Folding into a pure paraffin crystal.

The spaghettilike molecules of polyethylene have a remarkable ability to disentangle themselves and settle, as neatly folded chains, into the form of crystals. This chain-folding behavior, found to be characteristic of many flexible polymers, is still largely unexplained. Recently, researchers discovered that shorter molecular strands, containing as few as 150 carbon atoms, also fold when they crystallize.

"Such behavior can be considered characteristic of the crystallization of .. flexible chains in general," conclude Andrew Keller and his colleagues at the University of Bristol in England. Their paper appears in the July 26 SCIENCE.

Polyethylene molecules typically consist of 10,000 or so linked carbon atoms, each also bonded to two hydrogen atoms. To understand the behavior of polyethylene, Keller's group looked at a family of simple hydrocarbons called alkanes, also known as paraffins. Polyethylene is simply a large number of these alkanes strung together. Using a newly developed method for synthesizing alkanes with a uniform, precise number of carbon atoms in each chain, the researchers produced sets of alkanes containing 150 to 390 carbon atoms.

By measuring various properties of the crystals created when the molten form of these alkanes cooled or when the crystallized out of solution, the Bristol team found that chain folding occurs as long as 150 or more carbon atoms are present. In each case, the molecules fold exactly in half or into thirds, quarters or fifths. The folds also prove to be very tight.

In a typical thin, platelike crystal, the folded molecules sit almost perpendicular to the crystal's flat upper and lower surfaces. The tendency of pure alkanes to fold in fractions of the extended chain length indicates that chain ends are kept out of a crystal's interior, the researchers say. The end groups lie even with the top or bottom surfaces. This reduces the incidence of imperfections that disturb the orderliness of these crystals.

For longer chains and for mixtures of molecules with chains of different lengths, the picture is somewhat more tangled. In this instance, the regular, sharp folds normally found are disrupted by intervening neighbors or by the trapping of remote parts of the same chain elsewhere in the crystal. Formations like "loose loops," "hairs" and "tie molecules" would give fold surfaces a disorganized look despite an orderly crystalline interior.

"Yet," reports Keller's group, "the existence of such imperfections should not obscure the intrinsic trend of long chains to fold in a regular manner." Even complex biological polymers often show some chain folding.