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.
More
Read the research article, High-Performance
Polyethylene Fibers “Al Dente”: Improved Gel-Spinning of Ultrahigh
Molecular Weight Polyethylene Using Vegetable Oils, by Dr Paul Smith, Dr Theo Tervoorthere, Raphael Schaller and Kirill Feldman here