Bulk and solution properties of a thermo-responsive rod–coil block polymer based on poly(N-isopropylacrylamide)

Abstract

A series of thermo-responsive rod–coil diblock copolymers with well-defined molecular weights have been synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization. The rod–coil diblock thermo-responsive copolymer studied herein is poly(N-isopropylacrylamide)-b-poly[bis(N-hydroxyisopropyl pyrrolidone) 2-vinylterephthalate] (PNIPAm-b-PHIPPVTA), in which PHIPPVTA is a thermo-responsive rod-like polymer and exhibits an MW-dependent liquid crystalline (LC) phase behavior and MW-dependent thermo-responsive behavior. The bulk phase behaviors of the copolymers have been investigated by a combination of techniques including differential scanning calorimetry (DSC), polarized light microscopy (PLM), 1D and 2D wide-angle X-ray diffraction. The experiment results demonstrated that the phase structure of PNIPAm-b-PHIPPVTA is a hexatic columnar nematic phase when N₄₃bH_x and N₁₅₁bH_x with x ≥ 95 and 146, respectively. The thermo-responsive behaviors were investigated with turbidity measurements using UV-visible spectroscopy. The results showed that the cloud point of the diblock copolymers PNIPAm-b-PHIPPVTA can be adjusted by varying the length of the PHIPPVTA block. Only one phase transition temperature has been observed in the thermo-responsive rod–coil diblock copolymer PNIPAm-b-PHIPPVTA. Of special interest is the cloud point of block copolymers with longer flexible chain segment more strongly depending on the length of the semirigid segment compared to copolymers with shorter flexible chain segment.