Resistant Rubber made from Proteins
Inspired by nature, Chinese scientists have produced a synthetic analogue to vulcanised natural rubber that is as tough and strong as the original. At the same time, unlike conventional rubbers, the material is easy to recycle.
Caoutchouc (natural rubber) is a collective term used for elastic polymers from which rubber is made. The main part goes into car tyres; further application areas are the automobile industry and articles of daily use such as cold foam mattresses. Although certain synthetic caoutchoucs, polyisoprenes, have the same main chain structure as natural caoutchoucs, vulcanised natural caoutchoucs are much stronger and more durable. The cause is spontaneous "self-reinforcement", a reversible stiffening of the material under mechanical stress, known as strain crystallisation. Apparently, special polar components at the ends of the polymer chains play a role in the high load capacity.
Cross-linking of polymer chains
Making use of the chain ends could improve the mechanical properties of synthetic rubbers. So far, however, there has been a lack of suitable methods of synthesis. A team of researchers associated with Yun-Xiang Xu and Guangsu Huang from Sichuan University, Chengdu (China), have now succeeded in developing such an approach, which they will present in the journal "Angewandte Chemie". Using an already established catalyst system based on rare earths and special stabilised precursors, they succeeded in producing very long polymer chains from isoprene units with a high proportion of cis links within the backbone and a large number of side chains with hydroxyl groups as polar end groups. Similar to natural rubber, the idea was then to link biomolecules to these end groups and provide a physical cross-linking of the polymer chains.
Inspired by the high resistance and strength of spider silk, the researchers chose short protein chains consisting of four molecules of the amino acid alanine. Since the peptide and polyisoprene chains are immiscible, the peptide chains preferably stick together, which ensures the desired physical cross-linking of the polyisoprene chains. This greatly increases the strength and toughness of the new synthetic rubber without impairing its elasticity. In addition, the material showed a clear self-reinforcement through strain crystallisation, which means that its properties correspond to those of vulcanised natural rubber.
Since conventional vulcanisation can be omitted, the reusability of the new high-performance polyisoprene rubbers has been significantly improved. This way, vast quantities of rubber that are difficult to recycle could be reduced in the future.