Fold structure of solution grown crystals of alkyl‐branched copolymers of polyethylene

Abstract

AbstractAccording to the hypothesis that uncrystallizable units are excluded from the crystalline lattice into the surface of the crystals and concentrated in the folds, morphologies and textures of solution grown crystals of branched polyethylenes have been studied as functions of content and length of the branches by means of various techniques.Methyl, ethyl, and propyl branches were introduced into polyethylene chains by polymerization of mixtures of diazomethane and the corresponding higher diazoalkane. The crystals of these branched polymers grown from dilute solutions were treated with a selective oxidizing agent, fuming nitric acid. The degraded products were examined as regards the changes during the oxidation in molecular weight, weight‐loss, layer thickness (by low angle X‐ray studies), branch content (by IR analysis), unit cell dimensions, density and X‐ray crystallinities and morphology (by electron microscopy), with the aim of clarifying the nature of disordered material in polymer single crystals. It emerges that the thickness of the surface layer increases with the content and length of the branches and that the folds may be considered as irregular loose loops for crystals of polymers with a branch content beyond ca. one per 100 carbon atoms. The results also revealed less perfect order of the crystal lattice for polymers with higher degree of branching. Thus, the branches seemed to be concentrated in the loose loops in the surface layer and within the crystal defects, both of which are accessible to oxidation. The concentration of the branches within the folds was discussed in terms of the content and the length of the branches. It was also found that the thicker the surface layer, the poorer the morphology of the crystals.