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Hierarchical self‐assembly of novel para‐aryltriazole helical foldamersPfukwa, Rueben 03 1900 (has links)
Thesis (PhD)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: Hierarchical information transfer is investigated as a tool to prepare well‐defined
nanostructures with high aspect ratios, via the self‐assembly of helically folding poly(paraaryltriazole)
(P(p‐AT)) foldamers.
A novel ‘helicity codon’ based on the 1,4‐linkage geometry in 1,4‐aryl‐disubstituted‐1,2,3‐
triazoles is developed. Helical folding is induced exclusively by directing all triazole moieties
into a cisoid configuration. By linking the triazole rings in a para fashion about the aryl
moiety, this helicity codon codes for a helix with a large internal cavity of ~ 3 nm. One turn
of the putative helical conformation requires 14 repeat units and the helical pitch is ~ 0.38
nm. The aryltriazole backbone is appended with amphiphilic oligo(ethylene glycol) (oEG)
units which have the dual roles of imparting solubility as well as instigating a solvophobic
helical folding in solvents which poorly solvate the hydrophobic arytriazole backbone but,
solvate the side chains fully. The helix interior is hydrophobic and the exterior is amphiphilic.
A true polymer synthesis approach to the foldamer synthesis, based on the copper catalysed
azide‐alkyne cycloaddition (CuAAC) AB step growth polymerization system, is developed.
This is preceded by a facile synthetic protocol for the AB monomers. The subsequent P(p‐
AT)s have high molecular weights ensuring several turns in the helical foldamer. A DMF/H2O
good solvent/bad solvent system is established. Twist sense bias in the helical foldamers is
successfully imparted by installing enantiopure chiral oEG side chains. Spectroscopic
signatures for the solvent dependent coil to helix transition are established enabling the
tracking of the conformational transitions from primary to secondary and finally tertiary
structure. Conclusive evidence for the formation of stable, long stacked helical columns, in
the solution state, is provided via cryo‐TEM. The helical stacks are several microns long, but
of random lengths and do not intertwine but rather run parallel to each other. The helical
stacks, however, have indeterminate lengths.
Control over the length and chirality of the self‐assembled helical stacks is successfully
imparted by using a template which mimics the role of ribonucleic acid (RNA) in tobacco
mosaic virus (TMV). The template used is the hydrophobic α‐helical polypeptide poly(γ‐
benzyl‐L‐glutamate) (PBLG). Self‐assembly is driven by solvophobicity in a DMF/H2O system, the PBLG template being encapsulated inside the hydrophobic cavities of the stacked/selfassembled
helical foldamers. Information from the template, i.e. length and chirality, is used
to control the length and the chirality of the stacked/self‐assembled construct.
The templated self‐assembly process is solvent dependent. When carried out in the solvent
regime at the coil to helix transition mid‐point of the foldamer host, system operates under
a dynamic equilibrium. Under these conditions, the self‐assembly process is shown to take
place between two distinct states, the foldamer helices and the helical template, the
template threading through the foldamer helices. The resulting self‐assembled construct
has a pseudo‐rotaxane architecture.
Under dynamic equilibrium conditions, temperature induced dis‐assembly of the templated
assembled construct, is shown to be a cooperative process, whilst re‐assembly is
characterized by a large hysteresis. By increasing the volume fraction of water, the
solvophobic character of the system is increased and template assembled construct is better
stabilised. The assembly system, however, loses its dynamic equilibrium character and falls
into kinetic traps. Temperature induced de‐threading, of the foldamer helices, becomes less
favourable and loses its cooperative character although the hysteresis loop is reduced. / AFRIKAANSE OPSOMMING: Hiërargiese inligtingsoordrag is bestudeer as ‘n hulpmiddel om goed gedefinieerde
nanostrukture met ‘n goeie beeldverhouding voor te berei. Die nanostrukture word
voorberei deur middel van self‐samestelling van heliese vouing van poli(para‐arieltriasool)
(P(p‐AT)) ‘foldamers’.
‘n Nuwe heliese‐kodon gebaseer op die 1,4 koppelingsgeometrie in 1,4 arieldigesubstitueerde‐
1,2,3‐triasool is ontwikkel. Heliese vouing word uitsluitlik geïnduseer as al
die triasole in die sis konfigurasie is. Deur die triasole in ‘n para konfigurasie te bind, kodeer
die heliese kodon vir ‘n heliks met ‘n groot interne kanaal van ~ 3 nm. Een draai van die
heliks benodig 14 herhalende eenhede en die heliese gradiënt ~ 0.38 nm. Amfifiliese
oligo(etileen glikol) (oEG) eenhede is aan die arieltriasoolruggraat aangeheg. Hierdie
aanhegting van oEG eenhede bevorder oplosbaarheid en dit induseer ‘n solvofobiese
heliese vouing in oplosmiddels wat nie die hidrofobiese arieltriasoolruggraat oplos nie, maar
wel die sy‐kettings volledig oplos. Die binnekant van die heliks is hidrofobies en die
buitekant is amfifilies.
‘n Polimeersintese benadering tot die ‘foldamer’ sintese (gebaseer op die koper
gekataliseerde siklo‐addisie reaksie tussen ‘n asied en ‘n alkyn) AB stapsgewyse groei
polimerisasiestelsel, is ontwikkel. Dit is voorafgegaan deur ‘n geskikte sintetiese protokol vir
die AB monomere. Die daaropvolgende P(p‐AT) het ‘n hoë molekulêre massa wat verseker
dat daar ‘n hele paar draaie in die heliese ‘foldamer’ is. ‘n DMF/H2O goeie oplosmiddel/
swak oplosmiddel sisteem is vasgestel. Draaiing van die heliks na ‘n spesifieke kant alleenlik
is suksesvol geïnduseer deur die toevoeging van suiwer enantiomere van die chirale oEG sykettings.
Spektroskopiese handtekeninge van die oplosmiddel‐afhanklike ketting tot heliks
transformasie word vasgestel sodat die oorgangstoestande gevolg kan word vanaf primêre
tot sekondêre en uiteindelik tesiêre struktuur. Beslissende bewyse vir die formasie van
stabiele, lang gestapelde heliese kolomme in die opgeloste toestand is bewys met cryo‐TEM.
Die heliese stapels is verskeie mikron lank, maar het verskillende lengtes. Die heliese stapels
is parallel aan mekaar en oorvleuel nie. Die lengte van die heliese stapels is egter
onbepaalbaar. Beheer oor die lengte en chiraliteit van die self‐samestellende heliese stapels is verkry deur
gebruik te maak van ‘n templaat wat die rol van ribonukleïensuur (RNS) in die
tabakmosaïekvirus (TMV) naboots. Hidrofobiese α‐heliese polipeptied poli(γ‐bensiel‐Lglutamaat)
(PBLG) is gebruik as die templaat. Self‐samestelling word gedryf deur
solvofobisiteit in ‘n DMF/H2O stelsel, met die PBLG templaat wat dan geënkapsuleer word
binne die hidrofobiese holtes van die gestapelde/ self‐saamgestelde heliese ‘foldamers’. Die
lengte en die chiraliteit van die templaat word gebruik om die lengte en chiraliteit van die
gestapelde helikse te beheer.
Die templaatbemiddelde self‐samestellende proses is afhanklik van die oplosmiddel. Die
stelsel is by ‘n dinamiese ewewig wanneer, uitgevoer in ‘n oplosmiddel, die ketting na heliks
oorgang die middelpunt van die ‘foldamer’ gasheer bereik het. By hierdie omstandighede
vind die self‐samestellende proses plaas tussen twee afsonderlike toestande nl. die
‘foldamer’ helikse en die heliese templaat, en die templaat wat vleg deur die ‘foldamer’
helikse vleg. Die gevolglike struktuur het ‘n pseudo‐rotaxane argitektuur.
By dinamiese ewewigstoestande veroorsaak temperatuur dat die self‐samestellende
templaatstrukture weer disintegreer. Hierdie is ‘n koöperatiewe proses terwyl die hersamestelling
gekarakteriseer word deur ‘n sloerende proses. Deur die waterfraksie te
vermeerder, word die solvofobiese karakter van die sisteem verhoog en die templaat selfsamestellende
struktuur beter gestabiliseer. Die samestellingsproses verloor egter sy
dinamiese ewewigkarakter en val in kinetiese slaggate. Temperatuur geïnduseerde
disintegrasie van die foldamer helikse word minder gunstig en dit verloor die koöperatiewe
karakter alhoewel die sloering verminder is.
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The effect of the crystalline state on the properties of the dative bondVenter, Gerhard 12 1900 (has links)
Thesis (PhD)--University of Stellenbosch, 2005. / ENGLISH ABSTRACT: Density functional theory (DFT) has been used to investigate the effect of
the surrounding molecules on the structure of selected boron-nitrogen compounds.
It was found that a very limited number of molecules, orientated
according to the experimental crystal structure, are needed to successfully
reproduce the large changes in structure witnessed when HCN–BF3
and CH3CN–BF3 crystallises. Specifically, the addition of seven molecules
shortens the B–N distance by 0.735 °A in (HCN–BF3)8 and 0.654A° in (CH3CN–
BF3)8.
Accompanying the large changes in B–N bond length are equally large
changes in the N–B–F angle. Investigation of the structure of these complexes
in terms of localised electron pairs shows that the availability of lone pairs,
in close proximity to the B–N bond axis, plays an important role in the bond
change. Through delocalisation of the fluorine lone pairs the antibonding
σ
∗(B–N) orbital becomes increasingly occupied as the N–B–F angle lessens
and vice versa.
Further, an investigation of the specific effects of dipole-dipole interactions
was performed by applying uniform electric fields of varying strength
along the donor-acceptor bond axis of a series of compounds of the form X–Y;
X=H3N, HCN, CH3CN; Y = BF3, BH3, SO3. All complexes investigated show
sensitivity to the external electric field, however, only the compounds having
nitrile donors and acceptors with fluorine atoms produce large changes,
which in turn are dominated by a very sudden large change in B–N bond
length occurring in a very narrow range of changing field strength. Analysis
of the changes in bond character reveals that HCN–BF3 and CH3CN–BF3
have long bonds in the gas phase, formed primarily through electrostatic interaction
between the donor and acceptor. In the short bond in the condensed
phase the bond character changes considerably through the introduction of
strong electron sharing interactions, i.e. covalent or orbital interactions.
Fundamental changes in the nature of the bond, catalysed by surrounding
molecules, thus lie at the heart of the large phase-dependent changes in these
species. / AFRIKAANSE OPSOMMING: ’n Kohn-Sham elektrondigtheidsteorie (DFT) studie is gedoen op die effek
van die omliggende molekules in die kristalstruktuur van sekere molekules
wat boor-stikstof bindings bevat. Daar is gevind dat ’n klein aantal
molekules, georienteer soos in die eksperimentele kristalstruktuur, benodig
word om die groot veranderinge in stuktuur te veroorsaak wat eksperimenteel
waargeneem word wanneer HCN–BF3 en CH3CN–BF3 kristaliseer.
Spesifiek, die byvoeging van sewe molekules verminder die B–N bindingslengte
met 0.735 °A in (HCN–BF3)8 en 0.654 A° in (CH3CN–BF3)8.
Die groot veranderinge in B–N bindingslengte gaan saam met ewe groot
veranderinge in die N–B–F hoek. ’n Ondersoek van die struktuur van die
molekules in terme van gelokaliseerde elektronpare wys dat die beskikbaarheid
van alleenpare, wat naby die B–N bindingsas lˆe, ’n belangrike rol speel
in the verandering in bindingslengte. Deur delokalisasie van die fluoor alleenpare
word die antibindende σ
∗(B–N) orbitaal toenemend beset soos die
N–B–F hoek afneem en omgekeerd.
Verder is die spesifieke effek van dipool-dipool interaksies ondersoek
deur uniforme elektriese velde aan te lˆe langs the donor-akseptor bindingsas
van ’n reeks komplekse van die vorm X–Y; X = H3N, HCN, CH3CN; Y = BF3,
BH3, SO3. Al die komplekse toon sensitiwiteit teenoor die eksterne elektriese
veld, maar net die verbindings wat nitriel akseptore en fluoor atome aan the
donor fragmente het, toon groot veranderinge, wat op hulle beurt weer oorskadu
word deur ’n skielike verandering in the B–N bindingslengte in ’n
nou band van veranderende veldsterkte. Analise van die veranderinge in
bindingskarakter toon dat HCN–BF3 en CH3CN–BF3 lang bindings in die
gasfase het, wat hoofsaaklik gevorm word deur elektrostatiese interaksies
tussen die donor en akseptor fragmente. In die kort binding in die kristalfase
is daar ’n aansienlike verandering in the karakter as gevolg van die intrede
van sterk elektrondelingsinteraksies, m.a.w. kovalente of orbitaalinteraksies.
Fundamentele veranderinge in the manier wat die binding saamgestel
word, wat gekataliseer word deur omliggende molekules, is dus die oorsaak
van die groot faseafhanklike veranderinge.
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Synthesis and characterization of two novel urethane macromonomers and their methacrylic/urethane graft copolymersAlshuiref, Abubaker 03 1900 (has links)
Thesis (PhD (Chemistry and Polymer Science))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT:
Polymethacrylates are well known adhesives and can carry specific functionality, but have
the disadvantage that their flexible backbones impart limited thermal stability and mechanical
strength. Polyurethanes (PUs) are finding increasing application and use in many industries
due to their advantageous properties, such as a wide range of flexibility combined with
toughness, high chemical resistance and excellent weatherability. PUs do however have
some disadvantages, for instance, PU is considered an expensive polymer, especially when
considered for solvent based adhesives. the focus of this study was to consider a largely
unstudied area of PU chemistry, namely combining PUs with polymethacrylates.
Two types of linear urethane macromers (UMs) UM1 and UM2 were synthesized by the
polyaddition polymerization of 4,4'-methylenediphenyl diisocyanate (MDI) with ethylene
glycol (EG), and MDI with neopentylglycol (NPG), via a pre-polymer method, followed by
termination with 2-hydroxy ethylacrylate (2-HEA) and methanol (MeOH) to yield UMs having
specific urethane chain lengths, and which have to be predominantly monofunctional.
Structural identification of the UMs was verified by MALDI-TOF-MS, FTIR, 13C-NMR and 1HNMR
spectroscopy.Various percentages of the respective UMs (0_55 wt % of methacrylate monomers) were
then incorporated into polymethyl methacrylate (PMMA) and poly n-butyl methacrylate
(PnBMA) backbones via solution free-radical copolymerization. The resulting methyl
methacrylate-g-urethane and n-butyl methacrylate-g-urethane copolymers were
characterized by 1H-NMR,13C-NMR, FTIR, SEC with double detectors (UV and RI), light
scattering, UV-Vis, HPLC, TGA, DSC, DMA and TEM. Weight percentages of UM
incorporated into the methyl methacrylate-g-urethane copolymers were calculated using
FTIR, UV-Vis and 1H-NMR techniques. Phase separation which occurred between the
urethane segment and methacrylate segment in the graft copolymerization products was
investigated by DMA, DSC and TEM analysis.
Microphase separation occurred in all PMMA-g-UM1 and PnBMA-g-UM1 copolymers: two
glass transitions temperatures corresponding to the PMMA or PnBMA and UM1 fractions,
respectively, were observed. On the other hand, DMA and DSC results showed that in most
graft copolymer products the two respective component parts PMMA-g-UM2 or PnBMA-g-
UM2 were compatible, because only one Tg was observed. Two glass transitions occurred
for PMMA or PnBMA and UM2 when the amount of UM was increased to 55 wt % during
copolymerization and microphase separation was evident in DSC, DMA and TEM
measurements. Thermal stability and storage modulus (stiffness) of all the synthesized PMMA-g-urethane
and PnBMA-g-urethane copolymers increased as the concentration of urethane
macromonomer in the copolymerization feed increased, as confirmed in TGA and DMA
results. The surface and adhesive properties of the synthesized graft copolymer were studied
by measuring the static contact angle and peel strength. Adhesion increased as the content
of UMs increased in the graft copolymer. The graft copolymers prepared using a high UM2
feed for both PMMA and PnBMA showed improved in adhesion compared to the pure
methacrylate polymers. The adhesion was better for both leather and for vinyl. / AFRIKAANSE OPSOMMING:
Polimetakrilate is bekende kleefstowwe. Hulle het egter die tekortkoming dat hulle buigbare
ruggraat beperkte termiese en meganiese stabliteit besit. Poliuretane (PUs) word deesdae al
hoe meer gebruik in baie nywerhede as gevolg van hulle baie voordele, insluitend hul wye
buigsaamheid tesame met sterkte, hoë chemiese weerstand en uitstekende weerbaarheid.
PUs het egter ’n paar nadele: hulle is baie duur, veral wanneer hulle gebruik word in
oplosmiddel-gebaseerde kleefstowwe. Die doel van hierdie studie is om die kombinering van
PUs met polimetakrilate te bestudeer, 'n onderwerp wat tot dusver baie min aandag-getrek
het.
Twee tipes liniêre uretaanmakromere (UMs), UM1 en UM2, is gesintetiseer deur
gebruik te maak van poliaddisiepolimerisasie van 4,4'-metileendifeniel diisosianaat (MDI) met
etileenglikol (EG), en MDI met neopentielglikol (NPG), via ‘n prepolimeermetode, gevolg deur
terminering met 2-hidroksiëtielakrilaat (2-HEA) en metanol (MeOH). Die produk hiervan is
UMs met spesifieke kettinglengtes (hoofsaaklik monofunksioneel). Die samestelling van die
UMs is met behulp van die volgende gevorderde analitiese tegnieke bepaal: MALDI-TOFMS,
FTIR, 13C-NMR en 1H-NMR.
Verskillende hoeveelhede van die UMs (0_55 gewIing% metakrilaatmonomere) is dan
in die polimetielmetakrilaat (PMMA) en poli-n-butielmetakrilaat (PnBMA) ruggrate
geïnkorporeer deur middel van oplossing-vryradikaalpolimerisasie. Die samestelling van die
kopolimeerprodukte, metiel-metakrilaat-g-uretaan en n-butiel-metakrilaat-g-uretaan, is met
behulp van die volgende gevorderde analitiese tegnieke bepaal: 1H-NMR, 13C-NMR, FTIR,
SEC met dubbele detektors (UV en RI), ligverstrooiing UV-Vis, HPLC, TGA, DSC, DMA en
TEM. Die hoeveelheid UM geïnkorporeer in die metielmetakrilaat-g-uretaan kopolimere is
bereken deur gebruik te maak van FTIR, UV-Vis en 1H-NMR data. Die faseskeiding wat
plaasgevind het tussen die uretaansegment en die metakrilaatsegment in die produkte van
die entpolimerisasie is met behulp van DMA, DSC en TEM ondersoek.
In alle PMMA-g-UM1 en PnBMA-g-UM1 kopolimere het mikrofaseskeiding
plaasgevind: twee verskillende glasoorgangstemperature vir die PMMA of PnBMA en UM1
fraksies is waargeneem. Hierteenoor het DMA en DSC resultate getoon dat in die meeste
entkopolimeerprodukte (PMMA-g-UM2 of PnBMA-g-UM2) was die twee komponente
verenigbaar, aangesien net een Tg waargeneem is. In die geval van die kopolimere waar die
hoeveelheid UM in die kopolimerisasiereaksies tot 55 gew% verhoog is, is twee
glasoorgangstemperature vir PMMA of PnBMA, en UM2 waargeneem. Mikrofaseskeiding is
met behulp van DSC, DMA en TEM bewys.
Termiese stabiliteit en stoormodulus (styfheid) van alle gesintetiseerde PMMA-g uretaan en PnBMA-g-uretaan kopolimere het toegeneem namate die uretaankonsentrasie in
die kopolimerisasiereaksie toegeneem het soos deur middel van TGA en DMA resultate
bewys is. Die oppervlakte- en kleefeienskappe van die bereide entkopolimere is bestudeer
deur die statiese-kontakhoek en skilkrag te meet. Adhesie het toegeneem namate die UMinhoud
toegeneem het. Die entkopolimere berei met hoë PMMA en PnBMA inhoud het
uiteindelik beter adhesie getoon as die suiwer metakrilaatpolimere. Die adhesie was beter vir
beide leer en viniel.
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Mechanistic studies of reversible addition-fragmentation chain transfer mediated polymerizationCalitz, Francois Malan 03 1900 (has links)
Thesis (PhD)--Stellenbosch University, 2004. / ENGLISH ABSTRACT: To comply with the ever growing demands for materials with better properties and complex architectures,
polymer chemistry has resorted to the use of living free radical polymerization techniques. Despite the structural
control some of these techniques offer, major disadvantages do exist. For example, most require ultra-pure
reagents, hence only a small fraction of the monomers used in industry can be polymerized in this way. This
rendered these new living techniques less advantageous from a commercial point of view. Recently, a
revolutionary new living free radical process, namely the reversible addition-fragmentation chain transfer
process, or RAFT process, was developed that combines the control over the polymer produced with the
robustness and versatility of a free radical process.
However, the RAFT process is not without its problems. In some dithioester mediated polymerizations,
significant inhibition and rate retardation effects have been observed. Two main opposing opinions have been
proposed in recent literature to explain these phenomena observed. The main point of difference between these
two groups is the fate of the formed intermediate RAFT radicals, i.e., slow fragmentation of the formed
intermediate radicals together with possible reversible intermediate RAFT radical termination, or fast
fragmentation of the formed intermediate radicals together with possible irreversible intermediate RAFT radical
termination. Between these opposing two groups, there is a difference of six orders of magnitude for the rate of
fragmentation of the formed intermediate RAFT radicals.
The work presented in this thesis is an attempt to clarify some of the mysteries, i.e., inhibition and rate
retardation observed in some RAFT polymerizations. Experimental evidence to support or contradict the
theories of the above mentioned two opposing groups was investigated.
The concentration-time evolution of the intermediate radical concentration (cy), for styrene and butyl acrylate
polymerizations mediated by cumyl dithiobenzoate (COB) at 70°C and 90 °C, was followed via in situ electron
spin resonance spectroscopy (ESR). The concentration-time evolution profiles observed were ascribed to the
formation of very short chains during the early stages of the reaction. It was also found that the RAFT process is
not particularly sensitive to oxygen. The intermediate and propagating radical (cp) concentrations (and their
ratio) for the cumyl dithiobenzoate mediated styrene polymerizations were examined by ESR spectroscopy and
kinetics. The system showed strong chain length effects in kinetics, assuming all chains were of similar number
average molar mass (Mn). However, unusual behavior with respect to existing mechanistic knowledge was
observed in other aspects of the system. The central equilibrium "constant" (Keq) was found to be dependent on
both temperature and initial reactant concentrations. The observed intermediate radical concentrations were not
consistent with predictions based on existing literature models. It was also found that the time dependence of
the intermediate radical concentration varies significantly with the type of RAFT agent used. Unexpectedly, intermediate radicals were detected at very long reaction times in the virtual absence of initiator, enhancing the
belief of possible reversible termination reactions involving the intermediate radicals. An extra radical (nonpropagating
or intermediate) species was observed (via ESR spectroscopy) to form during some reactions. Its
concentration increased with time.
The combination of data from several analytical techniques provided evidence for the formation of dead chains
by the termination of intermediate radicals in the free radical polymerization of styrene, mediated by a cumyl
dithiobenzoate RAFT agent, at 84°C. Experiments done focused on the early stages of the reactions, targeting
very low final number average molar mass values, with high initiator concentrations. The formation of these
terminated chains did not occur to a significant extent until a large fraction of the chains reached a degree of
polymerization greater than unity. This corresponded to the occurrence of a maximum in intermediate radical
concentration.
In situ 1H nuclear magnetic resonance (NMR) and electron spin resonance spectroscopy was used to directly
investigate the processes that occur during the early stages (typically the first few monomer addition steps) of an
AIBN-initiated reversible addition fragmentation chain transfer polymerization of styrene, in the presence of a
cyanoisopropyl dithiobenzoate and cumyl dithiobenzoate RAFT agent, at 70°C and 84 °C respectively. 1H NMR
spectroscopy allowed the investigation of the change in concentration of important dithiobenzoate species as a
function of time. Identification and concentrations of the radicals present in the system could be inferred from
corresponding ESR spectroscopy data. An apparent "inhibition" effect was observed in both the cyanoisopropyl
and cumyl dithiobenzoate mediated polymerizations. This effect could be reduced by increasing the reaction
temperature to 84 °C. However, the use of cumyl dithiobenzoate as RAFT agent prolonged this effect. This
apparent "inhibition" effect was attributed to selective fragmentation of the formed intermediate radicals during
the early stages of the reaction, and to different propagation rate coefficients (kp) of the resulting (different)
radicals. A change in the equilibrium coefficient for the systems investigated was ascribed to possible
progressively decreasing addition and fragmentation rate coefficients of propagating and intermediate radicals
formed during the reaction. The increase in intermediate radical concentration, and thus possible intermediate
radical termination, was shown to also be a probable cause of the rate retardation observed in the RAFT
mediated systems investigated.
To conclude, probable causes of the observed inhibition and rate retardation in some dithiobenzoate mediated
systems were investigated. It was found that intermediate RAFT radical termination does occurs, albeit
reversibly or irreversibly. A maximum in the intermediate radical concentration, and thus possible intermediate
radical termination, was seen to occur during the observed rate retardation. An apparent inhibition effect
observed was ascribed to a possible change in termination kinetics, the formation of terminated intermediate
radical products and a rapidly changing kp of the propagating radicals. / AFRIKAANSE OPSOMMING: Om te voldoen aan die ewig groeiende aanvraag vir materiale met beter eienskappe en komplekse
samestellings, is in die polimeerchemie lewende vry-radikaal polimerisasietegnieke ontwikkel. Ten spyte van die
feit dat party van die polimerisasie tegnieke die strukuur van die gevormende polimere kan beheer, bestaan daar
tog nadele. Die meeste polimerisasie tegnieke benodig ultra suiwer reagense, dus kan net 'n klein fraksie van
die monomere wat deur die industrie gebruik word op so 'n manier gepolimeriseer word. Dus, vanuit 'n
komersieële oogpunt, is die nuwe lewende polimerisasietegnieke minder voordelig. Onlangs is 'n revolusionere
nuwe lewende vry-radikaal polimerisasieproses, naamlike die RAFT-(eng. reversible addition-fragmentation
chain transfer process) proses ontwikkel, wat die beheer oor die geproduseerde polimere, kombineer met die
robuustheid en veelsydigheid van 'n vry-radikaalproses.
Die RAFT proses is egter nie sonder probleme nie. Beduidende inhibisie en vertraging van die polimerisasie
tempo is in sommige dithioester-bemiddelde polimerisasies opgemerk. Daar is hoofsaaklik twee opponerende
opinies oor die redes vir die inhibisie en vertragings effekte. Die grootste verskil tussen die twee groepe lê in die
lot van die gevormde intermediêre radikaal, m.a.w. stadige fragmentasie van die gevormende intermediêre
radikale tesame met moontlike onveranderlike intermediêre radikaalterminasie, of vinnige fragmentasie tesame
met moontlike omkeerbare intermediêre radikaalterminasie. Tussen die twee groepe, is daar 'n verskil van ses
ordegrotes vir die groote van die tempo van fragmentasie van die gevormende intermediêre radikaal.
Die werk wat in die tesis weergee word, is 'n poging om sommige van die geheime van die RAFT proses, m.a.w.
inhibisie en vertraging van die polimerisasietempo, op te los. Die ondersoek was gerig op eksperimetele bewyse
om die teorieë van die twee opponerede groepe of te bevestig of teen te spreek.
Die konsentrasie tyd-verandering van die intermediêre radikaal konsentrasie vir stireen- en
butielakrilaatpolimerisasie, bemiddeled deur CDB (eng cumyl dithiobenzoate) by 70 oe and 90 oe, is gevolg deur
middel van in situ (lat. vir in die oorspronklike plek, m.a.w. binne-in die ESR masjien) elektronspin-resonans
(ESR) spektroskopie. Die vorm van die konsentrasie tyd-profiele is toegeskryf aan die vorming van baie kort
polimeerkettings gedurende die vroeë reaksietye. Dit is ook bepaal dat die RAFT-proses nie besonder sensitief
was vir suurstof nie. Die intermediêre en die propagerende radikaalkonsentrasie (en hulle verhouding) vir die
CDB bemiddelde stireen polimerisasies, is bepaal deur middel van elektronspin-resonans spektroskopie en die
kinetika van die sisteem. Die kinetika van die sisteem toon 'n sterk afhanklikheid teenoor die lengte van die
polimeerkettings, as aanvaar word dat al die kettings dieselfde numeriese gemiddelde molêre massa het. Des
nieteenstaande, is egter onverwagte gedrag in ander aspekte van die sisteem opgemerk. Dit was ook gevind
dat die sentrale ewewigs-"konstante" (Keq) afhanklik was van die temperatuur en die oorspronklike reaktant
konsentrasie. Die bepaalde intermediêre radikaalkonsentrasie het verskil van voorspelde waardes gebaseer op literatuur modelle. Dit is ook gevind dat die intermediêre radikaalkonsentrasie afhanklik is van die tipe RAFT
agent wat in die polimerisasie reaksies gebruik word. Intermediêre radikale is onverwags gevind na baie lang
reaksietye, wanner verwag is dat die konsentrasie van die afsetter, en dus ook die intermediêre radikale, baie
klein sou wees. Dit het die verwagting dat omkeerbare intermediêre radikaalterminasie kan plaasving, versterk.
'n Ekstra radikale spesie, wat gedurende die reaksie vorm en waarvan die konsentrasie groter word met tyd, is
ook deur ESR-spektroskopie geidentifiseer.
'n Kombinasie van verskillende skeikundige tegnieke is gebruik om bewyse te kry vir die vorming van dooie
kettings wat ontstaan deur middel van intermediêre radikale terminasiereaksies, in die vry-radikaalpolimerisasie
van stireen, wat deur 'n CDB RAFT-agent bemiddeled word by 84°C. Eksperimente is gedoen om die reaksie
tydens vroeë reaksietye te ondersoek. Baie hoë afsetter konsentrasies is ook gebruik, wat tot uiters lae
numeriese gemiddelde molêre massas van die polimeerkettings gelei het. Beduidende konsentrasies van die
dooie kettings is eers gevind nadat 'n graad van polimerisasie van groter as een bereik is. Dit het ooreengestem
met 'n maksimum in die konsentrasie van die intermediêre radikale.
In situ 1H kern magnetiese-resonans (KMR) en electronspin-resonans spektroskopie was gebruik om 'n RAFT
proses, wat gedurende die vroeë reaksie tye (tipies gedurende die eerste paar monomeer toevoegingstappe) te
bestudeer, wat deur AIBN (eng azo bis(isobutyronitrile)) afgeset word en bestaan uit stireen en CIDB (eng
cyanoisopropyl dithiobenzoate) en CDB RAFT agente onderskeidelik, en by 70°C and 84 °C reageer. 1H KMRspektroskopie
was gebruik om die veranderinge in die konsentrasie van die belangrike spesies te bepaal. Die
identifikasie en konsentrasie van die radikale kon bepaal word deur middel van ESR data. 'n Skynbare
'inhibisie-effek' is waargeneem in die reaksies wat bemiddeled word deur CIDB en CDB. Die effek is verminder
toe die reaksietemperatuur verhoog is na 84°C. Die gebruik van CDB as RAFT agent het egter die effek
vergroot. Die skynbare 'inhibisie effek' was toegeskryf aan die selektiewe fragmentasie van die intermediêre
radikale gedurende die vroeë reaksietye, en aan verskillende propagasie tempokoëffisiënte (kp) van die
verskillende radikale. Die veranderlike sentrale ewewigskoëffisiënte is toegeskryf aan die toevoegings en
fragmentasie tempokoëffisiënte van die propagerende en intermediêre radikale wat toenemend afneem. Die is
ook getoon dat die toename in die konsentrasie van die intermediêre radikale en dus moontlike intermediêre
radikale terminasie, 'n oorsaak kan wees van die vertraging van die polimerisasietempo in die RAFT-bemiddelde
reaksies.
Ter samevatting, die waarskynlike oorsake vir inhibisie en die polimerisasietempo vertraging opgemerk in sekere
dithiobenzoaat-bemiddelde sisteme, is ondersoek. Dit was gevind dat intermediêre radikaalterminasie wel kan
gebeur, of dit nou omkeerbaar of onveranderlik gebeur. 'n Maksimum in die konsentrasie van die intermediêre
radikale, en dus moontlike intermediêre radikaalterminasie, het voorgekom tesame met 'n vertraging in die
polimerisasietempo. Die skynbare inhibisie-effek wat opgemerk was kan toegeskryf word aan 'n moontlike verandering in die terminasie kinetika, die formasie van getermineerde intermediêre radikale en 'n vinnig
veranderende propagasie tempokoëffisiënt.
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The supramolecular chemistry of novel synthetic biomacromolecular assembliesNaidoo, Venthan B. 04 1900 (has links)
Dissertation (PhD)--Stellenbosch University, 2004 / ENGLISH ABSTRACT: Over the past decade peptide bola-amphiphiles have been the subject of much attention
because of their role as potential models of functionalised membranes and as new
generation surfactants. In the quest for new surfactants a peptidomimetic-based approach
was used to design a library of novel 'hybrid' bola-amphiphilic peptide surfactants
derived from sapecin B and a model symmetrical oligo-glycine bola-amphiphile. The
library was divided into different series, each one purpose-built; first, to investigate
hierarchal supramolecular architecture and, second, to investigate potential antimicrobial
activity. The bola-amphiphiles were synthesised using Fmoc-polyamide based solid phase
peptide synthesis and purified via high performance liquid chromatography. The peptide
hybrids were characterised using electro spray mass spectrometry, nuclear magnetic
resonance, different modes of electron microscopy, Fourier-transform infrared
spectroscopy and, in some cases, further studies were done using circular dichroism and
bioactivity tests.
The model bola-amphiphile suberamide(GGh was synthesised using peptide fragment
condensation based on solid phase peptide synthesis. The synthesis is bi-directional
(N~C and C~N) and versatile, making it possible to synthesis new dicarboxylic
oligopeptide bola-amphiphiles and other analogous compounds. The product,
suberarnide(GG)2, was purified using its inherent ability to self-assemble in an acidic
solution.
Novel asymmetrical bola-amphiphiles composed of dipeptide head groups linked via an
aliphatic (I)-amino acid, serving as a hydrocarbon spacer, were also synthesized. Two
small libraries of bola-amphiphiles were established - the first involved variation in
to-amino acid length and the other variation in the C-terminal amino acid. The bolaamphiphiles
were self-assembled in either 0.1% trif1uoroacetic acid or 0.1%
triethylamine. Electron microscopy revealed the formation of a variety of higher order
supramolecular architectures based on ~-sheet self-assembly. FT-IR spectrometry
indicated that interlayer and intralayer hydrogen bond networks, together with strong selfassociation,
promoted by the hydrophobic effect and, in certain instances, electrostatic
interactions, are responsible for the variety of supramolecular architectures. Variations in the higher order structures can be attributed to amino acid composition, specifically
length of m-amino acid, nature of the C-terminal amino acid and the optimised solvent
conditions used for the self-assembly process.
A third library of novel 'hybrid' bola-amphiphilic peptide surfactants, in which a cationic
tripeptide motif from antimicrobial peptides was combined in a hybrid molecule
containing a oi-amino acid residue, was established. These bola-amphiphiles displayed
potent antimicrobial activity against both Gram-positive and Gram-negative bacteria; the
analogues were as active or more active than the leader peptides yet, remarkably,
displayed little or no appreciable haemolytic activity. These organopeptide bolaamphiphiles
thus demonstrated selective toxicity towards bacteria. The hydrophobicity
imparted by the co-amino acid has contrasting effects on haemolysis and antimicrobial
activity of the peptide analogues. The other unique feature of these peptides and their
analogues is the fact they self-assembled into complex supramolecular architectures,
composed primarily of ~-sheets. Their self-assembly is primarily governed by
hydrophobic interactions together with inter and intralayer hydrogen bonding. Electron
microscopy clearly revealed higher order structures for both peptides and analogues. The
generation of higher order supramolecular architecture is dependent on optimisation of ~-
sheet self-assembly whereas antimicrobial activity is dependent on the balance between
net positive charge and optimum hydrophobicity of the peptide hybrids.
This study has demonstrated that it is possible to design hybrid peptide surfactants
capable of producing higher order supramolecular architecture and improving the
antimicrobial activity whilst reducing the haemolytic effect. The study and design of
these versatile 'purpose-built' bio-inspired surfactants heralds a novel approach, one that
shows tremendous potential. / AFRIKAANSE OPSOMMING: Die afgelope dekade het bola-amfifiliese peptiede baie aandag geniet weens hulle rolle as
potensiële modelle van gefunksionaliseerde membrane en as 'n nuwe generasie
surfaktante. In die soeke na nuwe surfaktante is 'n peptiedornimetiese benadering gevolg
om 'n biblioteek van nuwe "hibried" bola-amfifiliese peptiedsurfaktante van sapesien B
en 'n simmetriese oligoglisien bola-amfifil af te lei. Die biblioteek is in verskillende
reekse onderverdeel. Elke reeks is doelmatig vervaardig om ondersoek in te stel na twee
aspekte, nl. die rangorde van die supramolekulêre strukture en die potensiële
antirnikrobiese aktiwiteit. Fmoc-poliamied gebaseerde soliedefase-peptied-sin-tese is
aangewend vir die sintese van die bola-amfifile en hulle is met behulp van hoë
doeltreffendheid vloeistofchromatografie gesuiwer. Die peptiedhibriede is gekarakteriseer
met behulp van elekrosproei massaspektrometrie, kern-magnetiese resonansie,
verskillende modusse elektronrnikroskopie, Fourier-transform infrarooispektrometrie en,
in sommige gevalle is verdere studies met sirkulêre dichroïsme en bioaktiwiteitstoetsing
uitgevoer.
Die bola-amfifilsuberamiedtflfij--model is met behulp van peptiedfragment-konden-sasie
gesintetiseer gegrond op soliedefase-peptiedsintese. Dit sintese vind in twee rigtings plaas
(N~C en C~N) en is veelsydig aangesien dit die sintese van sowel nuwe dikar-boksielbola-
amfifile as ander analoë verbindings moontlik maak. Die produk, suber-arnied(GG)2,
is gesuiwer met behulp van die verbinding se inherente vermoë tot self-montering in suur
oplossings.
Nuwe assimetriese bola-amfifile, saamgestel uit dipeptiedkopgroepe, gekoppel via 'n
alifatiese ro-aminosuur, wat as koolwaterstofspasieerder dien, is ook gesintetiseer. Twee
klein bola-amfifilbiblioteke is saamgestel - die een het variasies in die ro-aminosuur se
lengte omvat en die ander een variasies in die C-terrninale aminosuur. Selfmontering van
die bola-amfifile het plaasgevind in of 0,1 % trifluorasynsuur Of 0,1 % trietielamien.
Elektronrnikroskopie het die bestaan van 'n verskeidenheid hoërorde supramolekulêre
strukture, gegrond op p-plaatselfmontering, aangetoon. Uit FT-IR-spektrometrie blyk dit
dat inter - en intralaag waterstofbinbdingsnetwerke en sterk selfassosiasie, 19. word
bevorder deur die hidrofobiese effek en, in sekere gevalle, elektrostatiese interaksies, is
verantwoordelik vir die verskeidenheid supramolekulêre strukture. Variasies in die hoërorde strukture kan toegeskryf word aan aminosuursamestelling, in besonder die
lengte van die ro-aminosuur, die aard van die C-terminale aminosuur en die geoptimiseerde
oplosmiddelkondisies wat gebruik is vir die selfmonteringsproses.
'n Derde biblioteek nuwe "hibried" bola-amfifiliese peptiedsurfaktante, waarin 'n kationiese
tripeptiedmotief uit antimikrobiale peptiede gekombineer is met 'n m-aminosuurresidu,
is geskep. Sommige van hierdie bola-amfifile het 'n kragtige antimikrobiese
aktiwiteit teenoor sowel Gram-positiewe as Gram-negatiewe bakterieë gertoon. Die
analoë strukture was aktief, of selfs meer aktief as die voorste peptiede maar het,
verbasend genoeg, nie 'n beduidende hemolitiese aktiwiteit vertoon nie. Hierdie
organopeptied bola-amfifil het dus 'n selektiewe toksisiteit teenoor bakterieë vertoon. Die
hidrofo-bisiteit, as gevolg van die ui-aminosuur, het 'n resiproke effek op hemolise en die
antimikrobiese aktiwiteit van die peptiedanaloë. Die ander uitstaande kenmerk van die
peptiede en hulle analoë is die vermoë om te selfmonteer en komplekse supramolekulêre
strukture, bestaande hoofsaaklik uit ~-plate, te vorm. Hierdie selfmontering word in
hoofsaak beheer deur hidrofobiese interaksies asook inter - en intralaagwaterstofbinding.
Elektronmikroskopie het duidelik hoërorde strukture getoon by sowel dié peptiede as
hulle analoë. Die ontwikkeling van hoërorde supramolekulêre struktuurvorms is afhanklik
van die optimalisering van die ~-plaatselfmontering. Daarteenoor is die antimikro-biese
aktiwiteit afhanklik van die balans tussen die netto positiewe lading en die opti-male
hidrofobisiteit van die peptiedhibriede.
Hierdie studie het getoon dat dit moontlik is om hibriedsurfaktante te ontwerp wat
hoërorde supramolekulêre strukture te produseer en om die antimikrobiese aktiwiteit te
verbeter terwyl die hemolitiese effek verminder word. Die studie en ontwerp van hier-die
veeldoelige, "doelmatig-gesintetiseerde" biogeïnspireerde surfakante stel 'n unieke
benadering daar, wat oor groot potensiaal beskik.
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Novel α-olefin polymer systemsSiphuma, Lufuno 03 1900 (has links)
Thesis (PhD)--Stellenbosch University, 2005. / ENGLISH ABSTRACT:
See fulltext for abstract / AFRIKAANSE OPSOMMING:
Sien volteks vir opsomming
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87 |
Synthesis and characterization of glycopolymer brushesFleet, Reda Ali 12 1900 (has links)
Thesis (PhD (Chemistry and Polymer Science))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: Please refer to full text for abstract
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88 |
Porous polymeric superstructures as in-growth scaffolds for tissue-engineered vascular prosthesesBezuidenhout, Deon 12 1900 (has links)
Thesis (PhD)--Stellenbosch University, 2001. / ENGLISH ABSTRACT:
Please refer to fulltext for abstract. / AFRIKAANSE OPSOMMING:
Sien volteks vir opsomming.
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Localisation and quantification of chemical functional groups on pulp fibresKlash, Abdalah 12 1900 (has links)
Thesis (PhD (Chemistry and Polymer Science))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: The distribution of different free chemical functional groups on wood and pulp
fibres was determined by means of atomic force microscopy (AFM) with
chemically modified tips. Because these functional groups show a higher
affinity to similar groups on the substrate surface during scanning, AFM
images were recorded using an additional digital pulsed-force mode (DPFM)
controller. This allowed the distribution of the chemical components to be
imaged and to a degree, also to be quantified. The investigated tip coatings
showed different sensitivities towards the major chemical components present
in wood fibres. These components were determined on spin-coated films as
well as wood fibres. It was possible to make a clear distinction between
cellulose and lignin in both cases. This technique could therefore be used to
differentiate between cellulose and lignin present on pulp fibre surfaces and to
confirm the successful removal of lignin by pulping.
The chemical composition of wood fibres and fibre surfaces of several acacia
and eucalyptus species, and hybrids originating from various growth sites in
South Africa, are compared. The objective was to determine the differences in
chemical surface composition due to genetics or site. The motivation for this
was to eventually facilitate a tailor-made supply of wood for pulping which
results in an optimal blend of fibres that can be pulped together with similar
yields. This, however, first requires a sound knowledge of the fibre properties.
The surface functionality on the single fibre level is a key property because it
determines how good inter-fibre bonding will be when paper is formed, which
in turn depends to a large degree on the amount of free hydroxyl groups that
are available and therefore on the cellulose content on the fibre surface.
The cellulose and lignin contents on the fibre surface were determined with
chemical force microscopy (CFM), a variation of AFM. CFM involves the use
of chemically modified tip using selected functional groups. Since, the general
bulk composition of the fibre and the surface composition differ, both
parameters were determined. Significant differences in the cellulose and lignin
content on fibre surfaces were found, with regard to genotype and site, respectively. In some, but not all, cases the surface composition of wood
fibres followed the bulk composition and differences were generally more
pronounced. Differences due to genotype were significant, especially with
regards to the surface lignin content - but variation due to site was also
distinctly recognisable.
This variation in surface functionality could be the reason why some pulpwood
blends result in a lower pulp yield and different quality. / AFRIKAANSE OPSOMMING: Die verspreiding van verskillende vrye chemiese funksionele groepe op hout
en pulpvesels is bepaal deur middel van atoomkragmikroskopie (AFM), met
chemies-gemodifiseerde punte (tips). Omdat hierdie funksionele groepe 'n
hoër verwantskap tot soortgelyke groepe op die substraat se oppervlak
gedurende skandering toon, kan AFM-beelde wat met 'n addisionele
digitalepulskragmodusbeheerstel bepaal word dit moontlik maak om die
verspreiding van die chemiese komponente uit te beeld en tot op ‘n sekere
vlak te kwantifiseer. Die ondersoekte punt-oppervlakmiddels het verskillende
sensitiwiteite teenoor die hoof chemiese komponente in die houtvesels en
spin-bestrykte films getoon. 'n Duidelike onderskeid kon in beide gevalle
tussen sellulose en lignien gemaak word. Hierdie tegniek kon dus gebruik
word om te onderskei tussen sellulose en lignien wat op die
pulpveseloppervlak teenwoordig was en om die suksesvolle verwydering van
lignien gedurende die pulpproses (pulping) te bevestig.
In hierdie studie is die chemiese samestelling van houtvesels en die
veseloppervlaktes van verskeie akasia en eucalyptus spesies, asook dié van
gekruisde spesies wat van verskeie werfliggings in Suid-Afrika afkomstig is,
vergelyk. Die doel was om te toets vir verskille in chemiese
oppervlaksamestellings, wat veroorsaak kan word deur genetika of werf
verskille, met die uiteindelike mikpunt om ‘n spesiaal-gemaakte voorraad van
hout vir pulping te fasiliteer, wat kan lei tot 'n optimale mengsel van vesels wat
saam gepulp kan word met soortgelyke opbrengs. Dit vereis natuurlik 'n goeie
kennis van die veseleienskappe. Die oppervlakfunksionering van enkel vesels
is ‘n kritiese eienskap omdat dit bepaal hoe goed interveselverbindings sal
wees wanneer papier gevorm word. Dit hang tot ‘n groot mate af van die
hoeveelheid vry hydroksielgroepe wat beskikbaar is en dus ook van die
sellulose inhoud op die veseloppervlak.
Die sellulose- en lignieninhoud op die veseloppervlak is bepaal met chemiese
kragmikroskopie – 'n variasie van atoomkragmikroskopie. Omdat die
algemene grootmaat samestelling van die vesel en die oppervlaksamestelling mag verskil, is altwee parameters bepaal. Beduidende verskille in die
sellulose- en lignieninhoud, met betrekking tot genotipe en werfligging, op
veseloppervlaktes is gevind. In sommige, maar nie alle, gevalle het die
oppervlaksamestelling van houtvesels ooreengestem met die
grootmaatsamestelling, en verskille was oor die algemeen meer beduidend.
Verskille as gevolg van genotipe was beduidend, veral met betrekking tot die
oppervlak lignieninhoud, maar variasie as gevolg van werfligging was ook
duidelik herkenbaar.
Hierdie variasie in oppervlakfunksionering kan die rede wees hoekom
sommige pulp–hout mengsels lei tot 'n laer pulpopbrengs en verskille in
kwaliteit.
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Polypropylene/filler nanocomposites by melt compounding and in situ polymerizationSoltan, Omar 12 1900 (has links)
Thesis (PhD (Chemistry and Polymer Science))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: The properties of polymer nanocomposites depend greatly on the
chemistry of the polymer matrices, the nature of the nanofillers, and the way in
which they are prepared. Understanding the synthesis–structure–property
relationship of nanocomposites is vital for the development of advanced
polymer nanocomposites with enhanced mechanical strength, stiffness and
toughness for structural engineering applications. To this end, the primary aim
of this study was to determine the impact that the preparation methods have
on the properties of PP/filler nanocomposites, with specific focus on the in situ
polymerization of propylene via the methylaluminoxane (MAO) activated
metallocene catalyst technique.
Two different fillers (Silica and Calcium carbonate) were used as
support for the metallocene catalysts. Different supporting methodologies for
the synthesis of the supported catalyst were examined. A C2 symmetric
metallocene catalyst ansa dimethylsilylbis(2-methyl benzoindenyl) zirconium
dichloride (MBI) was used in this study. The catalyst systems were then
evaluated for propylene polymerization.
The early observation shows that a direct adsorption of the metallocene
onto the filler has a diminishing effect on the catalyst productivity and the
fillers had to be treated with MAO in order to avoid catalyst deactivation by the
filler surface. Due to the low productivity of the supported active species, the
presence of soluble catalyst active species, besides the supported active
species is required in the synthesis of PP nanocomposites via in situ
polymerizations.
The syntheses of PP nanocomposites were carried out via in situ
polymerization in which different quantities of MAO treated fillers were reacted
with pre-activated catalyst solution. The effect of the addition of MAO-filler on
the polymerization kinetics and consequently on PP matrix microstructure was
investigated. Changes in the in situ polymerization kinetics, compared to
kinetics of homogeneous polymerization, were observed. Therefore, the microstructure of the polymer matrix was also influenced by the presence of
nanofillers in the polymerization media.
The influence of the different synthesis methods on the performance of
the nanocomposites was investigated using melt-mixed PP/filler
nanocomposites obtained using PP homopolymer. The dispersed phase
morphologies of the different nanocomposites were investigated by
transmission electron microscopy (TEM). Results show that PP
nanocomposites with improved filler dispersion were achieved by in situ
polymerization compared to melt-mixed nanocomposites.
The influence of the synthesis method on the crystallization behaviour
of PP nanocomposites was also investigated. It was found that, for the in situ
prepared nanocomposites the tacticity of the PP matrix plays the major role in
determining the degree of crystallinity. Results also show that when
nanocomposites with comparable PP matrices are compared, the overall
crystallization rate of the in situ polymerized nanocomposites is higher than
that of the melt mixed nanocomposites.
The mechanical properties of in situ polymerized PP and melt mixed
PP nanocomposite were also investigated and compared. Due to improved
nanoparticle dispersion in the PP matrix, in situ polymerized nanocomposites
show enhanced mechanical properties, especially tensile and impact
properties, compared to pure PP and melt mixed prepared nanocomposites
when a PP matrix of equivalent microstructure was used.
Finally, the melt compounding method was further investigated using
different fillers and commercial PP as a matrix. The effect of filler type, size
and applied surface coating on the flow and mechanical properties of PP
nanocomposites was studied. The aim of this part of this study is to obtain a
good trade-off between the processability and the mechanical properties and
to gain insight into the cause of the emergence of different properties for
nanocomposites prepared by melt compounding. / AFRIKAANSE OPSOMMING: Die eienskappe van polimeer nanokomposiete hang grotendeels af van
die chemie van die polimeer matriks, die wese van die nano-vullers, en die
manier waarop hierdie materiale berei word. Om die sintese-struktuureienskap
verwantskap te verstaan is noodsaaklik vir die ontwikkeling van
gevorderde nanokomposiete met beter meganiese eienskappe, styfheid en
taaiheid vir strukturele ingenieurstoepassings Die primêre doelstelling van
hierdie studie was dus om die impak van voorbereidingsmetodes op die
eienskappe van PP/vuller nanokomposiete te bestudeer, met spesifieke fokus
op die in-situ polimerisasie van propileen met metiel alumoksaan-geativeerde
metalloseen kataliste.
Twee verskillende vullers (silika en kalsium karbonaat) is gebruik as
ondersteuning vir die metalloseen kataliste. Verskillende metodiek is gebruik
om die ondersteunde kataliste te berei. ‘n C2 simmetriese metalloseen katalis
ansa-dimetielsiliel(2-metiel bensoindeniel) sirkonium dichloride (MBI) is in die
studie gebruik. Die katalissisteme is daarna evalueer vir propileen
polimerisasie.
Daar is oorspronklik vasgestel dat direkte adsorpsie van die
metalloseen op die vuller ‘n negatiewe effek op die katalis aktiwiteit gehad het,
en dat die vuller oppervlak eers met MAO behandel moes word om
deaktivering van die katalis deur die vuller-oppervlak te vermy. As gevolg van
die lae aktiwiteit van die ondersteunde aktiwe katalisspesies, is die
teenwoordigheid van opgeloste aktiewe katalis nodig vir die voorbereiding van
PP nanokomposiete via in situ polimerisasie-reaksies.
Die sintese van PP nanokomposiete is uitgevoer deur in –situ
polimersiasie waartydens verskillende hoeveelhede MAO-behandelde vullers
gereageer is met vooraf-geaktiveerde katalis oplossings. Die effek van die
byvoeging van MAO-vuller op die polimerisasie-kinetika en gevolglik op die
PP matriks mikrostruktuur is ondersoek. Dit is gevind dat die mikrostruktuur
van die polimeer-matriks beinvloed word deur die teenwoordigheid van
nanovullers in die polimerisasie-medium. Die invloed van verskillende bereidingsmetodes op die eienskappe van
die nanokomposiete is ondersoek deur smelt-vermengde PP/vuller
nanokomposiete te maak. Die dispersie-fase morfologie van verskillende
nanokomposiete is ondersoek deur transmissie elektron mikroskopie (TEM).
Resultate wys dat PP nanokomposiete met verbeterde vuller-dispersie berkry
is deur in situ polimerisasie in vergelyking met die smelt-vermengde materiale.
Die effek van die sintese-metode op die kristallisasie van die PP
nanokomposiete is ook ondersoek. Daar is gevind dat, vir die in situ bereide
nanokomposiete, die taktisiteit van die PP matriks die grootste rol speel in die
bepaling van die persentasie kristalliniteit. Resultate het ook gewys dat,
wanneer nanokomposiete met soortgelyke PP matrikse vergelyk word met die
in situ nanokomposiete, die laasgenoemde se tempo van kristallisasie hoer is
as vir die smelt-vermengde nanokomposiete.
Die meganiese eienskappe van die in situ bereide en smelt-vermengde
PP nanokomposiete is ook ondersoek en vergelyk. As gevolg van verbeterde
nano-partikel dispersie in die PP matriks, het die in situ bereide
nanokomposiete beter meganiese eienskappe openbaar, in vergelyking met
die smelt-vermenge nanokomposiete, veral trek- en slagsterkte. .
Laastens is die smelt-vermengings metode verder ondersoek deur
gebruik te maak van verskillende vullers en kommersiële PP as matriks. Die
effek van die tipe vuller, die grootte en die oppervlakbedekking van die
vullerpartikels op die vloei en meganiese eienskappe van die PP
nanokomposiete is ondersoek. Die doel van hierdie studie was om ‘n balans
te kry tussen prosesseerbaarheid en meganiese eienskappe en om insig te
verkry oor die verskille in eienskappe wat openbaar word wanneer smeltvermengde
nanokomposiete bereie word.
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