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

Abstract

Temperature/pH sensitive hydrogels were prepared by copolymerizing N-isopropyl acrylamide (NIPAAm) and acrylic acid (AAc). The influence of polyelectrolyte on the lower critical solution temperature (LCST) of temperature/pH sensitive hydrogels was investigated in the pH range of 2∼12. Swelling ratio of hydrogels as a function of pH at various temperatures was obtained by measuring the weight of the hydrogels in buffer solutions. Swelling ratio of the hydrogels in the presence of poly(allyl amine) (PAA) as a polyelectrolyte was also measured at the same conditions. The LCSTs of the poly( N-isopropyl acrylamide- co-acrylic acid) [P(NIPAAm– co–AAc)] hydrogels increased with pH value and disappeared above the p K a value of poly(acrylic 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 above the p K 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-AAc) 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.