The Special Characteristics of Sulfur Polymers

Researchers at the University of Liverpool are making progress in the development of new sulfur polymers. These materials offer an environmentally friendly alternative to traditional petroleum- based plastics and can be produced using sulfur that would otherwise be waste.

Royal Society Research Fellow and chemist Dr. Tom Hasell and his team have published two papers in Angewandte Chemie that document their practical and exciting developments for sulfur polymer technologies and applications. This research builds on their 2019 discovery of a new catalytic process for making polymers from sulfur.

Sulfur waste products from industrial processes have been studied for some time. Sulfur is plentiful and its polymers are more easily recyclable. Many commercial elastomers or “rubbers” contain polysulfides; they return to their original shape after being compressed or stretched.

The rigidity of these polymers comes from crosslinking between the polymer chains, produced during a process called vulcanization. The first vulcanization, when Charles Goodyear heated polyisoprene with sulfur, converted a sticky and unusable material into a product that could be made into useful products. Vulcanization is also used to create polychloroprene (Neoprene), styrene-butadiene, polyisoprene, and other classes of rubbers.

Led by Ph.D. student Peiyao Yan, the paper reports that adding urethane bonds increases the strength of the sulfur polymers more than 135 times. The controlled introduction of this second type of bonding allows the physical properties of the polymers to be carefully modified.

The strengthened sulfur polymers have shape-memory effects ‒ temporary deformations can be repeatedly reset by heating, a process applicable to soft robotics, medicine, and self-repairing items.

A second paper, published in Chemical Science, reports on the work of Dr. Hasell's group with researchers at Flinders University in Australia. They found that sulfur polymers could form rubber-like materials that can self-repaired within minutes by applying an amine catalyst to restore the bonds. This process requires little energy consumption and may offer a useful application for these sulfur polymers.

Dr. Tom Hasell said: "Both of these papers really show the potential of polymers made from waste sulfur to be a viable replacement material for some traditional petrochemical based plastics. Not only as a substitute material, but as one that is easier to recycle, and has exciting new properties for materials chemists to explore. We are excited to see what ideas researchers have for using these new findings, in particular the memory shape and "re-programming" properties."