A simple model of solvation is introduced to describe lower critical solution points in polymer solutions. Our theory predicts that physical bond formation between polymer segments and solvents can be a major cause of miscibility gaps showing a closed-loop or hourglass shape. Relative positions of upper and lower critical solution points are examined for the various molecular weights of the polymer. The result is compared with the observed phase diagram of polyethylene glycol in water.

• Received 20 December 1989

DOI:https://doi.org/10.1103/PhysRevLett.65.341