Spelling suggestions: "subject:"apolymer bcience"" "subject:"apolymer escience""
81 |
Synthesis and characterization of telechelic hydroxyl functional poly (N-vinylpyrrolidone)Pfukwa, Rueben 03 1900 (has links)
Thesis (MSc (Chemistry and Polymer Science))--Stellenbosch University, 2008. / Reversible addition fragmentation chain transfer (RAFT)-mediated polymerization has emerged as a
versatile method for preparing polymers with control over molecular weight and polydispersity.
Inherent in its mechanism is the retention of the chain transfer agent, the RAFT agent, at the
polymer chain ends. Typically RAFT agents are made up of two parts, the so called R (leaving) and
Z (thiocarbonyl thio, stabilizing) groups. These are retained as the a-and the w-end groups of the
final polymer, respectively. RAFT polymerization offers a ready method for preparing polymers
with well defined end functionalities.
The a-end functionality can easily be built into the R group. The Z group, however, is thermally
unstable and can impart color and smell to the polymer. Hence, two new methods for Z end group
removal were introduced. Both methods take advantage of the facile reaction between thiocarbonyl
thio compounds and radicals. By matching the functionalities of the R group (a-end group) with
that of the end modified w-chain end, both methods offer an easy route to accessing telechelic
functional polymers. End functional polymers have many important uses in industry and in the
biomedical field.
An alcohol functional xanthate RAFT agent was synthesized and successfully used to conduct the
RAFT-mediated polymerization of N-vinylpyrrolidone (NVP). Characterization by NMR and
MALDI ToF MS confirmed that a-hydroxyl-w-xanthate-functional PVP was easily produced.
In the first end group modification method radicals were generated as in atom transfer radical
addition (ATRA). A hydroxyl functional a-haloester was used as the ATRA initiator with a Cu
catalyst system. The alkyl radical produced by this ATRA initiator then replaced the Z group giving
a telechelic hydroxyl functional polymer. NMR analysis showed that the thiocarbonyl thio end
group was completely removed. The hydroxyl functionality was quantified by derivatizing with
trichloro acetyl isocyanate and subsequent analysis by NMR. MALDI ToF MS analysis, however,
was inconclusive. In the second method the thiocarbonyl thio end group was removed by simply heating the polymer
with hydrogen peroxide, thereby replacing the Z group with a hydroxyl end group at the w-chain
end, giving a telechelic functional polymer. The telechelic hydroxyl functional polymer was
subsequently crosslinked with a trifunctional isocyanate to make a PVP hydrogel. This confirmed
that the end-modified polymer was indeed telechelic. The swelling kinetics of this hydrogel were
determined in water at 37 oC.
|
82 |
Asymmetric flow field flow fractionation (AF4) of polymers with focus on polybutadienes and polyrotaxanesMakan, Ashwell Craig 03 1900 (has links)
Thesis (MSc)-- Stellenbosch University, 2012. / ENGLISH ABSTRACT: Over the past two decades, field flow fractionation (FFF), as a polymer characterization technique, has
become cutting edge technology. The demand for molar mass and size characterisation of complex
polymer systems has increased, especially in cases where classical calibration techniques such as
size exclusion chromatography (SEC) has shown several shortcomings. FFF is a technique
resembling chromatography. It has several significant advantages over SEC, especially for the
characterisation of ultrahigh molar mass (UHMM), branched and gel-containing polymers. In this
study, polybutadienes, which often contain the abovementioned species, were analysed by SEC and
asymmetric flow field flow fractionation (AF4). Both separation techniques were coupled to refractive
index and multi-angle laser light scattering detection. Similarly, polyrotaxanes, which are polymers
with complex and unique molecular architectures, were also investigated. Results showed that AF4
can explicitly be used as a superior tool over SEC. In the case of UHMM polybutadienes, much higher
molar masses could be detected by AF4, due to the absence of shear degradation which is often
encountered in SEC. Gel-containing species could be detected by AF4 as no filtering is required prior
to injection. Abnormal retention behaviour, a phenomenon often encountered in UHMM branched
polymers, was observed in SEC analysis of the polyrotaxanes materials. AF4 provided sufficient
separation from low to high molar masses, without out any irregularities. / AFRIKAANSE OPSOMMING: Gedurende die afgelope twee dekades het veldvloeifraksionering (FFF) as ‘n
polimeerkarakteriseringstegniek groot veld gewen. Die aanvraag na molekulêre massa en groottekarakterisering
van komplekse polimeersisteme het toegeneem, veral in die gevalle waar klassieke
kalibrasietegnieke soos grootte-uitsluitingschromatografie (SEC) etlike tekortkominge getoon het. FFF
is ‘n tegniek soortgelyk aan chromatografie, en het voorheen bewys dat dit oor ‘n redelike aantal
voordele bo SEC beskik, veral in die geval van ultrahoë molekulêre massa- (UHMM-), vertakte- en jelbevattende
spesies. In die huidige studie is polibutadieenpolimere, wat dikwels bogenoemde spesies
bevat, geanaliseer met behulp van SEC en onsimmetriese vloei-veldvloeifraksionering (AF4). Beide
skeidingstegnieke is gekoppel aan ‘n brekingsindeks en multihoek-laserligverstrooiingsdetektors. Op
dieselfde wyse is polirotaksane (polyrotaxanes) met komplekse molekulêre argitektuur bestudeer.
Daar is bewys dat AF4 uitsluitlik gebruik kan word as ‘n meer geskikte tegniek bo SEC. Baie hoër
molekulêre massas kon deur middel van AF4 vir UHMM polibutadieenpolimere raakgesien word as
gevolg van die verminderde afbrekende degradasie wat dikwels voorkom met SEC. Jel-bevattende
spesies is suksesvol geïdentifiseer met behulp van AF4 waartydens geen filtrering vir analise nodig
was nie. Abnormale retensie was sigbaar tydens SEC analise van monsters van polirotaksane, wat
dikwels voorkom in vertakte polimere. In teenstelling het AF4 bewys dat ‘n bevredigende skeiding van klein na groot molekulêre massas, sonder enige tekortkominge, moontlik is.
|
83 |
Poly(N-Vinylpyrrolidone) based biomimetic hydrogelsWilken, Celeste 03 1900 (has links)
Thesis (MSc)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: In the modern, fast moving society of today, there is a strong focus in research placed on
the elimination of lengthy invasive medical procedures. Target specific, and chemo selective
treatments are of great interest. Various advantages of this include patient comfort and over
and above all cost reduction of invasive procedures. This project focused on the development
of an injectable gel system with the use of biocompatible polymers. A system is regarded as
injectable if the functional polymers are in solution before administration, but gels when
added together. In order to design this system, one has to make use of polymers with
complimentary functional groups.
Telechelic amino-functionalized PVP with narrow molecular weight distribution was
synthesized via RAFT-mediated polymerization. The polymer was thoroughly characterized
and cross-linked with Poly(styrene-alt-maleic anhydride) to form a three dimensional
polymeric network capable of absorbing and retaining large amounts of water and or
biological fluid. Unfortunately the cross-linking needed to be performed in non-aqueous
solution due to hydrolysis of maleic anhydride as a competing reaction in water.
The gel was used in two model studies. The first model study focused on the attachment of
a synthetic polypeptide onto the gel. The second model study evaluated the cytotoxicity
effects of these gels when placed in direct contact with rodent cardiac myoblast and myocyte
cells. These studies rendered promising results for future biological applications. / AFRIKAANSE OPSOMMING: In die moderne, vinnig bewegende samelewing van vandag, word daar in navorsing, 'n sterk
fokus geplaas op die uitskakeling van lang indringende mediese prosedures. Doel spesifieke,
en chemo selektiewe behandelings is van groot belang. Verskeie voordele van hierdie is die
toename in gemak van die pasiënt en die verlaging van kostes verbonde aan hierdie
indringende prosedures. Hierdie projek het gefokus op die ontwikkeling van 'n inspuitbare gel
stelsel deur gebruik te maak van biologiese aanvaarbare polimere. ‘n Stelsel word beskou as
inspuitbaar, indien die funksionele polimere in oplossing is voor toediening, maar wel kan gel
wanneer bymekaar gevoeg word. Om hierdie tipe stelsel te kan ontwerp moet daar gebruik
gemaak word van polimere met beskikbare funksionele groepe.
Telecheliese amino-funksionele Poly(N-vinielpirollideen) met 'n smal molekulêre gewig
verspreiding is gesintetiseer deur middel van RAFT-bemiddelde polimerisasie. Die polimeer
is deeglik gekarakteriseer en daarna gekruis-koppel met P(STY-alt-MAnh) om 'n driedimensionele
polimeriese netwerk te vorm. Hierdie netwerk is dan ook in staat om groot
hoeveelhede water en/of biologiese vloeistof te absorbeer en te behou. Ongelukkig was
hierdie reaksie uitgevoer in ʼn nie-waterige oplossing as gevolg van die hidrolise van Maleïne
anhydride as 'n mededingende reaksie in water.
Die gel is in twee model studies gebruik. Die eerste model-studie het gefokus op die
beslaglegging van 'n sintetiese polipeptied op die gel. Die tweede model studie het die
sitotoksiese uitwerking van hierdie gels in direkte kontak met knaagdier hart myoblast en
lymfocyten selle geëvalueer. Hierdie studies het dan ook belowende resultate vir toekomstige
biologiese toepassings gelewer.
|
84 |
Synthesis and characterization of cationically and anionically modified poly(vinyl alcohol) microfibrilsChirowodza, Helen 03 1900 (has links)
Thesis (MSc (Chemistry and Polymer Science))--University of Stellenbosch, 2009. / In papermaking, the addition of filler can be detrimental to the properties of the resulting
paper hence the use of additives that enhance paper properties are of paramount importance.
Syndiotacticity rich poly(vinyl alcohol) (PVA) microfibrils were prepared for use as filler
retention aids. They were prepared via in situ fibrillation during the saponification of high
molecular weight poly(vinyl pivalate). The resulting fibers had high thermal stability and
crystalline melting temperature. They were not fully soluble in water even at 100 oC. In order
to make them less water resistant the syndiotacticity of the PVA microfibrils was varied by
copolymerizing vinyl pivalate with vinyl acetate and saponifying the resultant copolymer. It
was observed that changes in syndiotacticity had a significant effect on the crystallinity,
morphology and thermal properties of the resultant PVA.
The surfaces of the fibers were modified by first crosslinking using glyoxal (a dialdehyde),
and then attaching cationic and anionic groups by grafting and by carboxymethylation.
Crosslinking prior to modification was beneficial in minimizing the solubility of the fibers in
the aqueous media in which they were modified. Heterogeneous modification techniques were
employed so that fiber properties could be preserved. Carboxymethylation was carried out
using the two step Williamson’s ether synthesis. The first step involves the formation of a
highly reactive alkoxide by the reaction of PVA with a strong base and the second its
etherification using a functional alkyl halide. Poly(methacryloyloxy ethyl trimethyl
ammonium chloride) and poly(acrylic acid) were grafted from the PVA microfibrils using the
KPS/Na2S2O3 redox initiation system. Grafting was confirmed by FTIR and NMR
spectroscopy.
Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were carried
out on both modified and unmodified PVA microfibrils. The results showed that crosslinking
resulted in an enhancement of the thermal properties of the microfibrils. A decline in the onset
temperature for thermal degradation and crystalline melting temperature were observed, and
were attributed to the modification of the PVA microfibrils.
|
85 |
Investigating the effect of dyeing on the surface of wool fibres with atomic force microscopy (AFM)Abduallah, Abduelmaged 03 1900 (has links)
Thesis (MSc (Chemistry and Polymer Science))--University of Stellenbosch, 2006. / Dyeing has an influence on the characteristic properties of wool fibres. This can result in
changes in the final properties of the fibre including fibre elasticity, fibre strength and
breaking elongation of the wool fibres, especially in the case of dark colours. Damage
that occurs to the fibre surface due to the action of acid, alkali, dyestuff, water, heat, and
mechanical stress during the dyeing process can thus have an affect on the fibre breaking
elongation and the fibre strength. The aim of this project was to assess the effect of
dyeing with different colours (ranging from light to dark) on the surface of wool fibres
using atomic force microscopy (AFM).
The results indicated that four different types of surface damage can be discerned: scale
raising, scale chipping, fluting and pitting. The findings also indicate that the surface
damage to the fibres was greater and more noticeable (especially the scale raising) on
fibres dyed with dark colours than to fibres dyed with light colours. The same applied to
the fibre strength and elongation, where generally the fibre breaking extension and the
breaking load was smaller in the case of darker colours.
The effect of dyeing with different colour shades on the fibre surface was observed with
AFM. Several characteristic scale parameters of a statistical significant number of dyed
and undyed fibres were measured in order to determine the surface damage caused by
dyeing. The correlation between the surface damage, observed by AFM, and the loss in
tensile strength and fibre elongation, determined by tensile tests, were investigated. This
was done in order to determine the effect of the cuticle damage on the tensile stress/strain
behaviour. The results show that AFM is a viable tool to study the effects of different
dyes on the fibre surface and for detecting surface modifications with great accuracy.
It was found that dyeing with dark colours caused greater damage to the surface than
light colours, and it was possible to distinguish different lightness areas, which could be
regarded as light and dark colours.
|
86 |
The effect of molecular architecture on the properties of propylene impact copolymersBasson, N. C. 03 1900 (has links)
Thesis (MSc (Chemistry and Polymer Science))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: Impact polypropylene copolymers (IPPC) are important commercial materials, but their morphology and molecular architecture are not yet fully understood. In this study the focus was on selectively removing specific fractions from the original IPPC, recombining the remaining fractions, and studying the properties of these recombined polymers.
It was found that some properties of the samples changed remarkably, depending on the fraction of material that was removed before recombination. For example, the degree of phase separation and the crystalline morphology of the recombined materials varied noticeably.
During the study an effective way of staining samples for transmission electron microscopy (TEM) was developed. Furthermore, a comparison of fourier transform infrared spectroscopy (FTIR), with TEM and scanning electron microscopy (SEM) results, revealed a hitherto unreported relationship between phase separation.
Absorption bands appeared at 1100 cm-1 and 1080 cm-1 in the FTIR spectra and appear to be an indication of phase separation. It was further established that specific copolymer fractions present in the original polymer affect not only the morphology of the final polymer, but also the hardness and impact resistance. / AFRIKAANSE OPSOMMING: Impak polipropileen kopolimere (IPPK) is belangrike kommersïele materiale, maar die kennis met betrekking tot die morfologie en molekulere argitektuur van die materiale is nog gebrekkig. Tydens hierdie studie was die fokus op die selektiewe verwydering van spesifieke fraksies van die oorspronklike IPPK, herkombinering van die oorblywende fraksies, en die studie van die eienskappe van hierdie herkombineerde polimeriese materiale.
Daar is gevind dat sommige van die eienskappe van die herkombineerde materiale daadwerklik verskil van die oorspronklike materiaal, en dat die verskille direk verband hou met die spesifieke fraksie wat uit die oorspronklike materiaal verwyder is. Die mate van fase-skeiding asook die kristal-morfologie van die herkombineerde materiaal het opmerklik verskil van die oorspronklike.
Tydens die studie is n effektiewe manier ontwikkel om die materiale te vlek vir transmissie elektron-mikroskopie (TEM). Verder is daar ‘n verband tussen die resultate verkry deur Fourier Transform infrarooi spektroskopie (FTIS) en die verkry met TEM en skandeer elektron mikroskopie (SEM) vasgestel. Die verwantskap tussen FTIS en fase-skeiding is tot dusver nie in die wetenskaplike literatuur vermeld nie. Meer spesifiek is daar gevind dat absorbsie-bande sigbaar by 1100 cm-1 en 1080 cm-1 in die FTIS spektra ‘n aanduiding van fase skeiding kan wees. Dit was verder vasgestel dat spesifieke kopolimeer fraksies wat teenwoordig is in die oorspronklike IPPK, nie alleen die morfologie van die materiaal beinvloed nie, maar ook die hardheid en impak-weerstand van die materiaal.
|
87 |
The synthesis of 1-butene oligomers with vinyl endgroups and their use in further reactionsAl-Aeeb, Ahmed 03 1900 (has links)
Thesis (MSc (Chemistry and Polymer Science))--University of Stellenbosch, 2007. / This study comprises the synthesis, functionalization, and characterization of 1-butene
oligomers, as well as the synthesis of oligobutene-based macro-RAFT agent.
The directed oligomerization of 1-butene was carried out with a Cp2ZrCl2 as a catalyst,
activated with MAO as a co-catalyst (10% in toluene), in the ratio Al/Zr =1000/1.
Oligomers possessing vinylidene double bonds, with low molecular weight (Mw), ranging
between 800 and 2000 g.mol-1 as confirmed by gel permeation chromatography, were
obtained. The oligomers were successfully functionalized by adding hydroxyl
functionality to the vinylidene double bond using oxymercuration-demercuration
reaction, and as a result hydroxy-terminated oligobutenes were obtained.
Characterization techniques such as 1H NMR, 13C NMR, GC-MS and FTIR confirmed
the successful synthesis and functionalization of 1-butene oligomers.
The hydroxy-terminated oligobutenes were used to prepare an oligobutene-based
macro-RAFT agent. The synthesis of the macro-RAFT agent was carried out with an
esterification reaction between the hydroxy-terminated oligobutenes and an acid
functionalized RAFT agent. The successful synthesis of the macro-RAFT agent was
confirmed by 1H NMR, 13C NMR, FTIR, and UV spectroscopy. The chain transfer ability
of the macro-RAFT agent to induce living characteristics in free radical styrene
polymerization was investigated with respect to molecular weight control and kinetic
behaviour. The macro-RAFT agent was identified as suitable RAFT agent, yielding
polystyrene-b-oligobutnes with low polydispersities and molecular weight ranging from
3000 to 40000 g/mol.
|
88 |
RAFT mediated polysaccharide copolymersFleet, Reda 12 1900 (has links)
Thesis (MSc (Chemistry and Polymer Science))--University of Stellenbosch, 2006. / Cellulose, one of the most abundant organic substances on earth, is a linear polymer
of D-glucose units joined through 1,4-β-linkages. Cellulose is however not easily
processed without chemical modification. A number of techniques exist for the
modification of cellulose, of which the viscose process is one of the most widely
applied. Grafting of synthetic polymeric chains onto or from cellulosic materials is an
useful technique that can be used to combine the strengths of synthetic and natural
polymers dramatically, so changing the properties of cellulosic materials (pulp,
regenerated cellulose, cellulose derivatives).
In this study five model xanthate (Reversible Addition-Fragmentation chain Transfer
(RAFT)/Macromolecular Design through Interchange of Xanthates (MADIX)) agents,
namely, monofunctional, difunctional, trifunctional and tetrafunctional species of the
form S=C(O-Z)-S-R, with different leaving groups and different activating moieties,
were prepared and then studied to determine the feasibility of cellulose modification
via addition fragmentation processes. These agents were characterized by Nuclear
Magnetic Resonance spectroscopy (NMR), Fourier Transform Infrared spectroscopy
(FT-IR) and Ultraviolet spectroscopy (UV). Polyvinyl acetates (PVAc) in the form of
linear, three armed and four armed star shaped polymers were then successfully
synthesized in reactions mediated by these xanthate RAFT/MADIX agents
Xanthates were applied to polysaccharide materials using the viscose process
(xanthate esters were formed directly on a cellulosic substrate, with subsequent
alkylation) Grafting reactions were then conducted with the polysaccharides; cellulose
was modified with vinyl acetate, [this is an example of a surface modification of
natural polymers that is of interest in various industries, such as textiles and paper
manufacture].
Analysis of the graft copolymers was conducted via Size Exclusion Chromatography
(SEC), Liquid Adsorption Chromatography (LAC), Thermogravimetric Analysis
(TGA), and FT-IR.
Polyvinyl acetate was successfully grafted onto three polysaccharides (cellulosic
materials), namely Hydroxyl Propyl Cellulose (HPC), Methyl Cellulose (MC) and
cellulose. The study showed that the modification of cellulosic substrates with defined
grafts of vinyl acetate can be easily achieved through minor modifications to existing
industrial techniques.
|
89 |
Hydrophobic core/shell particles via miniemulsion polymerizationEtmimi, Hussein Mohamed 12 1900 (has links)
Thesis (MSc (Chemistry and Polymer Science))--University of Stellenbosch, 2006. / Hydrophobic core/shell latex particles were synthesized for use in barrier coatings using the
miniemulsion polymerization process. Particles with liquid or with hard cores were
successfully synthesized using miniemulsion as a one-step nanoencapsulation technique.
Different materials, including an oil (hexadecane, HD) and two different waxes (paraffin and
microcrystalline wax), were used as the core of the particles. The shell of the particles was
mainly made from a copolymer containing three relatively hydrophobic monomers, namely
methyl methacrylate (MMA), butyl acrylate (BA) and vinyl neodecanoate (Veova-10).
Before any further investigations could be carried out, it was important to determine the
morphology of the synthesized core/shell particles at the nanometer level. Particle
morphology was mainly determined by two different techniques: transmission electron
microscopy (TEM) and atomic force microscopy (AFM). TEM was used to directly visualize
the morphology of the investigated core/shell particles at the nanometer level, while AFM
was used to confirm the formation of these core/shell particles. AFM was a powerful
technique with which to study the particle morphology of the core/shell latices during the
film formation process.
As a second part of the study, the effect of various factors on the hydrophobicity and barrier
properties of the resulting films produced from the synthesized core/shell latices to water and
water vapour was investigated. This included the effect of: (i) the surfactant concentration,
(ii) the wax/polymer ratio for both waxes, (iii) the molecular weight of the polymeric shell,
(iv) the amount of the most hydrophobic monomer used (Veova-10), and (v) the degree of
crosslinking in the polymeric shell.
Results showed that all the above-mentioned factors had a significant impact on the water
sensitivity of the resultant films prepared from the synthesized core/shell latices. It was
found that the presence of wax materials as the cosurfactant, instead of HD, in the
miniemulsion formulation could significantly improve the hydrophobicity and barrier
properties of the final films to water and water vapour. In addition, increasing the amount of
wax, Veova-10, and the molecular weight of the resultant polymeric shell, led to a significant
increase in the hydrophobicity and barrier properties of the resultant latex films. In contrast,
hydrophobicity and water barrier properties decreased drastically as the quantity of surfactant
and degree of crosslinking increased in the final latex films.
|
90 |
Co-crystallization in polyolefin blends studied by various crystallization analysis techniquesSweed, Muhamed 03 1900 (has links)
Thesis (MSc (Chemistry and Polymer Science))--University of Stellenbosch, 2006. / Co-crystallization is the phenomenon by which chains of different crystallizabilities
crystallize at the same temperature. Co-crystallization is frequently observed in the blends of
different types of polyethylene. It is believed that co-crystallization can occur due to the
thermodynamically miscible parts of two components in the blends having similar
crystallization rate.
The study focused on the phenomenon of co-crystallization in polyethylene blends and how
by varying the crystallization conditions the co-crystallization region will change.
Three techniques have been used in this study. TREF was used to fractionate the polymers
and blends. Each of the TREF fractions was studied using both DSC and CRYSTAF to
determine whether the fraction contained both types of materials. It is shown that the
difference in the crystallization fractionation mechanisms between TREF, CRYSTAF and
DSC can be utilize to study co-crystallization effects in polyethylene blends.
Results also shows that by varying the heating and cooling rate profiles in DSC and
CRYSTAF the co-crystallization fractions will appeared as a single fraction or as two separate
fractions. Further, it was demonstrated how the co-crystallization area could be illustrated
using a unique 3-dimensional plot where the data from the prep-TREF fractionation, and the
DSC and CRYSTAF, were combined to give the “crystallization map” of the blend. These
plots give a quiche visual illustration of any co-crystallization regions in the blends as well as
how much the crystallization conditions effect the blend crystallization.
|
Page generated in 0.076 seconds