SynopsisThe influence of shearing history on the viscoelastic properties of low-density polyethylene (LDPE) is investigated. Swelling of extrudates from a melt indexer is measured for monitoring the variation in viscoelasticity. Continuous shearing of molten LDPE reduces the swelling ratio. The reduction is not due to molecular degradation, as evidenced by constancy of intrinsic viscosity. The rate of the reduction in the swelling ratio depends on the shearing conditions and characteristics of LDPE, but the swelling ratios finally attain a steady value. The swelling ratio reduced by the continuous shearing is completely recovered by solvent or heat treatment. The ratio of the completely reduced swelling ratio to the completely recovered one is defined as a new index representing the viscoelastic variation, the processing index (PI), and the relationship between PI and the primary molecular parameters of LDPE is investigated. It is concluded that the variation in the viscoelastic properties becomes more remarkable with increase in the weight-average molecular weight. The cause of the viscoelastic variation is also discussed from the rheological and thermodynamic points of view.
SynopsisLow-density polyethylene (LDPE) melts show anomalous rheological behavior; their viscoelastic properties vary with their shearing histories although their molecular structural parameters do not change. Capillary flow and die swell behavior were dependent not only on the experimental conditions such as temperature or shear stress but also on the processing index (PI), which was introduced in a preceding article in order to quantify the anomalous rheological behavior of LDPE melts. In addition, it was found that the flow activation energy a t constant shear stress also varied with the shearing histories. The experimental findings are discussed in terms of the rheological flow units of LDPE melts.
SynopsisAn investigation was performed to determine how optical properties of LDPE blown films changed when the material was subjected to extrusion shearing. In this study, shearing histories were given to the materials by designed extrusion shearing. Recognizable variations take place in haze and gloss of the blown film during the extrusion shearing. Such variations were expressed as a function of the processing index (PI), which was introduced in a preceding paper as a measure of the memory effect of shearing histories of LDPE. This means that the variations originate in a certain change in the cohesive state of the polymer molecules attributable to the shearing.
SynopsisViscoelastic properties of branched polymers vary with their shearing history; notwithstanding, their primary molecular parameters do not change. According to a recent study, such viscoelastic variation was believed to be observed only with crystalline long-chain branched polymers such as low-density polyethylene or polyacetal. That is, the origin of the viscoelastic variation was attributed to the presence of specific entanglements a t the branching points, which was formed during the crystallization process. However, the viscoelastic variation of the long-chain branched polymers is the phenomenon a t temperatures well above their melting points, namely, it is considered that whether the long-chain branched polymers are crystalline or not is not essential for the occurrence of the viscoelastic variation. Thus, the influence of the shearing history on the rheological properties of an amorphous long-chain branched polymer was investigated in this paper, and it was found that, irrespective of their crystallinity, the viscoelastic properties of long-chain branched polymers vary according fn their shearing history.
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