THERMOSENSITIVITY OF COPOLY(N-ISOPROPYLACRYLAMIDE ACRYLAMIDE) HYDROGELS AND THEIR INTERACTION WITH METHYLENE-BLUE

We have prepared hydrogels consisting of N-isopropylacrylamide (NIPAAm) and acrylamide (AAm) copolymer whose molar ratios were 100/0, 95/5, 90/10, 85/15, and 80/20. Equilibrium water content of each copolymer during stepwise heating (every 5-degrees-C) after each swelling had reached to its equilibrium in water at 10-degrees-C showed a curve with an inflection point attributed to its transition temperature, Ttr. However, each water content curve at temperature change (from 10-degrees-C to each constant temperature for measurement, that is, 30, 35, 40, 45, ... or 60-degrees-C) decreased with temperature as the above did and then resumed the almost original water content above the Ttr. The Ttr shifted to a higher temperature range with an increase in AAm composition. These experimental results suggest that the densely packed surface structure of the gel is immediately formed when the hydrogel is immersed in water at higher temperature than the Ttr. Based on this phenomenon, we could control the release of Methylene Blue (MB) molecules absorbed in the hydrogel by changing the temperature around the Ttr. Since diffusion coefficients (D) estimated from the permeation of MB through hydrogel membranes below Ttr have showed a different temperature (T) dependence from those above Ttr, we can determine their Ttr from the intersections of Arrhenius plots of log D vs 1/T. Diffusion of MB with hydrophobic and cationic nature into the hydrogel membranes showed the same temperature dependence and Ttr as those of neutral glucose used as a reference permeant, through the diffusion coefficients of glucose through the membranes were lower than those of MB. Further we pursued an adsorption of MB on the hydrogels and investigated an interaction of MB with hydrogel structures thermodynamically. MB had a strong hydrophobic interaction with hydrogel structures in the range below Ttr but not above Ttr because of the strong hydrophobic interaction between the gel structures themselves.