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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 ≈ 455 mM). The system was in the sol state at C CR ≤ 1.0 mM and 10.0 ≤ C CR ≤ 30.0 mM, while it underwent gelation at two C CR regions: 1.5 ≤ C CR ≤ 9.0 mM and 35 mM ≤ 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 ξ and the power-law exponent, n, in ICF have been extensively discussed.
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Tsujimoto et al. (2002) studied this question.
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