PNIPAM-based heteroarm star-graft quarterpolymers: synthesis, characterization and pH-dependent thermoresponsiveness in aqueous media

Abstract

Heteroarm star-graft quarterpolymers PS_n[P2VP-b-(PAA-g-PNIPAM)]_n bearing two different kinds of arms: a short polystyrene (PS) and a longer poly(2-vinyl pyridine)-b-poly(acrylic acid) (P2VP-b-PAA) block copolymer arm, grafted by PNIPAM chains onto the outer PAA segments, were synthesized by a three-pot multi-step reaction procedure. Thanks to the pH dependent ionization of P2VP and PAA blocks, as well as the thermo-sensitivity of the PNIPAM grafting chains, these star-graft quarterpolymers exhibited multifunctional responsive behavior in aqueous media. Upon heating the aqueous star-graft solutions, intermolecular hydrophobic association was observed above a critical temperature, defined as T_ass, which was affected by the electrostatic interactions exerted by the weak polyelectrolyte P2VP and PAA segments. This effect led to the appearance of a gel phase at concentrated solutions (C_P = 3 wt%). All the observed phenomena depend strongly on the solution conditions and the macromolecular characteristics, namely pH and ionic strength along with the number of arms and the PNIPAM grafting density. The promising preliminary results of this study show that aqueous formulations of these star-graft macromolecules, either in low or high polymer concentrations, could find potential applications as “smart” multi-compartmentalized nanocarriers and/or hydrogels.