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

Abstract

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.