Reentrant Phase Transition of Strong Polyelectrolyte Poly(N-isopropylacrylamide) Gels in PEG Solutions

Full Paper: The swelling behavior of a series of strong polyelectrolyte hydrogels based on N-isopropylacrylamide (NIPA) and 2-acrylamido-2-methylpropane sulfonic acid sodium salt (AMPS) was investigated in aqueous solutions of poly(ethylene glycol)s (PEG) of molecular weights 300 and 400 g/mol. Non-ionic hydrogels or hydrogels with 1 mol-% AMPS deswell with increasing PEG-300 volume fraction b in the external solution up to b = 0.6. As b is further increased, the hydrogels start to swell up to b = 1. The distribution of PEG inside and outside the gel phase changes with the gel volume; PEG chains first move from the gel to the solution phase from b = 0 to 0.6, while at higher b values they again penetrate the gel phase. In PEG-400 solutions, the contraction of the gels is jumpwise first-order phase transition at a critical b, while their reswelling occurs smoothly at higher values of b. Calculations using the Flory-Huggins theory indicate attractive interactions between PEG and PNIPA segments, which are responsible for the observed reentrant transition behavior. Variations of the swelling ratio (V/Vw) and the PEG concentration inside the PNIPA gels (m3) with the concentration of PEG-300 in the outer solution b. Calculations were using the FH theory for a series of values of f0, which is proportional to the amount of ionizable group incorporated into the network.