Poly(N‐isopropylacrylamide) Prepared by Free‐Radical Polymerization in Aqueous Solutions and in Nanocomposite Hydrogels

The molecular characteristics of poly(N‐isopropylacrylamide) (PNIPA) synthesized in aqueous solution were studied with respect to its polymer yield, molecular weight (MW), the polydispersity (the ratio of weight‐ to number‐average molecular weight: Mw/Mn), and intrinsic viscosity ([η]). Gel permeation chromatography (GPC) measurements were performed on unfractionated and fractionated PNIPA by selecting appropriate conditions and solvents, e.g., DMF/10 mM LiBr. It was observed that the MW, Mw/Mn and [η] of PNIPA change significantly depending on the monomer concentration ([NIPA]) in the reaction solution. Linear relationships were observed for the variations of log [NIPA] and Mw/Mn with log Mw. Also, the Mark‐Houwink‐Sakurada equation and the molecular weight exponent α were determined for PNIPA in DMF/10 mM LiBr solution (GPC measurements) and in methanol (light scattering measurements). Also, PNIPA chains were separated from nanocomposite hydrogels with different clay concentrations by decomposing the clay using an aqueous HF solution. The resulting PNIPA showed good correlation between Mw and [η] consistent with that for PNIPA synthesized in aqueous solution. The effects of fractionation and solvents on α and chain conformations, as well as the relationship between α and [η] are discussed.