Thesis (PhD)--Stellenbosch University, 2011. / ENGLISH ABSTRACT: Novel silicone nanocomposites were prepared using poly(acrylonitrile) (PAN) based reinforcing
fibres as well as multi-walled carbon nanotubes (MWCNTs). Compatibility of the fibre fillers with
the silicone matrix required the synthesis of novel amphiphilic, organic–inorganic graft
copolymers of PAN and poly(dimethylsiloxane) (PAN-g-PDMS). These fibre precursor materials
were synthesised via the “grafting through” technique using conventional free radical
copolymerisation. The PDMS macromonomer content in the feed was varied from 5 wt% to 25
wt% and the molecular weights of the macromonomer were 1000 g.mol-1 and 5000 g.mol-1. The
solvent medium of the precipitation reaction was optimised at a volume ratio of 98% benzene to
2% dimethylformamide (DMF). Successful incorporation of PDMS yielded graft copolymer blend
materials of PAN-g-PDMS, blended with PAN homopolymer and unreacted PDMS
macromonomer. A gradient elution profile was developed to track the successful removal of the
PDMS macromonomer via hexane extraction. The gradient profile showed that as the PDMS
content in the feed increased, the number of graft molecules in the blend increased relative to
the number of PAN homopolymer molecules. The crystallisability of the PAN segments was
shown to decrease as the PDMS content increased. The synthesised polymer was used as
precursor material for the electrospinning of fibre fillers. The electrospinning of the precursor
material was successfully achieved using 100% DMF as electrospinning solution medium. The
amphiphilic nature of the precursor material in DMF resulted in self-assembled aggregate
structures in the electrospinning solution. An increasing PDMS content was shown to affect the
aggregation of the precursor material, and resulted in an increase in the solution viscosity. The
“gel-like” solutions limited the achievable fibre morphological control when altering conventional
electrospinning parameters such as voltage, tip-to-collector distance, and solution
concentrations. The rapid evaporation and stretching of the solution during electrospinning,
combined with the phase segregated amphiphilic molecules in solution and the crystallisation of
the PAN segments resulted in (non-equilibrium morphology) fully porous fibres. The crystallinity
was shown to decrease after electrospinning of the fibre precursor materials. Successful
incorporation of surface oxidised MWCNTs into the electrospun fibres was achieved. The
content of nanotubes was varied from 2 wt% to 32 wt%. The MWCNTs reduced the mean fibre
diameters by acting as cross-linkers between the PAN segments and increasing the solution
conductivity. The nanotubes dispersed well throughout the porous structure of the fibres and
aligned in the direction of the fibre axis. Fabrication of silicone composites containing nonwoven
and aligned fibre mats (with 8 wt% MWCNTs in the fibres, and without) was successfully achieved. The compatibilisation of the PDMS surface segregated domains allowed excellent
dispersion and interaction of the PAN based fibre fillers with the silicone matrix. Mechanical
analysis showed improved properties as the PDMS content in the fibre increased. The highest
PDMS content fibres did, however, exhibit decreased properties. This was ascribed to increased
PDMS (soft and weak) content, decreased crystallinity and increased fibre diameter (lower
interfacial area). Dramatic improvements in strength, stiffness, strain and toughness were
achieved. The most significant result was an increase in strain of 470%. The mechanical results
correlated with results of SEM analysis of the fracture surfaces. The dramatic improvements in
properties were a result of the fibre strength and ductility, as well as the mechanism of
composite failure. / AFRIKAANSE OPSOMMING: Nuwe silikonnanosamestellings is berei deur gebruik te maak van poli(akrilonitriel) (PAN)
gebaseerde versterkende vesels wat multi-ommuurde koolstof nanobuisies bevat het.
Versoenbaarheid van die vesels met die silikonmatriks het die sintese van nuwe amfifiliese,
organies–anorganiese ent-kopolimere van PAN en poli(dimetielsiloksaan) (PAN-g-PDMS)
benodig. Die vesel voorlopermateriaal is deur middel van ‘n “ent-deur” vryeradikaalkopolimerisasie
gesintetiseer. Die inhoud van die PDMS makromonomeer in die reaksie het
gewissel vanaf 5% tot 25%. Die gebruik van twee verskillende molekulêre massas
makromonomere is bestudeer (1000 en 5000 g.mol-1). Die optimale oplosmiddelmengsel vir die
neerslagreaksie was 'n volume verhouding van 98% benseen tot 2% dimetielformamied (DMF).
Suksesvolle insluiting van PDMS het versnitmateriale van PAN-g-PDMS kopolimere gemeng
met PAN homopolimere en ongereageerde PDMS makromonomere gelewer. 'n Gradiënteluering-
chromatografiese profiel is ontwikkel om die suksesvolle verwydering van die PDMS
makromonomere via heksaanekstraksie te bepaal. Die gradiëntprofiel het aangetoon dat indien
die PDMS inhoud in die reagense verhoog is, die aantal entmolekules relatief tot PAN
homopolimeermolekules ook verhoog het. 'n Toename in PDMS inhoud het egter 'n afname in
kristallisasie van die PAN segmente tot gevolg gehad. Die gesintetiseerde polimeer is gebruik
as die beginmateriaal vir die elektrospin van veselvullers. Die elektrospin van die beginmateriaal
was suksesvol wanneer 100% DMF as elektrospinoplosmiddel gebruik is. Die amfifiliese aard
van die beginmateriaal in DMF lei tot outokonstruksie van aggregaatstrukture in die
elektrospinoplossing. Toenemende PDMS inhoud beïnvloed die outokonstruksie van die
molekules in oplossing en het gelei tot 'n toename in die oplossings se viskositeit. Die "gelagtige"
oplossings beperk die haalbare vesel se morfologiese beheerbaarheid wanneer
konvensionele elektrospin parameters soos elektriese spanning, punt-tot-versamelaar afstand,
en oplossingkonsentrasies gewysig word. Die vinnige verdamping en strek van die oplossing
tydens elektrospin, gekombineer met die fase-geskeide amfifiliese molekules in oplossing en die
kristallisasie van die PAN segmente, het gelei tot (nie-ewewig morfologie) volledige poreuse
vesels. Die kristalliniteit van die veselbeginmaterial het afgeneem nadat elektrospin toegepas is.
Die insluiting van die oppervlak-geoksideerde multi-ommuurde koolstof nanobuisies in die
elektrogespinde vesels was suksesvol. Die inhoud van die nanobuisies het gewissel van 2 wt%
tot 32 wt%. Die MWCNTs het die gemiddelde veseldeursnit verminder deur op te tree as
kruisbinders tussen die PAN segmente van die molekules. Die nanobuisies was goed versprei
deur die poreuse struktuur van die vesels en dit was gerig in die rigting van die vesel-as. Bereiding van die silikonsamestellings bestaande uit nie-geweefde en gerigte veseloppervlakke
(met en sonder 8 wt% multi-ommuurde koolstof nanobuisies in die vesel) was suksesvol. Die
versoenbaarheid tussen die oppervlak van die PDMS-geskeide gebiede en die silikonmatriks
laat uitstekende verspreiding en interaksie van die PAN-gebaseerde veselvullers met die
silikonmatriks toe. Meganiese analise het aangetoon dat die fisiese eienskappe verbeter het
namate die PDMS inhoud in die vesel vermeerder het. Die vesels met die hoogste PDMS
inhoud het egter verswakte eienskappe getoon. Dit is toegeskryf aan ‘n verhoogde PDMS
inhoud (sag en swak), ‘n afname in kristalliniteit en ‘n verhoogde veseldeursnit (laer
grensoppervlakke). Dramatiese verbeterings in sterkte, styfheid, verlengbaarheid, vervorming
en taaiheid is bereik. Die mees betekenisvolle gevolg was 'n toename in die verrekking van
470%. Die meganiese resultate is gekorreleer met SEM ontleding van die brekingsoppervlakke.
Die veselkrag en vervormbaarheid, sowel as die meganisme van die splyting van die
samestellings, het tot die dramatiese verbeterings in die meganiese eienskappe gelei.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/17875 |
| Date | 12 1900 |
| Creators | Bayley, Gareth Michael |
| Contributors | Mallon, Peter, Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Rights | Stellenbosch University |
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