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The aim of this study was to determine the linear viscoelastic properties of a series of ethylene/vinyl acetate copolymer/metallocene-catalysed polyethylene (mPEs) blends. Newtonian viscosity showed a pronounced positive deviation from the double reptation model, which assumes miscibility or, at least, cooperative relaxation between the mixed species. Enhanced values of steady-state compliance and...
This report describes complete density functional theory studies performed to elucidate ethylene polymerisation mechanisms of the Brookhart-type catalysts N,N'-(2,6-dimethylphenyl)ethylenediimine nickel (II) and N-(2,6-dimethylphenyl)pyridine-2-carboxaldiimine nickel (II). Both catalysts showed conformations that blocked the active site and thus hindered ethylene coordination. These conformations...
This report describes a computational DFT study, based on the hybrid density functional B3LYP method, designed to investigate the hydrogenolysis process that occurs in ethylene polymerisation catalysed by a nickel-bulk diimine catalyst (N,N'-bis(2,6-diisopropylphenyl)-2,3-butanediimine nickel(II) methyl cation [{(2,6-( i Pr) 2 Ph)N C(Me)-C(Me)=N(2,6-( i Pr) 2 Ph)}NiCH...
The rheological properties of metallocene-catalysed linear polyethylenes and those of blends prepared with ethylene-vinyl acetate copolymers were evaluated. The pure polyethylenes showed characteristic features of linear polymers in the melt state, but poor processability, as would be expected for materials of narrow molecular weight distribution. The characteristic sharkskin and slip-stick regimes...
This study was designed to determine the linear viscoelastic properties of a family of metallocene-catalysed propylene homopolymers in the 'plateau' and terminal regions. A low value of the plateau modulus, around 4.8x10 5 Pa, was obtained. Our results differ from those reported for conventional amorphous polypropylene in several recent studies. As previously undertaken for metallocene-catalysed...
A combined QM/MM study of the ethylene/1-hexene copolymerisation with bisiminepyridine iron(II) is presented. It has been found experimentally that these catalysts do not copolymerise ethylene with 1-hexene. Based on the mechanism of propagation and termination processes proposed by Deng et al., we have performed calculations for the ethylene and 1-hexene monomers in order to give a suitable explanation...
In this article, the linear viscoelastic properties of a family of metallocene-catalysed ethylene homopolymers and ethylene/1-hexene copolymers near the 'plateau' and in the terminal regions have been measured. The zero shear viscosity and relaxation time both scale to 3.41 power of the molecular weight, as is observed for most of the linear polymers. At the same time, no rheological signs associated...
Low-density polyethylene (LDPE) has been the subject of several rheological studies trying to correlate rheological features with molecular parameters. As the ethylene vinyl acetate copolymer (EVA-c) and LDPE are obtained through the same polymerization process, it has been long assumed that both materials would exhibit a similar viscoelastic response. However, a recent work has put forward the question...
DFT calculations have been carried out on the cationic species for the two different Brookhart's catalyst systems: [{ArN CH-HC NAr}NiR''] + (3a) and [{ArN CMe-MeC NAr}NiR''] + (3b) (where Ar={2,6-C 6 H 3 (Me) 2 } and R''=Me). These calculations reveal that the conformation of aryl groups attached to nitrogen atoms could provide a suitable explanation for...
Alkylamidinate complexes have been recently reported to be useful catalysts for olefin polymerisation as an alternative to metallocene systems. The present work reports theoretical calculations performed at DFT level for ethylene insertion reactions in zirconium and titanium alkylamidinate compounds with variable structural complexity. The energy barriers obtained for these reactions show that these...
Hydrogenolysis is a commonly used method to control the molecular weight or size of polymers obtained by Ziegler–Natta and related metallocene catalysts. However, the precise mechanism governing these controlling processes is still unknown. It is most accepted that the insertion of hydrogen molecule into the metal–alkyl bond of the catalyst active species competes favorably against the ethylene insertion...
A variety of techniques is used to describe the fine structure of a series of well defined linear polyethylenes, polymerized using the new generation of metallocene-based catalysts. The sample's molecular weight (M w ) is restricted to the usual values of processability of this material ranging from 14K to 460K. It will be clearly shown that the overall enthalpy of the rapidly crystallized...
A combined WAXD and d.s.c. study on blends of a series of commercial high density and low density (LD) polyethylene (PE) samples rapidly crystallized from the melt is presented. The melting curves of the materials are extensively analysed and compared to those exhibited by the individual components. The results, besides being indicative of a full compatibility of the two components, allow one to...
A study of the melting behaviour of polyethylene single crystals in a paraffin medium is presented. The diluent effect of the paraffin upon the polyethylene crystals is monitored by changes in the melting curves. The experimental depression values of the melting temperatures fit very well with those predicted by the combinatorial term of the Flory-Huggins equation. Kinetic effects associated with...
Melting point depression techniques are used to test the compatibility of poly(vinylidene fluoride) (PVDF) and poly(methyl methacrylate) (PMMA). The novelty of the experimental approach lies in the morphology of the melting lamellae, which is kept constant by using identical single crystals in various mixtures with PMMA. The values obtained for the Flory-Huggins interaction parameter are critically...
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