Session Y38: Polymeric Elastomers and Gels

Covalent Fusion of layered Incompatible Gels in Immiscible Solvents

We carry out dissipative particle dynamics (DPD) simulations to model a two layered stackable gel where the gels are incompatible and are present in immiscible solvent. The bottom layer of the gel is created first and then a solution of new initiators, monomers and cross-linkers is introduced on top of it. These components then undergo polymerization and form the second gel layer. We study all possible combinations of free radical polymerization (FRP) and atom transfer radical polymerization (ATRP) mechanisms with the two layers of the gel. For example, the bottom layer gel is created via ATRP, whereas the top layer gel follows FRP. Our focus is to do a systematic study of all these combinations and find out the factors responsible for combining two incompatible gels in immiscible solvents.   

Cyclic topology in polymer networks

Despite the ubiquity of applications, much of our fundamental knowledge about polymer networks is based on an assumption of ideal end-linked structure. However, polymer networks invariably possess topological defects: loops of different orders which have profound effects on network properties. Here, we demonstrate that all different orders of cyclic topologies are a universal function of a single dimensionless parameter characterizing the conditions for network formation. The theory is in excellent agreement with both experimental measurements of hydrogel loop fractions and Monte Carlo simulations without any fitting parameters. We demonstrate the superposition of the dilution effect and chain-length effect on loop formation. The one-to-one correspondence between the network topology and primary loop fraction demonstrates that the entire network topology is characterized by measurement of just primary loops, a single chain topological feature. Different cyclic defects cannot vary independently, in contrast to the intuition that the densities of all topological species are freely adjustable.   

Capturing dissipation and adhesion using transient network theory

Associative networks are prevalent in many fields and are of interest for applications where it is important that these materials are capable of adhering to their surroundings and/or provide a mechanism for dissipating energy in high-impact systems. However, little is known about the particular molecular behavior that differentiates these materials from their non-dissipative and non- adhesive associative counterparts. Here, we modify our previous work using the Smoluchowski equation to model the full network chain end-to-end distance distribution while tracking the population of individual chain conformations for chains that undergo multiple reaction intermediate steps. Thus, instead of the binary associated/dissociated states traditionally studied, we incorporate the ability for chains to partially dissociate and associate. This partial association/dissociation results in stress relaxation due to chain extension. In steady shear and start-up of steady shear, dissipation within the network becomes a function of both the elastically stored energy and the bond energy released during dissociation events.   

Dynamics of Bottlebrush Networks: A Computational Study

We study dynamics of deformation of bottlebrush networks using molecular dynamics simulations and theoretical calculations. Analysis of our simulation results show that the dynamics of bottlebrush network deformation can be described by a Rouse model for polydisperse networks with effective Rouse time of the bottlebrush network strand, $\tau_{R} =\tau_{0} N_{s}^{2} (N_{sc} +1)$ where, N$_{\mathrm{s}}$ is the number-average degree of polymerization of the bottlebrush backbone strands between crosslinks, $N_{\mathrm{sc}}$ is the degree of polymerization of the side chains and $\tau_{0} $is a characteristic monomeric relaxation time. At time scales $t$ smaller than the Rouse time, $t<\tau_{R} $, the time dependent network shear modulus decays with time as $G(t)\propto \rho k_{B} T(\tau_{0} /t)^{1/2}$, where $\rho $is the monomer number density. However, at the time scale $t$ larger than the Rouse time of the bottlebrush strands between crosslinks, the network response is pure elastic with shear modulus $G(t)=G_{0} $, where $G_{\mathrm{0}}$ is the equilibrium shear modulus at small deformation. The stress evolution in the bottlebrush networks can be described by a universal function of $t/\tau_{R} $.   

Bottlebrush and comb-like elastomers as ultra-soft electrical and acoustically active materials

Without swelling in a solvent, it is challenging to obtain materials with a modulus below 10$^{\mathrm{5}}$ Pa, which is dictated by chain entanglements. We show that macromolecules can be disentangled by dense grafting of side chains to long polymer chains. The bottlebrush and comb-like architectures demonstrate a unique combination of flexibility and network dilution, leading to significant decrease of the entanglement modulus ($G_{e})$ and increase of extensibility. Following theoretical predictions, it has been shown that the $G_{e}$ is controlled by the polymerization degrees of sidechains ($n_{sc})$ and grafting spacer ($n_{g})$ as $G_{e} \quad \approx $ ($n_{g}/n_{sc})^{1.5}$. Using the reduced entanglement density, we developed solvent-free elastomers with moduli on the order of 100 Pa and excellent extensibility. Using bottlebrush architectures we have developed PDMS dielectric actuators with high deformation at low electric field strength. Additionally strong acoustic adsorption leads to materials showing shape and volume control in light opaque environments.   

Controlling Phase Separation of Tough Interpenetrating Polymer Networks via Addition of Amphiphilic Block Copolymers.

Interpenetrating polymer networks (IPNs) offer a unique way to combine the mechanical properties of two thermoset systems. Often used to create a material that possesses both high toughness and tensile properties, here we use polydicyclopentadiene, cured via ring opening metathesis polymerization, to contribute high toughness and diglycidyl ether of bisphenol A cured via anhydride chemistry to contribute high tensile strength and modulus. As the uncompatibilized system reacts in the presence of one another, mesoscopic phase separation occurs and dictates the overall efficacy of combining mechanical properties. To control phase separation and drive the system towards more mechanically robust nanostructed IPNs, amphiphilic block copolymers of polybutadiene-$b$-polyethylene oxide, where one block possesses strong affinity to polyDCPD and the other the DGEBA, were added to the system. Here we present a systematic study of the influence of block copolymer composition in the overall blend on degree of phase separation and morphology using a combination of small-angle x-ray scattering (SAXS) and scanning electron microscopy (SEM) techniques. The resultant mechanical properties are then explored in an effort to link mechanical properties to blend morphology.   

Weak hydrogen bonding yields rigid, tough, and elastic hydrogels

Unlike living tissues, synthetic hydrogels are inherently soft and brittle, particularly when built of hydrogen bonds. It remains challenging to design hydrogels that combine high rigidity, strength at break, extensibility, high elasticity. Through free-radical copolymerization of $N,N$-dimethylacrylamide and methacrylic acid, we have designed a network system based on tunable composition of covalent bonds (permanent cross-links) and hydrogen bonds (sacrificial and recoverable crosslinks) with the following rationale: 1) Maintain a high total number of cross-links to ensure high modulus; 2) Introduce a high fraction of H-bonding to ensure high energy dissipation; and 3) Incorporate a small fraction of permanent cross-links to ensure shape control. By tuning the chemical composition and microstructure we have obtained materials with superb mechanical properties. The hydrogels contain 70 wt{\%} water (similar to living cartilage, skin, and ligaments), while display modulus of 28 MPa, strength of 2 MPa, fracture energy of 9300 J$\cdot $m$^{\mathrm{-2}}$, extensibility of 800{\%}, excellent fatigue-resistance, and great elasticity allowing for complete and fast strain recovery. The results agreed with theoretical predictions for modulus relaxation of dual networks with dynamic and permanent crosslinks.   

Cavitation of a Physically Associating Gel

Self-assembly of block copolymers in selective solvents form ordered structures such as micelles, vesicles, and physically crosslinked gels due to difference in their interaction with solvents. These gels have wide range of applications in tissue engineering, food science and biomedical field due to their tunable properties and responsiveness with changing environmental conditions. Pressurization of a defect inside a physically associating gel can lead to elastic instability (cavitation) leading to failure of the gel. The failure behavior involves dissociation of physical networks. A thermoreversible, physically associating gel with different volume fractions of a triblock copolymer, poly (methyl methacrylate)-poly (n-butyl acrylate)-poly (methyl methacrylate) [PMMA-PnBA-PMMA] in 2-ethyl 1-hexanol, a midblock selective solvent, is considered here. Mechanical properties were investigated using shear rheology and cavitation experiments. The experimental data is fitted with a constitutive model that captures the stiffening behavior followed by softening behavior of a physical gel. Finite element analysis has been performed on cavitation rheology geometry to capture the failure behavior and to calculate energy release rate during cavitation experiments.   

Tough Stretchable Physically-Crosslinked Hydrogel Fiber Mats from Electrospun Statistical Copolymers.

Nature uses supramolecular interactions combined with hierarchical structures to produce water-laden materials with combination of properties that are challenging to obtain in synthetic systems. Here we describe a simple method based on electrospinning of a self-associating amphiphilic copolymer. Immersion of the copolymer mats in water generates supramolecular hydrogels that are crosslinked by association of the fluorinated hydrophobic moieties in the copolymer. These robust hydrogel fiber mats exhibit extensibility greater than 225 {\%} and the elastic modulus can be comparable to the bulk hydrogel despite the porous structure of the as-spun mat. Moreover, the stress dissipation by re-arrangement of the physically associated network leads to coalescence of the fibers that propagates from the surfaces to the interior of the mat. Both the mechanical properties and this fiber coalescence behavior can be tuned by selection of the copolymer composition and the initial fiber dimensions. These tough, stretchable hydrogel fiber mats could find utility in a variety of biomedical applications due to their unique properties.   

Amphiphile-modified supramolecular hydrogels: optimized network structure and enhanced stiffness at "Goldilocks" amphiphile content

Hydrogels formed by hydrophobic physical crosslinks have high mechanical strength (larger than 100kPa).1 Surfactants, such as sodium dodecyl sulfate (SDS), can be used to control the mechanical strength and modulus of these hydrogels. Here we describe the change in the viscoelastic behavior of physically crosslinked copolymer hydrogels synthesized from N,N-dimethylacrylamide (DMA) and 2-(N-ethylperfluoro-octane sulfonamido) ethyl methacrylate (FOSM) by the addition of a SDS solution. Without confinement, SDS dissociates the FOSM micelle-like microstructure and facilitates swelling, which decreases the crosslink density of the hydrogel and reduces the modulus and strength of the hydrogel. With 1-dimensional macroscopic confinement, similar behavior was observed, but only for soaking times in the salt solution smaller than 15 h. For longer times (larger than 15 h), SDS improved the mechanical strength and modulus of the hydrogel presumably by reducing the imperfections in the hydrogel network and forming complexes with FOSM. 1. Hao, J.; Weiss, R. a. Mechanical Behavior of Hybrid Hydrogels Composed of a Physical and a Chemical Network. Polymer 2013, 54, 2174–2182.   

Advancing Reversible Shape Memory by Tuning Network Architecture

Recently, reversible shape memory (RSM) has been realized in conventional semi-crystalline elastomers without applying any external force and synthetic programming. The mechanism is ascribed to counteraction between thermodynamically driven relaxation of a strained polymer network and kinetically preferred self-seeding recrystallization of constrained network strands. In order to maximize RSM's performance in terms of (i) range of reversible strain, (ii) rate of strain recovery, and (iii) relaxation time of reversibility, we have designed a systematic series of networks with different topologies and crosslinking densities, including purposely introduced dangling chains and irregular meshes. Within a broad range of crosslink density ca. 50-1000 mol/m$^{\mathrm{3}}$, we have demonstrated that the RSM's properties improve significantly with increasing crosslink density, regardless of network topology. Actually, one of the most irregular networks with densest crosslinking allowed achieving up to 80{\%} of the programmed strain being fully reversible, fast recovery rate up to 0.05 K$^{\mathrm{-1}}$, and less than 15{\%} decrease of reversibility after hours of annealing at partial melt state. With this understanding and optimization of RSM, we pursue an idea of shape control through self-assembly of shape-memory particles. For this purpose, 3D printing has been employed to prepare large assemblies of particles possessing specific shapes and morphologies.   

Increasing the Mechanical Strength of Block Polymer Ion Gels Through the Stepwise Self-Assembly of a Thermoresponsive ABC Triblock Terpolymer

Blends of network-forming block polymers and ionic liquids have remarkable potential for solid electrolytes, as they allow the combination of desirable mechanical and electrical properties. While ABA triblock copolymers have successfully been implemented as the network component of ion gels, these networks contain looped defects, where the endblocks of the polymer loop back into the same spherical core instead of forming a bridge between two cores. We demonstrate that the ABC triblock terpolymer poly(ethylene-\textit{alt}-propylene)-\textit{block}-poly(ethylene oxide)-\textit{block}-poly(\textit{N}-isopropylacrylamide) (PEP-\textit{b}-PEO-\textit{b}-PNIPAm) in the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide forms a thermoreversible gel network with negligible looping defects. PEP-core micelles exist at all temperatures, while cooling causes association of the PNIPAm micelle corona to form a bridging network. Small-angle x-ray scattering and dynamic light scattering were used to characterize the high-temperature micelles. These gels show enhanced mechanical properties and the ability to form gels at lower concentrations than the corresponding thermoresponsive ABA triblock copolymers.   

Rheology and Relaxation Timescales of ABA Triblock Polymer Gels

When dissolved in a midblock selective solvent, ABA polymers form gels composed of aggregated end block micelles bridged by the midblocks. While much effort has been devoted to the study of the structure of these systems, the dynamics of these systems has received less attention. We examine the underlying mechanism of shear relaxation of ABA triblock polymer gels, especially as a function of chain length, composition, and concentration. Recent work using time-resolved small-angle neutron scattering of polystyrene (PS)-block-poly(ethylene-alt-propylene) (PEP) in squalane has elucidated many aspects of the dynamics of diblock chain exchange. By using rheology to study bulk relaxation phenomena of the triblock equivalent, PS-PEP-PS, we apply the knowledge gained from the chain exchange studies to bridge the gap between the molecular and macroscopic relaxation phenomena in PS-PEP-PS triblock gels.   

Non-continuum, anisotropic nanomechanics of random and aligned electrospun nanofiber matrices

Polymer nanofiber assemblies are widely used in cell culture and tissue engineering, while their nanomechanical characteristics have received little attention. In this study, to understand their nanoscale structure-mechanics relations, nanofibers of polycaprolactone (PCL) and poly(vinyl alcohol) (PVA) were fabricated via electrospinning, and tested via AFM-nanoindentation with a microspherical tip (R$\approx $10$\mu $m) in PBS. For the hydrophobic, less-swollen PCL, a novel, non-continuum linear F-D dependence was observed, instead of the typical Hertzian F-D$^{\mathrm{3/2}}$ behavior, which is usually expected for continuum materials. This linear trend is likely resulted from the tensile stretch of a few individual nanofibers as they were indented in the normal plane. In contrast, for the hydrophilic, highly swollen PVA, the observed typical Hertzian response indicates the dominance of localized deformation within each nanofiber, which had swollen to become hydrogels. Furthermore, for both matrices, aligned fibers showed significantly higher stiffness than random fibers. These results provide a fundamental basis on the nanomechanics of biomaterials for specialized applications in cell phenotype and tissue repair.