Effect of polyelectrolyte on the lower critical solution temperature of poly(N-isopropyl acrylamide) in the poly(NIPAAm-co-acrylic acid) hydrogel

Temperature/pH sensitive hydrogels were prepared by copolymerizing N-isopro pyl acrylamide (NIPAAm) and acrylic acid (AAc). The influence of polyelectr olyte on the lower critical solution temperature (LCST) of temperature/pH s ensitive hydrogels was investigated in the pH range of 2 similar to 12. Swe lling ratio of hydrogels as a function of pH at various temperatures was ob tained by measuring the weight of the hydrogels in buffer solutions. Swelli ng ratio of the hydrogels in the presence of poly(allyl amine) (PAA) as a p olyelectrolyte was also measured at the same conditions. The LCSTs of the p oly(N-isopropyl acrylamide- co-acrylic acid) [P(NIPAAm-co-AAc)] hydrogels i ncreased with pH value and disappeared above the pK(a) value of poly(acryli c acid) (PAAc) except hydrogel containing 10 mol% of AAc. The LCSTs of the P(NIPAAm-co-AAc) hydrogels in the presence of PAA could be observed even ab ove the pK(a) value of PAAc. At same pH, the LCSTs of the P(NIPAAm-co-AAc) hydrogels in the presence of PAA were lower than those of the P(NIPAAm-co-A Ac) hydrogels due to the polyelectrolyte complex formation between PAAc and PAA. The swelling ratio of P(NIPAAm-co-AAc) gel was the largest at 20 mol% of AAc content and decreased by polyelectrolyte complex formation with PAA . The swelling kinetics of P(NIPAAm-co-AAc) hydrogel was strongly dependent on pH value, temperature and polyelectrolyte solute. (C) 2000 Elsevier Sci ence Ltd. All rights reserved.