Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session P52: Structure and Rheology of HydrogelsFocus
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Sponsoring Units: DPOLY Chair: Samanvaya Srivastava, Vivek Sharma, University of California Los Angeles, University of Illinois at Chicago Room: LACC 512 |
Wednesday, March 7, 2018 2:30PM - 2:42PM |
P52.00001: Nanoparticle Diffusion During Network Formation of Tetra-poly(ethylene glycol) Hydrogels Russell Composto, Emmabeth Parrish Single particle tracking (SPT) of PEG grafted NPs was used to examine the gelation of succinimidyl glutarate (TPEG-SG) and amine (TPEG-A) terminated four arm stars, with mesh sizes of 3 to 6 nm for polymer concentration of 40 to 20 mg/mL. As concentration decreased from 40 to 20 mg/mL, gelation time, (i.e. sol-gel transition) increased. From SPT during gelation, NP mobility and NP spatial coverage increased as polymer concentration decreased in the sol state. Once in the gel state, NP mobility decreased, and NP motion became sub-diffusive and eventually localized in all concentrations. The mean square displacement and displacement distributions were investigated as a function of increasing time. Unexpectedly, in these relatively homogeneous gels, the onset of sub-diffusivity was marked by a rapid increase in dynamic heterogeneity followed by a decrease consistent with a homogeneous network. We propose a gelation mechanism in which clusters initially form a heterogeneous structure which fills in to form a fully gelled homogenous network. This work examines the kinetics of TPEG gelation and the homogeneity on the nanometer scale, which will aid in the implementation of these gels in biomedical or filtration applications. |
Wednesday, March 7, 2018 2:42PM - 2:54PM |
P52.00002: Selective Biomolecular Transport due to Structure, Affinity, and Diffusion in Nucleoporin-Like Hydrogels Danielle Mai, Yun Jung Yang, Bradley Olsen Nucleoporins are a class of hydrogel-forming proteins that occupy pores spanning the nuclear membrane, enabling high flux and high selectivity transport of proteins across the membrane. Selective biomolecular permeation has been replicated in recombinant protein hydrogels based on artificially engineered nucleoporin-like polypeptides (NLPs). NLPs consist of associative domains that promote gelation and formation of hydrogel structure, as well as affinity domains based on minimal consensus repeat sequences from the yeast nucleoporin Nsp1. NLP affinity domains can be modified to tune selective transport properties, thereby enabling systematic studies of sequence-property relationships. Biophysical characterization of NLPs using small-angle neutron scattering, fluorimetric binding assays, and forced Rayleigh scattering reveals the importance of entropic size exclusion, moderate binding affinity, and bound-state diffusion processes in the selective permeability of protein hydrogels. |
Wednesday, March 7, 2018 2:54PM - 3:06PM |
P52.00003: Impact of Molecular Weight on Fibrillar Methylcellulose Hydrogels Peter Schmidt, Paige Owens, Frank Bates, Timothy Lodge Methylcellulose (MC) is a partially methoxy-substituted cellulose ether. Aqueous solutions of MC undergo thermoreversible gelation upon heating. Although MC has been a commercial material for over 80 years, the mechanism of MC gelation has been a subject of debate. Recently it has been demonstrated that MC gelation is due to the formation of a fibrillar network, with fibril diameters of ca. 15 nm as measured by small-angle neutron scattering and cryogenic transmission electron microscopy (cryo-TEM). With this new understanding we have investigated the MC molecular weight dependence of the fibrillar network properties. Small amplitude oscillatory shear reveals that the gel modulus increases monotonically with increasing molecular weight. Utilizing cryo-TEM and small angle x-ray scattering (SAXS), we characterize the fibrillar structure. Fitting SAXS patterns to a semi-flexible cylinder model reveals that molecular weight does not impact the structure or concentration of fibrils. We therefore attribute the change in modulus to a decrease in fibrillar network heterogeneity with increasing molecular weight of MC. |
Wednesday, March 7, 2018 3:06PM - 3:42PM |
P52.00004: Relating Monomer Sequence, Self-Assembly and Mechanical Response in Dual Associative Protein Hydrogels Invited Speaker: Bradley Olsen The nanostructure and rheological response of block copolymer hydrogels are critical to engineering them for a wide variety of different applications. In our lab, we have developed a system of well-defined triblock copolymer gels based on associative protein midblocks and thermoresponsive endblocks that responsively transition from a shear-thinning state at low temperature to a reinforced state at high temperature. Small changes in the amino acid sequence in the thermoresponsive endblock can create large, qualitative changes in the mechanical response of the materials, such as the observation of tackiness. Here, a combination of scattering and mechanical testing is used to understand the relationships between molecular structure, self-assembly, and mechanical response. Thixotropic responses in shear deformation correspond to the formation of more highly ordered structures during deformation. The kinetics of structure formation as a function of temperature and concentration depend upon the relative mobilities of the mid and endblocks. Relaxation after the cessation of shear is characterized by two timescales corresponding to distinct relaxation processes in the materials. The substitution of glycine by alanine in specific positions within the protein endblocks leads to dramatic slowing of the endblock relaxation, which substantially changes the way that the structures deform under shear, leading to higher modulus and much higher toughness in the materials as a result of a minor chemical change in the polymer sequence. This shows how endblock engineering can be used as a tool to tune hydrogel properties. |
Wednesday, March 7, 2018 3:42PM - 3:54PM |
P52.00005: Internal mass segregation in PNIPAM hydrogels Jonas Cuadrado, Changwoo Do, Alberto Fernandez-Nieves Colloidal hydrogels swell and deswell depending on the environmental conditions. These size changes imply changes in the single-particle properties, including their internal structure, which can result in non-trivial morphologies. Here, we use PNIPAM-AAc ultra-low crosslinked particles at different temperature and charge, together with light and neutron scattering, to find that the particles are not homogenous. Instead, they are remarkably heterogeneous. We interpret the results in terms of phase separation using the Borue and Erukhimovich theory. |
Wednesday, March 7, 2018 3:54PM - 4:06PM |
P52.00006: Supramolecular Hydrogels Inhibit Water Crystallization at Cryogenic Temperature Bryan Vogt, Chao Wang, Clinton Wiener, Robert Weiss Ice crystallization with its large volume expansion of water is a destructive force that can reshape geological formations, cause costly structural damage, and limit the ability to cryogenically preserve biological media such as blood and organs. Controlling ice nucleation and growth is of great technological importance where additives including salts and aqueous polymers have been shown to delay nucleation and limit the crystallization rate, but their overall efficacy is limited. Inspired by the two methods by which nature manipulates the formation of ice (hydrogen bonding macromolecules and nanoconfinement between hydrophobic moieties), we report a family of supramolecular hydrogels with tunable supercooling capabilities. Through modulation of both interactions of the water with the hydrophilic base of the hydrogel and spacing of hydrophobic crosslinks, >99.0 wt% of the water in the hydrogel can be supercooled to 128 K and resist further crystallization on re-heating towards the melting point. A combination of SAXS and SANS is used to understand how these hydrogels evolve on cooling, while the water structure is probed with WAXS. This ability to inhibit ice formation in soft matter is unprecedented and provides new concepts in the quest to control water crystallization. |
Wednesday, March 7, 2018 4:06PM - 4:18PM |
P52.00007: Tough Liquid Crystal Hydrogels Shuang Zhou, Zhigang Suo Hydrogels are hydrophilic polymer networks swelling in water solutions. They are widely found in biological systems and used in engineering applications. Mechanical enhancement of them are done by modifying the polymer network, while chemical and biological functioning are realized through the water inclusion. These hydrogels are mostly isotropic and therefore has no optical functions. Here we present a new breed of liquid crystalline hydrogels that bears intrinsic molecular anisotropy by incorporating chromonic liquid crystals in the polymer network. They show strong photoelastic effects, temperature dependence of the mesogenic phases, and a remarkable toughness, making them readily applicable as optical strain sensors. The generic approach of fabricating the liquid crystal gels allows us to combine different polymers and chromonic liquid crystals. It also makes it possible to independently tune the mechanical and optical properties by through chemical composition, crosslink density and hydration rate. |
Wednesday, March 7, 2018 4:18PM - 4:54PM |
P52.00008: Strong, tough, or fragile: Brownian motion and the osmotic pressure of colloidal gels Invited Speaker: Roseanna Zia We interrogate via dynamic simulation the role played by the osmotic pressure in non-equilibrium phase separation in colloidal gels. In colloidal suspensions, attractive interparticle forces of order several kT lead to arrested phase separation and the formation of a bi-continuous network of reversibly bonded particles condensed into thick, glassy strands. The durable but temporary nature of the bonds permits ongoing structural age-coarsening that lowers the average potential energy of the gel: particles migrate from areas of fewer contacts to regions of more contacts, but thus migrate from regions of higher to lower hydrodynamic mobility. Particles thus undergo a Smoluchowski ratcheting to progressively deeper arrest. Concomitantly, the osmotic pressure evolves during quiescent aging from a more negative to a less negative value, indicating that the driver for further condensation or phase separation weakens continuously. These soft solids, also described as yield-stress fluids, yield mechanically under application of an external stress, force, or flow. Our recent work revealed that mechanical yield can be viewed as a non-equilibrium phase transition, where the liquid-to-solid transition sometimes re-arrests into a re-entrant solid material. To deepen our understanding of this as non-equilibrium phase behavior, we conduct a detailed examination of the role played by osmotic pressure in gels yielding under three modes: fixed stress, fixes strain-rate, and gravitational forcing, and show that an interplay between external perturbation and osmotic pressure temporarily release the gel from kinetic arrest, advancing it toward more complete phase separation. A theoretical model is advanced. |
Wednesday, March 7, 2018 4:54PM - 5:06PM |
P52.00009: Determination of Polymer Gel Viscoelasticity using Probe Rheology Simulations Rafikul Islam, Rajesh Khare In previous work, we showed that the viscoelastic moduli of unentangled and entangled polymer melts can be determined using the probe rheology technique in conjunction with molecular dynamics (MD) simulations. In order to assess the generality of the approach, we apply the technique to bead-spring models of polymer gel systems in this work. Active probe rheology simulations are carried out by subjecting a rigid particle that is embedded in a polymer gel matrix to an oscillatory external force. The observed probe motion is analyzed using the inertial generalized Stokes-Einstein relation (IGSER) to yield viscoelastic properties of the medium as a function of frequency. The results obtained by probe rheology simulations are compared with those obtained by the non-equilibrium molecular dynamics (NEMD) simulations. Results will also be presented for the effect of probe particle size and cross-link density of the gel on the observed viscoelastic properties of the gel medium. |
Wednesday, March 7, 2018 5:06PM - 5:18PM |
P52.00010: Viscolelastic Properties Controlled by pH of Telechelic Metal-Coordinating Hydrogels 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. Our study demonstrates that pH is the main driver for the hydrogel's viscoelastic properties, and that a specific metal concentration is not crucial to stable hydrogel formation. 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. |
Wednesday, March 7, 2018 5:18PM - 5:30PM |
P52.00011: Mesoscale Modeling of Swelling Kinetics of Hydrogels Shensheng Chen, Xin Yong Swelling kinetics of hydrogels critically determines whether gels can reach the thermodynamically favored states, thus plays a vital role in synthesis and processing as well as the materials properties of gel systems. To provide a microscopic understanding of the dynamics of gel swelling, we combine the many-body dissipative particle dynamics (MDPD) with Monte Carlo (MC) method to capture solvent exchange and dynamic motion of networks under a constant solvent chemical potential, constant pressure, constant temperature ( ensemble. To achieve pressure control, we apply a “Langevin piston” method to an extensive system. The chemical potentials equilibrium is reached by MC insertions and deletions of solvent. Our result shows the gel swelling as a result of the balance between the osmotic pressure and elastic force originated from the deformation of the network. We explore the effects of density of crosslinks, network topology, and bond styles on the equilibrium states of the hydrogels. Our model reveals the swelling kinetics of gels under experimentally relevant conditions in great detail. Moreover, this simulation tool is not limited to hydrogels but can be extended to simulate a wide range of complex fluid systems in chemical equilibrium under isothermal-isobaric condition. |
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