Differences in vibrational spectra of poly(N-isopropyl acrylamide) from water solution before and after phase separation

Abstract

Two poly(N-isopropyl acrylamide) (PNIPA) samples were prepared by drying the 5 wt% water solution of PNIPA either at 25 °C without phase separation (referred to as I-B) or at 38 °C after phase separation (I-C). PNIPA sample I-A was directly cast from its acetone solution. The IR absorbance of CH₃ asymmetric stretch related to the CH₂ symmetric stretch is higher for the samples I-C and I-A when compared with the same absorbance ratio for the sample I-B. At the same time, the vibration frequencies of amide I and II for I-C are similar to those for I-A, but obviously different from those for I-B. Raman scattering intensities corresponding to the CH₃ and CH₂ vibrations also depend on the sample preparation methods. A simple model simulation suggests that the above observations would be attributed to the configuration change in the side chain of the amide group. The Irans isomer is dominant in sample I-B and cis isomer dominant in samples I-A and I-C.