Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session F64: Polymeric Networks, Elastomers and GelsLive
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Sponsoring Units: DPOLY GSNP DSOFT Chair: Bradley Olsen, Massachusetts Institute of Technology MIT |
Tuesday, March 16, 2021 11:30AM - 11:42AM Live |
F64.00001: Coordinated experimental analysis of swollen polymer networks shows linear synthesis-swelling correlations and reveals fundamental inconsistencies in theoretical equations Nathan Richbourg, Nikolaos A Peppas Hydrogels are water-swollen polymer networks with physical properties that are closely associated with the concentration of polymer in the system at thermodynamic equilibrium. Equilibrium swelling theory estimates the extent to which hydrogels swell in water based on the structure of the polymer network. However, it cannot precisely predict the physical properties of real hydrogels. Here, we show that equilibrium swelling theory overestimates the importance of the equilibrium-swollen polymer volume fraction. Instead, by precisely characterizing the swelling of structurally controlled libraries of crosslinked poly(vinyl alcohol) hydrogels and end-linked poly(ethylene glycol) diacrylate hydrogels, we found that the relaxed polymer volume fraction, immediately following network synthesis, precisely and linearly correlates to the initial polymer volume fraction. Independent of this first relationship, the extent of swelling from the relaxed polymer volume fraction to the equilibrium-swollen polymer volume fraction precisely and linearly correlates to the expected molecular weight between crosslinks of the network. These independent, cross-validated relationships motivate fundamental reanalysis of equilibrium swelling theory. |
Tuesday, March 16, 2021 11:42AM - 11:54AM Live |
F64.00002: Bridging Dynamic Regimes of Segmental Relaxation and Center-of-Mass Diffusion in Associative Protein Hydrogels Ameya Rao, Helen Yao, Bradley Olsen Knowledge of molecular transport in crowded media is of central importance to understanding biophysical processes and designing soft materials with novel properties. Here, neutron spin echo (NSE) and forced Rayleigh scattering (FRS) are used in tandem to study the interplay between segmental and center-of-mass chain dynamics in a model protein hydrogel with strong binding interactions. The results provide evidence for several regimes of gel relaxation behavior with varying length scale, including a caging regime bridging submolecular relaxation and center-of-mass diffusion due to transient binding. On mesoscopic length scales, chains undergo two distinct regimes of apparent superdiffusion before terminal Fickian scaling. The combined NSE and FRS data are interpreted in the context of prior simulations of associating star polymers, revealing insight into structural length scales and binding kinetics governing the transition from segmental relaxation to self-diffusion in the protein gel. Finally, single-sticker tracer diffusion was measured to directly probe sticker association kinetics within the gel, the results suggesting that cooperative cluster motion may play a role in gel relaxation on larger length scales. |
Tuesday, March 16, 2021 11:54AM - 12:06PM Live |
F64.00003: Double-twist elastomers: mechanical untwisting and chiral buckling Andrew Rutenberg, Laurent Kreplak, Matthew Leighton Motivated by collagen fibrils, we adapt the theory of nematic elastomers to cross-linked double-twist cylinders. We first derive a small angle approximation for the mechanical response, and we show that it provides a good fit to experimental untwisting observed for collagen fibrils under extension. We then explore the mechanics of twisted fibrils under axial compression, and find that phase-separation occurs between highly strained and twisted regions and lightly strained regions. We call this phenomenon ‘chiral buckling’. |
Tuesday, March 16, 2021 12:06PM - 12:18PM Live |
F64.00004: Development of non-invasive shear wave elastography to assess the mechanical and fracture behavior of tough model gels Heiva Le Blay, Thomas Deffieux, Mickaël Tanter, Alba Marcellan Assessing biomechanical properties of soft tissues by ultrasound imaging is still a challenge to help physician to characterize pathologies. Benefiting from the recent progress in the field, the idea is to develop a new tool – being non-invasive and high time-resolution – devoted to the understanding of fracture processes in synthetic model gels. This approach gives new insights into soft matter fracture. |
Tuesday, March 16, 2021 12:18PM - 12:30PM Live |
F64.00005: Mechanochemistry as a Tool to Study Cavitation in Multiple-Network Elastomers Gabriel Sanoja, Xavier Morelle, Sylvie Castagnet, Costantino Creton Elastomers are often used as seals in connecting hardware for gas transport. Upon saturation with a gas followed by rapid decompression, elastomers suffer from cavity nucleation and growth. This phenomenon, known as rapid decompression, is well-known to damage elastomers and hamper their lifetime in use. As such, a method to non-destructively visualize this damage, particularly when the elastomer looks as-pristine, would enable the timely replace seals prior to failure. |
Tuesday, March 16, 2021 12:30PM - 12:42PM Live |
F64.00006: 3D printable ultrasoft solvent-free elastomers Shifeng Nian, Jinchang Zhu, Haozhe Zhang, Zihao Gong, Guillaume Freychet, Mikhail Zhernenkov, Baoxing Xu, Liheng Cai Additive manufacturing, or 3D printing, enables fabrication of structures inaccessible by conventional molding. Yet, existing feedstock is mostly stiff, fragile plastics, limiting their application as adaptive materials that comply with the shapes of objects they contact. Here, we report 3D printable ultrasoft elastomers by exploiting the self-assembly of a responsive bottlebrush-based triblock copolymer. The microphase separation of the chemically distinct blocks results in physically crosslinked networks that are stimuli-reversible. The bottlebrush architecture prevents the formation of entanglements, enabling elastomers with Young’s moduli low to ~102 Pa, 106 times softer than plastics and >100 times softer than all existing 3D printable elastomers. We demonstrate using the elastomers as inks for direct-write printing deformable 3D structures without the aid of external mechanical support or post-treatment. The design concept of 3D printable ultrasoft elastomers should be general and will enable the development of new soft materials for 3D printing adaptive structures and devices. |
Tuesday, March 16, 2021 12:42PM - 12:54PM Live |
F64.00007: Network Analysis of Triblock Copolymer Gels via Quasi-Static Tensile Experiments Kenneth Mineart Physically-crosslinked triblock copolymer gels are highly elastic and persist down to low copolymer concentrations (~1 wt%) due to the copolymer’s propensity to form nanoscale networks consisting of endblock domains connected by copolymer midblocks. Such materials’ have been extensively studied rheologically, and their resultant plateau modulus, G0, is found to be dominated by either the crosslinked network (low concentrations in which midblocks are not entangled) or midblock entanglements (higher concentrations). For styrenic triblock copolymers (e.g., poly[styrene-b-(ethylene-co-butylene)-b-styrene] (SEBS) in aliphatic solvents, the former case leads to moduli that scale linearly with the concentration of elastically effective midblocks, ceff, whereas the latter case yields supralinear behavior (i.e., G0 ∝ ceff2.0-2.4). Quasi-static tensile stress-strain data modeled using slip-tube network (STN) theory, on the other hand, enable modulus contributions from both the crosslinked network, Gc, and midblock entanglements, Ge, to be deciphered. This presentation will examine the relationship between these moduli and triblock copolymer concentration in a variety of gel systems. We anticipate that this data will provide new insights of the network structure in semi-dilute gels. |
Tuesday, March 16, 2021 12:54PM - 1:06PM Live |
F64.00008: Reinforced elastomers for dynamic applications: from non-linearities to Physics understanding Gaétan Grimaldi d'Esdra, Hélène Montes, François Lequeux Filled elastomers are materials with unic properties, and are now used in applications such as anti-vibratory systems. However, there is still debates about the Physics involved in the behaviour of the materials. A remaining question is the dependence of viscoelastic modulus upon the deformation amplitude, known as Payne effect. |
Tuesday, March 16, 2021 1:06PM - 1:18PM Live |
F64.00009: Predicting the flow of polymers under melt processing: from reaction kinetics to viscoelasticity Weizhong Zou, Amber Tupper, Nathan Rebello, Duminda S. Ranasinghe, William H. Green, Bradley Olsen, Christopher Couch When polymer molecules are functionalized by reactive extrusion, the presence of a radical-initiated complex reaction network makes the prediction of molecular topology based viscoelastic properties difficult. Using both computational chemistry and model compound studies, a mechanistic model for radical-mediated grafting of vinyl silane monomers by melt phase processing was developed. On the basis of the intrinsic kinetic dataset discerned from a hybrid quantum calculation procedure and NMR+GC/MS spectroscopy of model compound studies, quantitative relationships between the product properties and the reaction conditions are also revealed. As a demonstration on the predictive power of the model, a factorial design of experiment is performed with a number of industrial grade polyolefin samples being made from various combinations of reaction conditions and characterized by FTIR and linear rheology. By combining our recently developed viscoelastic model with the access to molecular architecture inferred from high temperature GPC, the predictions on both the yields of the graft content and the evolution of rheological moduli with respect to different processing conditions are found to be quantitively consistent with those of experimental measurements. |
Tuesday, March 16, 2021 1:18PM - 1:30PM Live |
F64.00010: Studying the Effect of Crosslinker Concentration on Structure, Dynamics, and Volume Phase Transition of Microgels. Kiril Streletzky, Andrew Scherer, Samantha Tietjen, Krista Freeman The effect crosslinker concentration on the structure and dynamics of polysaccharide microgels synthesized with surfactant was studied below and above volume phase transition. When relative amount of crosslinker was varied by a factor of a hundred, three apparent behavioral regimes emerged from static and dynamic light scattering measurements. At low crosslinker concentrations, microgel behavior was consistent with homogenously crosslinked microgels that displayed uniform deswelling above the transition temperature. The microgels became denser and more diffusive with temperature increase. At high crosslinker concentrations, microgels had an unusual temperature dependence and signs of inhomogeneous crosslinking. In this regime, microgels grew in size and became less dense and less diffusive with temperature increase. At intermediate crosslinker, microgels’ size and density didn’t significantly depend on a solution temperature. The apparent regimes are likely due to nonuniform crosslinker distribution in the microgel, which leads to a nonuniform density of microgel particles, especially at large crosslinker concentrations. Light scattering data was analyzed with Flory-Rehner model and compared to predictions for various microgel architectures. |
Tuesday, March 16, 2021 1:30PM - 1:42PM Live |
F64.00011: Theory of Side-chain Liquid Crystal Elastomers Luofu Liu, Rui Wang Liquid crystal elastomers, formed by incorporating liquid crystal moieties into polymer networks, have wide applications in smart materials due to their stimuli-responsive and energy storage properties. However, a detailed theoretical understanding of such materials is still lacking. Here, we develop a self-consistent field theory of side-chain liquid crystal elastomers by combining the affine network theory for polymer networks and freely-jointed chain model for liquid crystal polymers. Particularly, the coupling between the nematic ordering of the LC moieties and the orientation of chain segments is systematically treated by distinguishing the globalized contribution from the localized "hinge" effect. The theory relates nematic ordering and shape change with various molecular parameters. Through numerical computations, we investigate the effects of crosslink density, "hinge" configuration and external stress on nematic ordering and shape deformation upon phase transition. Furthermore, we predict the critical phase behaviors if the applied stress is strong enough. This work provides fundamental insight into the phase and elastic behaviors of liquid crystal elastomers, which plays a key step towards the rational design of such smart materials at the molecular level. |
Tuesday, March 16, 2021 1:42PM - 1:54PM Live |
F64.00012: Influence of Local Environment on Water Properties Within Physically Crosslinked Amphiphilic Copolymer Hydrogels Pablo Sepulveda-Medina, Bryan D Vogt The association of hydrophobic moieties provide effective crosslinks in amphiphilic statistical copolymers swollen in water, but also modulate the local properties of water through the nanostructure developed consisting of hydrated and hydrophobic regions. The characteristic nanostructures tend to confine water to sub-5 nm dimensions locally, which significantly influence the crystallization of water within the hydrogels. The influence of the dynamics of the hydrophobic component on freezing behavior is experimentally demonstrated with a family of copolymers of n-octadecyl acrylate (ODA) or ethylhexyl acrylate (EHA) as the hydrophobic comonomer and hydroxyethyl acrylate (HEA) as the common hydrophilic component. The crystallization of ODA leads to significant contrast in nanostructures at some compositions/processing between the family of hydrogels. The low Tg of EHA leads to a viscoelastic liquid for the swollen water copolymer, while hydrogels based on HEA-ODA copolymers are viscoelastic solids. Despite these structural and viscoelastic differences, both hydrogel families inhibit >90 % of the water from crystallization on supercooling at appropriate copolymer compositions. |
Tuesday, March 16, 2021 1:54PM - 2:06PM Live |
F64.00013: Dynamics of Anisotropic Nanoparticles Within Model Homogenous Hydrogels Katie Rose, Natalie Gotogsi, Jonathan H Galarraga, Jason A Burdick, Christopher B Murray, Daeyeon Lee, Russell John Composto Recently, it has been experimentally shown that nanorods, in comparison to chemically identical spheres, exhibit increased diffusion in mucus, the interstitial matrix of tumors and in polyacrylamide spheroids. Creating a model system where the nanoparticle confinement can be systematically varied is imperative in identifying key length scales for the enhanced dynamics of nanorods compared to their spherical counterparts. Using single particle tracking to examine the dynamics of quantum rods with increasing probe anisotropy during the gelation of homogenous hydrogels, we can examine the dynamics of the rods in response to the changing local environment. In this study, the diameter of the rods is comparable to the final mesh size, but the lengths of the rods are larger than the final mesh size. The difference between spherical and anisotropic probe dynamics for particles with comparable diameters during this dynamic confinement process is also evaluated. This work offers important insights for predicting and controlling nanoparticle dynamics in gels, applicable to drug delivery and nanoscale filtration. |
Tuesday, March 16, 2021 2:06PM - 2:18PM Live |
F64.00014: Analysis of the Gel Point of Polymer Model Networks by Computer Simulations Michael Lang, Toni Müller The gel point of end-linked model networks is determined from computer simulation data. It is shown that the difference between the true gel point conversion, pc, and the ideal mean field prediction for the gel point, pc,id, is a function of the average number of cross-links per pervaded volume of a network strand, P, and thus, exhibits an explicit dependence on junction functionality, f. In contrast, the amount of intramolecular reactions at the gel point is independent of f in a first approximation and exhibits a different power-law dependence on the overlap number of elastic strands as compared to the gel point delay, pc – pc,id. Therefore, pc – pc,id cannot be predicted from intramolecular reactions and vice versa in contrast to a long standing proposal in the literature. Instead, the main contribution to pc – pc,id for P > 1 arises from the extra bonds needed to bridge the gaps between giant molecules separated in space and scales roughly ∝ (P – 1)-1/2. Further corrections to scaling are due to nonideal reaction kinetics, composition fluctuations, and incompletely screened excluded volume, which are discussed briefly. |
Tuesday, March 16, 2021 2:18PM - 2:30PM On Demand |
F64.00015: Capturing the transient microstructure of poly(styrene)-poly(isoprene)-poly(styrene) gel subjected to temperature and large deformations Rosa Maria Badani Prado, Satish Mishra, Wesley Roth Burghardt, Santanu Kundu We present the real-time change in the microstructure of 10 and 20% (w/w) of poly(styrene)-poly(isoprene)-poly(styrene) [PS-PI-PS] gel in mineral oil, captured using small-angle X-ray scattering experiments and compared with shear rheology experiments. Here the polymer network consists of physically associated PS-blocks bridged by swollen PI-blocks. We capture the evolution of the network with temperature and relate it to the gelation mechanism. We probe the real-time change in microstructure at different temperatures during oscillatory shear and stress relaxation. At high oscillation amplitude, the microstructure splits into two arrangements after a particular strain, displaying circular and elliptical 2D-scattering patterns simultaneously. We characterize the split microstructure using a modified polydispersed core hard-sphere model. Our fitting results indicate a deviation in the stretch experienced by the PI-blocks than predicted by the affine deformation model. The orientation in the gel microstructure is quantified using the anisotropy factor. During relaxation, the anisotropy factor displays time-dependent decay, similar to the shear modulus. Using the stretched-exponential model, the microstructure restoration time and the stress-relaxation time are also compared. |
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