TEXTURE OF INJECTION-MOLDED POLY(ETHYLENE-2,6-NAPHTHALENE DICARBOXYLATE) PARTS

Evolution of structural gradients developed in injection moulded polye thylene naphthalate was studied using a variety of structural techniqu es including a newly developed micro-beam X-ray diffraction pole figur e technique. PEN, being a slowly crystallizing high temperature polyme r, forms a three layer structure: amorphous skin + shear crystallized intermediate layer + amorphous core in the interior at low mould tempe ratures. The thickness of the crystallized sublayers formed under the action of flow decrease with the increase of mould temperature as a re sult of reduction of stress history experienced by the Bowing polymer chains. Their thickness increases in the narrow passages of the cavity as a result of increased fraction of the extensional component of the flow field which locally causes increases in crystallization rates. T he crystalline structure of these layers were found to contain both al pha and beta phases. The hot stage-WAXS experiments indicate that the beta phase possess a melting temperature approximately 30 degrees C hi gher than the alpha form. In the shear crystallized regions, the polym er chains are oriented along the flow direction and the naphthalene pl anes that are flexibly linked together along the polymer chain tend to be oriented parallel to the broad surfaces of the part. The preferent ial alignment of the naphthalene planes results in highly layered macr ostructure in the shear crystallized zones with weak interlayer forces . As a result, these regions readily delaminate into plate-like struct ures observed in the SEM fracture surfaces. (C) 1997 Elsevier Science Ltd.