Cross-linked polymers (CPLs) are structures with large, linked molecular chains that exhibit exceptional mechanical properties and chemical resistance. However, their modification is not easy. Scientists at Tokyo Institute of Technology (Tokyo Tech) say they have developed a method that easily allows the fusion of different polymers, enabling the precise tuning of the properties of the final material by selecting appropriate base polymers and mixing them at the right proportion.
Certain CPLs exhibit outstanding properties due to their interlocked tridimensional structures. Motivated by the potential applications, a research team from Tokyo Tech led by Professor Hideyuki Otsuka reportedly managed to cross-link different CPLs together through an unprecedented approach.
“The development of a novel method to fuse different CPLs would bring a revolution in the field, as their mechanical properties can be easily and systematically tuned in an operationally simple process,” explains Otsuka.
For a CPL to have self-healing capabilities, which is very attractive for many applications, the polymers have to be joined by dynamic covalent bonds, according to Tokyo Tech. These bonds also allow for fusing different types of CPLs, however, the carbon molecules used in currently available linkers are prone to oxidizing, which complicates the fusion and processing of CPLs in bulk. The research team employed a linker molecule, called BiTEMPS, that cross-links polymers through a central sulfur–sulfur (S–S) covalent bond. This bond can be temporarily cleaved in half at temperatures higher than 80°C, which allows for exchange among different polymers at the free ends, called TEMPS radicals. Through this cleaving and re-joining process, different CPLs can be fused together. One main advantage of TEMPS radicals is that they are highly stable against oxygen, meaning that all the processing can be done without requiring oxygen care.
To prove the usefulness of their approach, the researchers cross-linked two types of CPLs, one much more elastic than the other. By hot-pressing their mixture, they fused the CPLs together. The mechanical properties of the final material were dependent on the ratio of the raw CPLs used.
“The mechanical properties of the fused samples could be widely tuned to make them as soft and elastic as desired. As the variety of available polymers is almost infinite, it should be possible to generate materials that exhibit a broad spectrum of physical properties using our method by judiciously choosing appropriate polymer compositions and mixing ratios,” concludes Otsuka.
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