Supercritical CO2 affects the copolymerization, LCST behavior, thermal properties, and hydrogen bonding interactions of poly(N-isopropylacrylamide-co-acrylic acid).

Radical copolymerizations of N -isopropylacrylamide (NIPAm) with acrylic acid (AA) initiated by azobisisobutyronitrile in supercritical carbon dioxide (scCO 2 , 55 °C, 27.6 MPa) were compared with those performed in MeOH at 55 °C. The AA compositions of the copolymers in scCO 2 were lower than those of the copolymers in MeOH and they increased upon decreasing the pressure of scCO 2 from 27.6 to 13.8 MPa. The glass transition temperatures of these copolymers deviated positively from those of respective homopolymers prepared in both media, with the copolymer obtained at a 50/50 feed ratio exhibiting the highest value of T g . FTIR spectroscopy revealed enhanced H-bonding between the amide group of the NIPAm and the carboxyl group of the AA in the copolymers in scCO 2 , suggesting that the much higher values of T g of these copolymers, relative to those in MeOH, arose presumably from the more highly alternating sequences of their NIPAm and AA units. [ABSTRACT FROM AUTHOR]