Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session X11: Polymeric Elastomers and Gels |
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Sponsoring Units: DPOLY Chair: Chelsea Davis, Purdue University Room: 270 |
Friday, March 17, 2017 8:00AM - 8:12AM |
X11.00001: Structure formation in nanocomposite hydrogels Dilip Gersappe, Di Xu We use Molecular Dynamics simulations to study structure formation in physically associating nanocomposite hydrogels. Nanofillers were modeled as rigid bodies of disk-like shapes and physical crosslinks were simulated by introducing a short-range attraction between the nanofillers and polymer chain ends. The structure, dynamics and mechanics of this polymer gel was studied as function of nanofiller volume fraction. We observe the formation of a percolated nework of these structures, with an ordered local structure but disordered globally, as we increase the filler fraction. The dynamics of polymers showed significant caging effects in the gel state. Stress autocorrelation and elongation results were analyzed as a function of the nano-filler concentration. [Preview Abstract] |
Friday, March 17, 2017 8:12AM - 8:24AM |
X11.00002: Rapid Self-healing Nanocomposite Hydrogel with Tunable Dynamic Mechanics Qiaochu Li, Sumeet Mishra, Brian Chapman, Pangkuan Chen, Joseph Tracy, Niels Holten-Andersen The macroscopic healing rate and efficiency in self-repairing hydrogel materials are largely determined by the dissociation dynamics of their polymer network, which is hardly achieved in a controllable manner. Inspired by mussel's adhesion chemistry, we developed a novel approach to assemble inorganic nanoparticles and catechol-decorated PEG polymer into a hydrogel network. When utilized as reversible polymer-particle crosslinks, catechol-metal coordination bonds yield a unique gel network with dynamic mechanics controlled directly by interfacial crosslink structure. Taking advantage of this structure-property relationship at polymer-particle interfaces, we designed a hierarchically structured hybrid gel with two distinct relaxation timescales. By tuning the relative contribution of the two relaxation modes, we are able to finely control the gel's dynamic mechanical behavior from a viscoelastic fluid to a stiff solid, yet preserving its rapid self-healing property without the need for external stimuli. [Preview Abstract] |
Friday, March 17, 2017 8:24AM - 8:36AM |
X11.00003: Toward a predictive model for the failure of elastomer seals. Nicola Molinari, Musab Khawaja, Adrian Sutton, Arash Mostofi Nitrile butadiene rubber (NBR) and hydrogenated-NBR (HNBR) are widely used elastomers, especially as seals in oil and gas industry. During exposure to the extreme temperatures and pressures typical of well-hole conditions, ingress of gases causes degradation of performance, including mechanical failure. Using computer simulations, we investigate this problem at two different length- and time-scales. First, starting with our model of NBR based on the OPLS all-atom force-field, we develop a chemically-inspired description of HNBR, where C=C double bonds are saturated with either hydrogen or intramolecular cross-links, mimicking the hydrogenation of NBR to form HNBR. We validate against trends for the mass density and glass transition temperature for HNBR as a function of cross-link density, and for NBR as a function of the fraction of acrylonitrile in the copolymer. Second, a coarse-grained approach is taken in order to study mechanical behaviour and to overcome the length- and time-scale limitations inherent to the all-atom model. The effect of nanoparticle fillers added to the elastomer matrix is investigated. Our initial focus is on understanding the mechanical properties at the elevated temperatures and pressures experienced in well-hole conditions. [Preview Abstract] |
Friday, March 17, 2017 8:36AM - 8:48AM |
X11.00004: The Role of Hydroxide and Metal Concentration on the Viscoelastic Properties of Metal Coordinated Gels Seth Cazzell, Niels Holten-Andersen Nature uses metal binding amino acids to engineer mechanical properties. An example of this engineering can be found in the mussel byssal thread. This acellular thread contains reversible intermolecular protein-metal bonds, which allows the mussel to robustly anchor to rocks, while withstanding the mechanically demanding intertidal environment. Inspired by this metal-binding material, we present a synthetic hydrogel designed to mimic this bonding behavior. The mechanical properties of this hydrogel can be controlled independently by manipulating the amount of metal relative to the metal binding ligand, and the gel's pH. Here we report how high metal to ligand ratios and low pH can be used to induce the formation of a strong, slow relaxing gels. This gel has potential applications as an energy dissipating material, and furthers our understanding of the bio-inspired engineering techniques that are used to design viscoelastic soft materials. [Preview Abstract] |
Friday, March 17, 2017 8:48AM - 9:00AM |
X11.00005: A Poroelastic Approach for Quantifying Gel Network Properties Edwin Chan, Nichole Nadermann, Katie Feldman, Eric Davis The unique chemical and structural properties of polymer gels has led to the application of these materials in various membrane-based technologies where selective transport is critical to device performance. Characterizing the chemical and structural properties of a gel is critical to understanding its transport behavior. yet quantifying these properties is nontrivial as it typically requires multiple measurement techniques. In this talk, we demonstrate poroelastic relaxation indentation (PRI) as a single measurement tool to characterize the swelling, mechanical and transport properties of model poly(ethylene glycol)-based hydrogel systems. By applying the appropriate thermodynamic polymer network model and the linear theory of poroelasticity, we are able to use the results from PRI to extract the thermodynamic parameters, elastic modulus, water permeability and mesh size of these gels. We validate these results with small angle neutron scattering to illustrate the applicability of the PRI measurement technique for studying these membrane-like materials. [Preview Abstract] |
Friday, March 17, 2017 9:00AM - 9:12AM |
X11.00006: Quantifying the impact of cyclic defects on polymer network elasticity Rui Wang, Bradley Olsen, Jeremiah Johnson Despite the ubiquity of applications, much of our fundamental knowledge about polymer networks is based on an assumption of ideal end-linked structure. However, polymer networks invariably possess topological defects: loops of different orders. Here, we develop a kinetic graph theory which demonstrates the universal cyclic topology of polymer networks. The theory is in excellent agreement with experimental measurements of hydrogel loop fractions without any fitting parameters. The one-to-one correspondence between the network topology and primary loop fraction demonstrates that the entire network topology is characterized by measurement of just primary loops. Different cyclic defects cannot vary independently. To study the correlations between the network topology and gel elasticity, we develop a real elastic network theory (RENT), a modified phantom network theory that accounts for the impacts of cyclic defects. We demonstrate that small loops (primary and secondary loops) have vital effect on the modulus; whereas this negative impact decreases rapidly as the loop order increases, especially for networks with higher junction functionalities. Loop effect can propagate to its neighborhood strands. RENT provides predictions that are highly consistent with experimental observations of polymer network elasticity, providing a quantitative theory of elasticity that is based on molecular details of polymer networks. [Preview Abstract] |
Friday, March 17, 2017 9:12AM - 9:24AM |
X11.00007: Linear and Nonlinear Elasticity of Networks Made of Comb-like Polymers and Bottle-Brushes H. Liang, A. Dobrynin, M. Everhart, W. Daniel, M. Vatankhah-Varnoosfaderani, S. Sheiko We study mechanical properties of networks made of combs and bottle-brushes by computer simulations, theoretical calculations and experimental techniques. The networks are prepared by cross-linking backbones of combs or bottle-brushes with linear chains. This results in ``hybrid'' networks consisting of linear chains and strands of combs or bottle-brushes. In the framework of the phantom network model, the network modulus at small deformations $G_{\mathrm{0}}$ can be represented as a sum of contributions from linear chains, $G_{\mathrm{0,l}}$, and strands of comb or bottle-brush, $G_{\mathrm{0,bb}}$. If the length of extended backbone between crosslinks, $R_{\mathrm{max}}$, is much longer than the Kuhn length, $b_{k}$, the modulus scales with the degree of polymerization of the side chains, $n_{\mathrm{sc}}$, and number of monomers between side chains, $n_{\mathrm{g}}$, as $G_{\mathrm{0,bb}}\propto $(n$_{\mathrm{sc}}$/n$_{\mathrm{g}}+$1)$^{\mathrm{-1}}$. In the limit when $b_{k}$ becomes of the order of $R_{\mathrm{max}}$, the combs and bottle-brushes can be considered as semiflexible chains, resulting in a network modulus to be $G_{\mathrm{0,bb}}\propto $(n$_{\mathrm{sc}}$/n$_{\mathrm{g}}+$1)$^{\mathrm{-1}}$(n$_{\mathrm{sc}}^{\mathrm{1/2}}$/n$_{\mathrm{g}})$. In the nonlinear deformation regime, the strain-hardening behavior is described by the nonlinear network deformation model, which predicts that the true stress is a universal function of the structural modulus, $G$, first strain invariant, $I_{\mathrm{1}}$, and deformation ratio, $\beta $. The results of the computer simulations and predictions of the theoretical model are in a good agreement with experimental results. [Preview Abstract] |
Friday, March 17, 2017 9:24AM - 9:36AM |
X11.00008: Molecular model for the diffusion of associating telechelic polymer networks Jorge Ramirez, Thomas Dursch, Bradley Olsen Understanding the mechanisms of motion and stress relaxation of associating polymers at the molecular level is critical for advanced technological applications such as enhanced oil-recovery, self-healing materials or drug delivery. In associating polymers, the strength and rates of association/dissociation of the reversible physical crosslinks govern the dynamics of the network and therefore all the macroscopic properties, like self-diffusion and rheology. Recently, by means of forced Rayleigh scattering experiments, we have proved that associating polymers of different architectures show super-diffusive behavior when the free motion of single molecular species is slowed down by association/dissociation kinetics. Here we discuss a new molecular picture for unentangled associating telechelic polymers that considers concentration, molecular weight, number of arms of the molecules and equilibrium and rate constants of association/dissociation. The model predicts super-diffusive behavior under the right combination of values of the parameters. We discuss some of the predictions of the model using scaling arguments, show detailed results from Brownian dynamics simulations of the FRS experiments, and attempt to compare the predictions of the model to experimental data. [Preview Abstract] |
Friday, March 17, 2017 9:36AM - 9:48AM |
X11.00009: Dynamics in weakly-ordered oppositely-charged polyelectrolyte complex gels Anand Rahalkar, Guangmin Wei, Samanvaya Srivastava, Matthew Tirrell, Vivek Prabhu Polyelectrolyte complexes (PEC) formed by ABA block copolymers form well-defined ordered phases through design of the block fraction, added salt, and polymer concentration. This tailored structure offers many opportunities for materials design through charge complexation. In this presentation, we extend our efforts to understand the dynamics in weakly-ordered PEC gels. We find that the formation of an equilibrium between micelles and clusters in the dilute phase provides insight into multiple relaxation modes observed in PEC gels by angular-dependent dynamic light scattering. Combining these results with small-angle neutron scattering, we establish the structure and dynamics on the temperature-composition plane for the model system of guanidinium and sulfonate functionalized poly(allyl glycidyl ether) end blocks with poly(ethylene glycol) middle block. [Preview Abstract] |
Friday, March 17, 2017 9:48AM - 10:00AM |
X11.00010: A model for structural changes of reconstituted fibroin gels during deformation Peiran Jin, Peter Olmsted Silk from silkworms has been used in the textile industry for thousands of years. Recently, a physical electrogel(e-gel) was made by reconstituting Bombyx mori silk into stable aqueous solutions and then applying small DC electric field [Tabatabai et al, Soft Matter 11 (2015) 756]. The e-gels exhibit distinctive strain hardening and are partially recoverable from strain. To explain these phenomena, we build a coarse grained model of fibroin protein polymers, which comprise crystallizable domains and amorphous domains. We find that the kinetics of unfolding and folding of crystalline domains changes the number and functionality of crosslinks in the physical network, and thus contributes to the strain hardening of the gel and the non-recoverable strain. [Preview Abstract] |
Friday, March 17, 2017 10:00AM - 10:12AM |
X11.00011: Synthesis of PEG-rich PLGA-PEG-PLGA for the PLGA-PEG-PLGA/laponite hydrogels with thermoresponsive sol-gel transitions Keishi Tanimoto, Tomoki Maeda, Atsushi Hotta Poly (D,L-lactide-co-glycolide)-b-poly (ethylene glycol)-b-poly (D,L-lactide-co-glycolide) (PLGA-PEG-PLGA) possesses moderate biocompatibility originating from the relatively shorter PEG block in its polymeric molecule. For the maximum utilization of the highly biocompatible PEG block, the PEG block should be relatively longer, and thus the PEG/PLGA ratio, the molecular weight ratio of PEG and PLGA, should be higher. In addition, for the wider use of PLGA-PEG-PLGA in the biological fields, the aqueous PLGA-PEG-PLGA solution should transfer from sol to gel states in response to the increase in temperature. It was reported, however, through the previous researches, that the PLGA-PEG-PLGA solution with a high PEG/PLGA ratio (above 0.5) would not exhibit thermoresponsive sol-gel transitions. In this work, PLGA-PEG-PLGAs with higher PEG/PLGA ratios were synthesized and the laponite, an inorganic nanoparticle, was added to the solutions to realize the thermoresponsive sol-gel transition. It was found that the PLGA-PEG-PLGA with the high PEG/PLGA ratio of 3.0 could exhibit the thermoresponsive sol-gel transition by adding laponite at 1.25 weight percent. The physical characteristics of the gel were also studied by the dynamic mechanical analysis (DMA) [Preview Abstract] |
Friday, March 17, 2017 10:12AM - 10:24AM |
X11.00012: Self-assembly of nematic liquid crystal elastomer filaments Wei-Shao Wei, Yu Xia, Shu Yang, A. G. Yodh In this work we investigate the self-assembly of nematic liquid crystal polymer (NLCP) filaments and their corresponding cross-linked elastomer structures. Specifically, by fine-tuning surfactant concentration, prepolymer chain length, and temperature within a background aqueous phase we can generate filaments composed of oligomerized LC monomers. Filaments with narrowly dispersed diameters ranging from one hundred nanometers to a few micrometers can be obtained. Using polarization optical microscopy, we show that the nematic LCs within the filaments have an escaped radial structure. After photo-cross-linking, nematic liquid crystal elastomer filaments are obtained with well-maintained directors and smooth surface structure. Since these materials are elastomers, the size and mechanical and optical response of the filaments can be "tuned" near the nematic to isotropic phase transition temperature. [Preview Abstract] |
Friday, March 17, 2017 10:24AM - 10:36AM |
X11.00013: The counter-ion condensation behavior of linear polyelectrolyte polymers and polyelectrolyte gels Honglei Guo, Takayuki Kurokawa, Masakazu Takahata, Wei Hong, Yoshinori Katsuyama, Yiwan Huang, Tasuku Nakajima, Takayuki Nonoyama, JianPing Gong Polyelectrolytes are polymers whose repeating units bear an electrolyte group. The charges on a polyelectrolyte chain will repel each other and adopt a more expanded conformation. From linear polyelectrolyte chains to polyelectrolyte networks, the flexibility of chains decreases and thus induces a decrease of counter-ions activity. In this study, we adopt microelectrode technique (MET) to study the counter-ions condensation of polyelectrolyte hydrogels, and compare the condensation effect with the linear polyelectrolytes. The results show that the condensation of counter-ions is affected by the conformation of polymer strands: the less flexibility of polymer strands has less mobile counter-ions. [Preview Abstract] |
Friday, March 17, 2017 10:36AM - 10:48AM |
X11.00014: Colloid Induced Deformation and Strain Stiffening of Biopolymer Networks Ashesh Ghosh, Kenneth Schweizer Recent experiments by Bharadwaj, Ewoldt, Braun and coworkers on colloid-biopolymer composites have shown strain stiffening of a large mesh network above a threshold value of colloid concentration and upon crossing a temperature where the particle undergoes a volume contraction. A qualitative hypothesis has been advanced that the driving force is the presence of a strong cohesive interaction between the colloid and biopolymer which induces strong filament bending upon particle shrinkage. We have formulated and studied a simple model for this phenomenon where the biopolymer is taken to be a semiflexible worm-like chain which experiences a short range square well attraction with the filament. The optimum (lowest free energy) structure of the (deformed) filament is then determined as a competition between bending and cohesive forces, which depends on particle diameter, network mesh size, and the attraction strength and range. An effective phase diagram for the emergence of a colloid-induced deformed filament contour is constructed. Model calculations are performed for the bending energy, cohesive energy and degree of modulus increase as a function of system parameters. Illustrative numerical applications to F-actin and fibrin networks will be presented. [Preview Abstract] |
Friday, March 17, 2017 10:48AM - 11:00AM |
X11.00015: The observation and evaluation of cross-linking inhomogeneity from the displacement of nanoparticles embedded in polymer network by small angle scattering under elongation Kengo Nishi, Mitsuhiro Shibayama We have investigated the effect of polymer/filler interaction on the displacements of silica nanoparticles in gels by introducing them into poly (N,N-dimethylacrylamide) gels (PDAM-NP gels), and polyacrylamide gels (PAM-NP gels). It is well known that PDAM chains in gels are strongly adsorbed onto silica nanoparticles while PAM chains are not. We carried out SAXS measurements on these gels under uniaxial elongation. Interestingly, we found that the SAXS scattering profiles of PDAM-NP gels and PAM-NP gels were totally different. A four-spot pattern was observed in the 2D structure factors of PDAM-NP gel and was assigned to a movement of the nanoparticles in an affine way. On the other hand, as for PAM-NP gels, sharp peaks was observed in much lower $q$ region than affine deformation, indicating that the peak corresponds to the correlation peak of high cross-linking region. Furthermore, we propose a new analysis method to evaluate concrete and detailed information from anisotropic scattering patterns. [Preview Abstract] |
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