Session W35: Dynamics in Polyelectrolyte Complexes and Associative Polymer Networks

Engineering hydrogel viscoelastic mechanics via bio-inspired supramolecular metal-coordinate dynamics

Growing evidence supports a critical role of metal-coordinate supramolecular crosslinking in governing stimuli-responsive properties of soft biological materials. As such, bio-inspired metal-coordinate crosslinking provides unique opportunities to further our understanding of how to directly correlate macroscopic hydrogel mechanics with microscopic crosslink dynamics. Recent insights generated from such fundamental studies of metal-coordinate supramolecular chemo-mechanical couplings in hydrogels will be presented.   

Experimental evidence of universal behavior in ion-induced volume phase transition in polyelectrolyte gels

Introduction of high valence counterions into polyelectrolyte solutions and gels results in a reversible volume phase transition. The volume change can be induced by a small change in the concentration of the equilibrium salt solution. In the present work new results are reported for the volume transition induced by calcium-sodium ion exchange in sodium polyacrylate gels made by osmotic swelling pressure and small angle neutron scattering measurements. We show that the threshold Ca²⁺ ion concentration at which the transition occurs increases with increasing NaCl concentration in the surrounding bath, decreases with increasing the concentration of the ionized groups on the polymer backbone and the temperature, and is practically unaffected by changing the crosslink density of the network. It is demonstrated that the normalized swelling data fall on a master curve, indicating that the volume transition exhibits universal behavior. These findings imply that gel volume transition is primarily governed by changes in the electrostatic interactions as a consequence of the divalent-monovalent ion exchange.   

pH modulated nanoparticle diffusion in silica-polyacrylamide hydrogels

Using single particle tracking (SPT), we investigate nanoparticle (NP) diffusion in polyacrylamide hydrogels containing immobile silica particles (0% - 10% volume). For low concentrations of silica particles, two distinct populations of diffusing NPs are observed, localized and diffusive, whereas only diffusive NPs are found in the neat hydrogel. Primarily diffusive behavior is recovered for high concentrations of silica, attributed to an incomplete formation of the mesh as indicated by rheology. A proposed mechanism for the localized behavior of the NP probes is the pH mediated attraction via hydrogen bonding interaction between the PEG brush grafted to the surface of the NP and the silanol groups on the silica surface. The extent of the PEG-silica interaction and its subsequent impact on NP dynamics is examined at different pH values using quartz crystal microbalance with dissipation (QCM-D). This study provides valuable insight into controlling the diffusion of NPs in hydrogels based on pH mediated interactions with an incorporated tertiary component, with implications in drug delivery and filtration.   

Length-scale dependent anomalous diffusion regimes in associative protein hydrogels

Associative polymer gels are of interest as tunable, responsive materials for biomedical and soft robotics applications. Although theories can describe the effect of crosslink reversibility on network viscoelasticity, diffusion of chains through the network is poorly understood due to the complex interplay between sticker association and strand relaxation. Here, we use forced Rayleigh scattering and neutron spin echo to study self-diffusion in model coiled-coil protein hydrogels over a range of 8 decades of length-squared. We show the first experimental evidence for multiple diffusion regimes spanning from the submolecular to the Fickian, including anomalous caging and superdiffusive regimes, which depend on the concentration and length scale probed. Interpreting these results in the context of Brownian dynamics simulations allows characterization of various diffusive modes and molecular parameters (e.g., sticker dissociation and strand relaxation time) governing transitions between these regimes. Finally, tracer diffusion of single coiled-coils is measured in a matrix of proteins containing 4 such domains to link quantitatively single-sticker dynamics to overall chain diffusion rates at various length scales.   

Anomalous diffusion in a model associative network with high sticker density.

The dynamic nature of the bonds in associative polymer networks has led to their use in the design of tough and self-healing hydrogels. The ability to self-heal in these networks relies on the reformation of the original network and is strongly affected by the timescale for self-diffusion. This work measured the self-diffusion of a model associative network, consisting of a linear polymer functionalized with histidine side-groups, using forced Rayleigh scattering. The effect of sticker density was investigated by synthesizing random copolymers through RAFT polymerization, with up to 15 histidine groups per chain. The polymers showed anomalous diffusion that persisted even for the highest sticker density. Earlier work has shown that anomalous diffusion as observed from FRS measurements in other associative networks, with up to four stickers per chain, primarily result from molecular hopping. These results indicate that molecular hopping could still be an important mode of self-diffusion in these networks, even at high sticker densities.   

Scattering Investigations of Structure and Dynamics of Triblock Polyelectrolyte Complex Hydrogels

Polyelectrolyte complex (PEC) hydrogels are physically crosslinked 3-D networks that form upon complexation between oppositely charged polyelectrolytes. The development and use of PEC hydrogels for diverse biomedical applications require an in-depth understanding of the thermodynamics and kinetics of hydrogel assembly as well as their mesoscale structures and bulk properties. In this talk, we introduce the intimate correlations between PEC domain morphologies and material relaxation timescales in hydrogels comprising oppositely charged ABA triblock polyelectrolytes. Polymer size and concentration as well as solution ionic strength were all found to dictate the PEC domain morphologies, size and arrangements. X-ray photon correlation spectroscopy (XPCS) investigations revealed two distinct relaxation modes of the nanoscale PEC domains. The polymer concentration-independent faster relaxation was expected to capture the dynamic processes inside the PEC domains, while the slower polymer concentration-dependent domain relaxation timescales were found to be inherently linked with the equilibrium hydrogel structure. Upon addition of salt, both relaxation modes became faster, indicating speeding of the intra-domain relaxations and smaller, faster-relaxing PEC domains.   

Stimuli-responsive polyelectrolyte gels and the role of ion and polymer solvation

Polyelectrolyte gels are essential components of living systems, since biological tissues are largely composed of polyelectrolyte gels providing a medium for the transport of ions and molecules more easily and effectively while at the same time providing structural integrity. The challenge of modeling the influence of solvation and ion partition on the swelling of gels is due to the coupling between the polyelectrolyte chain configurations and the spatial distribution of the ionic species in solution. We perform molecular dynamics simulations of a minimal model of a polyelectrolyte nanogel particle in solution with an explicit solvent and ions, where the relative strength of dispersion interactions between the solvent and the charged species defines the solvent quality and the position of the ion along the Hofmeister series. Our findings demonstrate that the solvent plays a crucial role in gel swelling and the sensitivity of swelling to the addition of salt and the ion partitioning between the gel and the surrounding solution. Overall, our findings provide a guideline for the development of a more predictive theory of the thermodynamic and transport properties of these complex systems.   

Influence of temperature, salt and molecular weight on the dynamics of polyelectrolyte complexes.

Oppositely-charged polyelectrolytes can spontaneously associate into either solid-like complexes or liquid-like coacervates based on the external salt concentration. The linear viscoelasticity of polyelectrolyte complexes/coacervates (PEC) can be influenced by a variety of factors, such as temperature, salt and pH. Due to the insufficient chain length of polyelectrolyte, few works have reported the entanglement behavior of PEC and only reptation time (τ_rep) has been found. Here, we prepared five pairs of PEC with different molecular weight and matched polycation/polyanion chain length. Rheology experiments were carried out for all PEC pairs at different temperatures and salt concentrations. Time-temperature (TTS) and time-temperature-salt (TTSS) superpositions were achieved with good fit to sticky association theory. Relaxation times for polymer partnering (τ_b), entanglement (τ_e) and reptation (τ_rep) were revealed directly from the TTS data. All these characteristic lifetimes were slowed by the sticky dynamics of Pol⁺Pol^- pairs. We found that the relaxation kinetics of temperature showed Arrhenius dependence, whereas changing the salt concentration impacted the lifetime of Pol⁺Pol^- pairs, the number of stickers per chain and the volume fraction of polymer.   

Water binding and mobility in polyelectrolyte complexes

Water is central in the assembly and the dynamics of charged macromolecular systems including polyelectrolyte assemblies. Nevertheless, it remains a challenge to resolve how water influences assembly characteristics and materials properties of these systems. We have examined via molecular modelling and interconnected experiments the role of water at molecular level on polyelectrolyte complexes and multilayers. We compare different polyelectrolyte systems under varying hydration and salt content and report that the hydration and water binding at especially intrinsic ion pairs connects with the thermal and mechanical response of hydrated polyelectrolyte assemblies. The findings rise attention to water and water partitioning as a control parameter of polyelectrolyte materials properties.

[1] P. Batys, S. Kivistö, S. M. Lalwani, J. L. Lutkenhaus, and M. Sammalkorpi, Soft Matter 15, 7823 (2019).
[2] P. Batys, Y. Zhang, J. L. Lutkenhaus, and M. Sammalkorpi, Macromolecules 51, 8268 (2018).
[3] Y. Zhang, P. Batys, J. T. O’Neal, F. Li, M. Sammalkorpi, J. L. Lutkenhaus ACS Central Science 4, 638 (2018).   

Water's effect on the glass transition and dynamic mechanical properties of polyelectrolyte complexes

Charged assemblies bearing opposite or complementary charges span natural (proteins, enzymes, DNA) to synthetic materials (surfactants, synthetic polyelectrolytes). Assembly is facilitated by electrostatic attraction and entropic release of counterions, and most often occurs in aqueous media. Notably decades ago, Michaels described synthetic polyelectrolyte complexes as brittle when dry but “leathery or rubberlike” when wet, which points to the strong effect of water on the mobility of a charged assembly. Here, the molecular origin of the glass transition is quantified for several charged macromolecular systems is investigated using calorimetry and molecular modeling as a function of water content. A general relationship is revealed as it holds for two completely different types of charged systems (pH- and salt-sensitive) and for both polyelectrolyte complexes and polyelectrolyte multilayers, which are made by different paths. This suggests that water facilitates the relaxation of charged assemblies by reducing attractions between oppositely charged intrinsic ion pairs. We further demonstrate the dual role of water and temperature in the dynamics of polyelectrolyte complexes by showing time-temperature and time-water superpositioning in a single polyelectrolyte complex system. This is accomplished by changing the relative humidity to adjust the water content in the complex during testing. Results indicate the existince of a water-based shift factor (a_w) that bears a log-linear relationship.   

Electric field-dependent metastable phenomena in polyelectrolyte solutions

We have recorded the response of various polyelectrolyte systems to applied voltages. Here, we explore the effects of chain length, architecture, and concentration on the charge transport properties of our solutions in order to determine the origin of nonlinear behavior demonstrated by these materials. Our findings suggest the existence of several mechanisms by which ions can flow through charged polymers, each of which can be observed within distinct voltage regimes and solution conditions. With these findings, we can understand similar characteristics seen in other charged networks.   

Electrospinning Coacervates – No Chain Entanglements Required

Electrospun fibers have utility across a range of fields. However, electrospinning traditionally requires long-chain, entangled polymer solutions. This combination of factors is necessary to create physical entanglements that relax slower than the timescale for electrospinning to prevent capillary breakup of the polymer jet. This requirement has also meant that spinning polyelectrolytes is particularly challenging as electrostatic repulsions along the polymer backbone dramatically increase the solution viscosity. We have reported a strategy for electrospinning charged polymers via complex coacervation. The coacervate liquid resulting from the complexation of oppositely-charged polymers can be used for electrospinning, resulting in solid fibers that are stable against dissolution in water and organic solvents. Here, we explore the potential for using the cooperative electrostatic interactions between polymer chains in place of physical entanglements. We investigated the electrospinnability of coacervates as a function of chain length. We demonstrated successful spinning of fibers using coacervates of oligomers of length <10. These results suggest the potential for using coacervates as a new class of electrospun materials where fiber formation is decoupled from chain length.   

Phase Separation and Gelation in Solutions of A–B Associative Polymers

An equilibrium theory for reversible network formation in two-component solutions of associative polymers is presented to account for the phase behavior due to hydrogen bonding, metal–ligand, electrostatic, or other pairwise associative interactions. We consider polymers of types A and B with many associating groups per chain and consider only A–B association between the groups. A simple analytical expression for the free energy is derived and is shown to be consistent with the classical Flory–Stockmayer gelation theory. It is shown that association and formation of a reversible network is always accompanied by a tendency for phase separation, even at good solvent conditions, a significant difference from self-associative polymers. Homogeneous networks are most easily stabilized near stoichiometric conditions between A and B associative groups, resulting in a sol–gel–sol transition as the overall composition is altered. Chemical incompatability between the A and B polymers drives a competition between attractively and repulsively-driven phase separation, leading to microphase formation and eutectic behavior.