Effects of synthesis conditions on formation of thermosensitive porous gels and swelling/shrinking properties

Effects of the synthesis temperature and the cross-linker concentration on the formation of thermosensitive N-isopropylacrylamide (NIPAM) porous gels which swell or shrink quickly in response to the change in temperature were examined systematically. The swelling and shrinking properties of the porous gels were also examined. Although phase separation occurs by polymerizing NIPAM at the temperatures above the transition temperature of polyNIPAM (about 32degreesC), with increasing the cross-linker concentration a structure composed of aggregated microgel particles and pores, i.e. the porous gel, was formed. The swelling rate of such a porous get was extremely large compared with that of a non-porous gel prepared with low concentration of cross-linkers. The swelling rate of gel composed of fully developed porous structure was almost inversely proportional to the square of the characteristic length of the gel in the swollen state. On the other hand, the shrinking of the gel was very fast compared with the swelling, and the gel prepared with a low concentration of cross-linker also shrank very fast, as did a porous gel prepared with a high concentration of cross-linker. The similarity is due to the fact that the gel prepared with a low concentration of cross-linker is also composed of a percolate structure formed by a phase separation, namely a porous structure, in the swollen state of the initial stage of shrinking process.