Morphological and mechanical changes during drawing of polyethylene of low density (highly branched Lupolen of relatively high molecular weight and linear ACX of extremely high molecular weight) were studied by small angle X-ray scattering, observation of true stress-strain curves, crystallinity determination from density and calorimetric investigation of the debris after fuming nitric acid treatment. The low density of the samples is partly caused by branching and partly by high molecular weight. Both effects interfere with crystallization. The SAXS shows the formation of a two point pattern, which turns into a four point diagram at higher draw ratio. The long period decreases to a constant new long period independent of the thermal treatment of the original film. The crystallinity of annealed film drops during drawing to a value similar to that of the quenched material. All these facts are good evidence for microfibril formation as was already found in high density PE.
The true stress--strain curves of the two PE types investigated yield a single master curve by multiplying the stress by a proper constant factor which, we believe, measures the ratio of intercrystalline link density of the two samples. But the curves differ significantly from those of high density PE. We believe that the differences are caused by the large differences in crystallinity and in the number of intercrystalline links. In particular, the large number of links increasing with molecular weight could be the cause for the low extensibility of polyethylene samples of low density compared to that of high density PE.
The true stress-strain curves allow calculation of the work necessary for deformation of the volume element, This differential work density seems to be composed of two components: one, decreasing with higher draw ratio, is the work necessary for destruction of the original microspherulitic structure and formation of microfibrils; the other, increasing with draw ratio, contains the work for further deformation of the fibre structure by sliding motion of the microfibrils.