The thermal transitions observed in hydrogels based on interpenetrating networks of poly(ethyl acrylate) and poly(hydroxyethyl acrylate) are explained in a simple thermodynamic framework based on the phase diagram that avoids making use of the hypothesis of different “kinds” of water present in the hydrogel. The features that can be explained on its basis include the melting temperature depression of water, the occurrence of crystallization on heating (even when it was absent upon cooling), and the existence of an amount of noncrystallizable water. Three water concentration regimes can be predicted to exist, which give rise to qualitatively different kinds of thermograms.

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