Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session A32: Polymer Networks, Gel, and Elastomers: Fabrication and ArchitectureFocus
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Sponsoring Units: DPOLY Chair: Frederick Phelan, National Institute of Standards and Technology Room: 504 |
Monday, March 2, 2020 8:00AM - 8:12AM |
A32.00001: Mechanophore with analogue force readout in a polymer network Kaikai Zheng, Yifan Zhang, Lingxiang Jiang, Jiang Zhao, Steve Granick Polymer networks are fundamental from materials science to cellular biology, however their intrinsic properties are normally characterized using common ensemble-averaged methods. In this work, we introduce a synthetic mechanophore to read out the local force, and to show the proof of concept we covalently link it into a PMA (polymethylacrylate) network as crosslink. We observe differing local intensities of different mechanophores as well as their differing evolution during stretch, and from this quantify locally non-affine responses. Interestingly, it seems to be the single-molecule response yet with analogue force readout inaccessible using conventional mechanophores based on irreversible bond scission mechanisms. |
Monday, March 2, 2020 8:12AM - 8:24AM |
A32.00002: Extending the Real Elastic Network Theory to Account for Cooperative Effect of Cyclic Defects Tzyy-Shyang Lin, Rui Wang, Jeremiah Johnson, Bradley Olsen The real elastic network theory, or RENT, has provided insights into how cyclic defects within a phantom network lead to significant lowering of the modulus of polymer networks. However, RENT is a linear theory derived under the ideal loop gas assumption, which is strictly true only in the limit of infinite dilution of defects, rendering RENT inapplicable for networks with nonnegligible loop fraction. To go beyond the linear regime, the cooperative effects of multiple loops are investigated. First, the multi-defect effect is studied through the Virial expansion, in which the behavior of adjacent loops is compared to isolated loops. In most cases, it was found that the Virial coefficients vanish identically except for the first order terms, indicating that the results for the linear approximation are exact. Next, a nonperturbative probability-based theory was developed to capture the nonlinear behaviors. As expected, the nonlinear theory gave predictions almost identical to that of the linear theory in the small loop fraction regime. However, as loop fraction is increased, the nonlinear theory predicts a significant negative deviation from the linear theory, qualitatively matching the behavior observed in experiments. |
Monday, March 2, 2020 8:24AM - 8:36AM |
A32.00003: WITHDRAWN ABSTRACT
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Monday, March 2, 2020 8:36AM - 9:12AM |
A32.00004: Reactivity-property relationships in photocontrolled polymer networks Invited Speaker: Julia Kalow In polymer networks based on dynamic covalent bonds, changes in reactivity can be translated into macroscopic responses. Light offers precise, tunable, and noninvasive spatiotemporal control over molecular reactivity. In polymer networks based on dynamic covalent bonds, these changes in reactivity can be translated into macroscopic responses. The Kalow lab has designed crosslinks that allow us to tune the thermodynamics and kinetics of dynamic covalent bonds with light, including visible light, based on the conformation of an adjacent photoswitch. When incorporated into polymer networks, the stability or lifetime of these dynamic covalent bonds can be tuned with light. I will discuss our efforts to elucidate the molecular mechanism underlying these macroscopic changes, as well as rational optimization of the photoswitch to enable applications in 3D cell cultures and recyclable elastomers. |
Monday, March 2, 2020 9:12AM - 9:24AM |
A32.00005: Belousov Zhabotinsky reaction systems: How “far” is far from equilibrium? Vandana Rajput, Pratyush Dayal Understanding the behaviour of dynamical systems that are far from equilibrium has been a challenge for science and engineering. Belousov Zhabotinsky (BZ) reaction is a system that exhibits chemical oscillations due to periodic oxidation/reduction of the metal ion catalyst. In essence, the oxidized catalyst concentration varies with time crossing its steady state (SS) concentration with each oscillation. Mathematically, the Limit Cycles (LC), which represent self-sustained oscillations occur when the equilibrium (or SS) loses its stability via Hopf bifurcation. The LC resulting from HB, typically, surrounds the equilibrium, which is analogous to variation of oxidized catalyst concentration around SS. Using the Oregonator model, however, we demonstrate that the equilibrium point can be outside the LC under special conditions. Further, we use the nonlinear stability analyses and calculate Lyapunov coefficients to quantify how “far” the LCs are from the corresponding “equilibrium”. In addition, we predict the amplitude and frequency of oscillations, which are in good agreement with our simulation results. These findings can not only be used to characterise behaviours of nonlinear dynamical systems but can also be used to design smart functional materials. |
Monday, March 2, 2020 9:24AM - 9:36AM |
A32.00006: General Approach to Photo-Crosslink Bottlebrush Polymers Renxuan Xie, Sanjoy Mukherjee, Veronica Reynolds, Christopher Bates, Michael L. Chabinyc Entanglement-free bottlebrush polymers can form exceptionally soft networks that are compelling in applications ranging from mimics of biological tissue to sensor skins. Controlled polymerizations provide a route to synthesize precisely defined bottlebrush polymers, but require crosslinking to form networks. We have developed a general strategy to photo-crosslink bottlebrush precursors with miscible bis-benzophenone-based additives in ambient conditions without solvent. We demonstrate that this approach is effective with a wide variety of different side-chain chemistries including acrylates, esters, and siloxanes. Current bottlebrush network models assume all crosslinks are elastically effective, which is inadequate for our randomly crosslinked networks formed with benzophenone-based additives. We present a modified Phantom network model that is capable of distinguishing elastically effective crosslinks from the ineffective ones. This model is validated by examining the moduli of networks formed from bottlebrush polymers with different molar masses and side-chain chemistries. We also experimentally verified the minimal amount of crosslinkers predicted to achieve the lowest network modulus and high gel fraction. |
Monday, March 2, 2020 9:36AM - 9:48AM |
A32.00007: Gel Formation in Urethane Liquid Oligomers Praveen Agarwal, Bob Sammler, Luigi Pellacani, Asjad Shafi, Praveenkumar Boopalachandran, David Reuschle The phenomenon of gel formation is ubiquitous for various materials including polymer solutions, ionomers, biomaterials, and consumer household products. Developing a physical understanding of the interactions that lead to gel formation is essential for rational material design, and controlling the gel formation characteristics is a topic of significant interest. We have investigated a physical gel formed in a urethane oligomer system dispersed in an organic solvent. The mechanism of gel formation was investigated by rheology, polarized optical microscopy and FTIR. We find evidence of hydrogen bonding and ordered structure formation in the gel. Additionally, a rheology-based technique was developed to predict the gel formation characteristics. |
Monday, March 2, 2020 9:48AM - 10:00AM |
A32.00008: Modelling Intermolecular Cross-Linking in Collagen Fibrils Matthew Leighton, Laurent Kreplak, Andrew Rutenberg Collagen fibrils are microscopic molecular ropes that are structural components in many animal tissues. We present an equilibrium coarse-grained model for the structural and mechanical properties of these cross-linked fibrils. We model enzymatic cross-links as anisotropic Gaussian chains, which allows us to approximate their free-energy contributions. We add additional terms in the free energy for the Frank elastic energy due to the orientation of collagen molecules within the fibril, for the surface tension, and phase-field crystal terms which account for the D-band density modulations observed along the length of collagen fibrils. We computationally minimize the sum of these free-energy terms with respect to imposed strain fields acting on the fibril to obtain equilibrium structures. Using this framework we investigate the effect of strain on various important structural and mechanical properties of the fibril, such as the molecular director field and the stress-strain curve. |
Monday, March 2, 2020 10:00AM - 10:12AM |
A32.00009: From Gels to 3-D Networks: Creating Multifunctional Polymer-silica Nanofiber based Aerogels tahira pirzada, zahra ashrafi, Wenyi Xie, Saad Khan Gels containing water or solvents abound in applications with considerable efforts being made to fabricate new ones with enhanced functionalities. An area that remains less explored is the creation of aerogels by replacing the liquid matrix of gels with air. Can we preserve the structure of these systems on liquid removal and can we create materials that are just as versatile by doing so? We present a facile and sustainable solid templating approach to fabricate highly porous, flexible aerogels of hybride nanofibers of cellulose acetate and silica which are produced via sol gel electrospinning. SEM micrographs demonstrate a hierarchical architecture consisting of large secondary pores (30-50 μm) interconnected by a network of entangled nanofibers with 2-5 μm primary pores. XPS and in-situ FTIR studies provide evidence that thermal treatment of as-prepared aerogels results in crosslinking the silica-CDA network therefore enhancing their mechanical stability and hydrophobicity without compromising their low bulk density (~10 mg.cm-3) and porosity (>98%). Thermal studies demonstrate highly enhanced thermal stability and flame retardancy. These functional features together with ease of processing make these aerogels just as promising, if not more, than their liquid-based analogs! |
Monday, March 2, 2020 10:12AM - 10:24AM |
A32.00010: Probing the distribution of localization lengths in amorphous solids via wavelength-dependent elasticity Boli Zhou, Rafael Hipolito, Paul Goldbart The amorphous solid state exhibited, for example, by randomly crosslinked macromolecular systems has two distinguishing features: (i) it arises via a continuous transition controlled by the crosslink density; and (ii) as a result of the intrinsic randomness, the state is described by a distribution of single-particle localization lengths. Owing to the continuity of the transition, in its vicinity the localization-length distribution is concentrated predominantly at intermediate lengthscales, i.e., lengthscales larger than microscopic but not truly macroscopic. We report on the development of an elasticity theory for the amorphous solid state that is valid not only in the limit of long-wavelength strains but also for strains at wavelengths corresponding to intermediate lengthscales. The corresponding wavelength-dependent shear modulus is sensitive to the distribution of localization lengths, diminishing monotonically with decreasing lengthscale -- a physical reflection of the idea that elasticity at a given lengthscale is primarily supported by particles localized on that or shorter lengthscales. The dependence of the shear modulus on wavelength therefore provides an experimental pathway to probing the distribution of localization lengths. |
Monday, March 2, 2020 10:24AM - 10:36AM |
A32.00011: Asynchronous Dynamics in Crosslinked Polymer Networks Ketan S Khare, Frederick Phelan Recently, we successfully applied time-temperature superposition to obtain master curves of the mean squared displacement (MSD) of atoms in a cross-linked epoxy using atomistic simulations. The resulting curves extend to a macroscopic time-scale (109 s) and can be quantitatively compared with creep compliance obtained from experiments. Here, we show that the MSD trends of the center of mass of molecular units can also be superposed using identical shift factors. Comparison of the master curves and time-scaling exponents of different molecular moieties shows that units which differ in their topological constraints exhibit asynchronous dynamics. We call this feature topology-induced asynchronous (TIA) dynamics and define it as the temporal difference in the dynamics of atoms in a network that vary in their topological constraints. TIA dynamics arise due to the interplay between the chemistry of the epoxy monomer and the cross-linker. We discuss the role of asynchronous dynamics in the thermomechanical behavior of polymer networks composed of non-Gaussian monomers, and how chemistry-specific topological details are important for controlling the polymer dynamics. |
Monday, March 2, 2020 10:36AM - 10:48AM |
A32.00012: Probing rheology and mechanics of compressed microgel suspensions Svetoslav Nikolov, Alberto Fernandez-Nieves, Alexander Alexeev
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Monday, March 2, 2020 10:48AM - 11:00AM |
A32.00013: Energy Renormalization Approach to Coarse-Grained Epoxy Resins Andrea Giuntoli, Zhaoxu Meng, Nitin Hansoge, Sinan Keten
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