Investigation of the fold‐surface problem in polyethylene single crystals with the nitric acid oxidation technique

Abstract

AbstractSingle‐crystal preparations of polyethylene were treated with the selective oxidizing agent, fuming nitric acid. The degraded products were examined as regards layer thickness (by low‐angle x‐ray studies), chemical and weight changes, recrystallization and annealing treatments, and by broadline NMR, as part of a systematic investigation aimed at clarifying the nature of disordered material in single crystals. It emerges that there is a disordered‐mobile region along the fold surface of the crystals in agreement with other parallel works along similar lines. In a more detailed analysis we can now decompose the nitric acid attack into components affecting the basal and side surfaces, respectively. Taking into account the recrystallization–annealing observations, we infer that the fold surface is heterogeneous with folds of more than one kind. These results were combined with a preliminary molecular weight distribution study by gel permeation chromatography. Taking into account all the available evidence, we are led to suggest a composite structure where surface looseness, coresponding to long loops and hairs, is superimposed on the more regular folded surface. This model is in the process of being tested. The problems concerning the assignment of a value to the amount of surface looseness are being discussed. In addition, a discontinuity in the thermal behavior of the crystals between 75 and 80°C. has been detected.