Morphology of high-strength transparent polyethylene prepared under controlled conditions

Abstract

Highly oriented transparent polyethylene exhibiting a sixfold improvement in tensile yield strength has been prepared under ordinary pressures by shearing molten polyethylene between two concentric cylinders at rates between 0.4 and 10.0 sec−1 and at temperatures between 129 and 135°C. Our results, based on observed changes in volume and shear stress, show that a transparent polyethylene sample was obtained only when crystallization occurred during the shearing process and further that transparency was greater for samples containing greater fractions of strain‐crystallized polyethylene. The major factor controlling the onset of crystallization was found to be the amount of total strain which increased with increasing crystallization temperature. To understand its transparency, electron microscopy and x‐ray studies were carried out on polyethylene of various degrees of transparency. Results showed that transparent polyethylene contains perpendicularly well‐oriented lamellae (380–400‐Å spacing) with limited lateral dimensions (400–2000 Å); whereas polyethylene of increasing opaqueness contains lamellae (285‐Å spacing) oriented more or less perpendicular to the flow direction, but with much larger lateral dimensions (1000–10 000 Å).