  We have discovered unusual behavior of polymer coils in a binary solvent
(nitroethane+isooctane) near the critical temperature of demixing. The
exceptionally close refractive indices of the solvent components make the
critical opalescence relatively weak, thus enabling us to simultaneously
observe the Brownian motion of the polymer coils and the diverging correlation
length of the critical fluctuations. The polymer coils exhibit a
collapse-reswelling-expansion-collapse transition upon approaching the critical
temperature. While the first stage (collapse-reswelling) can be explained by
the theory of Brochard and de Gennes, the subsequent expansion-collapse
transition is a new unexpected phenomenon that has not been observed so far. We
believe that this effect is generic and attribute it to micro-phase separation
of the solvent inside the polymer coil.
