The polymer network of a gel, under certain conditions, undergoes a discrete transition in equilibrium volume with changes in solvent composition or temperature. This Letter demonstrates that ionization of the gel network plays an essential role in the phase transition. The volume collapse is also observed when the $p\mathrm{H}$ within the gel is varied.

• Received 13 June 1980

DOI:https://doi.org/10.1103/PhysRevLett.45.1636