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Computer-built polyethylene spherulites for mesoscopic Monte Carlo simulation of penetrant diffusion:  influence of crystal widening and thickening
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
2007 (English)In: Polymer, ISSN 0032-3861, E-ISSN 1873-2291, Vol. 48, no 8, 2453-2459 p.Article in journal (Refereed) Published
Abstract [en]

An algorithm able to mimic crystal lengthening, branching, widening and thickening was developed in order to build spherulites similar to those observed in polyethylene. The ranges of volume crystallinity and crystal width-to-thickness ratio attainable were <= 40% and 8-35, respectively. An on-lattice Monte Carlo-based algorithm was used to generate penetrant trajectories in the built spherulites. Diffusivity was assessed from the mean-square displacement of the penetrant molecules, normalized with respect to the mean-square displacement of the penetrant molecules in a crystal-free system, and compared with the geometrical impedance factor calculated from the Fricke theory using morphological data samples in the simulated spherulites. The crystal blocking effect was greater in the tangential plane than along the spherulite radius. All data, except that for the highest crystallinity system (40%), conformed to a linear relationship between the geometrical impedance factor obtained from the diffusivity data and the geometrical impedance factor calculated from morphological data; the latter being calculated according to the Fricke model using averages based on the squares of the crystal width-to-thickness ratio data. This finding suggests that wide crystals had a more pronounced effect on the geometrical impedance factor than was indicated by their number fraction weight. The system with the highest volume crystallinity (40%) showed a markedly higher geometrical impedance factor than predicted by the Fricke theory using the two aforementioned modifications.

Place, publisher, year, edition, pages
2007. Vol. 48, no 8, 2453-2459 p.
Keyword [en]
Diffusion, Simulation, Spherulite
National Category
Polymer Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-6451DOI: 10.1016/j.polymer.2007.02.041ISI: 000246330200032Scopus ID: 2-s2.0-33947593017OAI: oai:DiVA.org:kth-6451DiVA: diva2:11168
Note
QC 20100906. Uppdaterad från Submitted till Published (20100906)Available from: 2006-11-29 Created: 2006-11-29 Last updated: 2010-09-06Bibliographically approved
In thesis
1. Small Molecule Diffusion in Spherulitic Polyethylene: Experimental Results and Simulations
Open this publication in new window or tab >>Small Molecule Diffusion in Spherulitic Polyethylene: Experimental Results and Simulations
2006 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

The diffusion of small-molecule penetrants in polyethylene is hindered by impenetrable crystals and by the segmental constraints imposed by the crystals on the penetrable phase. Liquid and vapour n-hexane sorption/desorption measurements were performed on metallocene catalyzed homogenous poly(ethylene-co-octene)s. It was shown that the fractional free volume of the polymer penetrable component increased with increasing amount of penetrable polymer. It also increased with the relative proportion of liquid-like component in the penetrable polymer fraction. The detour effect was found to increase with decreasing crystallinity. The experimental study of the morphology of the polymers showed that the geometrical impedance factor followed the same trend with increasing crystallinity as the data obtained from n-hexane desorption. The changes in phase composition and character upon n-hexane sorption were monitored with Raman spectroscopy, WAXS and NMR spectroscopy. Partial dissolution of the orthorhombic and the interfacial component was observed upon nhexane sorption. Changes in the character of the components were furthermore analyzed: an increase of the density in the crystalline component and a decrease of the density in the amorphous component were observed in the n-hexane-sorbed-samples.

Molecular dynamics simulations were used for studying diffusion of n-hexane in fully amorphous poly(ethylene-co-octene)s. The branches in poly(ethylene-co-octene) decreased the density by affecting the packing of the chains in the rubbery state in accordance with experimental data. Diffusion of n-hexane at low penetrant concentration showed unexpectedly that the penetrant diffusivity decreased with increasing degree of branching.

Spherulitic growth was mimicked with an algorithm able to generate structures comparable to those observed in polyethylene. The diffusion in the simulated structure was assessed with Monte Carlo simulations of random walks and the geometrical impedance factor of the spherulitic structures was calculated and compared with analytical values according to Fricke’s theory. The linear relationship between geometrical impedance factor and crystallinity in Fricke’s theory was confirmed. Fricke’s theory, however, underestimated the crystal blocking effect. By modelling systems having a distribution of crystal width-to-thickness ratio it was proven that wide crystals had a more pronounced effect on the geometrical impedance factor than is indicated by their number fraction weight.

Place, publisher, year, edition, pages
Stockholm: KTH, 2006. 62 p.
Series
Trita-FPT-Report, ISSN 1652-2443 ; 2006:41
Keyword
diffusion, polyethylene, poly(ethylene-co-octene), free volume, morphology, intermediate phase, spherulite, Monte Carlo simulation
National Category
Polymer Chemistry
Identifiers
urn:nbn:se:kth:diva-4196 (URN)91-7178-504-3 (ISBN)
Public defence
2006-12-08, Sal E2, KTH, Llindstedtsvägen 3, Stockholm, 10:40
Opponent
Supervisors
Note
QC 20100909Available from: 2006-11-29 Created: 2006-11-29 Last updated: 2010-09-09Bibliographically approved

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