Study of the volume phase transition of poly( N-isopropylacrylamide) gels in aqueous solutions of pyridine by depolarized Rayleigh and Raman scattering

Abstract

The volume phase transition of neutral poly( N-isopropylacrylamide) gels in aqueous solutions of pyridine was studied by means of depolarized Rayleigh and Raman scattering. The Rayleigh reorientational relaxation time, τ Ray, of pyridine, calculated from the half-width of the depolarized Rayleigh spectrum, attains a maximum at approximately x=0.2 at which the gel undergoes the abrupt volume change. As the gel shrinks, the ratio, I G/ I S, of the intensity of the spectrum of the gel to that of the aqueous solution abruptly decreases, reflecting the decrease in the number of free pyridine in the gel network. A new Raman band, which shifts by 130 cm −1, appears for the shrunken gels. This new band is assigned to the C–H stretching vibrations of pyridine bound to the PNIPA chains by the hydrogen-bonding interaction. As the gel shrinks, the intensity, I B, of the new Raman band abruptly increases, on the contrary, the intensity, I F, of the free pyridine band decreases. The intensity ratio, I F/( I F+ I B), which represents the number fraction of free pyridine in the network, excellently agrees with I G/ I S. The results show that the hydrophilic solutes such as pyridine in PNIPA gels are bound by the hydrogen-bonding interaction between the solutes and the chains.