Earlier, we developed multinetwork elastomer by using a covalent bond, hydrogen bond, and clay plane bond. By taking advantage of the characteristics of each cross-linking, the thermoplastic elastomer shows excellent compression set resistance, good flowability, and high tensile properties. In the previous report, it was thought that the crosslinked part was hydrogen bonded to the end face of clay. However, it was estimated that the crosslinked part also has electrostatic interaction with ammonium salts on the clay surface by TEM, IR, and AFM measurements, and new estimated structure was proposed. In addition, it was found that the addition effect of organic clay was a specific phenomenon in the multinetwork elastomer from the comparison with the hydrogen bond crosslinking elastomer. In a hydrogen bond crosslinking elastomer that expresses physical properties only by hydrogen bond, the interaction between the clay and the hydrogen bond crosslinking weakened hydrogen bond crosslinking between polymers to caused deterioration of physical properties. However, in the multinetwork elastomer with stable covalent crosslinking, crosslinking does not change, and the hydrogen bond crosslinking site interacted with clay surface to caused non-localization of the crosslinking part. Therefore, the physical properties have been improved.

Strain-induced crystallization which occurs at the crack tip is thought to be the origin of excellent strength of natural rubber (NR). However, the detailed mechanism of the reinforcement by the strain-induced crystallization is still unclear. By the X-ray diffraction mapping technique, we find that paraffine crystals are specifically formed around crack tip where strain-induced crystallization is observed in a carbon black filled NR sample. Dissipation of energy for the transportation of paraffine molecules from inside the sample to the surfaces is thought to partly contribute the reinforcement of the material.

We made an apparatus for measurement of mechanocaloric properties of rubber. It consists of a pair of rubber tapes put together and suspended in a temperature-stabilized space. The upper end of the sample is connected to a force gauge and the lower end to a motor-driven mechanical device that stretches the sample quantitatively and reproducibly. The temperature of the sample is measured with a thermocouple inserted between the rubber tapes relative to a reference point fixed in the sample space. The tensile force, extension and temperature are measured to a precision of 0.01 N, 0.1 mm and 0.05 mK and recorded on a PC at a sampling interval of 0.5 s. Experimental results on a natural rubber sample gave the entropy of extension and stress-strain relation up to 200% elongation. A negative term in the entropy of extension is given explicitly in distinction to the normal positive one. Further analysis based on the ideal rubber equations gave consistent values of the structural parameter characterizing the rubber. Energy dissipation taking place in the deformation, a correction term in the present analysis, has been determined quantitatively for a systematic study of the irreversible effects