Spin the thread stronger than a piano string
Piano wire has a strength of two gigapascals. The chemical bond between carbon atoms in a molecule is 200 times stronger. We have learned to obtain extremely plastic polymers from such macromolecules; we have shown that it is possible to obtain nanofibers stronger than steel, says Prof. Andrzej Gałęski, laureate of the 2018 FNP Prize.
Professor Andrzej Gałęski from the Centre of Molecular and Macromolecular Studies of the Polish Academy of Sciences in Łódź received the Foundation for Polish Science Prize 2018 in the field of Chemical and Materials Sciences for developing a new mechanism for plastic deformation of polymers. The professor`s work led to the development of super-durable plastics, for which the aviation and automotive industry had been waiting for years.
WHAT WANDERS THROUGH CRYSTALS
The physicist demonstrated that the mechanical behaviour of polymers depends greatly on the structure. Polymer crystals are very plastic. Natural crystals of salt or sugar can not be plastically deformed at home when used in the kitchen. They are simply too fragile and they break.
"Polymer plastics deform plastically, they can change shape without destroying the crystals. This happens due to defect generation that can wander through crystals. This discovery not only was scientifically fascinating, it also opened up almost unlimited application potential" - says Prof. Gałęski.
Over the years, researchers working under his supervision have learned to create, for example, polymers tapes - commonly used today - with the strength of good quality steel. That`s how extremely strong strings, tapes and ropes were developed. Polymeric materials were called plastics, because it was obvious that they were flexible and plastic, but scientists were the ones to explain why this was happening.
CAVITATION - SEARCHING FOR HOLES
Polymers subjected to mechanical deformation turn white. Research showed that the cause of this were tiny bubbles formed as a result of tearing the amorphous material surrounding the polymer crystals.
"The direct cause of white coloration is the scattering of light on these bubbles. By analogy to a similar phenomenon in water and other liquids, we call this phenomenon cavitation" - explains the physicist.
He notes out that the process of cavitation in polymers is more complicated than in water. Fragments of macromolecules form crystals, and where the fragments of macromolecules are disordered, voids appear. Nevertheless, the liquid analogy helped researchers make several further discoveries. They wanted to reduce the amount of voids and prevent the polymer from cracking. And they were in for a surprise.
"Cavitation bubbles are formed near small admixtures, additives and impurities. We call this +nucleation+ of cavitation. We therefore decided to remove any contaminants or additives from the polymers that could lead to cavitation. We thought we would get rid of the voids, and it turned out that there were even more of them!" - says Prof. Gałęski.
This was how he discovered that cavitation in polymers was initiated by something else. It is caused by tiny, empty spaces in the amorphous phase. Only filling them with an organic substance allows to suppress cavitation.
BEAT THE ENTANGLEMENT OF MACROMOLECULES
Another problem that Prof. Gałęski faced was a theory of tangles, on which the Nobel laureate Pierre-Gilles de Gennes also worked. According to that theory, entangled polymers differ significantly from those not entangled. When the Nobel Prize winner was working on his theory, there was only one known example of polytetrafluoroethylene that was not entangled after polymerisation.
"It was polymerised at a relatively low temperature and when it grew, the macromolecule chain crystallized immediately and did not have the chance to tangle with other growing macromolecules. We have achieved the same effect with polyethylene polymerised at room temperature or lower. Such polyethylene is completely different from the one that you can buy at plastic wholesalers. Today, we are also able to get rid of tangles by dissolving in solvent and precipitating"- explains the FNP Prize winner.
When there are much less tangles or there are none, scientists can stretch such a polymer up to 1000 times! As a result, a very long and thin thread - with nanometer thickness - is formed from a small polymer grain. It is extremely strong, because it uses the strength of carbon-carbon bond.
"This is one of the strongest bonds that occur in nature. If we tried to break it mechanically, it would turn out that it has a strength of 400 gigapascals. For comparison - piano wire, the highest quality steel, has a strength of 2 gigapascals. And when such macromolecules are arranged along the direction of deformation, such a thread can have a strength up to 200 times higher than the best steel" - the scientist says.
He adds that the benefits are twofold. Firstly, an unentangled polymer is easily deformed, secondly, the nanofibres have very high strengths. Polish scientists have patented the method of obtaining such fibres through deformation in an extruder - a standard processing machine. Flood control sand bags are still produced with this technology. They have also launched an installation for the production of tape from recycled bottles.
PLASTICS AND ECOLOGY
"We started researching biodegradable polymers 10 years ago. Together with the Institute of Biopolymers, we tried to synthesize aliphatic-aromatic polyester. We studied the properties of that polymer, we also made compositions that could have practical applications. We could make cups, forks, plates, bags. But it should be emphasised that biodegradation has its drawbacks and will not replace the collection of garbage. A polymer is biodegradable only when placed in compost, in a special industrial installation containing specially selected fungi and enzymes that destroy plastic. Biodegradable materials can disappear, but the energy used during their production also disappears. To recover it, it would be necessary to gasify and burn the material rather than biodegrade it" - he describes.
"Biodegradation will not solve our problems with environmental pollution and it will not replace collecting garbage" - concludes the physicist.
PAP - Science in Poland, Karolina Duszczyk
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