Synthesis of star thermoresponsive amphiphilic block copolymer nano-assemblies and the effect of topology on their thermoresponse

Abstract

Well-defined multi-arm star thermoresponsive block copolymer nano-assemblies of [poly(N-isopropylacrylamide)-block-polystyrene]_n [(PNIPAM-b-PS)_n] with n = 1, 2, 3 and 4 arms were synthesized by RAFT dispersion polymerization via polymerization-induced self-assembly. By carefully changing the degree of polymerization (DP) of the polystyrene (PS) block and modulating the polymerization conditions, (PNIPAM-b-PS)_n nano-assemblies with similar DP and similar chain density but different topology structures of the tethered poly(N-isopropylacrylamide) (PNIPAM) chains were obtained. By comparatively studying the thermoresponse via turbidity analysis, dynamic light scattering, variable-temperature ¹H NMR and rheological analysis, it was found that the phase transition temperature of the tethered PNIPAM chains in the (PNIPAM-b-PS)_n nano-assemblies correlates to the number of arms, n, demonstrating that the topology of the tethered PNIPAM chains exerts a significant influence on the thermoresponsive phase transition.