Computational modeling and experimental study of solvent transport in polyelectrolyte gels

Solvent transport with simultaneous network deformation in polyelectrolyte gels was studied through computer simulations and visualization experiments.; A multi-component, multi-dimensional model for the swelling of polyelectrolyte gels in salt solutions was developed and solved numerically. The model accounts for the effect of network stress, osmotic pressure, and electrical potential on the diffusive flux of the free species. Osmotic pressure and network stress were derived from the Helmholtz free energy of the system which was composed of additive mixing, elastic and electrostatic terms. A two-dimensional finite element code, with DASSL for the time integration, was used. One- and two-dimensional swelling in unconstrained and constrained slab and axisymmetric finite volume cylinder geometries, were simulated for a salt-solvent-polymer system. The transient and equilibrium fields of electrical potential, concentrations, deformation, and stress were obtained for different parameters and geometries. Transient overshoots were predicted for the electrical potential in the one-dimensional swelling and for the network stress components in two-dimensional swelling. Non-uniformities in the residual profiles of all the variables were predicted for the constrained geometry case.; To validate this model a technique was developed for the real-time visualization of material line movement during transparent gel deformation. This technique is based on covalently attaching to the gel network a caged photo-activated fluorophore which can be uncaged with UV light. Sodium polyacrylate and poly(acrylamide-sodium acrylate) were prepared by copolymerizing caged fluorescein acrylamide with the gel network. Using a UV laser, permanent fluorescent material lines were written on the gels. These lines were used to quantitatively monitor the swelling of gel slabs in two-dimensional constrained geometries. Tabulation of these material lines was used to extract the Finger deformation tensor components and the polymer concentration at different swelling times. These experimentally obtained profiles compared favorably with the transient simulations of gel swelling, showing that the model can be used for realistic predictions of the swelling kinetics and of the spatially resolved field values.; The model was used to study applications of solvent transport in polyelectrolyte gels which rely solely on transient processing. The mechanical response of gel swelling and deswelling under application of external solution composition fluctuations was simulated. The predicted average gel height at the periodic steady state was found to be different from the one corresponding to the average forcing (composition). The other dynamical process studied was swelling-controlled solute release. Simulations of solute release from finite volume gel cylinders of different aspect ratios with simultaneous gel swelling were performed. These simulations predicted that the time dependent solute release is a strong function of the gel geometry, swelling rate, and degree of swelling.