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991.
Summary: Diffusion of n-hexane in poly(ethylene-co-1-hexene)s with 15–75 wt.% crystallinity was studied by desorption experiments analyzing data using the Fickian equations with a concentration dependent diffusivity. The effect of the impenetrable crystalline phase on the penetrant diffusivity (D) is described by D = Da/(τβ), where Da is the diffusivity of the amorphous polymer, τ is the geometrical impedance factor and β is a factor describing the constraining effect of the crystals on the non-crystalline phase. For a polymer with 75 wt.% crystallinity, τβ varied markedly with penetrant concentration (v1a) in the penetrable phase: 1000 (v1a = 0) and 10 (v1a = 0.15). This penetrant-uptake had no effect on the gross crystal morphology, i.e. β must be strongly dependent on v1a. Samples saturated in n-hexane exhibited a penetrant-induced loosening of the interfacial structure, as revealed by an increase in crystal density that require an increased mobility in the interfacial component and by a decrease in the intensity of the asymmetric X-ray scattering associated with the interfacial component. The geometrical impedance factor has been modelled by mimicking spherulite growth and τ was obtained as the ratio of the diffusivities of the fully amorphous and semicrystalline systems. The maximum τ obtained from these simulations is ca. ten, which suggests that β in the systems with v1a = 0.15 takes values close to unity. The simulations showed that the geometrical impedance factor is insensitive to the ratio of the crystal width and the crystal thickness. A free path length scaling parameter characteristic of the amorphous phase correlated with τ. 相似文献
992.
993.
Abbas Razavi Vincenzo Bellia Yves De Brauwer Kai Hortmann Liliane Peters Sabine Sirole Stephan Van Belle Ulf Thewalt 《Macromolecular Symposia》2004,213(1):157-172
The stereochemistry of propylene insertion/propagation reactions with a variety of Cs symmetric fluorenyl- containing single site catalysts is discussed. Our recent results indicate that independent of the chemical composition of the ancillary ligand fragments, or nature of the transition metal, active sites with local Cs symmetry and enantiotopic coordination positions behave syndioselectively in the general context of chain migratory insertion mechanism. Perfect bilateral symmetry neither exists nor is required in these processes. In this context the mechanism of syndiospecific polymerization is revisited by taking into account the structural characteristics and catalytic behavior of the original metallocene based (η5-C5H4-CMe2-η5-C13H8) MCl2/ MAO; M = Zr ( 1 ), Hf ( 2 ) catalyst systems and new syndiotactic specific systems including (η5-C5H4-CPh2-η5-3,6-di-tBut-C13H6)ZrCl2 ( 3 ), η1,η5-(μMe2Si)(3,6-di-tBut-Flu)(t-ButN)MCl2/ MAO; M =Ti ( 4 ), Zr ( 5 ) and η1,η5-(μMe2Si)(2,7-di-tBut-Flu)(t-ButN)MCl2/ MAO; M = Ti ( 6 ), Zr ( 7 ). 相似文献
994.
Ulf Diederichsen 《Nachrichten aus der Chemie》2002,50(2):191-192
995.
996.
ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option. 相似文献
997.