Theory of solvation-induced reentrant coil–globule transition of an isolated polymer chain

This paper discusses the conformational phase change of an isolated polymer chain which is capable of forming physical bonds with solvent molecules. On the basis of the mean-field theory of Flory type, we derive a formula for the temperature dependence of the expansion factor of the chain. Our theory takes account of the extra mixing entropy change induced by the solvation. We predict that the physical-bond formation between polymer and solvent molecules causes a reentrant conformational change between coiled and globular state when temperature is varied. We also show that the polymer chain exhibits a sharp collapse near the Θ temperature as the temperature is increased to the lower critical solution temperature of the polymer solution. This behavior can be interpreted in terms of the breakup of the polymer–solvents complex. The result is compared with the observed coil–globule transition on poly (N-isopropylacrylalmide) in water.