Fold surface of polyethylene single crystals as assessed by selective degradation. I. Ozone degradation method

Ozone has been used as a selective oxidizing agent for degrading polyethylene single crystals at room temperature in order to confirm and extend results on surface structure obtained by use of fuming nitric acid at temperatures above 60°C. The surfaces of the crystals were rendered highly accessible to the ozone gas by preparing the crystalline material in a highly expanded form; the solvent in which the crystals were suspended was removed by sublimation from the solid state. The extent and nature of the reaction were studied by measuring the increase in weight and in density, by direct chemical analyses, and by making use of infrared spectroscopy and gel-permeation chromatography. It was found that the surfaces of the crystals are attacked at room temperature by ozone, with resulting chain scission, and the broad features of the chemical reaction were established. Some folded chains are found to be as long as the original thickness of the crystal, and once folds have been cut, continuing reaction shortens the chains. In the early stages of the degradation, during which most of the weight increase takes place, the density of the crystals increases, and the magnitude of the increase is that expected from the increase in weight alone, i.e., assuming no increase in effective volume.