Swelling dynamics of cross-linked poly(acrylic acid) and neutralized poly(acrylate-co-acrylic acid) in aqueous solutions of (hydroxypropyl)cellulose

When a polyelectrolyte gel is placed into a (hydroxypropyl)cellulose (HPC) solution, there is a change in the local HPC concentration around the swelling particle. The HPC concentration profile development around the swelling gel is recorded as a function of time by optical microscopy, using a novel technique involving an isotropic-to-anisotropic phase transition, with a spatial resolution of a few microns. Using this technique the kinetics of swelling of poly(acrylic acid) (PAAc) and neutralized poly(acrylate-co-acrylic acid) (PAA) gels when placed in aqueous (hydroxypropyl)cellulose solutions and the HPC concentration redistributions in these systems were studied. A 75% neutralized PAA gel swells by sorbing mainly water, leaving HPC outside. At the beginning of the swelling, the HPC concentration gradient can be as high as 2% mu m(-1), with a transient HPC concentration of 80%. By contrast, in the case of the PAAc gel, the HPC concentration inside the particle is greater than outside at equilibrium. The final equilibrium is attained in two steps. During the first the solvent penetrates into the particle, producing a large temporary increase of HPC concentration outside the gel. During the second step HPC slowly penetrates the hydrogel, due to preferential interactions between PAAc and HPC, as shown by capillary viscometry for the corresponding linear polymer mixtures.