Electrospinning of polyacrylonitrile nanofibres with additives : study of orientation and crystallinity

Greyling, Corinne Jean (Stellenbosch : University of Stellenbosch, 2010-03)

Thesis (PhD (Chemistry and Polymer Science))--University of Stellenbosch, 2010.

Thesis

ENGLISH ABSTRACT: Polyacrylonitrile, (PAN), polymers are of interest due to their many unique characteristics in comparison to most other fibre forming polymers. The focus of this study was the effect of electrospinning variables and additives on their unusual thermal properties and internal molecular morphology with the objective of improving the performance of this fibre in industrial application, or at least contributing to the understanding of how the fibre processing variables affect the thermal performance. The process of electrospinning was used to process samples of PAN fibres with additives in order to determine their effect on the molecular orientation and crystallinity of the resultant nano- and micronsized fibres. The effect of additives in the forms of low molecular weight oligomers, blends of conductive polymers and nanoparticles and conductive salts (mainly copper and lithium based) on the molecular structure and thermal properties is reported. In addition the effect of certain electrospinning processing variables such as the PAN polymer grade, the type of solvent, the temperature, the spinning voltage and the spinning distance were evaluated in terms of their effect on the resultant fibre molecular structure. The crystallinity and molecular orientation are quantified using various analytical techniques including: Fourier-Transform Infra-Red Spectroscopy (FTIR); Differential Scanning Colorimetry; (DSC) Thermal Gravimetric Analysis, TGA, Scanning and Transmission Electron Microscopy; Wide-Angle X-ray Diffractometry, XRD, Small-angle Neutron Scattering; SANS, Solid-state 13C Nuclear Magnetic Resonance Spectroscopy; NMR, and Electron-spin Resonance Spectroscopy, ESR. The researcher has ascertained in this study that the orientation of electrospun PAN is comparable to that in commercial wet-spun fibres when the electrospinning field strength is greater than 0,75 kV per cm. The process of electrospinning leads to chemical degradation of the PAN polymer only previously noted by other fibre scientists after heating PAN fibres to more than 280 C. This is due to the high mechanical stress on the high surface area electospun fibres spun into an oxygen rich air atmosphere. The addition of certain salts, particularly lithium chloride and copper nitrate, to the PAN electrospinning dope by the researchers produced fibres with improved thermal stability in comparison to the control PAN samples with no additives. In addition these salts lower the endotherm for the production of carbonized fibres from PAN fibres and will therefore make more structured, stronger, high tenacity carbon fibres. The XRD revealed that through the addition of certain salts to disrupt the dipole interations in PAN, that fibres with a 2D hexagonal crystal structure can be electrospun and have a structure similar to wet spun PAN. It is shown that fibres in a non-aligned electrospun mat have a 3D crystal structure and lower molecular orientation that wet spun fibres. Some industrial applications of electrospinning using PAN fibres are developed and reported.

AFRIKAANSE OPSOMMING: Polyakrilonitrielpolimere (PAN) is van belang weens die baie unieke eienskappe wat hulle het in vergelyking met die meeste ander vesel-vormende polimere. Die fokus van hierdie studie op die effek van verskeie veranderlikes en byvoegmiddels op hul buitengewone termiese eienskappe en interne molekulêre morfologie, met die doel om die werksverrigting van hierdie vesel in industriële toepassings te verbeter, of ten minste by te dra tot die begrip van hoe die vesel-prosesveranderlikes die termiese werksverrigting beïnvloed. Die elektrospinproses is gebruik om monsters van PAN met bymiddels te prosesseer om vas te stel wat die effek sou wees op die molekulêre oriëntering en kristalliniteit van die gelewerde nano- en mikrongrootte vesels. Verslag word gelewer oor die effek van bymiddels in die vorm van oligomere met „n lae molekulêre gewig, mengsels van geleidende polimere en verskillende nano-partikels en geleidende soute (meestal koper- en litium-gebaseerd). Die effek van sekere veranderlikes in die elektrospinproses, soos die PAN polimeer-graad, die temperatuur, die tipe oplosmiddel, die spin-spanning en spin-afstand, is geëvalueer in terme van hulle effek op die vesels wat gelewer word. Die kristalliniteit en molekulêre oriëntering word gekwantifiseer deur verskeie analitiese tegnieke te gebruik, onder meer: Fourier-Transform Infrarooispektroskopie; Differensiële Skandeerkolorimetrie; Termiese Gravimetriese Ontleding; Skandeeringelektronmikroskopie; Transmissie Elektron Mikroskopie; Wye-hoek X-straal Diffraktometrie; Klein-hoek Neutronstrooiing; Vaste-toestand 13C Kern Magnetiese Resonansiespektroskopie; en Elektronspin Resonansiespektroskopie. Dit is in hierdie studie vasgestel dat die oriëntering van elektro-gespinde PAN vergelykbaar is met kommersiële nat-gespinde vesels as die elektro-spin veldsterkte groter is as 0,75 kV per cm. Die elektro-spin proses lei tot chemiese degradasie van die PAN polimeer, wat voorheen slegs deur ander vesel-wetenskaplikes waargeneem is nadat PAN tot meer as 280 C verhit is. Dit word veroorsaak deur die hoë meganiese spanning op die hoë-oppervlakarea vesels wat in „n suurstofryke lug-atmosfeer gespin word. Die byvoeging van sekere soute, veral litiumchloried en kopernitraat, by die PAN elektro-spin spinlak (dope) deur die navorser het vesels gelewer met verbeterde termiese stabiliteit in vergelyking met die kontrole PAN-monsters sonder bymiddels. Verder verlaag hierdie soute die endoterm vir die produksie van gekarboniseerde vesels uit PAN-vesels, en dit sal daarom meer gestruktureerde, sterker koolstofvesels met hoë taaiheid lewer. X-straaldiffraksieanalise het gewys dat gewone elekrospin vesels ‟n driedimensionele struktuur het maar as gevolg van die byvoeging van sekere soute, wat die dipoolinteraksies in PAN opbreek, dat vesels wat 'n soortgelyke tweedimensionele struktuur het as dié van groot skaalse kommersiële nat-spin vesels, maklik d.m.v. die elektrospinproses berei kan word. Enkele nywerheidstoepassings van elektrospin is ontwikkel, en beskryf.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/3996
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