Molecular thermodynamic model for swelling behavior and volume phase transition of multi-responsive hydrogels

A new molecular thermodynamic model for swelling behavior and volume phase transition of multi-responsive hydrogels was developed. The swelling behavior of hydrogels results from three contributions, the mixing of polymer network and solvent, the elasticity of polymer network and the ionic effect containing Donnan equilibrium of ions distributed inside and outside hydrogels and electrostatic interactions between the charges carried by the polymer network and the counter ions. Two kinds of model parameters are included; they are the interaction energy parameters between polymer network and solvent or between the two solvents, and a size parameter which is the molecular mass of the network between two cross-linking points. They should be determined from the experimental swelling curves of hydrogels. It is shown that this model can describe the swelling behavior and volume phase transitions of pH-sensitive and ion-sensitive hydrogels by using fewer model parameters comparing with other models. The different swelling behaviors such as shrinking at low pH and swelling at high pH, swelling at low pH and shrinking at high pH, shrinking at middle pH and swelling at both low pH and high pH, the swelling behaviors of pH- and temperature-sensitive hydrogels or temperature- and solvent-sensitive hydrogels, can be satisfactorily described.