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

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, τRay, 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, IG/IS, 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, IB, of the new Raman band abruptly increases, on the contrary, the intensity, IF, of the free pyridine band decreases. The intensity ratio, IF/(IF+IB), which represents the number fraction of free pyridine in the network, excellently agrees with IG/IS. 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.