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Morphological interpretation of n-hexane diffusion in polyethylene
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.
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
2005 (English)In: Polymer, ISSN 0032-3861, E-ISSN 1873-2291, Vol. 46, 929-938 p.Article in journal (Refereed) Published
Abstract [en]

The diffusion of small-molecule penetrants in polyethylene is retarded by the detour caused by the crystals and by the segmental constraints imposed by the crystals on the penetrable phase. The earlier reported n-hexane diffusivity data for a series of homogeneous poly(ethylene-co-octene)s showed unexpectedly that the detour was greatest in the low crystallinity polymers. The crystal width-to-thickness ratio and the crystallinity were assessed by electron microscopy and differential scanning calorimetry and used in the Fricke model. The calculations showed that the geometrical impedance factor followed the same trend with increasing crystallinity as the data obtained from n-hexane desorption. The high geometrical impedance factor shown by the low crystallinity samples was due to the presence of crystals with an unusually high crystal width-to-thickness ratio. A unified relationship, including data for both linear and branched polyethylene was found between the fractional free volume and the phase composition of the penetrable phase including the liquid-like, interfacial liquid and the interfacial crystal core.

Place, publisher, year, edition, pages
2005. Vol. 46, 929-938 p.
Keyword [en]
polyethylene, diffusion, morphology
National Category
Polymer Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-6447DOI: 10.1016/j.polymer.2004.11.073ISI: 000226430600033Scopus ID: 2-s2.0-11844260039OAI: oai:DiVA.org:kth-6447DiVA: diva2:11164
Note
QC 20100909Available from: 2006-11-29 Created: 2006-11-29 Last updated: 2017-12-14Bibliographically 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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