Unusual solution phase transition trend of poly(N-isopropylacrylamide-co-bis(N-hydroxyisopropyl pyrrolidone) 2-vinylterephthalate)

A series of water-soluble thermoresponsive copolymers, i.e., poly[(N-isopropylacrylamide-co-bis(N-hydroxyisopropyl pyrrolidone) 2-vinylterephthalate] [P(NIPAm-co-HIPPVTA)], with well-defined comonomer content have been synthesized via conventional free-radical copolymerization at low conversion. The liquid crystalline behavior of the copolymers was investigated using polarized light microscope and differential scanning calorimetry. The results revealed that the P(NIPAm-co-HIPPVTA) exhibit liquid crystallinity when the NIPAm content is lower than 52.2 mol%, which was obviously different from that of previous papers. In addition, thermo-responsive behaviors of copolymer also demonstrated by dynamic light scattering. The cloud points of these copolymers can be tuned over a broad range (i.e., 9.9–81.2 °C) by simply altering the molar content of the HIPPVTA. It is noted that the cloud point of the copolymers showed unexpected trend (i.e., parabolicaly other than linearly or exponentially) with the increasing of HIPPVTA content. That is, with an increase of HIPPVTA molar content, the cloud point of copolymer decreases at first and then increases.