Abstract
The history of volume phase transition of responsive gels from its theoretical prediction to experimental discovery was described and the major role of mixing Gibbs energy function in theoretical models was stressed. For detailed analysis and fine tuning of the volume phase transition, the generalized Flory–Huggins model with concentration and temperature dependent interaction function coupled with Maxwell construction as a tool is very suitable. Application of expansive stresses can uncover the potential of various swelling gels for volume phase transition. Experimentally, the abrupt, equilibrium-controlled phase transition is often hard to achieve due to passage of gel through states of mechanical instability and slow relaxation processes in macroscopic objects.
Highlights
The volume phase transition (VPT) in gels is characterized by an “abrupt” change in the degree of swelling and by the possibility of coexistence of two gel phases differing in the degree of swelling
The swelling equilibrium is characterized by change of the Gibbs energy ∆Gsw considered as additive contributions due to mixing of the cross-linked polymer with the solvent, ∆Gmix, and due to deformation of elastically active network chains, ∆Gel
In cross-linked gels, the osmotic pressure generated by mixing is opposed by the force generated due to stretching of network chains and the respective contribution to the Gibbs energy for a network can be expressed as
Summary
The volume phase transition (VPT) in gels is characterized by an “abrupt” (discontinuous) change in the degree of swelling and by the possibility of coexistence of two gel phases differing in the degree of swelling. For the Flory–Huggins mixing and the Gaussian network elasticity case, such treatment [14] resulted in phase diagram showing an abrupt (discontinuous) change of the degree of swelling with temperature or other stimuli, the position and width of which was determined by fixed values of the concentration of elastically active network chains, degree of dilution and Flory–Huggins interaction parameter At that time, such prediction was quite puzzling because nothing like that was observed experimentally before and the picture of a possible coexistence of two phases differing in the degree of swelling within a macroscopic piece of covalently cross-linked gel seemed absurd. Concerning the aspect (a) we will show the usefulness of observation of the changes using Maxwell construction and using generalized interaction function in the Flory–Huggins theory
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