Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session S9: Tough Hydrogels IFocus
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Sponsoring Units: DPOLY GSOFT Chair: Hyun-Joong Chung, University of Alberta Room: 268 |
Thursday, March 16, 2017 11:15AM - 11:27AM |
S9.00001: Surface-attached orthogonal gradient hydrogels Pandiyarajan Chinnayan Kannan, Jan Genzer Gradient materials play a significant role in the creation of artificial implants due to their potential to reduce stress concentration when two or more structures with different mechanical properties are joined together, $e.g.,$ tendon, a fibrous protein that connects the soft and hard muscle tissues in our body. We employ free radical polymerization to synthesize random copolymers containing 90{\%} of N-isopropyl acrylamide (NIPAAm), 5{\%} photo-active methacrylyloxybenzophenone (MABP) and 5{\%} thermally-active styrenesulfonylazide (SSAz) crosslinkers. The presence of MABP and SSAz facilitates adjusting gel density on a flat support in two orthogonal directions by spatially and independently controlling UV dosage and temperature. The swelling behavior ($\alpha )$ of the gels in water and methanol is examined using a spectroscopic ellipsometry and the degree of swelling depends on the extent of crosslinking that ranges from $\alpha $ $=$ 1-1.2 (highly crosslinked gels) to $\alpha \quad =$ 4-5 (loosely crosslinked gels). We compare the network properties surface-attached gels and un-attached identical counterparts and confirm that the linear swelling ratio of surface-attached networks is higher than that of the corresponding un-attached gels. [Preview Abstract] |
Thursday, March 16, 2017 11:27AM - 11:39AM |
S9.00002: Modulation of the Mechanical Properties of Hydrophobically Modified Supramolecular Hydrogels by Surfactant-Driven Structural Rearrangement Chao Wang, Clinton Wiener, Bryan Vogt, R. A. Weiss Understanding the mechanical properties of hydrogels is critical to their use in most applications. In this work, we examine how a surfactant, sodium dodecyl sulfate (SDS), can stiffen or soften a hydrogel based on a random copolymer of N,N-dimethylacrylamide (DMA) and 2-(N-ethylperfluorooctane- sulfonamido)ethyl methacrylate (FOSM) through a combination of rheology and small angle neutron scattering (SANS) for assessing the relationship between mechanical properties and structure. The copolymer forms a network crosslinked by aggregates of FOSM when immersed in water. This supramolecular network is kinetically trapped by the relatively immobile FOSM groups as they aggregate to avoid contact with water. The addition of SDS leads to the formation of effectively mixed micelles as the crosslinks to enable rearrangement of the FOSM to increase the equilibrium swelling of the hydrogel by as much as three times, while simultaneously increasing the elastic modulus of the hydrogel. However above a critical concentration, SDS sufficiently solvated the FOSM aggregate crosslinks to mechanically compromise the hydrogel through the loss of the nanodomain structure to allow the hydrogel to break-up into small pieces that eventually dissolved. [Preview Abstract] |
Thursday, March 16, 2017 11:39AM - 11:51AM |
S9.00003: Study on requirements of the first network for tough double network gels Tasuku Nakajima, Takayuki Kurokawa, Jian Ping Gong Double network (DN) hydrogels have attracted much attention as extremely tough interpenetrating network (IPN) hydrogels. Tough DN gels satisfy the following two physical conditions; 1. the 1st network is much more “brittle” than the 2nd network; 2. the 1st network is “weaker” than the 2nd network. Brittleness of gels is related to maximum extensibility of their network strands, while strength is related to concentration of the network strands. When a DN gel having such contrasting double network is deformed, much energy is dissipated owing to internal fracture of the brittle 1st network prior to breakage of the stretchable 2nd network. This process leads high toughness (energy for fracture) of DN gels. In this presentation, we introduce the two strategies to obtain such brittle and weak 1st network, which is required for synthesis of tough DN gels. One is pre-stretching strategy which has been always adopted. The other is using gels having very short network strands prepared at very dilute concentration, which has been newly established by us. [Preview Abstract] |
Thursday, March 16, 2017 11:51AM - 12:03PM |
S9.00004: Structure and mechanical properties of supramolecular random copolymer hydrogels cross linked by hydrophobic aggregates. Bryan Vogt, Clinton Wiener, Chao Wang, Bob Weiss Stress dissipation mechanisms are critical to improving the toughness of hydrogels. The use of reversible hydrophobic associations for crosslnking of hydrogels provides such a mechanism for toughening, but can also lead to the creep of the hydrogel as the crosslinks break and reform. The morphology of the hydrophobic aggregates thus is critical to the mechanical properties of the hydrogels. In this work, we will demonstrate how the processing of these copolymers impacts the hydrogel structure and this structure is correlated with the mechanical properties through a combination of small angle scattering, rheology, and tensile measurements. The hydrophilic and hydrophobic chemistries in the copolymer can be used to tune the water content and strength of the crosslinks, while the copolymer composition provides the number density of crosslinks and also acts to modulate the swelling of the hydrogel. These copolymers as well as their hydrogels can in general use traditional polymer processing, but the details of this processing impacts both the nanoscale morphology and the resultant mechanical properties of the hydrogels. [Preview Abstract] |
Thursday, March 16, 2017 12:03PM - 12:15PM |
S9.00005: The Effect of Salt on the Biaxial Viscosity and Creep Behavior of Polyelectrolyte Complex Films Shawn Chen, Kazi Sadman, Kenneth Shull Abstract Oppositely charged polyelectrolytes can lead to a phase separation phenomenon that results in materials with a diverse set of properties ranging from solutions to gels to solid precipitates. In this work stoichiometric polyelectrolyte complex (PEC) films of poly(styrenesulfonate) (PSS) and poly(diallyldimethylammonium) (PDADMA) were investigated using a custom-built biaxial membrane inflation test. Circular PEC films in contact with a variety of aqueous salt solutions were deformed by applying a preferential differential across the membrane. In each case, the material response consists of (1) an instantaneous elastic deformation, (2) a transient creep regime, and (3) a steady-state flow regime. We use the measured elastic moduli, creep compliances and biaxial viscosity values to provide insights to the density and lifetime of intermolecular ionic complexes within the materials. [Preview Abstract] |
Thursday, March 16, 2017 12:15PM - 12:27PM |
S9.00006: Polymer Structure and Water States in Salt-Containing Polyampholyte Hydrogels Xinda Li, Janet A.W. Elliott, Byeongdu Lee, Hyun-joong Chung The phase behavior of water in hydrogels has broad impact on various applications, such as lubrication, adhesion, and electrical conductivity, as well as the hydrogel's low temperature properties. The status of the water molecules is correlated to the structure of the polymer chains in the hydrogel. In this study, the structure and water status of a model charge-balanced polyampholyte poly(4-vinylbenzenesulfonate-co-[3-(methacryloylamino) propyl] trimethylammonium chloride), were investigated by using differential scanning calorimetry (DSC) and small-angle x-ray scattering (SAXS). A globular network structure suggested by SAXS results dictated the depression of the freezing point of water in the hydrogel, as supported by the DSC results. The polyampholyte chains undergo an irreversible collapse during dialysis in deionized water. Such collapsed hydrogels are not able to prevent freezing of water molecules. The results of both synthesis condition and post-synthesis treatments for polyampholyte hydrogels provide us insights to design optimal polyampholyte hydrogels for low temperature applications. [Preview Abstract] |
Thursday, March 16, 2017 12:27PM - 1:03PM |
S9.00007: Physics and Mechanics of dual-crosslink gels Invited Speaker: Costantino Creton Dual crosslink hydrogels are swollen polymer networks containing a population of physical bonds dynamic and reversible), coexisting with a population of permanent covalent bonds. From the point of view of mechanical properties, the most interesting combination is a minority of sparse covalent bonds, providing extensibility, and a majority of dynamic bonds. At high strain rates the dynamic bonds provide additional stiffness while at low strain rates they are invisible. As the strain rate approaches the inverse of the relaxation time, the gel becomes markedly viscoelastic. We will present the physics and mechanics of several gels where the relative population of the two types of bonds and the lifetime of the dynamic bonds are systematically varied using complexation bonds. The dynamic properties will be characterized with linear rheology experiments while large strain and fracture experiments will be characterized with uniaxial tensile tests on notched and unnotched samples. Most notably we find that the presence of rapidly exchanging physical bonds can increase the extensibility at break, effectively delaying crack propagation, while having little effect on the large strain properties. We will discuss the mechanisms by which the cracks can be delayed and the connection between the relaxation time(s) of the gel in linear rheology and the fracture properties as a function of extension rate. [Preview Abstract] |
Thursday, March 16, 2017 1:03PM - 1:15PM |
S9.00008: Percolation is not the end of gelation Pasha Tabatabai, Benjamin Partlow, David Kaplan, Daniel Blair We present an experimental investigation of how covalently crosslinked silk fibroin gels polymerize by quantifying the mechanical properties using bulk oscillatory rheology and fluorescence spectroscopy. Gelation of these biologically derived polypeptides are highly susceptible to the inherent complexities that result from secondary interactions. We will show that simple models, based on percolation, do not capture the essential physics. Our results indicate that percolation guides the initial gel formation, but molecular weight moderates the existence of a secondary mode of modulus growth. We speculate that this additional growth mode originates from hydrogen bonding near covalent crosslinks but is sterically inhibited at low molecular weights. [Preview Abstract] |
Thursday, March 16, 2017 1:15PM - 1:27PM |
S9.00009: Fabrication of macroscopic composite hydrogels to increase fracture toughness Riku Takahashi, Daniel King, Takayuki Nonoyama, Tasuku Nakajima, Taolin Sun, Takayuki Kurokawa, Jian Ping Gong Hydrogels show characteristic properties such as high water content, high flexibility, permeability, low friction and so on. However, it is difficult to replicate the complex functions exhibited by biological materials with pure hydrogels. Therefore, we need to establish a novel approach to expand the properties of hydrogels. Recently, the development of hydrogel toughening methods enable us to fabricate more functional materials, such as composite materials. Typical examples of composite materials which are widely used are reinforced concrete, plywood, and fiber-reinforced plastics. These examples represent macroscopic composites, where the reinforcing phase consists of a macroscopic structure within a continuous matrix. When combined, the composite properties are superior to the properties of the individual components. Inspired by this concept, we try to fabricate macroscopic composite hydrogels using hydrogels as a soft matrix and a stiff reinforcing skeleton to increase mechanical properties. The resultant materials show high Young's modulus and high fracture toughness due to a multiple fracture process. We believe that these improved mechanical properties are caused by the combination of soft and brittle materials, similar to the effect seen in double-network hydrogels. [Preview Abstract] |
Thursday, March 16, 2017 1:27PM - 1:39PM |
S9.00010: Polycation Crosslinked Triblock Copolymer Hydrogels with High Toughness and Controllable Structures Yaoyao Chen, Kenneth Shull ABA triblock copolymers with hydrophobic endblocks [poly methyl methacrylate (PMMA)] and hydrophilic midblocks [poly methacrylic acid (PMAA)] can self-assemble into elastic hydrogels in water. This hydrogel network can be toughened by incorporating metal ions, for example, zinc ions, into swollen midblocks to form a transient network. Instead of small molecule species (e.g. zinc ions), we recently develop a new tough hydrogel system by using polycations to crosslink swollen midblocks via electrostatic interaction. The stretchability of this system is significantly increased compared to that of zinc crosslinked hydrogels due to breaking-reforming of the transient network, and the toughness is comparable to that of double network (DN) hydrogels. The incorporation of polycations with low mobility enables the self-recovery of the structure after deformation. In the meanwhile, our hydrogel structures can be easily controlled by changing polycation charge ratio and environmental conditions, such as pH and salt. [Preview Abstract] |
Thursday, March 16, 2017 1:39PM - 1:51PM |
S9.00011: The role of energy dissipation of the matrix in the synergistic toughening of fiber reinforced soft composites Yiwan Huang, Daniel R. King, Tao Lin Sun, Takayuki Kurokawa, Jian Ping Gong As a vital class of soft materials, tough hydrogels have shown strong potential as structural biomaterials. These hydrogels alone, however, still possess limited mechanical properties (such as low modulus) when compared to some load-bearing tissues, such as ligaments and tendons. Developing both strong and tough soft materials similar to soft load-bearing tissues is still a challenge. To overcome this obstacle, we have recently introduced a new material design strategy by combining tough hydrogels with woven fiber fabric to create fiber reinforced soft composites (FRSCs). The new FRSCs exhibit extremely high toughness and tensile strength, far surpassing the simple combination of the individual components, indicating a synergistic reinforcement. In this work, we focus on understanding the role of energy dissipation of the soft matrix in the synergistic toughening of FRSCs. By selecting a range of soft matrix materials, from \textit{tough} hydrogels to \textit{weak} hydrogels and even a commercially available \textit{elastomer}, the toughness of the matrix is determined to play a critical role in achieving extremely tough FRSCs. This work provides a good guide towards the \textit{universal} design of extremely tough composites from various soft materials. [Preview Abstract] |
Thursday, March 16, 2017 1:51PM - 2:03PM |
S9.00012: Mechanical properties of supermolecular nanocomposite hydrogels formed by solution assembly of graphene oxide with polyelectrolytes Yipin Duan, Chao Wang, Nicole Zacharia, Bryan Vogt In this work, a simple route to generate a family of GO-hydrogels from aqueous solution based assembly of GO without any secondary chemical crosslinking has been demonstrated. Assembly of polycationic poly(ethylenimine), PEI, with GO leads to hydrogels that exhibit the classical signatures of the Payne Effect in filled rubbers, even when the hydrogels contain more than 99 {\%} water. Upon compression, these hydrogels exhibit irreversible stiffening that can increase the storage modulus determined from shear rheology by more than 3 orders of magnitude. This stiffening behavior is generalizable to other anionic 2D materials such as clay nanosheets (cloisite), which suggests that the mechanical properties are driven by jamming of the 2D sheets. The extensional (tensile) properties of these hydrogels can be dramatically improved by the co-assembly with poly(acrylic acid), PAA. At intermediate concentrations of PAA, both the elastic modulus and maximum extensibility are significantly increased to produce a tough nanocomposite hydrogel that is not covalently crosslinked. These studies provide insight into routes to generate tough hydrogels through electrostatic assemblies of 2D materials with polyelectrolytes. [Preview Abstract] |
Thursday, March 16, 2017 2:03PM - 2:15PM |
S9.00013: The role of reinforcement geometry in toughening hydrogel composites Daniel King, Yiwan Huang, Tao Lin Sun, Takayuki Kurokawa, Jian Ping Gong Reinforcing hydrogels with a stiff ``skeleton'' provides a method to create soft composites which possess mechanical properties desirable for biological applications, including high stiffness, flexibility, and shock absorption, all while containing water. Our recent research has shown that the energy dissipation of the composite is strongly dependent on the toughness of the matrix, when the failure mechanism is pullout of the fiber from the hydrogel matrix. Here we attempt to study the influence of composite toughness on the geometry of the fabric ``skeleton'' reinforcing the composite. We see that the size of the process zone is controlled by a balance between the tensile strength of the fibers and the frictional strength resisting pullout. If the strength of the fibers exceeds the frictional strength, failure by fiber pullout occurs. On the other hand, if the frictional strength exceeds the tensile strength of the fibers, fiber fracture occurs. Increasing sample width increases the frictional strength, and causes a composite failure transition from fiber pullout to fiber fracture. Based on this theory, we show that varying the reinforcement geometry influences the process zone size and therefore the toughness of the composite. [Preview Abstract] |
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