Fluctuation-induced multiple transformations of polymer chains in the critical region of a binary solvent

We have observed unexpected multiple transformations of polymer chains close to the liquid-liquid critical point of a binary solvent. The exceptionally close refractive indices of the solvent components, nitroethane and isooctane, 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 by dynamic light-scattering. The polymer coils exhibit a sequence of collapse-reswelling-expansion-reshrinking transitions upon approaching the critical temperature in the macroscopically homogeneous one-phase region. While the initial collapse can be qualitatively explained by the theory of Brochard and de Gennes, which predicts a negative contribution to the excluded volume because of the growing correlation length, the subsequent expansion-reshrinking is a new phenomenon that has not been observed so far. We believe that this effect is generic and attribute it to micro-phase separation inside the polymer coils. We hypothesize that the formation of a poor-solvent droplet inside the coil may generate fluctuation-induced critical Casimir forces between the inside-coil polymer-poor droplet and the surrounding polymer-rich layer.