Quasielastic Neutron Scattering Study on Thermal Gelation in Aqueous Solution of Agarose

The dynamics of water and agarose molecules in an agarose aqueous solution has been studied by means of quasielastic neutron scattering (QENS). The dynamic structure factor S (Q,E) of the agarose aqueous solution was fitted well to the sum of the Lorentz and delta function. The former is attributed to the diffusive motion of water molecules and the latter to the local vibrational motion of agarose molecules. The self-diffusion coefficient D of water molecules was obtained from the Q-dependence of the width of the Lorentz function, while the mean square displacement <u²> of agarose molecules was obtained from the Q-dependence of the intensity of the delta term. In the cooling direction, both D and <u²> decreased with decreasing temperature and showed discontinuous changes around the thermal gelation temperature (around 314 K). In the heating direction, however, D and <u²> did not show the obvious change below 343 K, indicating a large hysteresis effect. The present results of <u²> and D revealed that the thermal gelation suppresses the motion of the polymer and accelerates the diffusion of water molecules. The activation energy E_a of the diffusion of water in the sol state is the same as that of bulk water, but the E_a in the gel state is clearly smaller than that of bulk water.