FRANKFURT, Germany—A team of Chinese scientists have developed a synthetic equivalent to vulcanized natural rubber, according to a news release by the Society of German Chemists.
The researchers, led by Yun-Xiang Xu and Guangsu Huang from Sichuan University, Chengdu, revealed the development in a recent edition of technical journal Angewandte Chemie.
The structure employs short protein chains on the side chains of the polymer backbone to ensure a stable physical crosslinking and provide a "self-reinforcing effect" under load. Although synthetic polyisoprene rubbers have the same main-chain structure as natural rubber, vulcanized NR is much stronger and more durable. This, according to the scientists, is due to the "spontaneous self-reinforcement" effect of vulcanized NR—a reversible stiffening of the material under mechanical stress, referred to as strain crystallization. Here, special polar components at the ends of the polymer chains—non-covalently bonded proteins and phospholipids—play a role in producing this high-resilience characteristic.
Functionalization of the chain ends also can improve the mechanical properties of the synthetic rubbers. Until now, however, suitable synthesis methods were not available to achieve this.
To address this challenge, the Chinese scientists employed an established catalyst system based on rare-earth elements and special stabilized precursors. This approach, they reported, has yielded very long polymer chains from isoprene units, with a high proportion cis-linkages within the backbone and a variety of side chains with hydroxyl groups at the end.
Inspired by natural rubber, the idea was to link biomolecules here, which are responsible for a physical crosslinking of the polymer chains, according to the release.
The team also modeled the high strength of spider silk: choosing short protein chains (oligopeptides) from four molecules of the amino acid alanine. The oligo-alanines form accordion-like β-sheet structures that make up the hard constituents of silk and give it strength and thermal stability, according to to report.
As the peptide and the polyisoprene chains are immiscible, the peptide chains preferentially aggregate together. This effect is said to provide the desired physical crosslinking of the polyisoprene chains.
According to the scientific report, the process "greatly" increases the strength and toughness of the new synthetic rubbers without compromising the elasticity of the material. Furthermore, it said, the synthetic rubber shows a significant self-reinforcement through strain crystallization, with features corresponding to those of vulcanized natural rubber.
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