Thermal-responsive poly(N-isopropyl acrylamide)/sodium alginate hydrogels: preparation, swelling behaviors, and mechanical properties

Temperature-responsive hydrogels with enhanced mechanical properties are desirable in areas such as intelligent device. In this contribution, a poly(N-isopropyl acrylamide)/sodium alginate (PNIPAM/Ca-alginate) hydrogel was prepared by a two-step strategy, obtaining a hydrogel with two kinds of cross-linking: chemical cross-linking and ionic cross-linking. The hydrogels exhibited improved mechanical properties compared with that of the PNIPAM/Na-alginate hydrogel and the compressive strength of PNIPAM/Ca-alginate hydrogels depended significantly on the cross-linking density. In particular, PNIPAM/Ca-alginate hydrogels with the cross-linking agent content of 0.036 g displayed the optimal compressive strength of 308.9 kPa among the hydrogel samples with varied amounts of cross-linking agents. The swelling tests at varying temperatures indicated that PNIPAM/Ca-alginate hydrogels exhibited negative temperature-sensitive swelling behaviors and the lower critical solution temperature (LCST) was determined to be in the temperature range of 30 similar to 34 A degrees C. The swelling kinetics study revealed that the mechanism of water uptake of the PNIPAM/Ca-alginate hydrogels followed a less-Fickian mode, while the entire swelling behaviors of hydrogels followed the pseudo-second-order dynamic equation.