Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session D32: Responsive Polymers, Soft Materials, and Hybrids IFocus
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Sponsoring Units: DPOLY DSOFT DBIO Chair: Jinhye Bae, University of California, San Diego Room: 504 |
Monday, March 2, 2020 2:30PM - 3:06PM |
D32.00001: Silk-inspiration: hierarchy, assembly, and mechanics in polyurea-polypeptide hybrids Invited Speaker: LaShanda Korley Inspired by spider silk, we have designed a series of polymer-peptide polyurethane/ureas to explore the hierarchical arrangement critical to energy absorption and mechanical enhancement. We have developed chain-extended and non-chain extended peptide-polyurea hybrids with tunable secondary structure, modulating extensibility, toughness, and stiffness. The sheet-dominant hybrid materials were typically tougher and more elastic due to intermolecular H-bonding, while the helical-prevalent systems generally exhibited higher modulus. We have also explored the impact of a molecular design strategy that overlays a covalent and physically crosslinked architecture in these hybrids, demonstrating that physical constraints in the network hybrids influences hydrogen bonding and morphology. More recently, tailored physical associations within the soft and hard phases were engineered as a function of peptide content, leading to a rheological response dictated by block ordering and highlighting their potential as structural and injectable hydrogels. New efforts in silk composites that exhibit shape memory behavior, and 'green' pathways for silk-inspired materials are of current focus. These structural features have enabled new thrusts in injectable gels and responsive actuators. |
Monday, March 2, 2020 3:06PM - 3:18PM |
D32.00002: Osmotic Swelling Behavior of Ionic Cylindrical Microgels Mohammed Alziyadi, Alan Denton Ionic microgels are cross-linked polymeric networks that ionize and swell in a good solvent. The swelling behavior of the particles can be tuned by adjusting temperature, pH, and ionic strength, enabling applications to drug delivery and tissue engineering. Unlike spherical microgels, cylindrical microgels can swell in both radial and axial directions. Equilibrium swelling can be triggered by varying the balance between electrostatic and gel contributions to the osmotic pressure inside and outside of the particles [1, 2]. Within a cell model, we derive an exact statistical mechanical theorem for the electrostatic osmotic pressure of a cylindrical microgel to study the dependence of microion distribution and osmotic pressure on equilibrium size. To validate the theorem, we implemented Poisson-Boltzmann theory and molecular dynamics simulations. Combining our theorem with Flory-Rehner theory of polymer networks, we predict radial and axial equilibrium swelling ratios of cylindrical microgels as a function of salt concentration. Our results can help guide the design of smart, responsive particles. |
Monday, March 2, 2020 3:18PM - 3:30PM |
D32.00003: Swelling-induced Morphological Deformation and Constitutive Relation of Soft Materials in Micro-patterned Hydrogel Jung Gun Bae, Won Bo Lee Soft materials are ideal for designing structures with useful properties. Especially, hydrogels have been extensively studied for their response to various external stimuli and reversibility. A gel under geometric confinements deforms anisotropically due to applied directional stresses, but otherwise it can be equilibrated and swollen by solvent homogeneously. In particular, wrinkling instability can be observed for line patterns of hydrogels bounded on a rigid substrate. Geometry and swelling ratio of the gel can decide a morphology – in the case of wrinkles, wavelength and amplitude - of the pattern. We controlled not only geometry factors of the micro patterns of hydrogl , but also the swelling ratio of the gel by varying the amount of Acrylamide (AAm) added during synthesis. |
Monday, March 2, 2020 3:30PM - 3:42PM |
D32.00004: Dynamics of 3D polymer gel with reversible linkers Santidan Biswas, Victor V Yashin, Anna Balazs We utilize the gel lattice-spring approach to develop the 3D computational model of polymer gels containing the temporary crosslinks. The polymer chains are assumed to incorporate the folded domains that encompass the reactive functional groups (cryptic sites). Under deformation, the domains unfold and expose the cryptic sites, which can then form labile bonds with the linker chains grafted to the network. Once the deformation is removed, the linkers detach from the cryptic sites, and unfolded domains go back to the folded configuration thus hiding the cryptic sites. The gel behavior under applied force is described by the equations of elasticity of the polymer network coupled to the chemical kinetics equations for the folding and binding transitions. The model equations take into account the effects of finite chain extensibility on the gel elasticity and mechanosensitive reaction rates. Elasticity of the transient network is introduced to the model through the Flory approach. We study the behavior of the system under uniaxial extension and compression, and determine the effect of temporary crosslinks on the bulk and shear moduli of the gel. We also compare the behavior of the gel with a similar gel not undergoing the transient binding of the linker chains. |
Monday, March 2, 2020 3:42PM - 3:54PM |
D32.00005: Phototunable Viscoelasticity in Hydrogels Through Thioester Exchange Benjamin Carberry, Varsha V Rao, Kristi Anseth The extracellular matrix in which cells reside offers elastic and viscoelastic mechanical cues important for directing cell behavior. Recently, cellular responses to viscoelastic and elastic mechanical cues have been studied; however, questions remain as to how cells identify and transduce these cues differently, sparking a need for materials that can interrogate these properties separately. We present a cell culture substrate based on thioester exchange chemistry where viscoelasticity of the biophysical environment can be modulated in situ with light and the photoinitiated thiol-ene ‘click’ reaction. With this method, stress relaxation in thioester hydrogels with relaxation times ranging from 200,000-1,500,000s can be switched off in the presence of cells without change to the elastic modulus. NIH 3T3 fibroblasts cultured for 48 hr on viscoelastic substrates that transition to elastic substrates after 24 hr display cell morphology and YAP/TAZ expression similar to the elastic control. Phototunable viscoelastic thioester hydrogels provide a tunable materials system to investigate time-dependent cellular responses to viscoelasticity and should prove useful for understanding the dynamics of mechanoresponsive cellular pathways. |
Monday, March 2, 2020 3:54PM - 4:06PM |
D32.00006: Toughening mechanism of tough and self-healing physical hydrogels KUNPENG CUI, Jian Ping Gong Recently, we have developed a new class of tough and self-healing physical hydrogels composed of polyampholytes (PA). Those PA gels possess high toughness, stiffness, fatigue resistance and self-healing. However, the toughening mechanism of these gels is still unclear. In this work, we used real time small-angle X-ray scattering (SAXS) to study the toughening mechanism of PA hydrogels. |
Monday, March 2, 2020 4:06PM - 4:18PM |
D32.00007: Anisotropic Hollow Microgels That Can Adapt Their Size, Shape, and Softness Anne Nickel, Andrea Scotti, Judith Houston, Jerome Crassous, Jan Skov Pedersen, Walter Richtering We have recently shown how to create hollow, anisotropically shaped thermoresponsive microgels, polymeric networks with a solvent filled cavity that are swollen in a good solvent.[1]Sacrificial elliptical hematite silica particles were utilized as a template for the synthesis of a cross-linked N-isopropylacrylamide (NIPAm) shell. We characterized these microgels using a combination of light, X-ray, and neutron scattering. New form factor models, accounting for the cavity, the polymer distribution and the anisotropy, have been developed for fitting the scattering data. With such models, we demonstrated the existence of the cavity and simultaneously the anisotropic character of the microgels. Finally, the effect of temperature and shell thickness was investigated, showing that changes in size, softness, and aspect ratio are triggered. We believe that these hollow anisotropic microgels represent an attractive model system for fundamental physics (e.g. phase behavior or ordering phenomena) and a variety of applications especially in respect to their responsivity to temperature. |
Monday, March 2, 2020 4:18PM - 4:30PM |
D32.00008: Osmotic Pressure of Permeable Ionic Microgels Alan Denton, Mohammed Alziyadi Ionic microgels are soft, permeable, colloidal particles made of crosslinked polymer networks that ionize and swell in a good solvent. Swelling of these particles is responsive to external stimuli and involves a balance of electrostatic and gel contributions to the single-particle osmotic pressure. The electrostatic contribution depends on the distributions of mobile microions and fixed charge on the polymer. Within the cell model, we derive the electrostatic contribution within Poisson-Boltzmann (PB) theory, by minimizing a free energy functional with respect to electrostatic potential, and extract the gel pressure from the pressure tensor. By varying the free energy with respect to microgel size, we also derive exact statistical mechanical relations for the electrostatic osmotic pressure for models of planar, cylindrical, and spherical microgels with fixed charge uniformly spread over their surface or volume. To validate these relations, we solve the PB equation and compute ion densities and osmotic pressures [1]. We show that microgel swelling depends on the electrostatic pressure profile inside the particle and discuss implications for interpreting experiments. |
Monday, March 2, 2020 4:30PM - 4:42PM |
D32.00009: WITHDRAWN ABSTRACT
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Monday, March 2, 2020 4:42PM - 4:54PM |
D32.00010: Tuning Diblock Copolymer Morphologies by Stimuli-Responsive Supramolecular Interactions Xiangyu Zhang, Jing Zong, Dong Meng Ability to tune the microstructures formed by block copolymers via easy-to-use physical approaches offers additional handles to the materials for practical applications. One common approach is through adding homopolymers, which induces morphological changes due to preferential partitioning of homopolymers into specific micro-domains. Recently, supramolecular forces that are chemistry-specific and stimuli-responsive have been exploited to enable stimuli-switchable morphologies. To offer microscopic insights into this process, here we present a simulation study of diblock copolymers blended with homopolymers that are associative to one of the blocks through supramolecular forces. By manipulating the manner of associations, we investigate structural changes induced by supramolecular complexations, and to elucidate the differences from the counterpart van der Waals (VDW) force-driven systems. It is found that the homopolymer-receiving microdomain exhibits non-monotonic size changes accompanied by cluster formation as preferential partitioning occurs. Dynamics analysis suggests that both morphologies and supramolecular binding kinetics exert significant influence on the diffusion of homopolymers across the microdomains, implicative of the extent of “responsiveness” of such materials. |
Monday, March 2, 2020 4:54PM - 5:06PM |
D32.00011: On the solvation of elastin-like polypeptides in aqueous mixtures Yani Zhao, Manjesh Kumar Singh, Kurt Kremer, Robinson Cortes Huerto, Debashish Mukherji The collapsed or expanded state of a polymer is determined by the quality of the solvents: a polymer collapses in a poor solvent, whereas |
Monday, March 2, 2020 5:06PM - 5:18PM |
D32.00012: Chemomechanical origin of directed gel locomotions driven by intenal chemical pulses Qingyu Gao, Lin Ren, Irving R Epstein Our previous work reported the directed locomotion of self-oscillating polymer gels in asymmetric environments, such as retrograde and direct wave locomotion, reciprocal migration, photophobic and phototropic movement. We now ask whether directed locomotions and their transitions can be generated only from intrinsic chemical dynamics and its modulation. We examine this question by simulating the locomotion of a responsive polymer gel in a homogeneous environment. We find that autonomous directional locomotion emerges in the absence of asymmetric interaction with the environment, and that a transition between modes of gel locomotion can be induced by adjusting the spatially uniform intensity of illumination or certain kinetic and mechanical system parameters. We find that the internal wave dynamics and modulation of the system act as the impetus for signal-driven active locomotion in a manner similar to the way in which an animal's locomotion is generated via driving by nerve pulses. |
Monday, March 2, 2020 5:18PM - 5:30PM |
D32.00013: Competition between Hydrophobic and Electrostatic Interactions determine pH-responsive Supramolecular Self-assembly Saikat Chakraborty, Christian M. Berac, Pol Besenius, Thomas Speck We present results from a molecular dynamics simulations study, inspired by experiments on pH-regulated self-assembly into filaments. Experiments are performed in aqueous media, with 1:1 binary mixture of amphiphilic peptide monomers having oppositely charged side chains. In neutral conditions, the monomers self-assemble into alternating copolymers. In acidic and basic conditions, selective screening of Coulombic interactions occurs. This leads to formation of homopolymers. Building on our model for homopolymers in implicit solvent1, we introduce charges to mimic the two monomers. Our simulations reveal that the consideration of pH-dependent hydrophobic interactions is essential to observe pH-switch in polymerization. Further, assembly kinetics is analyzed, comparing simulations to experimental data which are obtained from circular dichroism spectroscopy. We relate both to appropriate growth models. While the nucleation and elongation are key mechanisms in simulations, fragmentation is important in experimental kinetics. |
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