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Small Molecule Diffusion in Spherulitic Polyethylene: Experimental Results and Simulations
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
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 [en]
diffusion, polyethylene, poly(ethylene-co-octene), free volume, morphology, intermediate phase, spherulite, Monte Carlo simulation
National Category
Polymer Chemistry
Identifiers
URN: urn:nbn:se:kth:diva-4196ISBN: 91-7178-504-3 (print)OAI: oai:DiVA.org:kth-4196DiVA: diva2:11169
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
List of papers
1. Polyethylene morphology
Open this publication in new window or tab >>Polyethylene morphology
2004 (English)In: Advances in polymer science, ISSN 0065-3195, Vol. 169, 29-73 p.Article in journal (Refereed) Published
Abstract [en]

The morphology of polyethylene has been an important theme in polymer science for more than 50 years. This review provides an historical background and presents the important findings on five specialised topics: the crystal thickness, the nature of the fold surface, the lateral habit of the crystals, how the spherulite develops from the crystal lamellae, and multi-component crystallisation and segregation of low molar mass and branched species.

Keyword
polyethylene, morphology, review
National Category
Polymer Chemistry
Identifiers
urn:nbn:se:kth:diva-6446 (URN)10.1007/b94176 (DOI)000223226800003 ()2-s2.0-2942748165 (Scopus ID)
Note
QC 20100909Available from: 2006-11-29 Created: 2006-11-29 Last updated: 2010-09-09Bibliographically approved
2. Morphological interpretation of n-hexane diffusion in polyethylene
Open this publication in new window or tab >>Morphological interpretation of n-hexane diffusion in polyethylene
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.

Keyword
polyethylene, diffusion, morphology
National Category
Polymer Chemistry
Identifiers
urn:nbn:se:kth:diva-6447 (URN)10.1016/j.polymer.2004.11.073 (DOI)000226430600033 ()2-s2.0-11844260039 (Scopus ID)
Note
QC 20100909Available from: 2006-11-29 Created: 2006-11-29 Last updated: 2017-12-14Bibliographically approved
3. n-Hexane sorption in poly(ethylene-co-octene)s: effect on phase composition and character
Open this publication in new window or tab >>n-Hexane sorption in poly(ethylene-co-octene)s: effect on phase composition and character
2010 (English)In: European Polymer Journal, ISSN 0014-3057, E-ISSN 1873-1945, Vol. 46, no 3, 381-388 p.Article in journal (Refereed) Published
Abstract [en]

Diffusion of small-molecule penetrants in semi-crystalline polymers is retarded by two factors: penetrant detour bypassing impenetrable crystals and the constraining effect of the crystals on the amorphous component. Previous experiments have shown that the latter factor becomes much less important at higher penetrant concentration in the polymer. Structural changes in a series of poly(ethylene-co-1-octene)s occurring on saturation in n-hexane at 296 K, covering a wide range of crystallinity (17–75 wt.%), were studied by wide-angle X-ray scattering, Raman spectroscopy and NMR spectroscopy. Densification of the crystal unit cell and partial dissolution of the interfacial component on n-hexane sorption are the main experimental findings. The conclusion is that the penetrant molecules increase the mobility of the polymer chain segments adjacent to the crystal interface, enabling better packing of the crystal stems and importantly also causes a reduction in the constraining factor (β) for diffusion.

Keyword
Poly(ethylene-co-1-octene), n-Hexane sorption, Crystal interface, Penetrant-induced structure change, X-ray scattering
National Category
Polymer Chemistry
Identifiers
urn:nbn:se:kth:diva-6448 (URN)10.1016/j.eurpolymj.2009.11.011 (DOI)000276284200001 ()2-s2.0-76749159273 (Scopus ID)
Funder
Swedish Research Council, 621-2001-1621
Note

QC 20100909

Available from: 2006-11-29 Created: 2006-11-29 Last updated: 2017-12-14Bibliographically approved
4. Diffusivity of n-hexane in poly(ethylene-co-octene)s assessed by molecular dynamics simulations
Open this publication in new window or tab >>Diffusivity of n-hexane in poly(ethylene-co-octene)s assessed by molecular dynamics simulations
2007 (English)In: Polymer, ISSN 0032-3861, Vol. 48, no 17, 5174-5180 p.Article in journal (Refereed) Published
Abstract [en]

Molecular dynamics simulations have been used to study diffusion of n-hexane in wholly amorphous poly(ethylene-stat-octene)s with comonomer contents ranging from 0 to 11.5 mol%. The branches in the polymer increased the specific volume by affecting the packing of the chains in the rubbery state in accordance with experimental data. The diffusion of n-hexane at penetrant concentrations between 0.5 and 9.1 wt% was simulated within time-scales between 0.1 and 0.2 μs. The penetrant diffusivity unexpectedly decreased with increasing comonomer content. The penetrant molecule motion statistics showed that systems with high comonomer content showed a greater tendency for short distance motion (over a sampling period of 3 ps) whereas the systems with lower comonomer content showed penetrant motion over longer distances. It seems that the branches retarded local chain mobility of the polymer thereby trapping the penetrant molecules. All systems studied showed a minimum in penetrant diffusivity at ca. 1 wt% n-hexane and a marked increase in diffusivity at higher penetrant concentrations. The volumetric data for the different polymer–penetrant systems were consonant with additional volumes of the different components. Comparison between simulated diffusivities (for a wholly amorphous polymer) and experimentally obtained diffusivity data for semicrystalline polymers showed that constraining effect of the crystals were substantial for the highly crystalline systems and that it gradually decreased with decreasing crystallinity.

Keyword
Poly(ethylene-stat-octene), n-Hexane, Molecular dynamics simulation
National Category
Polymer Chemistry
Identifiers
urn:nbn:se:kth:diva-6449 (URN)10.1016/j.polymer.2007.06.051 (DOI)000249085000031 ()2-s2.0-34547405295 (Scopus ID)
Note
QC 20100909Available from: 2006-11-29 Created: 2006-11-29 Last updated: 2010-09-09Bibliographically approved
5. Mesoscale modelling of penetrant diffusion in computer-generated polyethylene-spherulite-like structures
Open this publication in new window or tab >>Mesoscale modelling of penetrant diffusion in computer-generated polyethylene-spherulite-like structures
2006 (English)In: Polymer, ISSN 0032-3861, Vol. 47, 5588-5595 p.Article in journal (Refereed) Published
Abstract [en]

Molecular dynamics simulations have been used to study diffusion of n-hexane in wholly amorphous poly(ethylene-stat-octene)s with comonomer contents ranging from 0 to 11.5 mol%. The branches in the polymer increased the specific volume by affecting the packing of the chains in the rubbery state in accordance with experimental data. The diffusion of n-hexane at penetrant concentrations between 0.5 and 9.1 wt% was simulated within time-scales between 0.1 and 0.2 μs. The penetrant diffusivity unexpectedly decreased with increasing comonomer content. The penetrant molecule motion statistics showed that systems with high comonomer content showed a greater tendency for short distance motion (over a sampling period of 3 ps) whereas the systems with lower comonomer content showed penetrant motion over longer distances. It seems that the branches retarded local chain mobility of the polymer thereby trapping the penetrant molecules. All systems studied showed a minimum in penetrant diffusivity at ca. 1 wt% n-hexane and a marked increase in diffusivity at higher penetrant concentrations. The volumetric data for the different polymer–penetrant systems were consonant with additional volumes of the different components. Comparison between simulated diffusivities (for a wholly amorphous polymer) and experimentally obtained diffusivity data for semicrystalline polymers showed that constraining effect of the crystals were substantial for the highly crystalline systems and that it gradually decreased with decreasing crystallinity.

Keyword
Poly(ethylene-stat-octene), n-Hexane, Molecular dynamics simulation
National Category
Polymer Chemistry
Identifiers
urn:nbn:se:kth:diva-6450 (URN)10.1016/j.polymer.2005.05.162 (DOI)000239644000044 ()2-s2.0-33745898521 (Scopus ID)
Note
QC 20100909Available from: 2006-11-29 Created: 2006-11-29 Last updated: 2010-09-09Bibliographically approved
6. Computer-built polyethylene spherulites for mesoscopic Monte Carlo simulation of penetrant diffusion:  influence of crystal widening and thickening
Open this publication in new window or tab >>Computer-built polyethylene spherulites for mesoscopic Monte Carlo simulation of penetrant diffusion:  influence of crystal widening and thickening
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.

Keyword
Diffusion, Simulation, Spherulite
National Category
Polymer Chemistry
Identifiers
urn:nbn:se:kth:diva-6451 (URN)10.1016/j.polymer.2007.02.041 (DOI)000246330200032 ()2-s2.0-33947593017 (Scopus ID)
Note
QC 20100906. Uppdaterad från Submitted till Published (20100906)Available from: 2006-11-29 Created: 2006-11-29 Last updated: 2017-12-14Bibliographically approved

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  • ieee
  • modern-language-association-8th-edition
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