Temperature hysteresis and morphology of volume phase transition of gels

Temperature hysteresis is theoretically studied for the volume phase transition of chemically cross-linked gels. The condition of the appearance of a ``surface skin layer'' of new phase is formulated and calculated. The result predicts that the surface skin layer is destabilized near the critical point of the transition due to volume fluctuations. We discuss the relevance of this result to inaccessibility to the Ising critical point [L. Golubovi {c} \'{c}{} and T. C. Lubensky, Phys. Rev. Lett. 63, 1082 (1989)].