06 September 2013

New plastic becomes stronger when stressed, drop-proof smartphones incoming

Plastics are amazingly versatile materials, and their usage in all manner of objects is only increasing. Virtually everything you own is at least partially composed of plastic, and that’s usually a very good thing. Plastic is light, inexpensive, and can be molded in any shape. However, it’s not always the strongest material. Many smartphone owners know the sorrow of dropping a phone and finding the fall cracked or chipped the casing. A new type of plastic developed at Duke University could change all that. This material actually gets stronger when it is stressed.

The carefully designed molecular structure of the material is what gives it this unusual property. Like all plastics, this one has a backbone composed mostly of carbon. However, the carbon atoms are arranged in a series of triangles extending down in long chains with two bromine atoms at one point. The researchers found that the unique structure of this compound could turn “destructive” energy into “constructive” energy. But how?

When the polymer chains are tugged or experience shock, they tear on one side. Other plastic polymers would not be so uniformly damaged, leading to structural failure. However, this is only the beginning of the transformation. The shearing force breaks the triangle into a longer chain, which also frees up bonding sites at the bromine locations for a second molecule to come in.

The researchers included a molecule called a carboxylate in this plastic to utilize those bonding sites. This cross-links multiple chains and increases the material’s strength at the site of damage. Because this material reacts to mechanical force instead of light, heat, or chemical exposure, it is called a mechanophore.

The Duke team conducted a variety of tests to ensure this new mechanophore was actually making new bonds consistently. On the large scale, the material was fed through an extruder, which forces plastic into a mold. The mechanophore went from being pliable to stiff as the mechanical force initiated structural changes. When measured on the microscopic scale through a technique called nanoindentation, scientists found that the hardness increased by 200 times after the extrusion process. It even works when the plastic is dissolved in a solution. Disturbing the molecules causes it to cross-link and become a plastic gel that forms a layer on the side of the container.

This isn’t just a cool science experiment, though. A versatile mechanophore like this could revolutionize a number of areas. For example, implantable medical devices like artificial heart valves could be made more durable and less likely to fail. Prosthetic limbs could also be constructed with stress-absorbing mechanophores in crucial areas. Perhaps one day you’ll even have to “season” a new phone by knocking it around a little bit.

This plastic may actually see use outside the lab, but the team is hoping to engineer a followup that would be even more useful. The goal now is to reverse the hardening when stress is removed from the plastic. That would allow shock-resistant plastics that are still soft and flexible. You may never have to worry about a plastic shopping bag tearing again.

Research paper: doi:10.1038/nchem.1720 - “Mechanochemical strengthening of a synthetic polymer in response to typically destructive shear forces”

Now read: Graphene used to make graphene-copper composite that’s 500 times stronger


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