Diverse nanostructures and gel behaviours contained in a thermo- and dual-pH-sensitive ABC (PNIPAM–PAA–P4VP) terpolymer in an aqueous solution

Abstract

A thermo- and dual-pH-sensitive ABC triblock copolymer composed of poly(N-isopropylacrylamide)-block-poly(acrylic acid)-block-poly(4-vinyl pyridine) (PNIPAM-b-PAA-b-P4VP, NAV) was synthesized via sequential reversible addition–fragmentation chain transfer (RAFT) polymerization and subsequent hydrolysis. NAV, which had the property of multi-stimuli, containing thermo-sensitive PNIPAM block and opposite pH-sensitive PAA and P4VP blocks, formed diverse nanostructures in an aqueous solution by varying the temperature and pH. Nanostructures including large compound micelles (LCMs), vesicles, spherical micelles and 3D topologies were observed using dynamic light scattering (DLS) and transmission electron microscopy (TEM). More significantly, the solation–gelation behaviour and morphology of the NAV system induced by temperature and pH were investigated by rheology and scanning electron microscopy (SEM), respectively. TEM and SEM revealed that the gelation behaviour of NAV resulted from the bridge effect of the ionized PAA chains between the hydrophobic P4VP cores (V-core) and PNIPAM cores (N-core) over the LCST of PNIPAM. The results obtained from the rheology measurements showed that the gelation temperature was independent of alkaline pH, but slightly tuned by the copolymer concentration or pH (only 6 to 7). Repeated sol–gel transition via temperature changes under alkaline conditions showed the good hydrogel recyclability of NAV.