Thesis (PhD (Chemistry and Polymer Science))--University of Stellenbosch, 2011. / ENGLISH ABSTRACT: Positron annihilation lifetime spectroscopy (PALS) is well established as a novel method
currently available for the study of polymers at a molecular level because of its sensitivity
to the microstructural changes in the polymer matrix. The technique provides unique, but
limited, information of the solid state structure – primarily on the nature of the free
volume (or unoccupied space) in the polymer due to the less dense packing of polymer
chains relative to in other solid materials.
In the case of completely homogeneous polymer materials the measurement and
interpretation of the positron annihilation parameters is relatively simple. However, in the
case of polymers with more complex morphologies the situation becomes less clear. This
is due to the possibility of the formation, localization and subsequent annihilation of o-Ps
(ortho-positronium) within different areas of the complex morphology. This can result in
more than one o-Ps lifetime component being present, and each of the different
components corresponds to areas with differing types and amounts of 'open spaces'.
In this study a detailed and systematic approach was taken to study the positron
annihilation parameters in various semi-crystalline polymers and to correlate these to the
chain structure and morphology of the materials. The study focused specifically on
polyolefin polymers as these are the most widely used semi-crystalline materials, but
more importantly, they offer the possibility to produce a variety of morphological
complexity by simple manipulation of the chain structure – while there is essentially no
difference in the chemical composition of the materials. The copolymers were selected to
study the influence of short-chain branching (amount and length), short-chain branching
distribution and tacticity on the morphology, and subsequent positron annihilation
lifetime parameters.
Three separate topics were addressed. First, preparative temperature rising elution
fractionation was used to isolate polymer samples that are homogeneously crystallisable
and to produce a series of polymers with differing chain structure and resultant
morphologies. Second, additional series were produced by removing specific
crystallisable fractions from the bulk materials. Third, the temperature variation of the
samples as they approach and go through the crystalline melting point was studied. All the raw positron data were found to be best fitted with a four-component
positron annihilation lifetime analysis. The longest lifetime (which is attributed to
annihilation of o-Ps in the amorphous phase of the materials) showed systematic
variations with the degree and nature of the short-chain branding, tacticity variation, a
combination of both short-chain branching and tacticity, and changes in the amorphous
phase as a result of heating. The third lifetime component (which is attributed to o-Ps
annihilation in or around the crystalline areas of the materials) showed less variation
across the sample series. Typically, greater variations were observed in the propylene
copolymers than in the ethylene copolymers, which are reflective of the more complex
chain structure and corresponding morphology in the propylene copolymer series. Direct
evidence for a contribution from the nature of the amorphous phase to the bulk
microhardness of the sample was also found. / AFRIKAANSE OPSOMMING: Positronvernietigingsleeftydspektroskopie (PALS) is goed gevestig as ‘n nuwe metode
vir die studie van polimere op molekulêrevlak agv die sensitiwiteit van die metode vir
mikrostrukturele veranderings in die polimeermatriks. Hierdie tegniek verskaf unieke,
maar beperkte, inligting aangaande die vastetoestandstruktuur – veral aangaande die aard
van die vryevolume (of onbesette spasie) in die polimeer as gevolg van die minder digte
verpakking van polimeerkettings relatief tot in ander vastestowwe.
In die geval van volledig homogene polimeriese materiale is die meet en
interpretasie van die positronvernietigingsparameters relatief eenvoudig. Maar in die
geval van polimere met meer komplekse morfologieë is die situasie minder duidelik. Die
rede hiervoor is die moontlikheid vir die formasie, lokalisering en gevolglike vernietiging
van o-Ps (orto-positronium) in die verskillende areas van die komplekse morfologie. Dit
kan tot gevolg hê dat meer as een o-Ps komponent teenwoordig is en waar elk van die
verskillende komponente ooreenstem met areas met verskillende tipes en hoeveelhede
'oop spasies'.
In hierdie studie is ‘n sistematiese, in-diepte benadering gebruik om die
positronvernietigingsparameters in verskeie semikristallyne polimere te bestudeer en
hulle te korreleer met dié van die kettingstruktuur en die morfologieë van die materiale.
Hierdie studie het spesifiek gefokus op poliolefiene aangesien hulle die mees algemene
semikristallyne materiale is wat gebruik word en, nog meer belangrik, hulle bied die
geleentheid om verskeie komplekse morfologieë te lewer dmv eenvoudige manipulasie
van die kettingstrukture – terwyl daar basies geen verandering in die
chemiesesamestelling van die materiale is nie. Die kopolimere is gekies om die invloed
van kort-ketting vertakking (lengte en hoeveelheid), kort-ketting vertakking verspreiding
en taktisiteit op die morfologie en vervolgens die positronvernietigingsleeftyd parameters
te bestudeer.
Drie onderwerpe is aangespreek. Eerstens, preparatiewe
temperatuurstygingelueringsfraksionering (prep-TREF) is gebruik om polimeermonsters
wat homogeenkristalliseerbaar is te isoleer om sodoende 'n reeks polimere met
verskillende kettingstrukture, en gevolglike morfologieë, te lewer. Tweedens, 'n addisionele reekse monsters is berei deur die verwydering van spesifieke kristalliseerbare
fraksies vanaf die grootmaatmonsters. Derdens, die temperatuurverandering van die
monsters wanneer die monsters naby aan die kristallyne smeltpunt is en wanneer hulle
deur die kristallyne smeltpunt gaan is bestudeer.
Daar is bevind dat alle rou positrondata ten beste gepas het in 'n vier-komponent
positronvernietigingsleeftydanalise. Die langste leeftyd (wat toegeskryf is aan
vernietiging van o-Ps in die amorfe fase van die materiaal) het sistematiese variasies
getoon met die volgende: hoeveelheid en aard van die kort-kettingvertaking, verandering
in taktisiteit, 'n kombinasie van beide kort-kettingvertakking en taktisiteit en veranderings
in die amorfiesefase as gevolg van verhitting. Die derde leeftyd komponent (wat
toegeskryf is aan die o-Ps vernietiging in of rondom die kristallyne areas van die
materiale) het minder variasie in hierdie reeks monsters getoon. Daar is tipies meer
variasie waargeneem in die propileenkopolimere as in die etileenkopolimere, wat ’n
weerspieëling is van die meer komplekse kettingstruktuur en ooreenstemmende
morfologie in die propileenkopolomeerreeks. Direkte bewys vir 'n bydrag van die aard
van die amorfe fase tot die grootmaat mikrohardheid van monsters is ook bevind.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/6597 |
| Date | 03 1900 |
| Creators | Sweed, Muhamed |
| Contributors | Mallon, Peter, University of Stellenbosch. Faculty of Science. Dept. of Chemistry and Polymer Science. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Rights | University of Stellenbosch |
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