D. Melekaslan et O. Okay, Reentrant phase transition of strong polyelectrolyte poly(N-isopropylacrylamide) gels in PEG solutions, MACRO CH P, 202(2), 2001, pp. 304-312
The swelling behavior of a series of strong polyelectrolyte hydrogels based
on N-isopropylacrylamide (NEPA) and 2-acrylamido-2-methylpropane sulfonic
acid sodium salt (AMPS) was investigated in aqueous solutions of poly(ethyl
ene glycol)s (PEG) of molecular weights 300 and 400 g/mol. Non-ionic hydrog
els or hydrogels with 1 mol-% AMPS deswell with increasing PEG-300 volume f
raction phi in the external solution up to phi = 0.6. As phi is further inc
reased, the hydrogels start to swell up to phi = 1. The distribution of PEC
inside and nut side the Eel phase changes with the gel volume PEG chains f
irst move from the gel to the solution phase from phi = 0 to 0.6, while at
higher phi 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 phi, while their reswelling occurs smoothly at higher values of ph
i. Calculations using the Flory-Huggins theory indicate attractive interact
ions between PEG and PNIPA segments, which are responsible for the observed
reentrant transition behavior.