Mechanical response and equilibrium swelling of thermoresponsive copolymer hydrogels

Covalently crosslinked thermoresponsive (TR) gels form a special class of hydrogels that swell noticeably below their volume phase transition temperature T_c and shrink above T_c. As the critical temperature is weakly affected by the preparation conditions and molar fractions of monomers and crosslinkers in the pre-gel solution, a facile method to modulate T_c (which is required for biomedical applications of TR gels and their use as temperature-triggered actuators) is to incorporate relatively small amounts of neutral monomers whose hydrophilicity differs from that of the basic monomers. Although observations on copolymer gels confirm the effectiveness of this method, the molar fractions of comonomers necessary for the tuning of T_c remain unknown. A model was developed for the mechanical response and equilibrium swelling of TR gels. Adjustable parameters in the governing relations were found by fitting equilibrium swelling diagrams for poly(N-isopropylacrylamide) homopolymer and copolymer gels. Good agreement is demonstrated between the experimental data and the results of simulation. Based on the model, an analytical formula is derived that expresses the volume phase transition temperature in terms of the molar fraction of comonomers. Its ability to predict the critical temperature is confirmed by comparison with observations on several copolymer gels.