Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session Q17: Networks, Gels, and ElastomersRecordings Available
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Sponsoring Units: DPOLY Chair: Ying Li, University of Connecticuit Room: McCormick Place W-184BC |
Wednesday, March 16, 2022 3:00PM - 3:12PM |
Q17.00001: Investigating the network structure and transport properties of physically-crosslinked lignin-based composites Keturah Bethel The biodegradability, antimicrobial properties, and chemical activity of lignin make it attractive for use in the fabrication of sustainable materials. However, the heterogenous nature of lignin has created a bottleneck in understanding how lignin concentration and molecular weight (MW) alter the network structure and ultimately the mechanical and transport properties of these soft composites. To address this issue, we have fabricated a series of lignin–poly(vinyl alcohol) (PVA) composites, via the freeze-thaw method, using both raw lignins with high dispersity, as well as fractionated lignins with prescribed MWs and low dispersity. Specifically, the lignin concentration and MW were systematically varied, ranging in lignin concentrations of 20 wt% to 60 wt% and lignin MWs ranging from ~5700 g/mol to ~19900 g/mol. The transport properties of these membranes were examined by probing the water uptake and permeabilities of model penetrants (e.g., methylene blue). Furthermore, the Young’s modulus, as well as the complex modulus were characterized via mechanical indentation and dynamic mechanical analysis, respectively. Moreover, lignin leaching was analyzed using UV-vis spectroscopy. Preliminary data indicates that the concentration, MW, and dispersity of lignin modulates the transport and mechanical properties of the soft composites. |
Wednesday, March 16, 2022 3:12PM - 3:24PM |
Q17.00002: Exploring the Rheological Properties of Entangled and Unentangled Crosslinked Polymer Nanocomposite Networks Yi Feng, Pinar Akcora We explored the effect of entanglements and particle loading on viscoelastic properties of crosslinked polymer nanocomposite networks. The permanently cross-linked entangled and unentangled networks of poly (methyl acrylate) were synthesized in the presence of nanoparticles. The viscoelastic properties measured in a strain-controlled rheometer showed that with particles larger than the network mesh size, the reinforcement was independent on particle loading in entangled composite networks. The recovery to the initial modulus after deformation occurred fast in entangled networks suggesting that even with highly attractive particles, chain relaxations were unperturbed. In the permanent composite network where chains cannot relax, the reinforcement depended on particle loading and the localized particle aggregation of particles within a dense network. To further investigate the chain architecture effect on reinforcement mechanism of nanocomposite networks, new polymer loop-grafted nanoparticles were prepared and blended with long and short chain matrices. Interpenetrating network formation through loops and entanglements will be discussed. |
Wednesday, March 16, 2022 3:24PM - 3:36PM |
Q17.00003: A Simple Relation between Stiffness and Swelling of a Hydrogel Yiwei Gao, Nickolas K.K. Chai, Negin Garakani, Sujit S Datta, H. Jeremy Cho From pasta to biological tissues to contact lenses, gel and gel-like materials inherently soften as they swell with water. In dry, low-relative-humidity environments, these materials stiffen as they de-swell with water. We use semi-dilute polymer theory to develop a simple power-law relationship between hydrogel elastic modulus and swelling. From this relationship, we can predict hydrogel stiffness or swelling at arbitrary relative humidities. This predictive capability can enable more rapid material discovery and selection for hydrogel applications in varying humidity environments. |
Wednesday, March 16, 2022 3:36PM - 3:48PM |
Q17.00004: Tuning the properties of surface-anchored polymer networks by varying the concentration of thermally-activated crosslinker, annealing time, and temperature in a one-pot reaction Jan Genzer, Sungoung Woo, C.K. Pandiyarajan We investigate the properties of surface-anchored polymer networks created via one-pot synthesis using thermally active 6-azidosulfonylhexyltriethoxy silane (6-ASHTES). 6-ASHTES is a bi-functional gelator that undergoes crosslinking and surface-anchoring reactions when annealed above 100°C. We employ a polyvinylpyrrolidone (PVP) with different molecular weights (10 - 1,300 kDa) as a model system to examine the effect of 6-ASHTES concentration, annealing time, and annealing temperatures on gel formation. A thin film of PVP/6-ASHTES mixture is deposited on a clean silicon wafer and annealed to form network layers. Spectroscopic ellipsometry measures the film thickness of the crosslinked layers from which the gel fraction and swelling ratio are determined. The gel fraction of PVP in the network can be “dialed in” by varying the annealing time, temperature, and concentration of 6-ASHTES in the PVP/6-ASHTES mixture. We use a simple Monte Carlo simulation model to describe crosslinking as a function of crosslinker concentration, reaction rate, reaction time, and polymer length. The trends obtained from the model simulations are in qualitative agreement with the experimental data. |
Wednesday, March 16, 2022 3:48PM - 4:00PM |
Q17.00005: Elucidating the Network Structure, Mechanical and Transport Properties of Poly(vinyl alcohol)–Lignin Soft Composites Containing Fractionated, Purified Lignins Nicholas Gregorich, Sagar Kanhere, Jaden Stutts, Graham Tindall, Tyler B Martin, Amod Ogale, Mark Thies, Eric M Davis Lignin-containing hydrogels have gained attention for use in a variety of aqueous-based separations as lignin is an abundant biopolymer with a high concentration of hydroxyl groups which can be utilized as crosslinking sites during fabrication. However, to date, little is understood regarding how the addition of lignins alters the network structure of composite hydrogels, which is of great importance as most lignin-based hydrogel investigations utilize highly disperse, heterogeneous lignins. Herein, a novel series of lignin–poly(vinyl alcohol) (PVA) composite hydrogels were synthesized utilizing ultraclean lignins of prescribed molecular weights (MWs) and low dispersity acquired via fractionation of highly disperse lignins. Mechanical properties of the hydrated composites were characterized via tensile strength, Young's modulus, and dynamic mechanical analysis. The network structure of the composites was characterized via small-angle neutron scattering and swelling measurements (molecular weight between crosslinks and water uptake). The permeabilities of various pollutants (e.g., hexavalent chromium, methylene blue, bovine serum albumin) through the hydrated composites were measured via ultraviolet-visible spectroscopy, where penetrant permeability was dependent on lignin MW. |
Wednesday, March 16, 2022 4:00PM - 4:12PM |
Q17.00006: Advanced fitting procedure for AFM single-molecule data Ramesh C Tripathi, Peter Hoffmann Atomic Force Microscopy (AFM) can be used in single-molecule binding studies to measure parameters such as off-rates and binding distances. However, interpretation of the obtained bond rupture data is marred by the need to use relatively simple models to describe a potentially very complex unbinding process. The unbinding process is typically not describable by a fixed 1-dimensional barrier height of a single bond but is characterized by a spectrum of barrier heights. Moreover, even if data is filtered to exclude clear multiple rupture events, situations where multiple bonds break simultaneously, thus appearing as single rupture events, further complicate the picture. We present a new data analysis method, using least likelihood fits, that takes some of these effects into account. The method is tested on artificial data generated by Monte Carlo simulations and then applied to measure molecular bond rupture data, using the avidin-biotin system as a model system. |
Wednesday, March 16, 2022 4:12PM - 4:24PM |
Q17.00007: Nanocomposite Hydrogels as Soil Additives Shoumik Saha, Dilip Gersappe Recent advancements in geotechnical engineering involve replacing pore water with chemical or biological-based solutions instead of cement to strengthen soil. One such candidate is a nanocomposite hydrogel which is formed from physical attractions between hydrophilic polymer chains and nanoparticles. They have enhanced properties and can retain higher amounts of water. Here, we use molecular dynamics simulations to study structure formation, dynamics and mechanics of these polymer gels as a function of nanofiller volume fraction under both equilibrium and non-equilibrium conditions. Nanofillers were modeled as bodies of disk-like shapes, with strong rigid bonds and a non-bending cosine harmonic potential between each sub-particle. Physical crosslinks were simulated by introducing a short-range attraction between the nanofillers and polymer chain monomers. We model gels both with a single polymer, as well as a binary mixture of polymers to study the effects of polymer chain interaction and assembly on structure formation and gelation of these systems. Our results show that assembly between the nanoparticles is an important determinant of the final properties of the gel which is mediated by the types and interactions of the polymer used. |
Wednesday, March 16, 2022 4:24PM - 4:36PM |
Q17.00008: Dynamic frictional behavior and dilatancy of hydrogels Jing Wang, Justin C Burton Hydrogels consist of hydrophilic polymer networks that swell dramatically when immersed in water or other solvents. Due to hydrogels' biphasic nature, their frictional properties often display non-monotonic and time-dependent behavior. During frictional sliding, hydrogels experience shear stresses that can induce dilatancy, commonly known as the Poynting effect. This effect is important since it may strongly affect the surface and bulk transport of solvent in the compressed hydrogel. Here we use a rheometer and a custom-built pin-on-disk tribometer to help examine hydrogels' time-dependent frictional behavior and dilatancy. We observe a positive Poynting effect in polyacrylic acid hydrogels at certain sliding velocities. The role of polymer relaxation was also explored by measuring the friction coefficient and normal stress over several hours, where the timescale for dilatant bulk relaxation is often much longer than the timescale for frictional relaxation. |
Wednesday, March 16, 2022 4:36PM - 4:48PM |
Q17.00009: Investigating patterned hydrogel matrix instabilities during swelling Caroline R Szczepanski, Alyssa VanZanten The flexible, cross-linked nature of hydrogels affords tunable water sorption, a trait crucial in designing drug delivery systems and other dynamic soft matter systems. The imbibement or uptake of water into a hydrogel usually leads to uniform swelling, however when hydrogel materials are macroscopically patterned with distinct domains of different cross-link density, the stresses induced by swelling can lead to rupture and failure events. Our ongoing work reveals that if this rupture can be delayed or avoided by material design, such as tuning cross-link density gradient across the domain interface, unique surface features during swelling arise that ultimately change interfacial behavior. These features represent complex surface instabilities that arise from a material that does not achieve an equilibrium swelling state. In this work, we present new methods of investigating these instabilities to better understand dynamic rupture and failure. This includes experimental approaches to estimate the compressive stress that causes surface buckling and the osmotic force associated with swelling of patterned domains. We discuss how understanding these dynamic processes will allow for more fine-tuned engineering of dynamic soft material systems and can inform material design. |
Wednesday, March 16, 2022 4:48PM - 5:00PM |
Q17.00010: Gel rupture and surface instabilities during dynamic swelling Michelle R Driscoll, Kelsey-Ann N Leslie, Shih-Yuan Chen, Caroline R Szczepanski, Robert Doane-Solomon, Srishti Arora, Alyssa VanZanten Hydrogels have had a profound impact in the fields of tissue engineering, drug delivery, and materials science. Due to the network architecture and hydrophilic character of these materials, imbibement with water often results in uniform swelling and isotropic expansion which scales with the degree of cross-linking. However, the development of internal stresses during swelling can have dramatic consequences including transient surface instabilities as well as rupture or failure events. Historically, imbibement with water has been probed by characterizing hydrogels in two distinct states: (1) unswollen, or (2) in an equilibrium swelling state. However, to fully characterize and leverage these instabilities and rupture events, this dynamic process must be investigated directly. To address this gap, our ongoing work focuses on rupture events in poly(ethylene glycol)-based networks that occur in response to swelling with water. In homogeneous samples, we find that rupture events follow a three-stage process that can be described based on changes in the material properties during swelling. We also highlight unique failure events that occur in macroscopically patterned hydrogels, where distinct variations in network rigidity are engineered. With these patterned samples, complex surface instabilities arise during swelling, and we discuss how this dynamic behavior can be leveraged to improve surface and bulk material performance. |
Wednesday, March 16, 2022 5:00PM - 5:12PM |
Q17.00011: Fracture toughness of two-dimensional networks Marcos A Reyes-Martinez, Edwin P Chan, Christopher Soles The relationship between mechanical properties, such as Poisson’s ratio, and the fracture toughness of disordered materials ranging from metallic glasses to polymers is not well understood. Recent studies on the fracture of disordered mechanical metamaterials indicate that the trend in fracture toughness as a function of Poisson’s ratio is not universal across different constituent materials. Rather, it is dictated by the local elastic constants of the networks, specifically by the magnitude of the angle bending resistance relative to bond stretching resistance (k_angle/k_bond). To demonstrate this concept, we designed 2D ordered networks that allow for systematic control of k_angle/k_bond through geometric considerations, and conducted tensile tests to show that the enhancement in stiffness and toughness are related to how the local elastic constants resist bond reorientation during global deformation. Additionally, we demonstrate how different constituent materials can couple with this geometric control of the load bearing elements to enhance the mechanical properties of the entire network. The insights gained from this study can provide new manufacturing routes to the design of mechanical metamaterials with controlled toughness. |
Wednesday, March 16, 2022 5:12PM - 5:24PM |
Q17.00012: Hydrogel Dehydration under Oil Flow Lukas Hauer, Philipp Baumli, Burkhard Dunweg, Doris Vollmer Hydrogels are present in many medical and consumable products because of their capability to hold water under a range of externally imposed stresses (e.g., mechanical). Here, we investigate the stability of two types of hydrogel coatings (thermoresponsive (poly-N-isopropylacrylamide, PNIPAm) and weakly charged (poly (acryl acid), PAA)) by steadily streaming silicone oil over them both, experimentally and numerically. We find that independent of the hydrogels polymer properties or the flow conditions, the swollen coating depletes linearly over time. The linearity of the depletion indicates that diffusion drives water molecules from the hydrogel into the oil flow. However, process timescales indicate that water is not provided instantaneous: Water molecules are retained in the hydrogel, leading to a local non-equilibrium at the shared interface which throttles depletion. We model the molecule flux over the interface, using Kramers’ theory, and implement the obtained (thermal) non-equilibrium flux in mean-field framework, which we solve numerically. Results of our simulation and experiments do match, well. These findings let us deduce that depletion is unaffected by the properties of the hydrogel (charge, elasticity, etc.) but only a consequence of the chemical potential of water and the flowing silicone oil. Depletion can be tuned by matching chemical potentials between water and silicone to avoid depletion. |
Wednesday, March 16, 2022 5:24PM - 5:36PM |
Q17.00013: Nanostructural Evaluation of Styrenic Block Copolymer Gels Across a Diverse Formulation Parameter Space Kenneth P Mineart The potential nanostructures of styrenic block copolymer gels have been shown to mimic those in pure block copolymers including lamellar, gyroid, cylindrical, and spherical morphologies. In pure block copolymers, the domain sizes within these various morphologies have been quantitatively linked to the copolymer's molecular characteristics, but a similar understanding has yet to be fully developed in block copolymer gels owing to a greatly expanded parameter space. The present study focuses on the size and spacing of spherical domains within gels wherein the styrenic blocks comprise the spherical domains and aliphatic blocks and an aliphatic-selective solvent comprise the continuous domain. Gels composed of triblock copolymers with different molecular weights and block compositions and at various concentrations will first be discussed. Next, gels containing pairs of styrenic block copolymers, including various triblock-triblock and triblock-diblock combinations, across a concentration matrix will be appraised. The structural results from all copolymer gels will be evaluated to determine if universal principles dictating domain sizes can be established. |
Wednesday, March 16, 2022 5:36PM - 5:48PM |
Q17.00014: Design of bio-inspired complex active systems from polymer hydrogels through 3D finite element modeling and simulations Priyanka Nemani, Ravi Sastri Ayyagari, Pratyush Dayal Biomimetic engineering inspires materials to derive the innate abilities of living organisms to achieve complex and multi-functional properties. Active complex materials that harness chemo-mechanical transduction can be futuristic materials for developing biomimetic technologies. In this work, we develop a generic finite element-based framework to simulate the mechanical response of active polymer gels powered by self-oscillating Belousov Zhabotinsky (BZ) reaction. Our approach combines the Flory Huggins theory to include polymer-solvent interactions, and non-Gaussian statistical mechanics, with limited chain extension, to account for large elastic deformations in polymer gels. Through our 3D simulations, we capture the dynamics of active BZ gels that transform to inactive polymer hydrogels in the limiting case. In addition, our simulations quantify the role of friction and the incompressibility of the polymer gels. Our findings can be utilized for designing a variety of bio-inspired systems of complex geometries from active soft materials including actuators, robots, sensors, etc. |
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