Session E30: Polymer Networks, Gels, and Elastomers I: Dynamics

Soft ionic diodes formed at the interface of ionic liquid networks

Soft elastic materials capable of conducting ionic charge carriers offer numerous opportunities for the design of new classes of highly deformable electronic devices. Our group has recently studied the formation of ionic ‘diodes’ at the interface between two single-ion conducting elastomers with oppositely signed charge carriers formed by polymerization and crosslinking of ionic liquid monomers. We characterize the properties of these interfaces using a variety of techniques, in particular AC impedance spectroscopy, and show that they act as stretchable rectifying junctions based on capacitive, rather than electrochemical, effects. Further, such soft ionic diodes possess a number of potentially useful characteristics, including strain sensitive electrical signals and reversibly switchable electro-adhesion. Finally, we have explored templating methods to generate high surface area ionic diodes, which are expected to yield further improvements in properties.
  

Neutron Spin Echo Insight into Dynamic Networks Formed by Ionomers

Association of ionic groups strongly affects the rheology of ionomers in melts and in solutions. The constraint arises from formation of ionic clusters affects the macroscopic dynamics. While the network is constrained, the chains remain dynamic on the length scale of the network mesh. Using NSE we probe chain dynamics in networks formed by polystyrene sulfonate (PSS) as the physical crosslinks are perturbed by addition of ethanol. Solutions of 20Wt% PSS, 11,000 g/mol (PD=1.02) with 3 and 9% mole sulfonation were studied. S(q,t) measured across a broad temperature range, was analyzed in terms KWW and characteristic times extracted. At length scales that correspond to the ionic domains, the motion is constrained in both solvents; however, ethanol enhances the dynamics. While the ionic domains expand, the dynamics remain restricted, pointing to formation of “ionic clouds” where the ionic clusters are swollen with alcohol, but remain segregated, retaining the physical crosslinks of the network. At smaller dimensions, the dynamics becomes significantly faster and no differences are observed as ethanol was added. Increasing the temperature results in equal acceleration of the motion across all q values, indicative of retaining the dynamic, swollen, physical crosslinks.   

Solvent Polarity Effects on Segmental Dynamics in Ionic Polymer Networks: Quasi Elastic Neutron Scattering Study

Incorporating ionic groups into polymers drives formation of physical networks that affect the dynamics of the macromolecules. Here, using quasi elastic neutron scattering (QENS), segmental dynamics of slightly sulfonated (3mol%) polystyrene (PSS) networks formed in cyclohexane, a non-polar environment and a theta solvent for PS was studied, followed by tracking the polymer motion as the association of the sulfonated groups is disrupted by ethanol. Specifically, a 10%(w/w) PSS in cyclohexane/ethanol solutions was studies in the q range of 0.3 Å^-1 to 1.3 Å^-1 corresponding to 5 Å -21 Å. The relaxation times and segmental motion were extracted from a Kohlrausch-Williams-Watts (KWW) analysis. As expected for PSS in cyclohexane, the dynamics is constrained at larger dimensions but the polymer remains mobile on smaller length scales. Addition of less than 5%(v/v) of ethanol is enough to release the constrains. Surprisingly however, further increase in ethanol results in decrease in segmental motion. This reduction could be attributed to overall plasticization of the polymer in presence of ethanol.   

Dynamics of Ionomer Networks Studies by Pulse Field Gradient (PFG) NMR

Ionomer networks are formed in solutions at extremely low concentrations. Tuning these networks offers a means to control the structures that remain trapped as the solvents evaporate to form membranes. Here, we probe the diffusion of the polymers and the solvents in networks formed by polystyrene sulfonate (PSS) by pulse field gradient NMR. The diffusion of the polymer reflects the properties of the network and that of the solvent. Specifically, PSS networks with sulfonation levels of 0, 3 and 9 mole % were studied in toluene and cyclohexane neat and mixed with ethanol. Toluene is a good solvent for polystyrene and cyclohexane is a theta solvent at room temperature. We find that with increasing sulfonation levels, both the polymer and the solvent motions decrease in both solvents. Regardless of sulfonation level, the diffusion of PSS is almost same in cyclohexane and toluene at room temperature. With increasing temperature, the diffusion of all components increases in both solvents however it is more pronounced in cyclohexane. The solvents diffuse faster than the polymers but follow similar trends. While perturbing the ionic clusters affect the overall network dynamics, the polar solvent also affects the hydrophobic network rigidity.   

Solvent Effects on the Structure of Sulfonated Polystyrene Networks

Driven by ionic association, ionomers form dynamic sparse networks in solutions at extremely low ionic content. Here we probe the factors that affect the structure of these types of networks formed by lightly sulfonated polystyrene (PSS) using small angle neutron scattering (SANS). Measurements were carried out in 0.25-10Wt% PSS in toluene/ethanol solutions, across a broad temperature range. In toluene, a good solvent for the PS, the SANS patterns consist of a broad signature at intermediate q, attributed here to an average network mesh size, determined by the distance between ionic physical crosslinks. At low q, a network characteristic upturn is observed. With increasing concentrations, the characteristic dimension, calculated from R_g of a Gaussian chain, decreases by ~1nm due to increasing cluster size and numbers. The long-range correlations of ~20nm, were captured by the Beacuge model. The PS chains assume an overall Gaussian configuration around the ionic clusters. Surprisingly, while the long-range correlations diminish with addition of ethanol, the mesh size decreases. While the ionic clusters become less defined, the PS chains are concurrently affected, assuming a more constrained chain configuration.   

Dissipative Particle Dynamics Computational Modeling of Structurally Tailored and Engineered Macromolecular (STEM) Gels

Structurally tailored and engineered macromolecular (STEM) gels are polymer networks containing latent initiator sites available for post synthesis modifications, with the networks acting as a backbone grafted with secondary polymer side chains. Here we use dissipative particle dynamics (DPD) simulations to study the mechanical response of the modified STEM gels under compression. We observe lower compressional modulus of the networks after adding secondary side chains and the mechanical properties are tunable by varying the grafting density and side chain length. To gain insight into the microscopic origin of observed mechanical behaviors, we measure the chain entanglements density, conformational entropy change, and conduct 3D structural domain analysis during the compression process. Furthermore, we show that attaching the initiator sites to the primary networks through labile links allows the links to break/rearrange, and thus relieves local stress concentrations.   

Internal Fracture in Tough Double Network Hydrogels Revealed by Various Modes of Stretching

The cyclic stretching measurements in various geometries including uniaxial, planar, unequal and equal biaxial extension, reveal the distinctive features of the internal fracture in the double network (DN) hydrogels. The modulus reduction, dissipated energy (D), dissipation factor (Δ, the ratio of dissipated energy to input strain energy) in each loading-unloading cycle are evaluated as a function of the imposed maximum elongation (λ_i,m) in i-direction (i=x,y) in each cycle. The modulus reduction and Δ depend on the stretching mode when compared at the same λ_x,m, but each of them exhibits a universal relation independently of the stretching mode when the magnitude of left Cauchy-Green deformation tensor is used as a variable. In contrast, Δ in filled elastomers shows the corresponding universal relation when the first strain invariant is used as a variable (Mai et al., Soft Matter 13, 1966–1977, 2017). The difference in governing variable indicates that the influence of the cross-effect of strains (λ_iλ _j; i,j =x,y,z and i ≠ j) on the dissipation factor is pronounced in the DN gels whereas it is minimal in the filled elastomers. (Mai et al., Macromolecules 51, 5245–5257, 2018)   

Design and Control of Finite Conformational Changes in Mechanical Networks

Conformational changes in physical networks play a crucial role in many systems, enabling error correction in DNA replication, cooperativity in hemoglobin, and mechanical capacities in metamaterials. Important work has begun to delineate the relationship between network structure and instantaneous conformational change. However, these efforts have failed to address finite conformations, which are critical for the successful function of most physical networks. Here we establish a simple framework for the design and control of mechanical spring networks in 2 and 3 dimensions. Specifically, for a set of nodes with arbitrarily specified initial and final positions, we characterize all bipartite networks with zero energy at these positions, demonstrate transitions between these positions, and design multi-stable networks for information storage. Finally, we use hysteresis and bi-stability to design networks demonstrating cooperativity.   

Quantifying force-induced bond dissociation in metal-coordinate gels under steady shear flow

The dynamic nature of the bonds in associating polymer networks has led to its use in the design of tough and self-healing hydrogels. While the ability of the materials to regain its original stiffness after a recovery period has been documented, such data provides no information on the molecular level processes occurring as the network is damaged, and subsequently as it heals. In this work, the non-linear response of a model associative network, consisting of linear side-functionalized chains, with nickel-terpyridine complexation as the crosslinking group was investigated. With a custom-built setup, force-induced bond dissociation was quantitatively measured through fluorescence measurements, as the network was strained under steady shear. The measured fraction of dissociated bonds is compared to predictions of several models from transient network theory.   

The Dynamics of Bulk Polymers with Metal-ligand Coordination Crosslinking

The dynamics of polymer networks consisting of metal-ligand coordination as crosslinks have been widely studied in solution and gel systems. However, understanding the dynamics of bulk polymers with metal-ligand coordination crosslinking remains a challenge because it is hard to decouple the polymer relaxation and association/dissociation of the dynamic bonds. To address this problem, we carefully designed both the polymer structure and the intramolecular dynamic bond in the system. A series of copolymers containing ethyl acetoacetate ligands have been synthesized by RAFT polymerization. The ethyl acetoacetate ligands can form coordination complexes with different metal ions (Ni(II), Cu(II), and Zn(II)) and act as reversible crosslinks. We investigated the dynamic responses of these polymer networks by rheometer and DMA. These lightly crosslinked networks have behavior that varies both with choice of metal and quantity of metal, thereby revealing the kinetic differences due to the metal-ligand coordination bonds in bulk polymers. We show that these non-covalent dynamic bonds in the bulk polymer networks play an important role in determining their mechanical properties.