Tensile properties of two responsive hydrogels

Temperature-responsive hydrogels were prepared from N-vinyl caprolactam/ethylene glycol dimethacrylate (PVCL) or from hydroxypropylcellulose/divinyl sulfone (HPC). Both gels undergo reversible volume shrinkage between room temperature and 50 °C, and for both, the tensile stress–strain behavior in the collapsed state above the temperature-induced transition is qualitatively different from that at room temperature. At the higher temperature, PVCL gels become stiffer, more ductile, and more viscoelastic. HPC gels, on the other hand, have lower initial tangent moduli in the high-temperature state. Possible molecular mechanisms are suggested, and implications for the design of temperature-responsive actuators (‘artificial muscles’) from these materials are discussed.