Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Y16: Elastomers and Gels II |
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Sponsoring Units: DPOLY Chair: Ryan Hayward, University of Massachusetts Room: 317 |
Friday, March 20, 2009 8:00AM - 8:12AM |
Y16.00001: Deuterium NMR studies of segment orientation in PDMS unimodal and bimodal endlinked networks. Claude Cohen, Geoffrey Genesky, T. Michael Duncan Polymer segment orientation in elastomers is revealed by solid state deuterium NMR spectra: earlier work has focused on the frequency split between the peaks of the spectra from stretched elastomers rather than the details of the lineshape. The split has classically been interpreted as a measure of polymer segment order parameter S caused by excluded volume interactions between neighboring segments. We synthesized deuterated PDMS chains of about 5000 g/mol and 80,000 g/mol to probe the segmental orientation of each component separately in bimodal networks. Even in the unstretched state, the spectra for the labeled short chain networks show an evolving lineshape with varying short chain content. We compute the average absolute value of the frequency shift of the entire spectrum to better account for the highly aligned segments. This method allows us to probe the chain segment alignment with increasing strain in both unimodal and bimodal networks and confirms Monte Carlo simulation results. [Preview Abstract] |
Friday, March 20, 2009 8:12AM - 8:24AM |
Y16.00002: Volume-phase Transitions and Confined Water in Surface-tethered Poly(N-isopropylacrylamide) Networks Ajay Vidyasagar, Ryan Toomey We present a simple approach for studying volume-phase transitions in thin, mechanically anchored responsive polymer networks. The approach is based on the photo- cross-linking of copolymers synthesized from N- isopropylacrylamide NIPAAm) and methacroyloxybenzophenone(MaBP). We monitored the swelling of poly(NIPAAm-co-MaBP) networks in contact with aqueous solution as a function of cross-link density with both neutron reflection and ATR-FTIR. Neutron reflection reveals that the volume-phase transition of tethered poly(NIPAAm) networks coincides with the miscibility gap of linear poly(NIPAAm) solutions. Whether or not the poly(NIPAAm) network undergoes a continuous or discontinuous collapse depends on its degree of cross-linking. At cross-link densities above 5 mole$\%$, the network remains in the single-phase region of the linear solution phase diagram and shows only a continuous transition between the swollen and collapsed states. Moreover, in the collapsed state, 30-35$\%$ by volume of water remains in the network, which is independent of cross-link density. The relative position of the FTIR absorption peaks during the collapse follows very closely the amount of water in the layer and can be related to the local dielectric environment within the network. [Preview Abstract] |
Friday, March 20, 2009 8:24AM - 8:36AM |
Y16.00003: Strain induced alignment of particles in an elastomer host Ben Spott, Jeremy Neal, Peter Palffy-Muhoray The properties of composite materials consisting of anisometric nanoparticles dispersed in a host depend on the orientation of the nanoparticles. It is interesting to ask how mechanical strain of the composite affects their orientation. We have carried out experiments stretching both two- and three-dimensional rubber samples containing rigid rod-like particles and measured their orientational order parameter as a function strain. We discuss these results, and make connections with theoretical models. [Preview Abstract] |
Friday, March 20, 2009 8:36AM - 8:48AM |
Y16.00004: Dynamics of uniaxially oriented elastomers using dielectric spectroscopy HyungKi Lee, Daniel Fragiadakis, Darren Martin, James Runt We summarize our initial dielectric spectroscopy investigation of the dynamics of oriented segmented polyurethanes and crosslinked polyisoprene elastomers. A specially designed uniaxial stretching rig is used to control the draw ratio, and the electric field is applied normal to the draw direction. For the segmented PUs, we observe a dramatic reduction in relaxation strength of the soft phase segmental process with increasing extension ratio, accompanied by a modest decrease in relaxation frequency. Crosslinking of the polyisoprene was accomplished with dicumyl peroxide and the dynamics of uncrosslinked and crosslinked versions are investigated in the undrawn state and at different extension ratios. Complimentary analysis of the crosslinked PI is conducted with wide angle X- ray diffraction to examine possible strain-induced crystallization, DSC, and swelling experiments. Quantitative analysis of relaxation strengths and shapes as a function of draw ratio will be discussed. [Preview Abstract] |
Friday, March 20, 2009 8:48AM - 9:00AM |
Y16.00005: Shape-Memory Polymers Based on Fatty Acid-Filled Elastomeric Ionomers Elise Izzo, Robert Weiss Shape memory polymers (SMPs) have applications as medical devices, actuators, sensors, artificial muscles, switches, smart textiles, and self-deployable structures. All previous design of SMPs has involved synthesizing new polymers or modifying existing polymers. This paper describes a new type of SMP based on blends of an elastomeric ionomer and low molar mass fatty acids or their salts (FAS). Shape memory elastomers were prepared from mixtures of a sulfonated EPDM ionomer and various amounts of a FAS (e.g., zinc stearate, zinc oleate, and various aliphalic acids). Nanophase separation of the metal sulfonate groups provided the ``permanent'' crosslinks, while sub-microscopic crystals of the low molecular weight FAS provided a physical crosslink needed for the temporary shape. The material was deformed above the melting point of the FAS and the new shape was fixed by cooling the material while under stress to below the melting point of the FAS. Polar interactions between the ionomer and the FAS stabilized the dispersion of the FAS in the polymer and provided the continuity between the phases that allowed the crystals of the FAS to provide a second network of physical crosslinks. The temporary shape was erased and the material returned to the primary shape by heating above the melting point of the FAS. [Preview Abstract] |
Friday, March 20, 2009 9:00AM - 9:12AM |
Y16.00006: Dynamics of Segmented Polyurethane Elastomers Using Dielectric Spectroscopy James Runt, Daniel Fragiadakis, Alicia Castagna, Taeyi Choi This investigation focuses on the molecular dynamics of segmented polyurethane copolymers with different hard segment contents (30 to 52 wt percent) and soft segment chemistries. Methylene bis(p-phenyl isocyanate) and 1,4-butanediol constitute the hard segments in all materials under investigation, while soft segments include poly(tetramethylene oxide) and a 80-20 mixture of poly(dimethylsiloxane) and poly(hexamethylene oxide). The dynamics of these materials were explored over a wide temperature and frequency range using dielectric spectroscopy. In addition to investigating the details of segmental and local processes, three dielectric relaxations above Tg were observed for the first time in segmented polyurethanes, and their origin discussed in the presentation. For example, the highest temperature process is assigned to Maxwell-Wagner-Sillars interfacial polarization. The strength of the MWS process is a sensitive indicator of the change in microphase-separated character. It disappears at a temperature similar to that at which the small-angle X-ray scattering maximum disappears, indicating the transformation to the single phase state. [Preview Abstract] |
Friday, March 20, 2009 9:12AM - 9:24AM |
Y16.00007: Polydomain to monodomain transition in nematic liquid crystal elastomers Badel Mbanga, Fangfu Ye, Jonathan Selinger, Robin Selinger Liquid crystal elastomers are crosslinked polymer networks covalently bonded with liquid crystal mesogens. In the nematic phase, due to strong coupling between mechanical strain and orientational order, these materials display strain-induced instabilities associated with formation and evolution of orientational domains. Deformation of an initially polydomain nematic elastomer film induces a transition to the monodomain configuration. We model this phenomenon using a recently developed finite element elastodynamics simulation method. We study the rate-dependent material response upon uniaxial extension and resolve the textures that form along with the associated stress-strain behavior. Our simulations yield qualitative agreement with experimental observations. This model allows us to explore the fundamental physics governing dynamic mechanical response of nematic elastomers and also provides a potentially useful computational tool for engineering device applications. [Preview Abstract] |
Friday, March 20, 2009 9:24AM - 9:36AM |
Y16.00008: Liquid crystal elastomers: Bent core flexo-electricity Martin Chambers, Rafael Verduzco, Samuel Sprunt, James T. Gleeson, Antal Jakli We report on the swelling of calamitic liquid crystal elastomers (LCE) with bent-core mesogens (BCM); this swelling took place at a temperature where both materials were in their isotropic phase. The BCM used varied in the degree of saturation of their hydrocarbon tails, which affects both viscosity and phase behaviour. We determined both swelling magnitude and dynamics. The host LCE systems homogeneously imbibe BCM up to 30-40 {\%} mol. Based on differential scanning calorimetry, shape change anisotropy, and optical birefringence measurements, the swollen elastomers are all found to exhibit nematic phases, with some possessing a lower temperature smectic phase. Bent-core liquid crystal elastomers and swollen calamitic LCE in BCM were investigated for the flexoelectric properties by inducing a mechanical deformation. The value of the bend flexoelectric constant, e$_{3 }$of the swollen BCM containing LCE systems is comparable of that of the neat bent-core liquid crystal. [Preview Abstract] |
Friday, March 20, 2009 9:36AM - 9:48AM |
Y16.00009: Photoinduced distortions of polydomain liquid crystal elastomer samples Wilder Iglesias, Peter Palffy-Muhoray We have studied the effects of illumination of a dye doped polydomain nematic elastomer. The sample was on a glass substrate, and its free surface was illuminated by polarized light from an Ar+ laser. The intensity of the reflected light in the far field was studied to probe photoinduced deformations. Above a threshold intensity, a target pattern appeared in the far field, indicating sample deformation. We discuss the experimental observations, and relate these to the processes such as photoinduced surface relief. Experimental results are compared with the predictions of simple theory. [Preview Abstract] |
Friday, March 20, 2009 9:48AM - 10:00AM |
Y16.00010: Dynamic Aspect of Electro-Opto-Mechanical Effects in Swollen Nematic Elastomers Kenji Urayama, Atsushi Fukunaga, Toshikazu Takigawa, Antonio DeSimone, Luciano Teresi The nematic elastomers swollen by low molecular mass liquid crystals exhibit a macroscopic deformation with a significant change in birefringence in fast response to electric fields. We experimentally characterize the dynamics of this ``electro-opto-mechanical effect.'' The optical and mechanical rise times (in response to field-on) decrease in nearly proportion to the square of field strength, while the corresponding decay times (in response to field-off) are almost independent of field strength. The optical rise and decay times are about one order of magnitude smaller than the mechanical ones. We also propose a minimal model to describe the main features of both static and dynamic characteristics of this phenomenon observed experimentally. 1) Urayama, K., Honda, S., Takigawa, T., \textit{Macromolecules}, \textbf{2006}, $39$, 1943. 2) Fukunaga, A., Urayama, K., Takigawa, T., DeSimone, A., Teresi, L., \textit{Macromolecules}, in press. [Preview Abstract] |
Friday, March 20, 2009 10:00AM - 10:12AM |
Y16.00011: Analysis of Diffusion through Dynamic Network Polymers using Multi-photon Fluorescence Recovery after Photobleaching Jiahui Li, Kelley Sullivan, Edward Brown, Mitchell Anthamatten Multi-Photon Fluorescence Recovery after Photobleaching (FRAP) techniques are utilized to study small molecule transport through polymer networks containing multiple hydrogen-bonding functional groups. Experiments involve uniformly dispersing small dye molecules (fluorophores) into functionalized polymers and networks. Polymer samples are then locally bleached, and the recovery of fluorescence is studied using 2-photon fluorescence microscopy. By curve-fitting fluorescence recovery curves to a model, diffusion coefficients are obtained. We have investigated series of polymer networks containing H-bonding interactions with different compositions at different temperatures. The diffusion coefficient through these polymer networks shows an expected Arrhenius-like temperature dependence. The influence of hydrogen bonding and network architecture on transport activation energies will be discussed. [Preview Abstract] |
Friday, March 20, 2009 10:12AM - 10:24AM |
Y16.00012: Gelling Mechanism of Aluminum Di-Soaps in Oils Xiaorong Wang, Mindaugas Rackaitis This work demonstrates that aluminum di-soaps form nano-sized spherical micelles in the oils and that the aggregation of these micelles forms a network that gives rise to a gel formation -- thereby refuting a long-held belief that the gel formation was the result of linear polymeric chains of aluminum association. The discovery of such aluminum nano-particles could expand application of these materials to new technologies because these materials are chemically inert, odorless and nontoxic, and have been widely used in greases, paints, gels, cosmetics, drugs and foods. [Preview Abstract] |
Friday, March 20, 2009 10:24AM - 10:36AM |
Y16.00013: Surface Plasmon Resonance Effects in Responsive Polyelectrolyte/Gold Nanoparticles Hydrogel Thin Films Ihor Tokarev, Iryna Tokareva, Venkateshwarlu Gopishetty, Sergiy Minko In this study, we explored localized surface plasmon resonance excited in gold nanoparticles coupled with continuous and macroporous stimuli-responsive hydrogel thin films. The 100-nm-thick porous hydrogel film with vertically aligned cylindrical pores decorated with spherical Au nanoparticles (synthesized in the hydrogel) and placed on Au islands (prepared on a glass substrate) enabled the highly-sensitive optical detection of changes in the swelling degree of hydrogel induced by an external stimulus (pH). The strong optical response of our sensing platform is attributed to the electromagnetic coupling between the nanoparticles and islands that is highly sensitive to the inter-particle spacing. The transformation of a chemical signal into the optical effect can be used for analytical applications. [Preview Abstract] |
Friday, March 20, 2009 10:36AM - 10:48AM |
Y16.00014: Gelation, Dynamics and Mechanics of Associating Polymers: Exploring Parameter Space Robert S. Hoy, Glenn H. Fredrickson We present simulation studies of associating polymer melts, where a fraction of the monomers are ``sticky.'' A coarse grained hybrid MD/MC model is used to accurately model polymer dynamics and qualitatively capture chemical kinetics. The effects of varying sticky bond strength, sticky monomer concentration and placement along chains, chain length N, temperature T, and chemical kinetics are examined. All have independent and interesting effects on the dynamics and mechanics. We focus on the ``reversible'' gel regime where almost all chains are instantaneously connected to the network, yet chains are delocalized and the system displays complex time and strain- dependent properties. [Preview Abstract] |
Friday, March 20, 2009 10:48AM - 11:00AM |
Y16.00015: Step Cycle Deformation Processing of Elastomers and Gels Based on Semicrystalline Polyolefin-based Block Copolymers Fanny Deplace, Zhigang Wang, Nathaniel A. Lynd, Atsushi Hotta, Glenn H. Fredrickson, Edward J. Kramer, Jeffrey M. Rose, Anna E. Cherian, Geoffrey W. Coates, Hisashi Ohtaki, K. Hirokane, F. Yamada, Yong-Woo Shin, Fumihiko Shimizu Recent catalysts have enabled the synthesis of block copolymers with semicrystalline syndiotactic and isotactic polypropylene endblocks and amorphous ethylene-r-propylene midblocks. In these copolymers, the crystals play the role of physical crosslinks which can deform plastically under stretching. Neat elastomers, gels in mineral oil and gels from which mineral oil has been extracted have been subjected to step cycle tensile tests. The incremental plastic deformation of the crystals has dramatic effects on the true stress versus extension ratio curves. Moreover, small and wide angle X-ray scattering experiments during step cyclic tests revealed the evolution of the microstructure of the crystalline blocks: crystal fibrils and crystals in the fibrils are oriented parallel to the tensile direction at large strains and once the stress has decreased to zero, the fibrils return to being randomly oriented again. [Preview Abstract] |
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