Glycerol contact angle measurements on exterior and interior surfaces of polyethylene rods: Difference between surface and bulk morphology

Abstract

AbstractA difference between surface and bulk morphologies of extruded low density polyethylene (LDPE) has been observed through measurement of the contact angle of glycerol on both exterior and interior surfaces of the extrudate. The sessile drop contact angle on the exterior surface of LDPE rods prepared in a Brabender plasticorder increasea with increasing screw speed (40–180 rpm) and correspondingly increased die pressure at each extrusion temperature (110–130°C). On the other hand, the sessile drop contact angle on the interior surface of LDPE rod, exposed by grinding, decreased with increased screw speed. Qualitative trends were identical with exterior contact angle measured by the Wilhelmy plate method or with the interior surface exposed by fracture. Contact angle variations were interpreted in term of density or crystallinity to suggest that the bulk or interior crystallinity increased with increasing die pressure (even at constant screw speed), while the crystallinity of the exterior decreased with increasing screw speed (even at nearly constant die pressure). The difference between surface and bulk morphologies was consistent with that deduced from DICUP absorption kinetics and optical microscopy [J. Appl. Polym. Sci., 29, 2383 (1984)], although differences were noted in the effect of screw speed on surface morphology.