(53a) Collapse and Swelling of Polymer Chains in Mixed Solvents Near the Critical Point

The anomalous behavior of polymer chains in a binary solvent, nitroethane-isooctane, near the critical temperature of demixing has been investigated by dynamic light-scattering. The components of the solvents have exceptionally close refractive indices, which makes the critical opalescence relatively weak and thus enabling us to simultaneously observe the Browning motion of the polymer chains and the diverging correlation length in the critical region. We present results of a dynamic light-scattering study of three polystyrene samples (25, 000 g mol^-1, 50, 000 g mol^-1, and 123, 000 g mol^-1). The polymer chains exhibit a collapse-reswelling-collapse transition when the temperature is reaching the critical temperature. The effect is fully reproducible. While the first stage of the transition (collapse) can be explained by the classical theory of Brochard and de Gennes, the subsequent reswelling-collapse transition is a phenomenon that has not been observed so far. We have modified the original theory of Brochard and de Gennes by accounting for microscopic phase separation of the binary solvent inside the polymer chain.