Swelling pressure induced phase-volume transition in hybrid biopolymer gels caused by unfolding of folded crosslinks: a model.

A thermodynamic model is proposed describing swelling changes and swelling transitions of hybrid gels in which domains of folded chains are chemically built in as cross-links. These folded domains can be unfolded to random coils by osmotic forces produced by the synthetic gel matrix. Uncoiling takes place if the osmotic force acting on the cross-links exceeds the critical value. By unfolding, a new interacting surface is exposed to interactions and affects the swelling pressure. The chains of the folded domains may have ionized groups. The model is based on mean-field statistical-thermodynamic treatment of swelling of polyelectrolyte gels with finite extensibility of network chains. This study is related to hybrid hydrogels with built in protein motifs. A continuous change in external variables increasing the degree of swelling of the hydrogel brings about an abrupt increase in volume (transition) of the gel. The position and magnitude of the transition depend on structural parameters of the hybrid gel, such as fraction of the folded domains in the gel, degree of ionization of chains in the domain, presence of additional chemical cross-links, or degree of dilution at gel formation. Two options for reversibility of the changes are considered: (a) unfolding is irreversible and deswelling proceeds along other curve than swelling and (b) swelling is reversible when the osmotic force decrease below the critical value. In the latter case, swelling changes are described by a closed loop with two transitions. Under certain conditions (high dilution at network formation and sufficiently high degree of ionization of chains of the folded domains), a transition appears known as the collapse transition induced by balance of hydrophobic and hydrophillic interactions. This collapse transition induces the folding transition by which the folded domains are reformed.