crystals
Image: P. MANDAL/University of Sheffield

The process by which polymers crystallize is important to many industries: such materials, which contain molecules made of long carbon-based chains, are found in clothing, car bumpers and other common products. And in this week's issue of Physical Review Letters, a team from the University of Sheffield describes a relatively simple polymer that crystallizes in an unusual way, perhaps shedding light on the reaction in general. By using a short polymer, the 200-carbon chain of the n-alkane C198 H398 (right), the authors avoided complications that arise from working with polydisperse samples--those having molecules of many different lengths--and found compelling evidence for the idea that crystallization occurs by way of many different steps.

Goran Ungar and his colleagues studied the effects of concentration on crystallization rates. They found that at low concentrations, the rate increased with concentration, as was expected. At concentrations between 1 percent and 3 percent polymer weight, however, the rate decreased nearly to zero. Above 3 percent, it again picked up. Their explanation, based on observations made under the microscope, is as follows: Below 3 percent concentration, the crystals are made up of extended chains that have lined up with one another. But as the concentration rises to between 1 and 3 percent, some of those long flexible chains begin to fold, and these once-folded chains interfere with other chains lining up with the existing extended chains. Only when the concentration rises above 3 percent are there enough folded chains sticking onto one another that they begin to dominate the pattern of crystal growth. Earlier work by this group hinted at the interference caused by folding chains, which they called "self-poisoning." Now they have proof. Future studies should demonstrate whether self-poisoning happens in all polymer crystallizations.