Porous carbon nanofibers containing silica-coated iron oxide nanoparticles by carbonisation of electrospun amphiphilic copolymer nanocomposites

Van Niekerk, Marthinus Gerhardus Mynhardt (2019-03)

Thesis (MSc)--Stellenbosch University, 2019.

Thesis

ENGLISH ABSTRACT: Amphiphilic PAN-g-PDMS copolymers were synthesized by conventional free radical polymerization of acrylonitrile monomer and PDMS macromonomer. Three chain lengths of PDMS macromonomer grafts were employed, each forming a series for which the monomer:macromonomer feed ratio was varied. SEC results showed a wide range of molar masses of the synthesized copolymers. This is to be expected given the uncontrolled nature of conventional free radical polymerization reactions. ¹H NMR measurements proved that a direct proportionality exists between the PDMS macromonomer feed and the incorporation thereof. Electrospinning of the synthesized PAN-g-PDMS copolymers from DMF was done at three different TCD’s. In some cases, the fiber diameters could be explained by considering the PDMS graft content in conjunction with the molar mass, though in many cases the reasons for patterns in fiber diameters could not be clarified. A possible reason for this is that the expected phase segregation was, to some extent, inhibited by the method of solution preparation that entailed stirring at 50 °C for an hour followed by sonication for an hour. It is noteworthy that copolymers containing 5 kDa PDMS grafts showed severe gelation of the polymer solution. DSC analysis performed on the as-synthesized powders as well as on the electrospun fibers revealed that the current samples do not follow any clear trends w.r.t. PDMS graft content. When considering the cyclization exotherm which is a necessary process in order to convert the precursor fibers into carbon fibers, an interesting observation was made for the three series of copolymers containing 5 kDa grafts: as the PDMS graft content is increased a shoulder forms which ultimately separates into a double peak in the thermogram. This is presumably due to severe phase separation of domains rich in PAN homopolymer and domains rich in PDMS grafts undergoing the cyclization reaction at different temperatures. Co-precipitation of IONPs yielded nanoparticles with a diameter of 10.2 ± 2.4 nm as determined by TEM analysis. Silica coating of the IONPs initially produced multi-core nanoparticles with a diameter of 62.6 ± 12.5 nm containing 72.2 wt.% silica determined gravimetrically. The amount of TEOS added as reagent was decreased in order to form single-core nanoparticles of 22.2 ± 4.3 nm containing 29.6 wt.% silica and thus a thinner shell thickness. Thermal pre-treatment of single-core nanoparticles at 800 °C produced nanoparticles that appear very similar to untreated single-core nanoparticles. Electrospun fiber-particle nanocomposites showed nanoparticle agglomeration in the case of multi-core nanoparticles and good dispersion in the case of single-core nanoparticles and thermally treated single-core nanoparticles. Fiber-particle nanocomposites containing thermally treated single-core particles electrospun after 18 h of solution ageing showed some particle agglomeration. Carbonisation of the fiber-particle nanocomposites containing thermally treated nanoparticles produced intact carbon fiber-particle nanocomposites that showed very little fiber breakage. Sorption capacity measurements revealed hydrophilic behavior which was substantiated by the presence of an absorption peak associated with silica in the ATR- FTIR spectrum of the carbon fiber membrane.

AFRIKAANSE OPSOMMING: Amfifiliese PAN-ent-PDMS kopolimere was gesintetiseer deur konvensionele vrye radikaal polimerisasie reaksies van akrilonitriel monomere en PDMS makromonomere. Drie ketting lengtes van PDMS makromonomeer ente was gebruik waarvan elk ‘n reeks vorm waarvoor die monomer:makromonomeer verhouding varieer was. SEC resultate het ‘n wye verskeidenheid van molêre massas van die gesintetiseerde kopolimere getoon. Hierdie resultaat is te verstaande, gegewe die onbeheerde natuur van konvensionele vrye radikaal polimerisasie reaksies. ¹H KMR metings het bewys dat ‘n direkte eweredigheid bestaan tussen die PDMS makromonomeer ente en die inkorporasie daarvan. Gesintetiseerde PAN-ent-PDMS kopolimere was ge-elektrospin vanaf DMF teen drie verskillende TCD’s. In sommige gevalle kon die vesel diameters verklaar word deur die PDMS ent inhoud tesame met die molêre massa te in ag te neem, alhoewel die vesel diameters in vele gevalle nie verklaar kon word nie. ‘n Moontlike rede hiervoor is dat die verwagte fase- segregasie tot ‘n mate inhibeer was deur die metode van voorbereiding van die oplossing wat roering teen 50 °C vir ‘n uur gevolg deur sonikasie vir ‘n uur behels. Dit is noemenswaardig dat kopolimere wat 5 kDa PDMS ente bevat hewige jellering van die polimeer oplossing getoon het. DSC analise wat op die voorbereide poeiers asook op die ge-elektrospinde vesels uitgevoer was, het gedui dat die huidige monsters geen duidelike tendens met betrekking tot PDMS ent inhoud het nie. Deur die sikliserings reaksie eksoterm, ‘n nodige proses om die voorloper vesels in koolstof vesels om te skakel, te analiseer was ‘n interessante observasie was gemaak vir die drie reekse van kopolimere bevattende 5 kDa ente: soos die PDMS ent inhoud vermeerder vorm ‘n skouer wat uiteindelik skei en ‘n dubbel-piek in die termogram vorm. Dit is vermoedelik die gevolg van hewige fase-segregasie van domeine ryk in PAN homopolimeer en domeine ryk in PDMS entpolimere wat die sikliserings reaksie by verskillende temperature ondergaan. Ko-presipitasie van IONPs het nanoparticles met ‘n diameter van 10.2 ± 2.4 nm gelewer soos bepaal deur TEM analise. Silika bedekking van die IONPs het aanvanklik multi-kern nanopartikels met ‘n diameter van 62.6 ± 12.5 nm en ‘n silica inhoud van 72.2 wt.%, gravimetries bepaal, geproduseer. Die aantal TEOS bygevoeg as reagent was verminder om sodoende enkel-kern nanopartikels van 22.2 ± 4.3 nm bevattende 29.6 wt.% silika, en dus ‘n dunner omhulsel, te produseer. Termiese voor-behandeling van die enkel-kern nanopartikels teen 800 °C het nanopartikels gelewer wat baie eenders voorkom as die onbehandelde enkel-kern nanopartikels. Geëlektrospinde vesel-partikel nanokomposiete het nanopartikel agglomerasie getoon in die geval van die multi-kern nanoparticles en goeie verspreiding in die geval van die enkel- kern nanopartikels en die termies voor-behandelde enkel-kern nanopartikels. Vesel-partikel nanokomposiete bevattende termies behandelde enkel-kern partikels ge-elektrospin na 18 h van oplossingsveroudering het weinig partikel agglomerasie getoon. Verkoling van die vesel-partikel nanokomposiete bevattende termies behandelde nanopartikels het ongeskonde koolstof vesel- partikel nanokomposiete gelewer wat baie min vesel verbreking getoon het. Metings van sorbsie kapasiteit het hidrofiliese gedrag ontbloot wat gestaaf was deur die teenwoordigheid van ‘n absorpsie piek geassosieer met silika in die ATR-FTIR spektrum van die koolstof vesel membraan.

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