Properties of gels topologically cross-linked with physical restraints

Molecular net gels (MN gels) are polymer gels constructed based on a topological structural design principle. The MN gels are prepared by penetrating polymerization of N-isopropylacrylamide (NIPAAm) in the presence of molecular nets (MNs), which are soluble ultra-high-molecular-weight three-dimensional network polymers. In this study, we investigated the mechanical properties of MN gels prepared with various MN concentrations that provide “cross-linking space”. The MN gels exhibited highly flexible mechanical properties, and as the MN concentration increased, the elastic modulus, fracture stress, and toughness increased, suggesting that MN could function as a “cross-linker”. Interestingly, even with an increase in MN concentration—i.e., an increase in elastic modulus—the fracture strain was almost constant. Furthermore, since MN gels are composed of temperature-responsive PNIPAAm, we investigated the temperature dependence of their mechanical properties. Above the lower critical solution temperature (LCST) of PNIPAAm, a significant increase in elastic modulus and fracture stress was observed compared to below the LCST, without accompanying large volume shrinkage. In addition, stress relaxation and plasticity of the MN gels were also investigated. The results revealed that the MN gels exhibit plastic behavior comparable to that of un-cross-linked polymers. These observations that the MN gels exhibit both elastic and plastic responses depending on the timescale of deformation are likely due to their dynamic cross-linking structure. Thus, the MN gels represent a novel class of gel materials with unprecedented properties.