Temperature-, pH- and CO₂-Sensitive Poly(N-isopropylacryl amide-co-acrylic acid) Copolymers with High Glass Transition Temperatures.

A series of poly(-isopropylacrylamide--acrylic acid) (PNIPAAm--PAA) random copolymers were synthesized through free radical copolymerization in MeOH. The incorporation of the acrylic acid units into PNIPAAm tended to enhance the glass transition temperature (), due to strong intermolecular hydrogen bonding between the amide groups of PNIPAAm and the carboxyl groups of PAA, as observed using ¹H nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectroscopic analyses. The lower critical solution temperature (LCST) increased upon increasing the pH of the aqueous solution containing PNIPAAm--PAA because the COOH groups of the PAA segment dissociated into COO groups, enhancing the solubility of the copolymer. In addition, high-pressure differential scanning calorimetry revealed that the LCSTs of all the aqueous solutions of the copolymers decreased upon increasing the pressure of CO₂, suggesting that CO₂ molecules had displaced H₂O molecules around the polar CONH and COOH groups in PNIPAAm--PAA, thereby promoting the hydrophobicity of the copolymers in the aqueous solution. In addition, the values of of a film sample increased upon treatment with supercritical CO₂, implying that intermolecular interactions in the copolymer had been enhanced after such treatment.