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

- Copolymers of NIPAm and AA synthesized in both scCO2 and MeOH.
- LCST of copolymer from scCO2 was lower than that from MeOH.
- More alternating sequences of NIPAm and AA units in copolymers from scCO2.
- Increased Tg behavior of copolymer in both scCO2 and MeOH.

Radical copolymerizations of N-isopropylacrylamide (NIPAm) with acrylic acid (AA) initiated by azobisisobutyronitrile in supercritical carbon dioxide (scCO2, 55 °C, 27.6 MPa) were compared with those performed in MeOH at 55 °C. The AA compositions of the copolymers in scCO2 were lower than those of the copolymers in MeOH and they increased upon decreasing the pressure of scCO2 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 Tg. FTIR spectroscopy revealed enhanced H-bonding between the amide group of the NIPAm and the carboxyl group of the AA in the copolymers in scCO2, suggesting that the much higher values of Tg of these copolymers, relative to those in MeOH, arose presumably from the more highly alternating sequences of their NIPAm and AA units.

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