Elasticity at Swelling Equilibrium of Ultrasoft Polyelectrolyte Gels: Comparisons of Theory and Experiments

We have studied the elasticity of low volume fraction hyaluronic acid polyelectrolyte gels in swelling equilibrium as a function of salt concentration and strand length. By measuring the shear modulus at different salt concentrations for a constant volume fraction, and for different volume fractions at constant salt concentrations, we uniquely identify the physical parameters responsible for swelling, the chain hydrophobicity, χ, and the degree of ionization of the network, α, for our experimental conditions. Using these parameters, we predict the complex relationship between the elasticity of the network and the polymer volume fraction. The relationship is nonmonotonic due to the internal pressure of the system, and we predict that even in the equilibrium linear-elastic regime, polyelectrolyte gels stiffen as they swell in low salt solutions. For higher salt concentrations, the relationship is reversed, and the modulus increases with polymer volume fraction as expected. Using dynamic light scattering and water permeation experiments, we find that the longitudinal modulus deduced from gel relaxations is proportional to the shear modulus measured directly from rheological experiments. We find that classical formulas of swelling equilibrium are adequate to capture the measured properties of swollen polyelectrolyte gels and that the various scaling predictions for the moduli are inadequate. The present work explains polyelectrolyte gel behavior using existing swelling equilibrium theories and establishes an experimental protocol for the determination of the physical parameters required to fully describe highly swollen polyelectrolyte hydrogels in high salt environments

Conformation of Methylcellulose as a Function of Poly(ethylene glycol) Graft Density

Low molecular weight thiol-terminated poly­(ethylene glycol) (PEG) (<i>M</i> ≈ 800) has been grafted onto a high molecular weight methylcellulose (MC, <i>M</i>_w ≈ 150000) by a facile thiol–ene click reaction; graft densities varied from 0.7% to 33% (grafts per anhydroglucose unit). Static and dynamic light scattering reveals that the overall radius of the chain increases systematically with graft density, in a manner in excellent agreement with theory. As the contour length remains unchanged, it is apparent that grafting leads to an increase in the persistence length of this semiflexible copolymer, by as much as a factor of 4. These results represent the first experimental verification of the excluded volume theory at low grafting densities, and demonstrate a promising synthetic platform for systematically increasing the persistence length of a model semiflexible, water-soluble polymer

Gelation, Phase Separation, and Fibril Formation in Aqueous Hydroxypropylmethylcellulose Solutions

The thermoresponsive behavior of a hydroxy­propyl­methyl­cellulose (HPMC) sample in aqueous solutions has been studied by a powerful combination of characterization tools, including rheology, turbidimetry, cryogenic transmission electron microscopy (cryoTEM), light scattering, small-angle neutron scattering (SANS), and small-angle X-ray scattering (SAXS). Consistent with prior literature, solutions with concentrations ranging from 0.3 to 3 wt % exhibit a sharp drop in the dynamic viscoelastic moduli <i>G</i>′ and <i>G</i>″ upon heating near 57 °C. The drop in moduli is accompanied by an abrupt increase in turbidity. All the evidence is consistent with this corresponding to liquid–liquid phase separation, leading to polymer-rich droplets in a polymer-depleted matrix. Upon further heating, the moduli increase, and <i>G</i>′ exceeds <i>G</i>″, corresponding to gelation. CryoTEM in dilute solutions reveals that HPMC forms fibrils at the same temperature range where the moduli increase. SANS and SAXS confirm the appearance of fibrils over a range of concentration, and that their average diameter is ca. 18 nm; thus gelation is attributable to formation of a sample-spanning network of fibrils. These results are compared in detail with the closely related and well-studied methylcellulose (MC). The HPMC fibrils are generally shorter, more flexible, and contain more water than with MC, and the resulting gel at high temperatures has a much lower modulus. In addition to the differences in fibril structure, the key distinction between HPMC and MC is that the former undergoes liquid–liquid phase separation prior to forming fibrils and associated gelation, whereas the latter forms fibrils first. These results and their interpretation are compared with the prior literature, in light of the relatively recent discovery of the propensity of MC and HPMC to self-assemble into fibrils on heating