An abrupt change in the polyzwitterionic swelling ratio as a function of the temperature, sodium chloride concentration and chemical crosslinking density is established. These results are reasonably explained by the model presuming zwitterionic cluster formation in the segments between the chemical junction points. The zwitter-ionic clusters, produced at low chemical crosslinking density only, act as temperature- and salt concentration-dependent physical junction points. An irreversible temperature-stimulated swelling-shrinking behaviour of the polyzwitter-ionic hydrogels (swelling ratio-temperature hysteresis loop) at low chemical cross-linking density is also observed and analysed.
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Georgiev et al. (2001) studied this question.