Fracture and Fiber Formation of Polyethylene Crystals

Abstract

Deformation experiments with substrate free polyethylene single crystals are employed to study the plastic deformation and the formation of fibers from polyethylene single crystals. The observations by transmission electron microscopy and electron diffraction indicate that substrate-free polyethylene crystals fail by brittle fracture at all temperatures between 77 K and 388 K and all strain rates between 10⁻³ and 10⁻¹ sec⁻¹. The formation of fibers and their molecular structure were studied by drawing fibers from substrate-free polyethylene single crystals. The results suggest that the formation of fibers occurs by a two-step process. The first step is the breaking off of single blocks of folded chains from the single crystals so that a “string of pearls” structure is obtained. If the temperature is sufficiently high, this process is followed by the thermally activated rearrangement of the molecules in the drawn fibers so that a “bamboo structure” results.

The effect of lattice defects present in the drawn polyethylene single crystals on the formation of fibers was studied by introducing crosslinks into the single crystals. The crosslinks were generated by irradiating the polyethylene crystals with 60 kV electrons prior to fiber drawing. It was observed that the introduction of crosslinks results in a network of interconnected fibers the “mesh-size” of which decreases with increasing crosslink density so that, finally (at a dosage of about 200 Mrad), drawing results in a thin continuous film (approximately 15 A thick).