Clarification of the inner microenvironments in poly(N-isopropylacrylamide) hydrogels in macrogel and microgel forms using a fluorescent probe technique

Temperature dependence of cross-linked poly(N-isopropylacrylamide) (PNIPAAm) gels in particle microgel and macrogel forms with different densities of crosslinking was studied by fluorescence measurements of 8-anilino-1-naphthalene sulfonic acid (ANS) doped into the hydrogels. The experiments for each sample were carried out for some weeks, by making sure that equilibrium was established at each temperature. The fluorescence intensity ratio of ANS at 460 and 540 nm was found to be useful as a measure of the polarity of the environment around ANS molecules in a gel. The results indicate that the environment of PNIPAAm chains in macrogel and microgel forms of different crosslinking densities are the same. Fluorescence depolarization of ANS first clarified that, after volume phase transition, the collapsed state of PNIPAAm gels is unlike a polymer film, but has free spaces where ANS can rotate freely because of the water molecules escaping from the PNIPAAm chains. The information of these free spaces produced by desorption of water molecules would be useful in the processing and utilization of PNIPAAm particle gels.