Effects of ionization on the phase behavior of poly(N-isopropylacrylamide-co-acrylic acid) and poly(N,N-diethylacrylamide-co-acrylic acid) in water

Phase transitions of poly(N-isopropylacrylamide-co-acrylic acid) (PiPA-AA) and poly(N,N- diethylacrylamide-co-acrylic acid) (PdEA-AA) in water have been investigated by means of turbidimetry, Fourier transform infrared (FTIR) spectroscopy and differential scanning calorimetry (DSC). The phase transition temperatures (T _p) of these copolymers increase with the degree of ionization (α) of the acrylic acid (AA) units, which in turn is dependent on the pH of the solutions. Apparent values of pK _a for the AA units, determined from the pH dependencies of T _p, are 4.7 and 5.4 for PiPA-AA and PdEA-AA, respectively. Differences between T _p for PiPA-AA and T _p for PiPA homopolymer (ΔT _p) are +1.5 and −0.2 °C/mol% of AA at α=1 and 0, respectively. The values of ΔT _p for PdEA-AA are +2.6 (ionic) and −0.5 (nonionic)°C/mol%, indicating that the incorporated AA units have a larger effect on PdEA than on PiPA. DSC measurements performed with each of these copolymers at different pH values show a linear relationship between T _p and the enthalpy of transition (ΔH). IR measurements of PiPA-AA show that the profiles of IR bands from both iPA and AA units exhibit critical changes at T _p of the copolymer. Heating the solution above T _p leads to shifts of the amide II, C–H stretch, and C–H bend bands from the iPA units toward lower wavenumbers, as well as a shift of the amide I band from the iPA units toward higher wavenumbers. A decrease in the intensity of the symmetric C=O stretch IR band from carboxylate anions (1560 cm⁻¹), and an increase in the intensity of the C=O stretch band from COOH groups (1705 cm⁻¹) suggest that a partial protonation of the carboxylate groups (COO⁻+H⁺→COOH) takes place upon the phase transition.