unconstrained dimensions. Our scaling theory predicts a crossover from theta solvent behavior to marginal solvent behavior upon stretching gels that are in concentrated regime at swelling equilibrium -- a phenomenon that was experimentally observed long ago, but not understood theoretically. For gels that are in the semidilute good solvent regime at swelling equilibrium, we predict a crossover to theta solvent behavior upon compression and a crossover to marginal solvent behavior upon stretching. Our theory reproduces the previously known results for equilibrium swelling degree as well as known deformation characteristics in theta and athermal solvents. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D21.00009: Hydrogen Bonding in Poly(butyl acrylate) Melts and Elastomers Mitchell Anthamatten, Christopher Lewis Hydrogen bond strength and density are critical design variables that influence the formation of supramolecular networks from linear polymers. Larger, higher strength H-bonding groups, tend to be more susceptible to aggregation, phase segregation and stacking. However, even weak, monovalent and bivalent hydrogen bonding groups can increase melt viscosity, compatiblize binary blends, and introduce hierarchal structure into amorphous melts. We prepared a series of poly(butyl acrylate) copolymers with different amounts and types of hydrogen bonding side-groups (HBG's). Copolymers containing ``weak'' HBG's behaved as unentangled melts, with no indication of network formation. Copolymers bearing strong hydrogen bonding groups (UPy) behaved as soft, elastic solids. The rheologically distinct behavior of UPy-containing copolymers is attributed to dimer lifetimes exceeding the experimental timescale. Results are relevant to developing adaptable materials including shape memory polymers, self-healing materials, damping materials and adhesives that utilize hydrogen bonding to influence bulk mechanical properties. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:30PM |
D21.00010: Stress--Strain Relationship of Highly Stretchable Dual Cross-Link Gels: Separability of Strain and Time Effect Costantino Creton, Koichi Mayumi, Alba Marcellan, Guylaine Ducouret, Tetsuharu Narita We studied the stress--strain relation of model dual cross-link gels having permanent cross-links and transient cross-links over an unusually wide range of extension ratios $\lambda $ and strain rates $d\varepsilon $\textit{/dt} (or time t $=$ ($\lambda $ -- 1)/$ (d\varepsilon $\textit{/dt)}). We propose a new analysis method and separate the stress into strain- and time-dependent terms. The strain-dependent term is derived from rubber elasticity, while the time-dependent term is due to the failure of transient cross-links and can be represented as a time-dependent shear modulus which shows the same relaxation as in small strain. The separability is applicable except for the strain stiffening regimes resulting from the finite extensibility of polymer chains. This new analysis method should have a wide applicability not only for hydrogels but also for other highly viscoelastic soft solids such as soft adhesives or living tissues. [Preview Abstract] |
Monday, March 3, 2014 4:30PM - 4:42PM |
D21.00011: Fresh Insights on the mechanical response of methylcellulose hydrogels Joseph Lott, John McAlliser, Frank Bates, Timothy Lodge The thermoreversible gelation of aqueous solutions of methylcellulose (MC) at elevated temperatures is well established. However, it has only recently been determined conclusively that the structure of such gels is fibrillar in nature and the rheological properties observed are a result these structures. Cryo-transmission electron microscopy (cryo-TEM) and small-angle neutron scattering (SANS) provide detailed insight into the fibrillar network's size scales, growth with temperature, and composition. In light of this new understanding, we explore the possibility of reevaluating the rheological behavior of MC gels under the paradigm of the mechanics of filament networks. [Preview Abstract] |
Monday, March 3, 2014 4:42PM - 4:54PM |
D21.00012: Effect of polydispersity on the phase behavior of soft microgel suspensions Andea Scotti, Urs Gasser, Emily Herman, Akiti Singh, L. Andrew Lyon, Alberto Fernandez-Nieves Microgel suspensions with a majority of small particles and a small fraction of big particles with about double diameter can form crystals without defects caused by the large particles (A. St. John Iyer and L.A. Lyon, Angew. Chem. Int. Ed. 48, 4562-4566, 2009). However, no hard sphere crystals form at size-polydispersities higher than 12\%. We study the role of size-polydispersity in suspensions of fully swollen poly(N-isopropylacrylamide) (pNIPAM) microgel particles with controlled polydispersity ranging from 10\% up to 25\%. Crystals appear in samples with polydispersity as high as 17\%. Using small-angle neutron scattering and contrast matching with samples composed of small deuterated particles and large protonated particles, we directly measure the form factor and shrinkage of the large particles in concentrated samples. The large particles are found to shrink to about the size of the small particles when the effective volume fraction of the suspension approaches 1. These results suggest a different role of size-polydispersity in soft sphere systems. [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D21.00013: Shapes of drying hydrogel cylinders Etienne Reyssat, Vincent Etienne Most materials change shape upon drying or wetting. Inhomogeneity of the drying and wetting processes lead to the development of internal stresses. As a result, a solid sample undergoes complex deformations or fracture. We present experimental work on the evolving shapes of drying hydrogel cylinders. We show that depending on the initial aspect ratio of the cylinder, a gel sample undergoes a variety of shape changes. We show qualitative analogies with the drying of wood and with instabilities of cylindrical shells or plane membranes. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D21.00014: Advantages of Using Soft Materials in Scanning Probe Lithography Keith A. Brown, Daniel J. Eichelsdoerfer, Mary X. Wang, Chad A. Mirkin Scanning probes based upon soft materials provide new capabilities and insights into the science of scanning probe lithography. Specifically, we have explored a cantilever-free architecture that consists of an array of sharp probes on an elastomeric film on a glass slide. This architecture allows every probe in an array to be in simultaneous, gentle contact with a surface, allowing one to perform lithography with millions of probes in parallel. Here, we describe three recent developments in cantilever-free scanning probe lithography that were enabled by the elastomeric material. 1) As the mechanical properties of elastomers can be readily tuned, it is possible to tailor the spring constant of the probes.$^{\mathrm{1}}$ 2) The high coefficient of thermal expansion of elastomers means that local heating can be used to physically actuate individual probes allowing for arbitrary patterning.$^{\mathrm{2\thinspace }}$3) Solvents retained in the elastomer can mediate molecular printing and allow a user to pattern hydrophilic and hydrophobic materials in totally dry environments. $^{\mathrm{1}}$D. J. Eichelsdoerfer, \textit{et al}., Nano Lett. \textbf{13}, 664 (2013). $^{\mathrm{2}}$K. A. Brown, \textit{et al}., Proc. Natl. Acad. Sci. USA \textbf{110}, 12921 (2013). [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D21.00015: Dilute and Semidilute Solutions of a Nonionic, Rigid, Water-soluble Polymer Paul Russo, Wayne Huberty, Donghui Zhang The solution physics of random polymer chains was established largely on the behavior of commercial polymers such as polystyrene for organic solvents or nonionic poly(ethyleneoxide) for aqueous solvents. Not only are these materials widely available for industrial use, they can be synthesized to be essentially monodisperse. When it comes to stiff polymers, good choices are few and less prone to be used in industrial applications. Much was learned from polypeptides such as poly(benzylglutamate) or poly(stearylglutamate) in polar organic solvents and nonpolar organic solvents, respectively, but aqueous systems generally require charge. Poly(N$_{\mathrm{\varepsilon }}$-2-[2-(2-Methoxyethoxy) ethoxy]acetyl-L-Lysine) a.k.a. PEGL was pioneered by Deming and coworkers. In principle, PEGL provides a convenient platform from which to study stiff polymer behavior---phase relations, dynamics, liquid crystal formation and gelation---all with good molecular weight control and uniformity and without electrical charge. Still, a large gap in knowledge exists between PEGL and traditional rodlike polymer systems. To narrow this gap, dynamic and static scattering, circular dichroism, and viscosity measurements have been made in dilute and semidilute solutions as necessary preliminaries for lyotropic liquid crystalline and gel phases. Supported by NSF DMR 1306262. [Preview Abstract] |
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