A new model for the thermally induced volume phase transition of gels

A theoretical model that considers hydrophobic interaction is proposed for the thermally induced discontinuous shrinkage of hydrogels. In this model, the free energy of a hydrogel is divided into four parts: the elastic free energy of networks formulated according to the theory which takes the limit of elongation into account, the free energy arising from osmotic pressure of dissociated counterions in the gel, the free energy of interactions except for the hydrophobic interaction represented by a virial-type volume interaction equation, and the free energy of the hydrophobic interaction. By the former three terms, the thermally induced swelling of gels and the effects of aspect ratio of segment are expressed. The incorporation of the fourth hydrophobic interaction term makes it possible to explain the thermally induced shrinkage of gels, and suggests the possibility of an explanation of "convexo"- type volume phase transition by means of the hydrophobic interaction.