Integrating cutting-edge facilities and advanced chemistry, we have developed SHINAYAKA (flexible and tough) polymers based on novel molecular concept strengthening polymers in ImPACT (Impulsing Paradigm Change through Disruptive Technologies) Program by CAO (Cabinet Office), Government of Japan. To achieve difficult targets, we adopted novel matrix management system for Industry-University Corporation to make up a dream team for polymers to industrialize beyond conventional collaboration between industry and university. Specifically, a company is assigned as each project manager and connected with various technologies from different universities. In addition, we clarified novel molecular bonding mechanisms to yield tough polymers and develop innovative structural materials for automobile. As a final goal, we constructed a concept car with developed SHINAYAKA polymers to verify concepts toughening various polymers and demonstrate lightness and safety by applying SHINAYAKA polymers to vehicles.

Bridgestone Corporation took part in ImPACT (Impulsing PAradigm Change through disruptive Technologies) research program, which is nationally funded five-year R&D activity operated by the Cabinet Office, Government of Japan. In ImPACT program, Bridgestone tackled to realize resource saving, light weight, tires through innovative tough rubber compound with maintaining energy efficiency. Comprehensive and multi-faceted analyses for fracture mechanisms of rubber were carried out. The overall research framework, derived innovative material design concept and practical realization methodology with double network concept are reported in this paper.

We investigate experimentally various aspects of the crack propagation phenomena in elastomers by means of “dc/dt” method. Propagation speed (v), shape of crack tip, and local strain distribution near the crack tip are examined as a function of imposed tearing energy (G). In particular, we focus on a discontinuous transition of v at a characteristic value of G(G_c) between slow and fast modes. We correlate the G dependence of v and crack-tip shape with macroscopic nonlinear viscoelasticity of the elastomers. The change in the crack-tip shape accompanying the velocity transition is explained by weakly nonlinear fracture mechanics with the ratio of the first- and second order elastic moduli. The power law exponent for the G dependence of v (G ~ v^a) in fast mode has a close correlation with the exponent for the viscoelastic spectrum in glass-rubber transition regime (G(t)~t^-k). We find G_c is governed by the product of the fracture toughness and the ratio of the first- and second order elastic moduli for the elastomers. We also show that the local strain distribution near the crack tip in dynamic crack revealed by DIC technique involves the strong effect of nonlinear elasticity.

Crack propagation on sheet materials is a useful phenomenon to characterize material toughness. In particular, it has been known that crack-propagation speed jumps abruptly as a function of applied strain. We have proposed a model which reproduces the velocity jump. This model uncovers the physical origin of the velocity jump as a glass transition that occurs in the vicinity of a crack tip. This theory suggests that the velocity jump can be universally observed for a certain class of viscoelastic materials. In fact, we recently succeed in detecting the velocity jump in semicrystalline porous polypropylene sheets. This work clarifies the importance of the dynamic test, in which sheets is extended with a constant speed during crack propagation. We further explore the potential of crack-propagation tests in industry on the basis of our recent findings.

Crack velocity in rubbers, which in general monotonically increases with increasing applied strain, can exhibit a peculiar jump (discontinuous increase) at a critical strain. This phenomenon, which is called velocity jump or mode transition, has been known for decades and considered to be an important and relevant issue both scientifically and industrially. Nevertheless, until recently the mechanism of crack mode transition in rubbers had remained unclear. Our recent work of numerical simulation (Sci. Rep., 2017, 7, 42305) revealed that a non-monotonic temporal development of stress the near crack tip is produced due to viscoelastic nature of rubber-like materials, which causes the velocity jump. This interpretation of the phenomenon turned out to be consistent with findings based on a mathematically solvable model by Sakumichi and Okumura (Sci. Rep. 2017, 7, 8065), suggesting that the slow-fast transition is caused by change in behavior at the crack tip from rubbery to glassy nature. Consistency of the implication of these theoretical studies with experimental observations about the mechanical properties of rubbers is also discussed.

In 2003, we developed Double Network (DN) hydrogels that achieved toughening by a completely different principle from previously existed rubbery materials. DN gel is composed of two interpenetrated networks with highly contrasting topological structure. One network is rigid and brittle from highly pre-stretched and densely crosslinked polymer, and the other network is soft and stretchable from coiled and sparsely crosslinked polymer. Interestingly, DN gel exhibits toughness comparable to natural cartilage despite of low internal friction of the material. In this review, we describe the points that have been clarified about the toughening mechanism of DN gel, after that, we also outline the latest research trends on the novel tough materials development based on the double network concept.

The rotational relaxation time of a fluorescent molecule dispersed in rubbery polymers was characterized by time-resolved fluorescence anisotropy (TRFA) measurement and an attempt was made to quantitatively combine it with the segmental relaxation time of the corresponding polymer obtained by dielectric relaxation spectroscopy. Then, we show that the segmental relaxation time extrapolated to higher temperatures using the Vogel-Fulcher-Tammann law could be superimposed on the rotational relaxation time, resulting in a single curve. This behavior was common for polyisoprene and acrylonitrile/butadiene copolymer, implying that the rotational dynamics of a fluorescence probe is a useful tool for the characterization of polymer dynamics. Once this was established, TRFA was applied to a model filler interface combining with the evanescent wave excitation. Also, sum-frequency generation spectroscopy, which provides the best depth resolution among available techniques, was used. We found the presence of the dynamics gradient of chains in the interfacial region with the SiO₂ surface and tried to assign it to the two kinds of adsorbed chains, namely loosely and strongly adsorbed, at the interface. The segmental relaxation of chains in the strongly adsorbed layer at the interface could be slower than that of bulk chains by more than 10 orders.