Electrospun carbon nanotube filled composite nanofibers by non-covalent compatibilization

Siebert, Piere-Andre John (2016-03)

Thesis (MSc)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: This study covers the successful production of poly(acrylonitrile) (PAN)/multi-walled-carbon-nanotube (MWCNT) fibrous composites by the incorporation of noncovalently functionalized MWCNTs in various loading fractions. In maintaining the sp2 hybridization as opposed to the conventional covalent functionalization, the conjugated MWCNT structure is maintained and in doing so preserves the mechanical and conductive properties commonly associated with these fillers. This allows for the optimum transference of desired property/properties to the produced composite assuming sufficient filler dispersion. MWCNTs are functionalized by polymeric compatibilizers of mono- and multi pyrene functional character. The pyrene moiety allows for the noncovalent 𝜋-𝜋 stacking interaction to the conjugated surface of MWCNTs by strong van der Waals forces. Mono pyrene functional poly(acrylonitrile) (Py-PAN) polymers were synthesized via Atom Transfer Radical Polymerization (ATRP) from presynthesized pyrene functional initiators namely 1-pyrenemethyl-2-bromoisobutyrate and 1-pyrenebutyl-2-bromoisobutyrate respectively. The synthesis of PAN polymers of α-chain end pyrene functionality was also attempted by the relatively unexplored redox initiated polymerization by catalytic cerium ammonium nitrate (CAN) from 1-pyrenemethanol and 1-pyrenebutanol initiators respectively. The synthesis proved challenging from the proposed initiators and with relatively low yields even after an extensive method development in an effort to achieve optimum reaction conditions. Multi pyrene functional PAN polymers (PAN-co-PyMMP) was successfully prepared via Conventional Free Radical (CFR) copolymerization of conventional acrylonitrile (AN) monomers and presynthesized (1-pyrene)methyl-2-methyl-2-propenoate (PyMMP) macromonomers in producing a PAN based polymers of protruding pyrene moieties along the polymer chain. The multi pyrene functional copolymer is successfully prepared to three predetermined PyMMP compositions (or mol% functionality) by variation of the monomer feed ratio. The synthesized initiators as well as the monomer and functional polymers were analyzed by 1H-NMR analysis confirming the isolation of the hypothesized products. 1H-NMR was also implemented to determine the average composition of the pyrene moiety incorporated into both of the structurally alternate functional PAN polymers presented as the mol% pyrene functionality. Mono- and multi-functional variations of the PAN polymer were exposed to increasing amounts of MWCNTs and analyzed by 1H-NMR following reported instances of a quenching phenomenon being seen in the 1H-NMR spectra of signals representing protons taking part in 𝜋-𝜋 stacking interactions (protons protruding from the pyrene moiety). The analysis produced a quenching trend of the polymers as the MWCNT loading is increased. These trends are confirmed by the more common method of tracking these noncovalent 𝜋-𝜋 stacking interactions by the quenching phenomenon apparent from the UV-fluorescence analysis of the pyrene moieties. Trends observed are presented to be linear as the mono-functional polymer is exposed to increasing amounts of MWCNTs where as the multi-functional polymer shows a greater initial quenching followed by what seems to be an inversion point at a 20 wt% to 30 wt% MWCNT composition after which a linear trend emerges upon a further increase of MWCNT loadings comparable to what is seen for the mono-functional polymer. The quenching analysis suggests a stronger MWCNT/functional-polymer interaction for the multi-functional polymers at similar mol% pyrene functionalities. As envisioned, electrospinning is used to prepare the fibrous composite PAN/MWCNT materials that were prepared by the noncovalent functionalization of pristine MWCNTs via the mono and multi pyrene functional PAN compatibilizers. This lead to the successful dispersion of the MWCNT filler throughout the fiber matrix in an aligned state along the fiber axis, as shown by scanning electron microscopy (SEM) and transition electron microscopy (TEM) analysis. A preliminary study was undertaken to produce PAN/graphene fibrous nanocomposites using the novel compatibilizers, synthesized in this study. Association analysis with compatibilizers indicated weaker 𝜋- 𝜋 stacking interactions when compared to the MWCNT fillers. Further analysis sugest poor graphite exfoliation and filler dispersion within the fibrous matrix. The carbonization of noncovaently funtionalized fibrous MWCNT/PAN composites was sucsessful as shown by SEM analysis.

AFRIKAANSE OPSOMMING: Hierdie studie dek die suksesvolle sintese van poli(akrilonitriel) (PAN)/multi-wand-koolstof-nanobuisies (“multi walled carbon nanotubes (MWCNT)”) veselagtige samestellings deur die byvoeging van nie-kovalente gefunksionaliseerde MWCNTs van verskillende hoeveelhede. In teenstryding met kovalente funksionalisasie – sp2 hibridisasie van MWCNTs word behou deur die gebruik van ‘n nie-kovalente funksionalisasie metode. Die gekonjugeerde MWCNT struktuur word dus behou, en meganiese en geleidende eienskappe geassosieer met hierdie materiale word nie belemmer nie. Indien hierdie nie-kovalente gefunksionaliseerde MWCNTs gebruik word as vullers sal die eienskappe geassosieerd daarmee optimaal oorgedra word na die samestelling; indien die vuller eweredig in die samestelling versprei is. In hierdie studie is MWCNTs gefunksionaliseer met mono- en multi-pyrene om eweredige verspreiding toe te laat. Die pyrene eenheid heg aan die gekonjugeerde MWCNT struktuur deur ‘n nie-kovalente 𝜋- 𝜋 stapel interaksie met behulp van Van der Waals kragte wat teenwoordig is. Mono pyrene funksionele poliakrilonitriel (Py-PAN) polimere was sintetiseer deur gebruik te maak van Atoom Oordrag Radikale Polimerisasie ("Atom Transfer Radical Polymerization (ATRP)”). Pyrene funksionele inisieerders wat vooraf gesintetiseer is, naamlik 1-pyrenemetiel-2-bromoisobuteraat en 1-pyrenebutiel-2-bromoisobuteraat, is hiervoor gebruik. In ‘n poging om PAN polimere met ‘n α-ketting einde pyrene funksionaliteit te sintetiseer, was ‘n relatief onbekende redoks geïnisieerde polimerisasie ondersoek. Hierdie polimerisasie maak gebruik van serium ammonium nitraat (“cerium ammonium nitrate (CAN)”) as katalisator en beide 1-pyrenemetanol en 1-pyrenebutanol as inisieerders. Lae opbrengste is verkry deur gebruik te maak van die bogenoemde inisieerders. Die metode was daarom aangepas en ontwikkel in ‘n poging om optimale reaksie kondisies te vind, maar gewenste opbrengste is nie verkry nie. Suksesvolle sintese van multi pyrene funksionele PAN polimere (PAN-co-PyMMP) is bereik deur gebruik te maak van Konvensionele Vrye Radikaal (“Conventional Free Radical (CFR)”) ko-polimerisasie. ‘n PAN gebaseerde polimeer met pyrene eenhede wat uitsteek langs die lengte van die polimeer-ketting is dus gesintetiseer, en is bereik deur gebruik te maak van konvensionele akrilonitriel (AN) monomere en vooraf gesintetiseerde (1-pyrene)metiel-2-metiel-2-propenoaat (PyMMP) makro-monomere. Die multi pyrene funksionele ko-polimeer is suksesvol gesintetiseer met drie verskillende voorafbepaalde hoeveelhede van PyMMP (mol% funksionaliteit) deur die verhouding van monomeer wat toegevoeg is te verander. Die gesintetiseerde inisieerders, makro-monomeer en funksionele polimere is deur 1H-KMR analiseer, en isolasie van die voorafgestelde produk is hierdeur bevestig. 1H-KMR analise was ook gebruik om die gemiddelde hoeveelhede van die pyrene eenhede wat teenwoordig is in die mono- en multi-pyrene polimere te bepaal; waar die hoeveelhede as mol% pyrene funksionaliteit noteer is. Daar is berig dat pieke van protone wat in die 1H-KMR spektrum voorkom se intensiteit kleiner raak wanneer die protone deelneem aan 𝜋- 𝜋 stapel interaksies. Gevolglik is mono- en multi-funksionele variasies van die PAN polimeer met toenemende hoeveelhede van MWCNTs gemeng, en daarna deur 1H-KMR geanaliseer. In die 1H-KMR analise is daar opgelet dat die pieke van die protone, wat by die pyrene eenheid ‘uitsteek’, se intensiteit kleiner raak soos die hoeveelheid MWCNTs toegevoeg meer raak. Hierdie tendens is ook bevestig deur gebruik te maak van die meer gewilde UV fluoressensie ontledings-metode, waar die π-π stapel interaksies bevestig is deur pieke vir die pyrene eenhede wat se intensiteit kleiner raak. Lineê tendense is waargeneem vir die mono-funksionele polimeer, waar die intensiteit van die pieke kleiner raak hoe meer MWCNTs bygevoeg is. Vir die multi-funksionele polimeer word daar ‘n groot aanvanklike verlaging in die intensiteit gesien, gevolg deur ‘n inverise punt by ‘n gewigs fraksie van 20-30 (“weight persentage (wt%)”) MWCNT waarna 'n lineêre tendens na vore kom. Vergelykbare verhoudings van MWCNTs is by beide die mono- en multi-funksionele polimere bygevoeg. Die bogenoemde resultate dui daarop dat daar ‘n sterker interaksie tussen die multi-funksionele polimeer en die MWCNTs voorkom in vergelyking met die mono-funksionele polimeer en die MWCNTs, waar dieselfde funksionele polimere teenwoordig is. Die saamgestelde veselagtige PAN/MWCNT materiale is sintetiseer deur die nie-kovalente funksionalisasie van MWCNTs met mono- en multi-pyrene funksionele PAN kompatibiliseerders waarna die samestelling ge-elektrospin is Dit het gelei tot die suksesvolle en eweredige verspreiding van MWCNT vullers deur die vesel matriks. MWCNTs wat in lyn is met die lengte van die vesels is gesien in SEM en TEM beelde.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/98299
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