Lots of applications from tires to wetsuits, stretch clothing to silicone implants use elastomers. These polymers can be flexed and stretched without breaking and return to their original form afterwards. There is always room for improvement in any given class of materials. Improve elastomer strength usually involves building more cross-links between polymeric strands. Now, researchers from the University of California at Santa Barbara's Materials Research Laboratory (MRL) have taken inspiration from the byssal strands that anchor marine mussels to rocks and ship's hulls to make better elastomers. Their approach overcomes the inherent trade-off between strength and flexibility in elastomers. [Valentine et al. Science (2017) 358, 502-508; DOI: 10.1126/science.aao0350]
"In the past decade, we have made tremendous advances in understanding how biological materials maintain strength under loading," explains team member Megan Valentine. "In this paper, we demonstrate our ability to open exciting lanes of discovery for many commercial and industrial applications."
Mussels have previously inspired wet, soft systems such as hydrogels. The UCSB team has now exploited knowledge of the iron coordination bonds found in the byssal strands to make dry polymers. Such materials might replace stiff but brittle materials in impact- and torsion-related applications.
"We found that the wet network was 25 times less stiff and broke at five times shorter elongation than a similarly constructed dry network," explains team member Emmanouela Filippidi. That is interesting but was anticipated. "What's really striking is what happened when we compared the dry network before and after adding iron. Not only did it maintain its stretchiness but it also became 800 times stiffer and 100 times tougher in the presence of these reconfigurable iron-catechol bonds."
To build these polymer networks, the team synthesized an amorphous, loosely cross-linked epoxy network and then treated it with iron to form dynamic iron-catechol cross-links. The presence of the iron provides a self-healing mechanism for the materials. With iron-catechol coordination bonds present, should any cross-link break it can reform albeit in a different place within the network. Physically, stretching the iron-catechol network does not lead to an accumulation of energy in the system. When the tension is released, the material does not then bounce back like a stretched rubber band it slowly assumes its original shape in a way akin to squashed memory foam. Such "energy-dissipative plastics" could be used to make more resilient casings for smart phones and other valuable but fragile electronics devices.
"The difference between response in wet and dry systems is huge and makes our approach a game-changer in terms of synthesizing useful engineering materials for high-impact applications," Valentine adds.
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase.