The non-covalent compatibilization of carbon nanotubes for use in polymeric composite materials

Scharlach, Kerstin (2014-04)

Thesis (MSc)--Stellenbosch University, 2014.

Thesis

ENGLISH ABSTRACT: Since the discovery of carbon nanotubes (CNTs), a large interest has developed around the incorporation of these into polymeric matrices in order to introduce the excellent mechanical, thermal and electrical properties of CNTs into the resultant composites. Nanocomposites of polymer/CNT composition allow for the favourable combination of the physical properties of the polymeric matrix and of the CNT filler. The biggest existing challenge of producing such nanocomposites is presented by the tendency of CNTs to occur in bundles or aggregates which are difficult to break up and to disperse in solution which leads to non-uniform distributions within the polymeric matrix. This problem has been combated through the use of CNT surface functionalization. However, a disadvantage exists with this solution. Since covalent functionalization of the CNT surface disrupts the electronic π-electron cloud which is responsible for the excellent electronic properties which CNTs are often desired for, an alternative method of functionalization must be employed in order to maintain the excellent intrinsic properties of CNTs yet create uniform dispersion of the nanotubes upon compatibilization with the polymeric matrix. Two alternative methods for the production of noncovalent compatibilization of multi-walled carbon nanotubes (MWNTs) with polystyrene were investigated and compared. These two methods involved the synthesis of a pyrene-functional macroinitiators for reversible addition fragmentation chain transfer (RAFT) and atom transfer radical polymerization (ATRP). Both of these methods allow for the controlled polymerisation of pyrene functional polystyrene chains. For comparison, the direct covalently functional MWNTs were also synthesised first by oxidation of the MWNT surface and conversion of the MWNT into the multifunctional RAFT and ATRP macroinitiator in which the styrene chains were controllably directly grafted from the surface of the MWNTs. The interaction of the pyrene chains with MWCNTs was monitored by using NMR, TGA and fluorescence spectroscopy. The NMR results showed the broadening and weakening of the pyrene protons as well as the polystyrene (PS) protons. TGA showed the loss of the pyrene-functional PS portion throughout the heating process. Fluorescence provided the conclusive result that the noncovalent compatibilization had occurred through the quenching of the emission and excitation signals as a result of electron transfer being facilitated by the π-stacking interactions. Finally, the MWNT nanocomposite polymer nanofibres are produced via the electrospinning technique with the various covalent and non-covalent compatibilized MWNT. The fibre morphology for the different compatibilization methodologies is compared as a function of the MWNT content. Distinct differences are observed for the different composites.

AFRIKAANSE OPSOMMING: Sedert die ontdekking van koolstof-nanobuisies (KNBs), het ʼn groot belangstelling ontwikkel rondom die betrekking van KNBs in polimeriese matrikse om samestellings met uitstekende meganiese, termiese en elektriese eienskappe te vervaardig. Nanosamestellings van polimeer/KNB komposisie laat toe dat gunstige kombinasies van fisiese eienskappe van die polimeer en die KNB vuller gerealiseer kan word. Die grootste uitdaging van die vervaardiging van sulke nanosamestellings is die neiging van KNBs om gebondelde formasie te vorm wat baie moeilik is om op te breek. Dit maak hulle verspreiding in oplossings en in polimeer matrikse oneweredig. Hierdie probleem word deur funksionalisering opgelos. Nogtans, ʼn nadeel van hierdie oplossing is dat kovalente funksionalisering verander die elektroniese struktuur van die KNB oppervlakte deur die ontwrigting van die π-elektron wolk wat vir die uitstekende elektroniese eienskappe verantwoordelik is. Dus moet ʼn alternatiewe funksionalisering metode gebruik word om die inherente eienskappe van die KNBs te behou en terselfde tyd ʼn uniforme verspreiding te bewerkstellig gedurende die vermenging met die polimeer matriks. Twee alternatiewe metodes vir die vervaardiging van nie-kovalente gefunksionaliserde multiommuurde koolstof-nanobuisies (Eng: MWNTs) met polistireen (PS) was ondersoek en vergelyk. Hierdie twee metodes was uitgevoer deur die sintese van ʼn pyreen-funksionele omkeerbare addisie-fragmentasie-kettingoordrag (OAFO) en atoomorrdragradikaaladdisie (AORA) makromiddel. Al twee van hierdie metodes lei tot ʼn gekontrollerde polimerisasie van pyreen-gefunksionaliserde stireen. Vir vergelyking was ʼn kovalente- gefunksionaliserde MWNT vervaardig deur die oksidasie van die MWNT oppervlakte en die daaropvolgende immobilisasie van dieselfde AORA en OAFO middel aan hierde aktiewe punte. Daarvan af was stireen gekontroleerd gepolimeriseer deur middel van die AORA en OAFO middel. Die interaksie was gekarakteriseer deur TGO, KMR en fluoressensie spektroskopie. Die KMR resultate het seine gewys van die verspreiding en verswakking van die pyreen en PS protone. TGO het die verlies van die pyreen-funksionele PS deel van die nie-kovalente produk gewys. Fluoressensie het beslissende bewyse gelewer dat die nie-kovalente funksionalisiering plaas gevind het deur die onderdrukking van die stralende en opwekkings seine as ʼn gevolg van die elektron oordrag wat deur die π-stapel interaksies gefasiliteer word. Uiteindelik was die nanosamestellings vermeng met PS en geelektrospin. Die vesel morfologie vir die verskillende gefunksionaliserde MWNT nanosamestellings metodes was vergelyk as ʼn funksie van MWNT inhoud. Duidelike verskille is waargeneem vir die verskillende samestellings.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/86354
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