Concept of secondary heterogeneous structure of long‐chain branched polyethylene

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Abstract
The mechanism of formation of surface roughness and extrusion swelling of the extrudate and the steady‐shear viscous flow behavior in the region of high shear rate for branched polymers were investigated using two low‐density polyethylenes and their sheared samples. These two polyethylenes varied in their degree of branching, molecular weight, and molecular weight distribution but were similar in their melt flow index. The effect of molecular parameters, especially long‐chain branching, on viscoelastic properties in the molten state was also considered. Samples of various degree of shearing level were prepared by passing them repeatedly through an extruder. Results of intrinsic viscometry, gel permeation chromatography, and infrared spectroscopy of the original and the sheared samples indicate that no appreciable variation between them takes place in the molecular parameters during the process of extrusion shearing. Both surface roughness and extrusion swelling of the extrudate diminish with increase in the extent of shear. The extrusion shearing affects the surface roughness and extrusion swelling of the extrudate as well as the capillary entrance effect more markedly for the highly branched polymers with considerably higher molecular weight than for the less branched species with bell‐type molecular weight distribution. These results demonstrate that heterogeneity becomes more conspicuous with the degree of long‐chain branching level, and therefore the role of long‐chain branching in the development of the heterogeneity is particularly important. It is suggested that the secondary heterogeneous structure arises through phase separation or from the heterogeneous formation of strongly entangled network at the branching point of the long‐chain branching in the manufacturing process of the low‐density polyethylene and that its presence causes the distinctive viscoelastic properties of long‐chain branched polymer melts.