Abstract
Reentrant shrinkage and swelling of poly (N-isopropylacrylamide), PNIPA, gels, with changes in temperature and solvent composition (methanol/water) were studied by the positron annihilation lifetime technique and fluorescent technique, the latter detecting rotational diffusion motion of fluorescent probe molecules. The data obtained with these techniques were compared in order to correlate the volume phase transition of gel polymer to the variations of size, contents, and size distribution of free volumes. The free volume of average size 0.27 nm was bifurcated drastically into large and small volumes of sizes 0.34 nm and 0.18 nm at 33°C, suggesting that pick-off reaction of ortho-positronium were proceeded in two different free volumes possibly generated in the loosely connected water molecules and those tightly coupled to gel-networks of polymer. Average free volume sizes of PNIPA gels were similar to those of methanol/water mixtures, suggesting that solvent molecules were incorporated into networks of the polymer gel and moved along in accordance with those of surrounding solvent molecules, whereas differences were recognized in the behavior of free volume contents between free solvent molecules and solvent molecules incorporated into gel-networks.
