Phase transition in amphiphilic poly(N-isopropylacrylamide): controlled gelation

Thermally reversible gelation of polymers is of converging interest in both the fundamental research and practical biomedical or pharmaceutical applications. While the block structure is widely reported to favor gelation, there are few studies regarding the behavior of amphiphilic random copolymers. Herein, hydrophobically modified poly(N-isopropylacrylamide) (pNIPAM) polymers were designed and synthesized by reversible addition-fragmentation chain transfer (RAFT) copolymerization of NIPAM and butyl acrylate (BA). A library of polymer systems was created by varying the BA : NIPAM ratio, molecular weight (Mw) and concentrations. While a coil-to-globule transition induced microphase separation occurred in the dilute solution, diverse phase behaviors were observed by phase diagram study. A transparent gel phase was identified in p(NIPAM-co-BA) systems, which was missing in its block counterpart pNIPAM-b-pBA, and existed over a wider temperature range with increased BA content, Mw and concentrations. A dynamic rheological analysis revealed that the gel properties were strongly dependent on temperature, which regulated the interchain hydrophobic association, and the gel proved to be highly elastic, stable, reversible and self-healable under the optimized conditions. The p(NIPAM-co-BA) system will be highly desirable for injectable in situ forming hydrogel materials, and the study demonstrated here can be potentially extended to other amphiphilic pNIPAM copolymers.