Modulation of the volume phase transition temperature of thermo-responsive gels

Thermo-responsive (TR) gels swell substantially below their volume phase transition temperature T_c and shrink above this temperature. Applications of TR gels in controlled drug delivery and their use as biosensors and temperature-triggered soft actuators require fine tuning of T_c. As the critical temperature is independent of the preparation conditions and molar fractions of monomers and cross-linkers, it is modulated by incorporation of (neutral or ionic) monomers and polymer chains into pre-gel solutions for TR gels. A model is developed for the mechanical response and equilibrium swelling of TR gels. Analytical formulas are derived for the effect of molar fraction of comonomers on the volume phase transition temperature T_c in copolymer gels and gels with semi-interpenetrating networks. Adjustable parameters are found by fitting equilibrium swelling diagrams on poly(N,N-diethylacrylamide) gels. Good agreement is demonstrated between predictions of the model and experimental data.