Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session C31: Polymeric Elastomers and Gels |
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Sponsoring Units: DPOLY Chair: Jens Glaser, University of Minnesota Room: 339 |
Monday, March 18, 2013 2:30PM - 2:42PM |
C31.00001: How water content determines small-molecule mobility in hydrogels Sung Chul Bae, Ah-Young Jee, Steve Granick Surprisingly little is known from quantitative physical study about dynamics within hydrogels, in spite of the fundamental importance of solvent concentration in the theory and application of nonpolar polymer gels. We have prepared model hydrogels of different kinds and studied the diffusion within them of fluorescently-labeled solutes. Comparison of translational and rotational diffusion shows remarkable dependence, on water content, of translation-rotation correlations. [Preview Abstract] |
Monday, March 18, 2013 2:42PM - 2:54PM |
C31.00002: Unexpected water screening in gel-encapsulated terbium systems Tetyana Ignatova, Juan G. Duque, Stephen K. Doorn, Slava V. Rotkin Terbium (Tb) salts and their compounds are used as bio-labels and dyes due to their unique photoluminescence (PL) properties. Our study focuses on PL of Tb ions in crowded surroundings, mimic to living cells. Silica gel with SWNT dispersed with sodium deoxycholate (DOC), silica gel only, and DOC water solution were chosen as prototypes for bio-environment. Time resolved and steady state spectroscopy was used to monitor the behavior of terbium in different enclosing. We observed significant increase of PL lifetime in the gel in comparison with aqua solution, which indicates the reduction of OH groups in the co-ordination shell of the Tb ion. PL spectra of Tb in water and in DOC micelles and gels confirmed structural changes during encapsulation process. [Preview Abstract] |
Monday, March 18, 2013 2:54PM - 3:06PM |
C31.00003: Molecular origins of reinforcement in responsively nanostructured, shear thinning double network hydrogels Matthew Glassman, Jacqueline Chan, Bradley Olsen Triblock copolymers containing associative protein midblocks and thermoresponsive endblocks have recently been shown to form reinforceable, nanostructured hydrogels. Triggered self-assembly of orthogonal physical crosslinks causes a reversible transition from a shear thinning material at low temperatures to a toughened state at high temperatures with resistance to creep, erosion, and failure in uniaxial compression. In this study, properties of the individual networks were varied to investigate the relationships among association density in the protein network, nanostructure formation, and ultimate mechanical reinforcement that could be realized in this double network architecture. Through a broad survey of materials, large changes in static and dynamic mechanical properties were identified, some leading to a 14-fold increase in plateau modulus and a decrease in creep compliance by more than two orders of magnitude over the range from 5-50$^{\mathrm{o}}$C. Detailed investigation of the structure and relaxation behavior of the underlying network of micelles with associative coronae reveals important parameter constraints for achieving high performance in these double network gels. [Preview Abstract] |
Monday, March 18, 2013 3:06PM - 3:18PM |
C31.00004: ABSTRACT WITHDRAWN |
Monday, March 18, 2013 3:18PM - 3:30PM |
C31.00005: Large-amplitude oscillatory shear of methylcellulose solutions through the sol-gel transition John W. McAllister, Joseph R. Lott, Frank S. Bates, Tim P. Lodge Methylcellulose (MC) is a chemically modified polysaccharide that is partially substituted by methoxy groups. Aqueous MC solutions undergo gelation and phase separation (LCST) upon heating, which is attributed to the assembly of molecules into fibrillar structures noted by cryo TEM images and small angle neutron scattering. The transition from a strain-softening solution to a strain hardening gel upon heating has been probed using large-amplitude oscillatory shear (LAOS). In addition to strain hardening, MC solutions exhibit positive normal stresses (pressing the plates of the rheometer apart) while MC gels exhibit negative normal stresses (contracting the plates together) at stresses larger than 10 Pa. Nonlinear rheological responses are a useful probe to monitor structure-property relationships as MC transitions from a solution to a gel. [Preview Abstract] |
Monday, March 18, 2013 3:30PM - 3:42PM |
C31.00006: Analysis of the biaxial stretching of Tetra-PEG gel Takuya Katashima, Ung-il Chung, Takamasa Sakai, Kenji Urayama Non-linear stress-strain relationships that elastomers exhibit are governed by the strain energy density function (W). Although many types of W models were examined, full understanding of W still remains incomplete due to the two problems; the limitation in deformation range and the inhomogeneities in polymer networks. In this study, we perform various types of biaxial stretching for Tetra-PEG gels, which is a near-ideal network. We found that (1) the Neo Hookean (NH) model, which has been considered as a model for ideal networks, fails to describe the biaxial date; (2) the stress ratio $\sigma_{\mathrm{y}}$ / $\sigma_{\mathrm{x}}$ (where x and y are the stretching and constrained directions, respectively) in pure shear is larger than the expectation of the models with no strain-coupling term, and $\sigma _{\mathrm{y}}$ / $\sigma_{\mathrm{x}}$ increases with an increase in polymer fraction. These results indicate that the two effects, i.e., finite extensibility and strain-coupling should be introduced in W. We extend the Gent model, which considers the finite extensibility on the basis of the NH model, by adding a linear I$_{\mathrm{2}}$ term. This model successfully describes the whole data with all fractions. [Preview Abstract] |
Monday, March 18, 2013 3:42PM - 3:54PM |
C31.00007: Low Modulus Silicone Elastomer Networks with Desirable Viscoelastic Properties for Cell Mobility Studies Julie N. L. Albert, Jan Genzer Biocompatible silicone elastomer networks provide a versatile platform for studying the effect of compliance on cell movement. In conventional network formation schemes, poly(dimethylsiloxane) (PDMS) is cross-linked via reactive end groups, and the modulus of the material is controlled by the ratio of polymer to cross-linker. However, low modulus networks fabricated in this manner are imperfect and insufficiently cross-linked with high soluble fractions and reduced elasticity, especially as the network modulus approaches that of soft tissues (on the order of 10 kPa). In order to overcome these limitations, we synthesized PDMS chains in which vinylmethylsiloxane units were incorporated every $\approx $15-20 kDa along the polymer backbone. We then cross-linked the polymer through the vinyl groups using hydrosilylation chemistry. The resultant networks exhibited lower soluble fractions and lower viscous dissipation/greater elasticity as compared to equivalent-modulus networks fabricated by the conventional end-group cross-linking scheme. We attribute the mechanical properties of our networks to the presence of network-bound free chain ends that effectively plasticize the network to lower the modulus without compromising network elasticity. [Preview Abstract] |
Monday, March 18, 2013 3:54PM - 4:06PM |
C31.00008: Coarse grain modeling of imperfect networks and gels Yelena Sliozberg, Tanya Chantawansri, Timothy Sirk, Jan Andzelm, Randy Mrozek, Joseph Lenhart There is a strong interest in chemically and physically cross-linked entangled polymer networks and gels due to their tailorability in respect to both mechanical and structural properties. Even so, these properties are sensitive to imperfections in the polymer networks, such as dangling ends and loops. Computational modeling is a viable tool to understand the effects of these imperfections on properties in a controlled environment, in which specific defects can be systematically created and varied. In this study, we have employed generic bead-spring models of flexible chains to study a chemically and physically cross-linked network. Our results will show the importance defects, such as dangling ends and loops, on the mechanical and structural properties of these networks. We will also discuss the effects of these defects on the time-dependent elastic modulus. The simulation results qualitatively agree with experimental results and the other theoretical predictions. [Preview Abstract] |
Monday, March 18, 2013 4:06PM - 4:18PM |
C31.00009: The Interesting Influence of Nanosprings on the Viscoelasticity of Elastomeric Polymer Materials: Simulation and Experiment Jun Liu, Liqun Zhang, Dapeng Cao Among all carbon nano-structured materials, helical nanosprings or nanocoils have attracted particular interest. Here, carbon nanosprings are directed to adjust the viscoelasticity and reduce the resulting hysteresis loss (HL) of elastomeric polymer materials. Two kinds of nanosprings filled elastomer composites are constructed: system I is obtained by directly blending polymer chains with nanosprings, while system II is composed of the self-assembly of the tri-block structure (chain-nanospring-chain). Through coarse-grained molecular dynamics simulation, we find that the incorporation of nanosprings prominently improve the mechanical strength of the elastomer matrix, and importantly, decrease considerably the hysteresis loss. Furthermore, the spring constant of nanosprings and the interfacial chemical coupling between chains and nanosprings both play a crucial role. It is inferred that elastomer/carbon nano-structured materials with good flexibility and reversible mechanical response (nanosprings, nanocoils, nanorings and thin graphene sheet) may possess both excellent mechanical and low HL properties, which could open a new avenue to fabricate high performance automobile tires, and facilitate the large-scale industrial application of these materials. [Preview Abstract] |
Monday, March 18, 2013 4:18PM - 4:30PM |
C31.00010: High-Strain Rate Mechanical Response of Cured Epoxy Networks Timothy Sirk, Ketan Khare, Mir Karim, Joseph Lenhart, Rajesh Khare, Jan Andzelm Chemically cross-linked polymer networks are increasingly common in high performance composites, adhesives and other applications involving high-impact loading conditions or ballistic collisions. The mechanical behavior of epoxy and other polymer networks exhibit a strong dependence on strain rate near the glass transition temperature (Tg); however, the elastic modulus at strain rates greater than 10$^5$ 1/s is difficult to capture with experimental techniques. We present computational results of Di-Glycidyl Ether of Bisphenol A (DGEBA) and Jeffamine diamines (D230) from molecular dynamics simulation, which is intrinsically well-suited to model material deformation at high strain rates. Our results show that the experimental Tg can be reproduced from molecular dynamics, and the Williams-Landel-Ferry equation is useful in rationalizing the shift of Tg due to fast annealing and high strain rates. Temperature sweeps of elastic modulus show the glass-rubber transition to occur over a significantly wider temperature range compared with experimental measurements at low strain rates. [Preview Abstract] |
Monday, March 18, 2013 4:30PM - 4:42PM |
C31.00011: Mechanical and Thermal Properties of Cross-Linked Phenolic Resins Using Molecular Dynamics John Lawson, Joshua Monk, Justin Haskins, Charles Bauschlicher To gain insight into the design of materials, it is valuable to understand how the chemical make-up at the nano-scale can influence the thermal and mechanical bulk properties. An atomistic computational study allows us to manipulate the structural make-up of individual phenolic chains as well as generate various cross-linked (or cured) systems. In this study, molecular dynamics simulations of bulk phenolic systems were performed with the software LAMMPS. An all-atom force field was chosen to investigate how the strength and thermal conductivity of the phenolic material varies as a function of the degree of cross-linking and chemical make-up of the phenolic chains. Small-scale mechanical tests were performed to compute various moduli for the phenolic systems above and below the glass transition at varied degrees of cross-linking. The thermal conductivity was obtained using the Green-Kubo approach for the virgin phenolic system as well as the strained systems. [Preview Abstract] |
Monday, March 18, 2013 4:42PM - 4:54PM |
C31.00012: Investigation of the Melting Point Depression of 12-Hydroxystearic Acid Organogels Using the Flory Diluent Model Kevin Cavicchi, Brian Lipowski This talk will focus on the gelation behavior of 12-hydroxystearic acid (12-HSA) in organic solvents. Thermo-reversible gelation occurs by crystallization of 12-HSA in organic solvent to form 3-D fibrillar networks. The melting point vs. composition for 12-HSA in a range of solvents has been measured. The liquidus lines could be fit with the Flory-diluent model that takes into account the non-ideal free energy of mixing and the disparity in the size of the solvent and 12-HSA molecules. The fits indicated that the effective molar volume of 12-HSA increased as the hydrogen bonding Hansen solubility parameter $\delta_{\mathrm{h}}$ of the solvent decreased. This is attributed to the hydrogen-bonding driven aggregation of the 12-HSA in the liquid state based on previous observations that 12-HSA forms aggregated structures in non-polar solvents (e.g. dimers and tetrameters). These results indicate that the stabilization of the solid phase in 12-HSA solutions has contributions from both variations in the entropy of mixing as well the enthalpy of mixing. The importance of both these factors for designing small molecule gelators will be discussed. [Preview Abstract] |
Monday, March 18, 2013 4:54PM - 5:06PM |
C31.00013: Structural analysis and mechanical properties of syndiotactic polypropylene (sPP) gels formed at different cooling temperatures Keita Takaesu, Atsushi Hotta The effects of the cooling temperature on the mechanical properties and the microstructure of the syndiotactic polypropylene (sPP) gel were investigated. sPP/decahydronaphthalene gels were prepared at different cooling temperatures followed by the compression testing to evaluate the mechanical properties. To analyze the microstructure of the gels, optical microscopy observation, differential scanning calorimetry (DSC) analysis, Fourier transform infrared spectroscopy (FTIR) analysis, and small angle X-ray scattering (SAXS) analysis were carried out. It was found that the sPP gel prepared at the lowest cooling temperature using liquid nitrogen (named Gel LN) showed highest mechanical properties. The sPP gels cooled at relatively high temperatures of 38$^{o}$C (Gel 38), 25$^{o}$C (Gel 25), and 0$^{o}$C (Gel 0) became more brittle. DSC analysis and FTIR analysis revealed that the crystal amount, which acted as the crosslinking points of sPP gels, of Gel LN was largest among other sPP gels. In addition, SAXS analysis suggested that the size of each crystalline domain in Gel LN was about 15 nm. It was concluded from these results, that Gel LN exhibited the highest mechanical properties due to its homogeneous and dense crystalline network structures. [Preview Abstract] |
Monday, March 18, 2013 5:06PM - 5:18PM |
C31.00014: Mechanical Measurement of Gels: Pre-stress and Failure Sami Fakhouri, Shelby Hutchens, Alfred Crosby A recently developed technique, Cavitation Rheology (CR), provides a means of measuring the mechanical properties of soft materials on length scales from $\sim 0.1$ $\mu$m to mm at a specific location. CR involves inflation of a small bubble at the tip of a syringe needle which has been inserted into a material. After insertion, the pressure in the syringe is raised until a critical point where the material fails, resulting in rapid inflation of a cavity at the syringe tip. The critical pressure for failure can provide information about the materials properties of the system such as the elastic modulus, E, the critical strain energy release rate, $G_c$, and the surface energy, $\gamma$. Modulus measurements by CR have been made in many synthetic gels and biological tissues with similar accuracy to shear rheology. However, as CR requires insertion of a needle into the subject material, measurements are inherently made in a pre-stressed state. In this work, we have examined the pre-stress associated with needle insertion and the influence of this stress on failure in a synthetic gel of PMMA-PnBA-PMMA triblock copolymer in 2-ethylhexanol. [Preview Abstract] |
Monday, March 18, 2013 5:18PM - 5:30PM |
C31.00015: First and second order volume-phase transitions in photo-cross-linked poly(cyclopropylacrylamide) and poly(N-vinylisobutyramide) coatings Ryan Toomey, Leena Patra The temperature-dependent swelling of thin (100 nm) coatings of photo-cross-linked poly(cyclopropylacrylamide), or poly(CPAAm), and poly(vinylisobutyramide), or poly(NVIBAm) was characterized. Both polymers contained 3 mole{\%} of methacroylaminobenzophenone (MnBP) as the photo cross-linking unit. Poly(CPAAm-co-MnBP) showed a continuous, 2nd order deswelling transition between 10 and 70 $^{\circ}$C with no hysteresis. Poly(NVIBAm-co-MnBP), on the other hand showed a discontinuous, 1st order deswelling transition at 45 $^{\circ}$C with hysteresis. The differences in the swelling transitions can be interpreted within the context of the cloud-point measurements of the uncross-linked polymers. Whereas poly(NVIBAm-co-MnBP) has a significant off-zero critical point (\textgreater\ 10 wt{\%} polymer) at 36 $^{\circ}$C, poly(CPAAm-co-MnBP) has a critical point at zero concentration and 23 $^{\circ}$C. Concurrent measurements of the infrared vibrations of the amide groups in both polymers further revealed that the amide group in poly(CPAAm-co-MnBP) maintains a constant hydrogen-bonding environment throughout the volume-phase transition. Poly(NVIBAm-co-MnBP), on the other hand, has a concentration-dependent hydrogen bonding environment around the carbonyl group, which is consistent with an off-zero concentration in the cloud point curve. [Preview Abstract] |
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