AMHERST, Mass. – A team of University of Massachusetts polymer scientists has challenged a longstanding theory regarding how plastics harden, perhaps offering scientists finer control over the flexibility or rigidity of specially produced plastics. The findings, by Professor Murugappan Muthukumar and former graduate student Paul Welch, were published recently in the journal Physical Review Letters. The work was funded by the National Science Foundation.
Polymers are extremely long chains of linked small molecules called monomers, Muthukumar explains. Polymers of a material such as polyethylene, from which plastic bags are made, can be as many as 10,000 monomers long.
When plastics are manufactured, the material is heated and then cooled, so that it will harden, or crystallize. The UMass finding has to do with the way in which polymers crystallize. Essentially, they fold back and forth in tight layers, producing a wide and very thin crystal, perhaps just 10 nanometers thick – about 10,000 times thinner than a human hair. A previous theory, the Lauritzen-Hoffman theory, suggested that even the lengthiest of polymers would eventually crystallize entirely if given enough time. It was a theory that couldn’t be tested in a laboratory, because the polymers under discussion were so long, they would have taken – in theory – infinite time to crystallize. “Whether polymers of this size would ever completely crystallize has been a puzzle for 60 years,” said Muthukumar. “How to control crystallinity is a big issue in industry. If a material is too crystalline, it may be too brittle for its intended use. This finding gives a potential way of controlling flexibility.”
Muthukumar and Welch, who completed his Ph.D. and now works in industry, tested the theory by conducting computer simulations of polyethylene crystallizing. They found that when very lengthy polymers harden, they never actually achieve total crystallinity. This is because they actually reach a state of equilibrium before all of the necessary folding and assembling of the crystal is completed. “We have shown that finite crystallinity is actually the equilibrium state.”
The finding may also lead to a better understanding of certain aspects of what biochemists call “the protein-folding problem.” Proteins are biological polymers, Muthukumar says; they are long strings of linked amino acids. In order for a cell to function properly, a cell’s proteins must fold themselves precisely into very complex configurations. Scientists are trying to learn how proteins manage to fold themselves with such precision.
“The way the amino acids interact has an effect on how they fold,” said Muthukumar, “and the way in which the individual amino acids are connected tweaks final shape of the protein. Scientists want to know what the effect of chain connectivity is in protein folding before addressing the chemical subtleties. Polyethylene crystallization is the simplest protein-folding problem.”
The journal article offers another insight into the crystallization process, as well. Growth of a crystal has previously been believed to be governed by a barrier at the growth front for addition of other molecules. But, Muthukumar says, the crystal’s growth is in fact spontaneous and is not controlled by any barrier, again opening a new way of thinking.
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