FTIR spectroscopic and calorimetric studies of the phase transitions of N-isopropylacrylamide copolymers in water

The phase transitions of copolymers of N-isopropylacrylamide (iPA) with acrylamide (PiPA-AAm) or acrylonitrile (PiPA-AN) in water have been observed by means of Fourier transform infrared (FTIR) spectroscopy and differential scanning calorimetry (DSC). The effects of both incorporation of these comonomer units and addition of salts to their aqueous solutions on the phase behaviors have been investigated. The positions of IR bands due to iPA and the comonomer units critically shift at lower critical solution temperatures (LCST) of these copolymers. The phase transition leads shifts of the amide II, C-H stretching, and C-H bending bands to lower wavenumbers and a shift of the amide I band to a higher wavenumber. The C equivalent toN stretching band (v(C equivalent toN)) of AN units in PiPA-AN shifts to a lower wavenumber. These shifts of the IR bands indicate that corresponding chemical groups experience dehydration to some extent upon the transition. The v(C equivalent toN) band is composed of two components due to the C equivalent toN groups that form a hydrogen bond with water (2247 cm(-1)) and dehydrated C equivalent toN groups (2241 cm(-1)). Analysis of the band by using a curve fitting method shows that almost all C equivalent toN groups form hydrogen bonds with water in the coil state and 90% of the C equivalent toN groups are dehydrated in the globule state. The hydration of the C equivalent toN group in the globule state is quite different from that of the amide C=O group, 85% of which forms hydrogen bonds with water even in the globule state. DSC measurements show that the heat of phase transition (DeltaH) linearly decreases with an increase in the LCST of the copolymers, meaning that AH is essentially determined by the LCST of the copolymer, which in turn depends on its composition. The structure of water surrounding the polymer chains as well as the balance between hydrophilicity and hydrophobicity of the polymers as a whole has importance to the phase transition behaviors of iPA copolymers.