Dynamic Light Scattering in Transient Reversible Gels

Aqueous gelatin slowly cooled from the sol state (60 °C) to different temperatures around the gel point has been investigated using dynamic light scattering. Three modes of correlation decay have been observed and their dependence on scattering vector ( q ), concentration ( c ), and temperature interpreted in the light of existing theories. The c dependence of the fast mode's q -independent diffusion coefficient gives information about excluded volume effects and cross-link density. The slowest mode is related to the transient network lifetime. Close to the sol−gel transition, its q -independent relaxation time increases by orders of magnitude and its amplitude decreases. The intermediate mode, the frequency of which increases linearly with q 3, is assumed to be due to cross-link density heterogeneities which are present in the nonideal gel. The slowing down of this mode on approaching the gel point gives rise to quenched fluctuations of cross-linking density which may be responsible for the increase in “extra” scattering intensity observed at small q vectors in permanent gels (Picot−Benoît effect).