Crystallization of polyethylene under molecular orientation

Abstract

In order to know the mechanism of crystallization under molecular orientation, a systematical study has been made on the isothermal crystallization of slightly crosslinked polyethylene films stretched to various extents in the molten state. Crystallization kinetics have been studied by measuring the changes in volume and tensile stress during crystallization, keeping the length of the film constant. The resultant texture has been examined by X-ray diffractions at wide and small angles and by electronmicroscopy. The stress relaxation in the molten state was remarkable and the results were not reproducible. However, reproducible results were obtained by conditioning the specimens mechanically in the molten state. It was found that in all the cases studied under various conditions the crystals formed were lamellar, the lamellae were developed perpendicularly to the stretched direction and piled up in parallel with the stretched direction, irrespectively of the draw ratio. The molecular tilt from the stretched direction, i. e., from the normal to the lamellar surface, changes continuously from ca. 70° (the so calleda-axis orientation) to 0° (thec-axis orientation) as the draw ratio is increased. The tendency to thec-axis orientation is facilitated by irradiating at higher doses, by extracting free branched chains from the whole polymer and by giving no mechanical conditioning prior to crystallization. The measurement of tensile stress reveals that the stress decreases in the initial stage of crystallization and turns to rise at a certain time depending on the degree of orientation prior to crystallization. The dilatometry shows that the initial stage of crystallization is remarkably facilitated by molecular orientation in the molten state but the later stage of crystallization is protracted by the molecular orientation. In the initial stage theAvrami's equation is applicable withn=1 and the later stage can be expressed by the same relation as that given for the process called “secondary crystallization” in the case of crystallization from the isotropic state. A mechanism underlying this sort of crystallization was proposed in the light of the results obtained on the crystallization kinetics and the resultant textures, which seems to afford a possible interpretation on the essential feature of the chain folding on crystallization of polymers.