Elastic polymers, or elastomers, are desirable materials because they can repeatedly de-form and re-form. But this softness often comes at the expense of mechanical strength, which the materials need to resist fracture under heavy loads. A new kind of tough elastomer that remains malleable is now on the horizon, thanks to a two-part polymerization strategy reported in Science (2014, DOI: 10.1126/science.1248494).
First developed for strengthening hydrogels, the strategy was long thought to be limited to these water-based polymers, says Costantino Creton of ESPCI ParisTech, in France, who led the research.
Creton’s route to strong yet soft elastomers is a welcome one, comments Zhigang Suo, a materials scientist at Harvard University. He says the resulting elastomers could be useful in established applications, such as sealants or tires, as well as in emerging ones, such as robotics, environmental monitoring, and energy-harvesting systems.
Until now, elastomers for demanding applications, such as automobile tires, have been toughened up with additives such as carbon black nanoparticles. “However, adding these fillers may cause complications in processing,” Suo says.
For their work, Creton’s team used the so-called hydrogel double-network strategy. First, the researchers polymerized short chains of the elastomer ethyl acrylate. Then they swelled the polymerized ethyl acrylate as much as possible using the monomers of a second elastomer, methyl acrylate. Finally, they polymerized the methyl acrylate.
The first swollen and stretched elastomer provides the material’s tensile strength, Creton explains. Meanwhile the monomers of the second, intercalated elastomer provide softness, because the monomers have polymerized normally and thus the material isn’t stretched to its maximum.
The fracture energy, the unit of strength for elastomers and other materials, for each of the component polymers used in this hybrid elastomer is low: approximately 1 J/m2 and 10 J/m2, respectively. But the fracture energy of the hybrid is about 1,000 J/m2. The results indicate that at normal temperatures the hybrid elastomer is just as strong as standard elastomers with nanoparticle-embedded reinforcements. But at higher temperatures, above 100 °C, at which many standard reinforced elastomers fail, the hybrids may have an edge, Creton says.
When making the new elastomer, Creton’s team also devised a general strategy that will be “valuable for studying stiffening and toughening mechanisms,” Suo says. The key is a butanediol diacrylate linker (called BADOBA) incorporated into the first, stretched ethyl acrylate network. When the network is subjected to mechanical force, these links break and fluoresce with blue light. This visualization strategy, which could also work in double-network hydrogels, gives materials scientists a long-awaited way to characterize elastomer networks, Creton says. Other analytical techniques, such as optical and electron microscopy, are not applicable to such materials, he notes.