Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session A42: Polymeric Elastomers and Gels |
Hide Abstracts |
Sponsoring Units: DPOLY Chair: Edwin Chan, National Institute of Standards and Technology Room: 214B |
Monday, March 2, 2015 8:00AM - 8:12AM |
A42.00001: Tailoring Phase Behavior and Mechanical Properties in Thermoplastic Elastomers through Block Sequence and Macromolecular Architecture Adam Burns, Richard Register Block copolymers exhibit unique properties which depend not only on the identities of the constituent blocks but also the block sequence and macromolecular architecture. Thermoplastic elastomers (TPEs) are a prime example. In TPEs the arrangement of glassy end blocks flanking a long rubbery midblock gives rise to a physically cross-linked, elastic solid. Exchanging the glassy blocks for crystalline blocks can improve the processability and solvent resistance, but adversely affects the mechanical performance. The block sequence crystalline-glassy-rubbery-glassy-crystalline has been developed to combine the advantages of both crystalline and glassy blocks. Careful selection of block lengths produces materials in which the order-disorder transition temperature lies below the melting point of the crystalline block, ensuring that the melt will be homogeneous above the melting point. Access to single-phase melts provides a large reduction in viscosity and elasticity over conventional TPEs, which remain microphase-separated in the melt. Inserting the glassy blocks between the crystalline and rubbery blocks produces a vitreous layer surrounding the crystalline domains, which improves the room-temperature mechanical performance. Incorporating the crystalline-glassy-rubbery motif into the arms of star block copolymers adds another level of control. The star architecture introduces a permanent cross-link at the center of the star without appreciably affecting the phase behavior. [Preview Abstract] |
Monday, March 2, 2015 8:12AM - 8:24AM |
A42.00002: Shear Induced Morphology Evolution and Dynamic Viscoelastic Behavior of Binary and Ternary Elastomer Blends Xia Dong, Xianggui Liu, Wei Liu, Charles C. Han, Dujin Wang The morphology evolution and rheological response of a near-critical composition polybutadiene /polyisoprene blend and solution-polymerized styrene-butadiene rubber/polyisoprene/silica ternary composites after various shear flow were in situ studied with the rheological and rheo-optical techniques. The relationship between the morphology of the blend during the relaxation after the cessation of steady shear with different shear rates and their corresponding rheological properties was successfully established. It was found that the different shear-induced morphologies under steady shear would relax to the equilibrium states via varied mechanisms after the shear cessation. The storage modulus G' increased significantly in the breakup process of the string-like phase. In long time scale, silica slowed down the succeeding breakup of the string-phase domains and simultaneous coalescence of broken droplets, and then effectively reduced the droplets size and stabilized the morphology. [Preview Abstract] |
Monday, March 2, 2015 8:24AM - 8:36AM |
A42.00003: Structure and mechanical properties of isotactic polypropylene (iPP) gels formed at different cooling temperatures Ryusuke Okoshi, Atsushi Hotta The effects of the cooling temperature on the crystalline network formation and the mechanical properties of isotactic polypropylene (iPP) gels were evaluated. iPP/decahydronaphthalene gels were prepared at different cooling temperatures varying from 25 degrees C to -196 degrees C. Tensile test was carried out to measure the mechanical properties of the gels. Scanning electron microscopy (SEM) and differential scanning calorimetry (DSC) analyses were conducted to observe the density and the homogeneity of the network structures. It was found that the iPP gel quenched at -196 degrees C was highly elastic, exhibiting the highest fracture strain and stress of 2500{\%} and 230 kPa. The SEM analysis revealed that nano-crystals were formed, which acted as crosslinkers that were distributed throughout the gels by quenching below -40 degrees C. By contrast, spherulites were observed to have grown inhomogeneously by cooling above -20 degrees C. It was found by DSC that the amount of crystals was increased by decreasing the cooling temperatures, indicating an increase in the density of the network structures. From these results, it was therefore concluded that iPP gel quenched at -196 degrees C possessed the highest mechanical property due to its dense and homogeneous network structures. [Preview Abstract] |
Monday, March 2, 2015 8:36AM - 8:48AM |
A42.00004: Multiple Stages of Crosslinking and Scission in Coarse-Grained Polymers Joanne Budzien Coarse-grained polymer chains were crosslinked, deformed, crosslinked a second time, and deformed again with stress measured at each deformation. Scissioning of crosslinks occurred at various deformations. By varying the level of scissioning and crosslinking at the deformation states, information is gathered about effective crosslink density that includes contributions from physical entanglements. [Preview Abstract] |
Monday, March 2, 2015 8:48AM - 9:00AM |
A42.00005: Pure shear deformation of the chemical gels with precisely tuned network structure Takuya Katashima, Kenji Urayama, Ung-il Chung, Takamasa Sakai We investigate the effects of the structural parameters on the cross effect of the strains under the pure shear deformation. The cross effect is the strength of the influence of the strain in one direction on the stress in the other direction. To clarify the molecular origin of this effect, we performed the biaxial stretching for the Tetra-PEG gels with various network strands, tuned connectivity and the unattached guest chains. We found that the cross effect of the strains increased with an increase in the network fraction including the elastically effect strands and dangling chains, regardless of the network strand length and the amount of guest chains. These results suggest that the cross effect of the strains may reflect the mutual interference between the neighboring chains like the nematic interaction or the topological effect. These findings will help the fundamental understanding of the rubber elasticity. [Preview Abstract] |
Monday, March 2, 2015 9:00AM - 9:12AM |
A42.00006: Wide bicontinuous compositional windows from co-networks made with telechelic macromonomers Gregory Tew Phase-separated and self-assembled co-network materials offer a simple route to bicontinuous-like morphologies, which are expected to be highly beneficial for applications such as ion, charge, and oxygen transport. Despite these potential advantages, the systematic definition of co-network structures has not been achieved, largely due to the lack of well-controlled chemistries for their preparation. Here, a thiol-ene end-linking platform enables the systematic investigation of phase-separated poly(ethylene glycol) (PEG) and polystyrene (PS) networks in terms of the molecular weight and relative volume fractions of precursor polymers. The ion conductivity and storage modulus of these materials serve as probes to demonstrate that both phases percolate over a wide range of compositions, spanning PEG volume fractions from $\sim$ 0.3 - 0.65. These findings demonstrate that this approach to thiol-ene co-networks is a versatile platform to create bicontinuous morphologies. [Preview Abstract] |
Monday, March 2, 2015 9:12AM - 9:24AM |
A42.00007: Squeezing a gel to establish network structure-transport property relationships Edwin Chan, Nichole Nadermann, Kelly McLeod, Greg Tew Gels are used in many applications, ranging from drug delivery to water purification, where regulating transport of a particular permeant is critical. The structure of the gel determines its transport properties but developing the gel structure-transport property relationships often require multiple measurement techniques. In this work, we demonstrate poroelastic relaxation indentation (PRI) as a single measurement tool to establish the relationships between the polymer network structure and the transport properties of well-defined hydrogel networks synthesized via a thiol-norbornene click reaction of poly(ethylene glycol) (PEG) chains. We use PRI to quantify the mechanical and transport properties of a series of ``click'' hydrogels with different crosslink densities. By applying various thermodynamic network swelling models to the describe the mechanical response of these gels as measured from PRI, we are able to extract thermodynamic parameters of these hydrogels including the Flory chi parameter and the mesh size. We validate our approach by comparing the thermodynamic parameters obtained from PRI with results from neutrons scattering studies of the same series of hydrogels. [Preview Abstract] |
Monday, March 2, 2015 9:24AM - 9:36AM |
A42.00008: Tensile Deformation and Morphological Evolution of Precise Acid Copolymers Luri Robert Middleton, Steve Szewczyk, Eric Schwartz, Jason Azoulay, Dustin Murtagh, Joseph Cordaro, Kenneth Wagener, Karen Winey Acid- and ion-containing polymers have specific interactions that produce complex and hierarchical morphologies that provide tunable mechanical properties. We report tensile testing and in situ x-ray scattering measurements of a homologous series of precise poly(ethylene-co-acrylic acid) copolymers (pxAA). Upon variation of the number of backbone carbons (x $=$ 9, 15, 21) between pendant acrylic acid groups along the linear polyethylene chain, these materials exhibit pronounced changes in both their tensile properties as well as their morphological evolution during deformation. The hierarchical layered acid aggregate structure coincides with the onset of a strain hardening mechanism and was observed in both a semi-crystalline sample (p21AA) as well as an amorphous sample (p15AA). The polymer with the shortest spacing between acid groups (p9AA) maintains a liquid-like distribution of acid aggregates during deformation, exhibiting low tensile strength which we attribute to facile acid exchange between acid aggregates during deformation. Our results indicate that the formation of the hierarchical layered structure, which coincides with polymer strain-hardening regime, originates from the associating acid groups cooperatively preventing disentanglement. [Preview Abstract] |
Monday, March 2, 2015 9:36AM - 9:48AM |
A42.00009: Nonlinear behavior of ionically and covalently cross-linked alginate hydrogels SeyedMeysam Hashemnejad, Mahla Zabet, Santanu Kundu Gels deform differently under applied load and the deformation behavior is related to their network structures and environmental conditions, specifically, strength and density of crosslinking, polymer concentration, applied load, and temperature. Here, we investigate the mechanical behavior of both ionically and covalent cross-linked alginate hydrogel using large amplitude oscillatory shear (LAOS) and cavitation experiments. Ionically-bonded alginate gels were obtained by using divalent calcium. Alginate volume fraction and alginate to calcium ratio were varied to obtain gels with different mechanical properties. Chemical gels were synthesized using adipic acid dihdrazide (AAD) as a cross-linker. The non-linear rheological parameters are estimated from the stress responses to elucidate the strain softening behavior of these gels. Fracture initiation and propagation mechanism during shear rheology and cavitation experiments will be presented. Our results provide a better understanding on the deformation mechanism of alginate gel under large-deformation. [Preview Abstract] |
Monday, March 2, 2015 9:48AM - 10:00AM |
A42.00010: Nonlinear Elasticity and Cavitation of a Triblock Copolymer Gel Santanu Kundu, Seyed Meysam Hashemnejad, Mahla Zabet, Satish Mishra Polymer gels are subjected to large-strain deformation during their applications. The gel deformation at large-strain is non-linear and can often lead to failure of the material. Here, we report the large-strain deformation behavior of a physically cross-linked, swollen polymer gel, which displays unique strain-stiffening response at large-strain. Investigations were performed using large amplitude oscillatory shear (LAOS) and custom developed cavitation rheology techniques. Gent constitutive model, which considers finite extensibility of midblock, was fitted with the LAOS data, therefore, linking the estimated parameters from LAOS analysis to the structure of the gel. Cavitation experiments were conducted as a function of temperature. Both analytical method and finite-element based modeling have been implemented to capture the pressure response in cavitation experiments. Our results provide a critical understanding of gel failure mechanism at large-strain. [Preview Abstract] |
Monday, March 2, 2015 10:00AM - 10:12AM |
A42.00011: Nonlinear Elasticity of Bottlebrush Networks and Gels Andrey Dobrynin, Zhen Cao, Jan-Michael Carrillo, Sergei Sheiko Bottlebrush networks are examples of supersoft elastic materials that demonstrate highly nonlinear stress-strain behavior leading to material hardening with increasing deformation. Using molecular dynamics simulations and theoretical analysis we studied correlations between mechanical properties of bottlebrush networks and molecular parameters. Our simulations showed that both the network shear modulus G and the elongation at break decrease (onset of finite extensibility) with increasing the degree of polymerization (DP) of the side chains. The finite extensibility behavior is ascribed to the increase of the backbone elongation ratio $\beta $ with DP of the side chains. Simulation results are in a good agreement with experimental observation of progressive softening of bottlebrush elastomers with increasing length of side chains and predictions of the nonlinear network deformation model which provides universal relationship between nonlinear network deformation modulus as a function of the first deformation invariant I1, bottlebrush backbone elongation ratio $\beta $, bottlebrush effective bending constant K and concentration of crosslinks. [Preview Abstract] |
Monday, March 2, 2015 10:12AM - 10:24AM |
A42.00012: On the role of geometric non-linearities in the mechanics of nematically ordered semi-flexible networks Louis Foucard, Jordan Kazuo Price, William Klug, Alex Levine Extending previous studies on the affine-nonaffine transition of nematically ordered semiflexible networks, we investigate numerically the effect of geometric non-linearities on the mechanical response of anisotropic networks of elastic filaments. We find that the strong dependence of buckling susceptibility on segment length has a pronounced effect on the nonlinear elastic behavior of anisotropic networks; contrary to isotropic networks, highly ordered ones show an important softening of the shear modulus at nonlinear (finite) strains. We compute the spatial correlation of the Conti/McKintosh buckling order parameter and show that the particularity of the nonlinear response of nematic networks resides in the cooperativity of the buckling events. We also show that dependence of the shear modulus on the nematic order parameter can be predicted using the assumption of affine deformation at small shear, and in terms of a generalized floppy mode analysis of the nonaffine mechanics at larger deformation. [Preview Abstract] |
Monday, March 2, 2015 10:24AM - 10:36AM |
A42.00013: Using stability analyses to predict dynamic behaviour of self-oscillating polymer gels Vaibhav Palkar, Gaurav Srivastava, Olga Kuksenok, Anna C. Balazs, Pratyush Dayal Use of chemo-mechanical transduction to produce locomotion is one of the significant characteristics of biological systems. Polymer gels, intrinsically powered by oscillatory Belousov-Zhabotinsky (BZ) reaction, are biomimetic materials that exhibit rhythmic self-sustained mechanical oscillations by chemo-mechanical transduction. Via simulations, based on the 3D gel lattice spring model, we have successfully captured the dynamic behaviour of BZ gels. We have demonstrated that it is possible to direct the movement of BZ gels along complex paths, guiding them to bend, reorient and turn. From a mathematical perspective, the oscillations in the BZ gels occur when the gel's steady states loose stability by virtue of Hopf bifurcations (HB). Through the use of stability analyses, we predict the conditions under which gel switches from stationary to oscillatory mode and vice versa. In addition, we characterize the nature of HB and also identify other types of bifurcations that play a critical role in governing the dynamic behaviour of BZ gels. Also, we successfully predict the frequency of chemo-mechanical oscillations and characterize its dependency on the model parameters. Our approach not only allows us to establish optimal conditions for the motion of BZ gels, but also can be used to design other dynamical systems. [Preview Abstract] |
Monday, March 2, 2015 10:36AM - 10:48AM |
A42.00014: Shape Actuation of Competitive Networks Yuan Meng, Jisu Jiang, Mitchell Anthamatten We demonstrate a single phase, two-way shape actuator that, in the absence of an external load, elongates upon cooling and reversibly contracts upon heating. In a simple and straightforward process, a partially crosslinked, semi-crystalline PCL network is melted, stretched to several hundred percent strain, and further crosslinked. Upon removal of the applied load, the elastic ``double network'' adopts a ``state-of-ease'' that retains part of its former strain. When cooled, internal stress-induced crystallization causes further elongation of the configurationally biased chains; and when heated, crystallites melt, and the sample returns to its equilibrium state-of-ease. Under optimized conditions, reversible actuation of over 15 percent strain can be reproducibly achieved, and samples can be cycled multiple times with highly uniform actuation with no observable creep. The mechanism behind such actuation was further confirmed via calorimetry and X-ray scattering. [Preview Abstract] |
Monday, March 2, 2015 10:48AM - 11:00AM |
A42.00015: Shape Memory Polymers from Blends of Elastomers and Crystalline Small Molecules Kevin Cavicchi, Nicole Brostowitz, Brent Hukill, Heather Fairbairn This talk will present work on the fabrication of shape memory polymers (SMPs) by swelling natural with molten fatty acids. By this method a SMPs with excellent shape fixity and recovery can be obtained during free recovery after uniaxial deformation to 100{\%} strain. Experiments to measure the shape memory properties under both stress and strain controlled conditions will be reported and compared. This fabrication method offers a number of advantages for preparing SMPs. First, it utilizes natural rubber as the base material for the SMP, which capitalizes on a high performance, commodity elastomer. Second, by blending a commercial polymer with a small molecule additive no additional chemistry is needed for the preparation of the SMP. Third, this route inverts the typically processing steps by crosslinking the permanent network prior to formation of the physically crosslinked reversible network. This offers a means to potentially generate a SMP from any preformed elastomeric article. [Preview Abstract] |
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