|December 17, 2015|
No tangled web
|BULLETPROOF jackets, yacht sails and tow lines are made of fabrics and ropes designed to withstand enormous force. Those fabrics and ropes are, in turn, woven or twisted from fibres made of artificial polymers, such as polyethylene, specially prepared in ways that make them strong.|
Unfortunately, this preparation uses inflammable and toxic solvents. That makes it hazardous for workers and potentially bad for the environment. But this may change if a team of materials scientists led by Paul Smith and Theo Tervoort at ETH Zurich has its way. As they write in Macromolecules, Dr Smith and Dr Tervoort have been trying to make strong polymer fibres using less-nasty solvents. Not only have they succeeded, their virtue has been rewarded by the discovery that this approach creates even stronger materials than the old and noxious one.
Polymers are long, chainlike molecules. Each link in the chain is either an identical chemical unit (as in the case of polyethylene) or one of a small set of such units (as in the case of nylon, which has two sorts of link). Such chains tend to intertwine in a disorderly fashion when part of a solid. Strength, however, requires order. To make a strong material the individual molecules should, as far as possible, be stretched out in parallel with one another, thus forming an elongated crystal, and the crystals should then be similarly aligned in a fibre as that fibre is being drawn.
If fibres are drawn directly from liquid polymers, they will solidify in a disorderly way. In a solution, though, the molecular chains can slip past each other, straightening themselves out and aligning themselves in the same direction, thus forming crystals. Then, when a thread is drawn, these crystals will line up along its axis.
The established method of doing all this is to use something good at dissolving the polymers in question---typically, a type of chemical known as a non-polar organic solvent. These substances are generally hydrocarbons of the sort petrol is made of. They are notoriously dangerous and bad for the environment.
Dr Smith and Dr Tervoort, however, wondered if it might be possible to coax polymer molecules into aligning themselves using more benign materials, such as peanut oil and even extra-virgin olive oil. These materials do not dissolve plastics like polyethylene at room temperature. If they did, such plastics could not be used as containers for cooking oils, or in many other familiar ways. But the researchers did not work at room temperature. By heating mixtures of polyethylene and their chosen solvents to 230°C, they got the polymer to dissolve.
They then cooled the mixture down, letting crystals form, drew fibres from the result, and tested what they had to see how it compared with fibres prepared using a non-polar solvent called decalin. The results astonished them. They had hoped the new fibres might be as good as decalin-made ones. In fact, they were better.
Tensile strength is measured in pascals. High-quality steel has a strength of one gigapascal (GPa). The two researchers' decalin-made fibres had a strength of about 2GPa. The best of those made using peanut oil and olive oil came in at 4GPa. Why is not yet clear. But something about the interaction between polyethylene molecules and the new solvents seems to encourage the formation of more perfect crystals than those that emerge from a decalin solution. If this applies to other polymers, and the process can be scaled up, Dr Smith and Dr Tervoort will have created a process for making strong fibres that is not only greener, but more effective.