AbstractAbstract
[en] Morphology, dielectric breakdown and mechanical properties have been studied in a number of model polyolefin systems. The TEM investigation of the morphology of a propylenelethylene copolymer containing a clarifying additive showed that the addition of the clarifier generally results in enhanced nucleation and the consequent formation of a more uniform structure. However, the morphology of the copolymer is shown to change abruptly from a fine to a coarse structure when the crystallisation temperature exceeds 128 deg. C. This is associated with a marked reduction in nucleation efficiency of the additive, which permits the growth of distinct spatially separated morphological features. These morphological features have further been clarified by investigation of the morphology of the pure copolymer, which was nucleated using self-seeding technique. A number of characteristic forms of lamellar aggregates present either in the clarified or pure copolymer are evidenced as a result of different directions in which they are viewed. Two distinct types of lamellae are found: lath-like crystals and cross-hatching lamellae. The former results from primary crystallisation, whereas the later is associated with secondary crystallisation. The different populations of lamellae correspond to multiple melting peaks observed in DSC traces. The electric strength of the clarified propylenelethylene copolymer exhibits a clear dependence on sample morphology. The parallel trend is also found in the mechanical ultimate failure stress and strain of the material, as recorded at the relatively high strain rate of 100 mm/min. The electrical tree growth in this material is significantly dependent on morphology. At the same electrical treeing condition, a branch-type tree forms in the quenched sample with a uniform structure, whereas a bush-type tree develops in the 134 deg. C crystallised sample with a coarse morphology. A CCD camera shows that the branch-type tree is electrically conducting, whereas the bush-type tree is non-conducting. The tree growth rate as a function of applied voltage in the two samples also exhibits different forms. The morphology of a linear/branched polyethylene blend has been investigated by SEM. The morphology of this system can be controlled appropriately by varying the thermal history. Open banded spherulites are formed at lower crystallisation temperatures, which impinge upon one another, whereas higher temperature crystallisation results in sheaf-like lamellar aggregates, which are distinct and separated by a low crystallinity matrix. The mechanical properties of this system are also examined, and are found to vary with the morphology of the material. Tree channels in the polyethylene blend are very clearly revealed. The location of the channels is significantly dependent on the microstructure of the material. Most of channels are distributed in low crystallinity regions, particularly, at the boundaries of spherulites. The structure of channels themselves is also revealed. Two types of channel are found; one, with a shell, appears electrically conducting, whereas, the other, with no shell, appears non-conducting. The formation of the different types of channel depends on both the treeing conditions and the morphology of the material. (author)
Source
Mar 2000; [vp.]; Available from British Library Document Supply Centre- DSC:DXN035512; Thesis (Ph.D.)
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Miscellaneous
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Thesis/Dissertation
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Descriptors (DEC)
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