Pliable plastic makes like a snake


By Lila Guterman A THIRTY-YEAR controversy over the behaviour of molten plastics may be at an end. Researchers in Delaware claim liquid polymers slither like snakes when two chunks of polystyrene are fused. Plastics such as polystyrene consist of thousands of carbon-rich units linked together in a chain. But how these chains intermingle when two pieces are welded to make a bottle, for example, has been hard to understand. Some people have suggested that the “head” and “tail” of each chain snake across into the adjacent piece of plastic, followed by the middle bit. But attempts over the past 30 years to prove this “reptation” theory have met with little success. Now Richard Wool, a chemical engineer at the University of Delaware in Newark, thinks he has cracked it at last. His team made two types of polystyrene by adding deuterium, a heavy isotope of hydrogen, to the ends of the chains in one case and to the midsection in the other. Then they melted the edges of the two pieces of plastic together. Although both types of polymer had the same amount of deuterium, the researchers report in Macromolecules (vol 31, p 4915) that one side of the joint ended up with substantially more deuterium than the other. The best explanation for the distribution of deuterium, says Wool, is that the heavy head and tail of one type of polymer snake across the interface first, leaving the deuterium-free midsections alone on the original side. The light head and tail of the other type of polymer slither across from the opposite side of the interface to join these midsections. “It’s very clear from this experiment that the reptation theory is the winner,” Wool says. Tom McLeish, a physicist studying reptation at the University of Leeds, agrees the data are compelling. “This adds considerable support to the model,
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