Gel Formation and Molecular Characteristics of Poly(N-isopropylacrylamide) Prepared by Free-Radical Redox Polymerization in Aqueous Solution

The molecular characteristics of poly(N-isopropylacrylamide) (PNIPA), prepared by free-radical polymerization using an aqueous redox initiator and reaction conditions comparable to those used in the synthesis of nanocomposite gels, were investigated by altering the monomer concentration ([NIPA]) and the polymerization temperature (T-p) across the transition temperature (LCST). When T-p<LCST, there is a critical [NIPA] (=n*) above which PNIPA partially forms gels in the absence of a chemical crosslinker, and the gel fraction increases with increasing [NIPA] and decreasing T-p. In the range of n<n*, the molecular weight of soluble PNIPA correlated well with [NIPA]. When T-p>LCST, gels were not formed regardless of [NIPA]. The structure and mechanism of formation of self-crosslinked PNIPA gels are discussed.