Sequence length distribution affects the lower critical solution temperature, glass transition temperature, and CO2-responsiveness of N-isopropylacrylamide/methacrylic acid copolymers

Abstract

In this study we investigated the effects of the sequence length distributions in random and block copolymers of N-isopropylacrylamide and methacrylic acid (MAA)—PNIPAm-co-PMAA and PNIPAm-b-PMAA, prepared through free radical copolymerization and reversible addition fragmentation chain transfer (RAFT) polymerization, respectively—on their lower critical solution temperature (LCST), glass transition temperatures (Tg), and CO2-responsiveness. The insertion of the PMAA segment into PNIPAm enhanced the thermal properties (e.g., higher values of Tg) because of strong hydrogen bonding between the carboxylic acid units of the PMAA segment and the amide units of the PNIPAm segment, as determined using Fourier transform infrared and 1H NMR spectroscopy. The LCST increased upon increasing the pH for both the random and block copolymers, because the COOH units of the PMAA segments dissociated to form COO− groups, which improved the solubility in aqueous solutions. Furthermore, the variation in the LCST of PNIPAm93-b-PMAA7 with respect to pH was greater than that of PNIPAm-co-PMAA, due to the sequence length distribution effect. Treatment with supercritical CO2 (scCO2) also caused the values of Tg to increase for PNIPAm-co-PMAA but decrease for PNIPAm93-b-PMAA7, presumably because CO2 in the micelle structure of the block copolymer had a plasticization effect. Reversible CO2-responsiveness of the block copolymer in aqueous solution was evidenced by the appearance and disappearance of cloudy aqueous solutions upon alternating bubbling with CO2 and N2; this behavior was not observed for the random copolymers.