Sticky ends allow different cross-linked polymers to fuse together

Cross-linked polymers (CPLs) are structures where large molecular chains are linked together, producing materials with exceptional mechanical properties and chemical resistance. But modifying them has always proved tricky. Now, scientists at the Tokyo Institute of Technology in Japan have developed a method for easily fusing different CPLs together, allowing the properties of the final material to be tuned precisely by selecting appropriate base polymers and mixing them in the right proportions. The scientists report their method in a paper in Angewandte Chemie Intenational Edition.

Polymers – large molecular chains composed of small repeating subunits – can be found all around us and also within us: DNA and proteins are some familiar natural polymers. In contrast, synthetic polymers, such as plastics, were first produced about a century ago, but have since found their way into our everyday lives because of their amazing properties. Polymers can be tailored according to their constituent subunits to possess many desirable characteristics, such as mechanical strength, stretchability, permeability, and so on.

Another way to obtain even more functionality in polymers is by using special cross-linker molecules to form CPLs, which can exhibit outstanding properties due to their interlocked tridimensional structures. Motivated by the potential applications, a research team from Tokyo Institute of Technology, led by Hideyuki Otsuka, has recently achieved a breakthrough in this field, by managing to cross-link different CPLs together using 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.

The researchers achieved this feat by employing a novel cross-linker molecule. For a CPL to have self-healing capabilities, which is very attractive for many applications, the polymers have to be linked by what are known as dynamic covalent bonds, which also allow different types of CPLs to be fused together. But the carbon molecules used in currently available linkers are prone to oxidizing, which complicates the fusion and processing of CPLs in bulk.

So the research team employed a linker molecule called BiTEMPS, which 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 to produce cleaved ends known as TEMPS radicals. These cleaved ends can then stick back together, but in different arrangements, offering a way to link different CPLs to each other. One of the main advantages of TEMPS radicals is that they are highly stable against oxygen, meaning that all the processing can be done without needing to protect the CPLs from oxygen.

To prove the usefulness of this novel approach, the researchers cross-linked two types of CPLs, one of which was much more elastic than the other. By hot-pressing the mixture, they managed to fuse the CPLs together, and the mechanical properties of the final material were dependent on the ratio of the two component CPLs.

"The mechanical properties of the fused samples could be widely tuned to make them as soft and elastic as desired," says Otsuka. "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." This innovative method will significantly advance the field of CPLs, allowing for the development of highly tailored materials for specialized applications.

This story is adapted from material from Tokyo Institute of Technology, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.