Dislocations in Polyethylene Single Crystals

Abstract

Dislocations have been observed and characterized in polyethylene single crystals by means of moiré patterns produced by superimposed lamellas. Analysis of the dislocations by dark and bright field electron microscopy has revealed that they are partial (imperfect) dislocations. The partials were shown to involve terminating {110} planes and to have Burgers vectors of 〈½a, ½b, γc〉. Most of the dislocations involved {110} planes containing chain folds. The occurrence of the dislocations as isolated partials rather than paired ones implies a low stacking fault energy for the polyethylene lattice. This conclusion is not surprising when one considers that the ``faults'' consist mostly of chains which simply have been rotated from their normal positions. Annealing of single crystals increased approximately one-thousand fold the dislocation density, and in addition produced, in dark field, bands parallel to the long diagonals of the crystals. These bands were found predominantly at the crystal edges. Selected area electron diffraction of the annealed crystals revealed that a monoclinic crystal phase, previously observed in stressed polyethylene, was concentrated near the crystal edges. The fact that the reported chain packing in the monoclinic phase is similar to that of a possible stacking fault in the orthorhombic lattice implies a definite relationship between the formation of partial dislocations and the occurrence of the monoclinic phase. These results suggest that dislocations play an important part in the annealing of polyethylene single crystals and perhaps of other polymers as well.