A New Way to Toughen up Elastomers

Elastomers (elastic polymers, or rubbers) are capable of recovering their original shape after being stretched. The polymer chains in the elastomers are randomly coiled. When you stretch elastomers, the molecules straighten in the direction in which they are pulled, and when you release them, the polymer chains return to their original, irregular coiled state.

An example of a natural elastomer is rubber, produced as latex (an aqueous suspension of small particles of cis-polyisoprene) by the rubber tree (Hevea brasiliensis). Alternative plant sources, such as dandelions, have been explored. Natural rubber, obtained by drying the latex, has been used for many centuries.

Natural rubber, which is soft and tacky in its original state, was transformed in the mid-19th century with the vulcanization process, which hardens it into a durable industrial product by crosslinking its polymer chains. Vulcanized rubber was used for tires with the development of automotive industry, although synthetic rubber became predominant in the 20th century.

In addition to thermosets, which require vulcanization, modern synthetic polymer elastomers include thermoplastics. Thermoplastics, such as polyurethanes, copolyesters, polyamides, and polyolefin blends, can be injection molded, extruded, blow-molded, or thermoformed. But the high elasticity of elastomers comes at the price of low mechanical strength. Usually this is solved by reinforcing elastomers with particulate fillers, such as carbon black or silica. The smaller the particle size, the greater the strengthening.

A New Approach

Now a totally different and very elegant approach has been used to obtain tough elastomers. A team of scientists from France and Belgium, led by Dr. Constantino Creton, has designed an elastomer with interpenetrating polymer networks, stretched to different degrees. They achieved this using sequential polymerization.

First, they produced a cross-linked rubbery network by UV polymerization of the monomer (ethyl-or methylacrylate) in a solvent. After solvent evaporation, the obtained “single network” was swollen in more monomer, UV initiator, and a crosslinker. The polymer chains in the first network were isotropically stretched from swelling, and the second network was polymerized until all monomer was used, resulting in a “double network.”

To stretch the chains in the first network even farther, the swelling in the monomer and polymerization step was repeated to obtain a “triple network” in which the first network was highly stretched, the second moderately stretched, and the third network was coiled. The interpenetrating networks were interconnected due to the chain transfer during acrylate polymerization. The highly stretched network is sacrificed, or breaks, first during the impact, dissipating energy and preventing crack propagation, as confirmed by incorporating chemiluminiscent crosslinkers that are activated upon breakage. The publication in Science summarizes:

Using sacrificial bonds, we show how brittle, unfilled elastomers can be strongly reinforced in stiffness and toughness (up to 4 megapascals and 9 kilojoules per square meter) by introducing a variable proportion of isotropically prestretched chains that can break and dissipate energy before the material fails. Chemoluminescent cross-linking molecules, which emit light as they break, map in real time where and when many of these internal bonds break ahead of a propagating crack. The simple methodology that we use to introduce sacrificial bonds, combined with the mapping of where bonds break, has the potential to stimulate the development of new classes of unfilled tough elastomers and better molecular models of the fracture of soft materials.

The concept of sacrificial bonds is used in many tough natural hydrogels and other natural highly elastic natural materials, such as spider’s silk. Understood and applied synthetically, it opens exciting possibilities. Now imagine how useful elastomers can be if we combine sacrificial bonds, interpenetrating networks, and self-healingproperties! Yes, we learn from Nature.