Hydration States of Poly(N-isopropylacrylamide) and Poly(N,N-diethylacrylamide) and Their Monomer Units in Aqueous Solutions with Lower Critical Solution Temperatures Studied by Infrared Spectroscopy

The coil-to-globule transition of poly(N-isopropylacrylamide) (PNiPA) and poly(N,N-diethylacrylamide) (PdEA) in aqueous solutions has recently received substantial interest in its drastic change in the hydration state from a hydrated random coil to a hydrophobic globule for practical applications: drug delivery and tissue engineering. In this report, the hydration states of PNiPA and PdEA in aqueous solutions were investigated by IR spectroscopy in the amide I, N H and C H stretching band regions as compared with those of their repeat units, N-isopropylpropionamide (NiPP) and N,N-diethylpropylacrylamide (dEP) in aqueous and cyclohexane solutions in combined with their phase diagrams. The IR spectral changes in the amide I and C H stretching band regions of N-alkylamides and N,N-dialkylamides including NiPP and dEP in aqueous solutions with varying concentration was characteristic due to different amide amide interaction; the amide amide interaction for N-alkylamide (C=O center dot center dot center dot H-N hydrogen bond) is stronger than that for N,N-dialkylamide (dipolar interaction). It is found that almost all amide groups of PNiPA in aqueous solution forms the intramolecular C=O center dot center dot center dot H-N hydrogen bond even in the coil state and that the amide group of PNiPA is less hydrated than that of PdEA in spite of the similar degree of hydration to alkyl groups. The IR spectral changes in the amide I and C H stretching band regions of PNiPA and PdEA in aqueous solutions with heat are ascribed to the dehydration of the amide and alkyl groups from the coil state to the globule one.