Suicidal Crystals
Crystalline polymers are found in materials for clothing (polyester), vehicle bumpers (polypropylene), and presentation transparencies (Mylar), so the plastics industry has a high stake in understanding the crystallization process. In the 13 November PRL, a British team reports on a simple polymer that crystallizes in an unusual way: For a range of concentrations, the 200-carbon chain molecules decrease their crystallization rate with increasing concentration, contrary to all other known materials. The authors present a simple explanation which highlights the complexity of polymer crystallization compared with that of small molecules and supports the view that polymers crystallize by a multistep process.
Polymer scientists are used to working with so-called polydisperse samples–ones that contain molecules with a range of lengths–because it is often unnecessary to make uniform samples for practical applications. But the heterogeneity can complicate fundamental polymer physics experiments, says Goran Ungar of the University of Sheffield in the UK. “The whole polymer physics has been done on an impure system,” he says. A decade ago a team at the University of Bristol in the UK came up with a way to make polymer samples of a specified length, up to 390 units long, with no molecule-to-molecule variations.
Short chains crystallize by stretching out and packing parallel to one another, like thousands of pencils in a box from the factory. But since polymers are flexible, they can also crystallize as once-folded chains, making the crystal “lamella” half as thick, or as twice-folded, for lamella one-third the thickness of the extended chain lamella. Crystals of long, polydisperse polymers may contain a mix of chains with different numbers of folds, so uniform-length molecules simplify the analysis.
Ungar and his colleagues at Sheffield and the University of Manchester looked at the effects of concentration on the n-alkane At low concentrations, the crystallization rate increased with concentration, as expected, but between 1% and 3% polymer by weight, the rate decreased almost to zero. Above 3% concentration, the rate went up again. The team identified the crystals as the extended chain type below 3% concentration and the once-folded type above 3%, based on the different appearance under the microscope of the two types of crystals. Although some fairly complex reactions are known to show such “negative order kinetics”–decreasing rate with increasing concentration–this is the first simple example; the reaction involves exactly one chemical species.
According to the researchers’ model, as concentration increases, the folded molecules begin sticking to the extended chain crystals and interfering with the attachment of new extended molecules. This interference process was suggested by their previous work, and they call it “self-poisoning.” As concentration increases further, so many folded chains stick that extended chain crystal growth ceases, and folded chain crystals begin to form at still higher concentrations.
Ungar says the experiments confirm the view, which is “increasingly difficult to ignore,” that polymer crystallization occurs by a “long, torturous process,” where a molecule needs to encounter a surface many times before sticking in the correct conformation. The folded chains slow down this process. Crystals of small molecules have only one conformation and so have a much easier time crystallizing.
Co-author Paul Higgs of Manchester says the self-poisoning process probably happens in all polymer crystallization, but it only became apparent with monodisperse molecules. Stephen Cheng of the University of Akron in Ohio agrees that the work confirms the self-poisoning model and is surprised that it can completely stop crystal growth.