Dynamic light scattering study on reentrant sol-gel transition of poly(vinyl alcohol)-Congo Red complex in aqueous media

The poly(vinyl alcohol)-Congo Red (PVA/CR) complex in aqueous solutions exhibited a reentrant sol-gel transition at ambient temperature by increasing the CR concentration, C-CR, when the PVA concentration was near the chain overlap concentration (C-PVA approximate to 455 mM). The system was in the sol state at C-CR less than or equal to 1.0 mM and 10.0 less than or equal to C-CR less than or equal to 30.0 mM, while it underwent gelation at two C-CR regions: 1.5 less than or equal to C-CR less than or equal to 9.0 mM and 35 mM less than or equal to C-CR. This reentrant transition has been investigated microscopically by means of dynamic light scattering. Speckle patterns appeared exclusively in the gel phase. The reentrant sol-gel transition was also demonstrated by a shape change of the time-intensity correlation function (ICF), which varied from a stretched-exponential function for sol states to a power-law function for gel states. The variations of the correlation length and the power-law exponent, n, in ICF have been extensively discussed.