A Model for a Thermally Induced Polymer Coil-to-Globule Transition

A semiquantitative mean-field model for the thermally induced (heating-induced) polymer coil-to-globule transition (HCGT) is developed with no adjustable parameters. The transition temperature Θ is given for a long chain by the equation Θ = 2 T p * [1 − ρ̃ B (Θ)], where T p * is the characteristic temperature of the polymer and ρ̃ B (Θ) is the bulk solvent density at the transition temperature. The variables ρ̃ B and T p * are obtained by invoking the Sanchez−Lacombe (S−L) equation of state. Calculated HCGT temperatures show good agreement with experimental lower critical solution temperatures (LCSTs). The predicted globular state is characterized by the dominance of attractive polymer self-interactions over excluded volume interactions. There is a critical value of the ratio of polymer to solvent S−L characteristic temperature below which no HCGT transition is predicted for an infinite chain. This model can be easily generalized to treat cross-linked gels and their contraction−expansion characteristics.