Effect of the macromolecular architecture on the thermoresponsive behavior of poly(N-isopropylacrylamide) in copolymers with poly(N, N-dimethylacrylamide) in aqueous solutions: Block vs random copolymers

Three samples from each type of poly(N-isopropylacrylamide)-block-poly(N,N-dimethylacrylamide) (PNIPAM-b-PDMA) block copolymers and poly(N-isopropylacrylamide-co-N,N-dimethylacrylamide) (P(NIPAM-co-DMA)) random copolymers with similar chemical composition were synthesized by RAFT polymerization. The effects of DMA on the thermoresponse of its two kinds of copolymers with NIPAM were comparatively investigated by turbidity analysis, variable-temperature H-1 NMR and dynamic light scattering with PNIPAM homopolymer as a control. It was discovered that the phase transition temperature, the temperature response window and the hysteresis values of the PNIPAM-b-PDMA and P(NIPAM-co-DMA) counterpart were significantly different. In particular, the temperature response window, which represents the sensitivity of response, is smaller for random copolymers than that of block copolymers in aqueous solution, indicating that the temperature response of random copolymers is more sensitive than that of block copolymers. The possible reasons leading to different thermo-responsive phase transition characteristics between block copolymers and random copolymers are discussed, and the formation of different hydrophobic structures with the temperature increasing is ascribed.