Takamasa Sakai1
The University of Tokyo1
Vulcanization is a chemical process that links polymers containing natural rubber and forms tetra-branched crosslinks, creating a three-dimensional (3D) polymer network in which the positions of the constituent polymer chains are fixed. Even when subjected to large deformations, the polymer chains return to their original position when the stress is removed. This is the molecular origin of the elasticity of rubber. Such rubbery materials have become essential in our daily lives, as their elasticity and flexibility provide a functionality range that cannot be supplied by hard materials. Despite the usefulness of these materials, the environmental effect of plastic pollution has been revealed; rubbery materials are a source of microplastic pollutants, and the additives used can also cause environmental pollution. Thus, there is a need to design new rubbery materials that are more environmentally friendly. A methodology for making rubbery materials stronger, without any additives, should help meet this demand.<br/>Although crosslinking is a well-known chemical process, its effect on the ultimate mechanical properties of rubbery materials is still not fully understood. One reason for the poor understanding is the invisibility of the polymer network: it is impossible to visualize the polymer network in rubber because the covalent bonds between the carbon atoms remain almost impossible to visualize using any state-of-the-art microscope. The crowding of the polymer chains also introduced challenges; polymer networks in rubbery materials are far removed from schematic mesh; instead, they resemble entangled spaghetti. In fact, the chain entanglements govern the mechanical properties of the rubber and make it difficult to investigate the effects of the crosslinked structure.<br/>One means of decreasing the number of chain entanglements is adding a diluent to the polymer network. The resultant swollen polymer network is called a polymer gel. In polymer gels, the polymer network architecture plays a vital role in determining the physical properties. Researchers have realized the advantages of such an approach in this century, and many polymer gels with advanced mechanical properties have been developed. These include gels with slidable crosslinks, gels consisting of two coexisting independent networks, homogeneous gels, and self-healing gels. Further, concepts developed in gel science have proven to be translatable, allowing the successful design and fabrication of excellent condensed rubbery materials. In this respect, gel science provides innovative methods for developing advanced rubbery materials.<br/>In this study, we focused on a single simple problem of elucidating how the branching factor—the number of strands connected to each branch—influences the ultimate mechanical properties of polymer networks. Previous studies have been mostly focused on tetra-branched networks, and systematic comparison with other branching factors has been limited to weak deformation regimes. In particular, little is known about tri-branching, which is the lowest branching factor that can form a network. We show that tri-branching, which entails the lowest branching factor, results in a large elastic deformation near the theoretical upper bound. This ideal elastic limit is realized by reversible strain-induced crystallization, providing on-demand reinforcement. The findings indicated that the polymer chain highly orientates along the stretching axis, where enhanced reversible strain-induced crystallization was observed in the tri-branched rather than the tetra-branched network. A mathematical theory of structural rigidity explains the difference in chain orientation. Although tetra-branched polymers have been preferred since the development of vulcanization, we believe that our findings, highlighting the merits of tri-branching, will prompt a paradigm shift in the development of rubbery materials.