Kinetics of swelling and shrinking of poly (N-isopropylacrylamide) hydrogels at different temperatures

Abstract The swelling ratio of dried poly(N-isopropylacrylamide) hydrogels in distilled water was kinetically studied at different temperatures. The water content in the gel little increases after 30 min at higher temperatures than 33°C (LCST), while it increases for the initial 30 min with time. Between 25 and 33°C, the water content continues to increase for 8 hours, keeping the higher increase ratio at lower temperature. On the other hand, the hydrogel swollen in distilled water at 4°C starts to shrink after the hydrogel being moved into the distilled water kept at different temperatures between 25 and 40°C. There observed two types of shrinking behavior above and below LCST. In the initial 30 min of the shrinking process of the hydrogel, the water content decreases more obviously at higher temperatures. Over 3 h later, however, the water content kept in the hydrogel little decreases at 40°C, and the decreasing ratio during a unit time is highest at 35°C. At 37°C, the decreasing ratio of the water amount during a unit time gradually decreases with time after 1 h. Therefore, the water content kept in the hydrogel after the shrinking process of the swollen hydrogel for 24 h is lowest at 35°C, while that after the swelling process of dried hydrogel for 24 h is lowest at 37 and 40°C. From these findings, it is considered that there are at least three types of water molecules in the hydrogels, as follows: (i) water molecules adsorbed on the hydrophobic polymer chains in the tight skin phase formed by the shrinkage of the hydrogel at 37 and 40°C; (ii) water molecules movable in the looser polymer network in the skin phase formed around LCST and those adsorbed on the polymer chains; (iii) water molecules connected to each other with hydrogen bonds, adsorbed on the hydrophilic polymer chains, and movable in the polymer matrices.