The preparation of polyolefin nanofibres by solution electrospinning

Keulder, Liesl (2013-03)

Thesis (PhD)--Stellenbosch University, 2013.

Thesis

ENGLISH ABSTRACT: Solution electrospinning is a technique used to produce polymer micro- or nanofibres. Recently a great deal of research has been done on the application of polymer nanofibres produced by this method. The solution electrospinning of polyolefins have not been researched in-depth mainly due to the difficulty in dissolving these polymers in suitable electrospinning solvents. We managed to electrospin polypropylene copolymers at room temperature, obtaining both polymer micro- and nanofibres. A suitable solvent system was developed (cyclohexane/DMF/acetone) that allowed for the room temperature solution electrospinning of these crystalline polypropylene copolymers. It was also shown that using propylene-1-alkene copolymers with a low comonomer content was a facile way of producing crystalline polyolefins nano – and microfibers. Similar attempts to electrospin isotactic polypropylene were unsuccessful, even though lower molecular weight materials were used than in the case of the copolymers. This lead to an investigation of solution melting temperature by SCALLS. The copolymers showed great variance in their solution melting temperatures despite the fact that they all had more or less the same crystallinity and amount of comonomer, indicating that the type of comonomer played an important role in the solubility of the copolymer. The effect of different collectors was investigated, but in the end it was found that between spinning unto ice, foil on ice of just foil, foil still seemed to be the best collector. Comparing crystallinity of the polymer powders with that of the polymer fibres by DSC and WAXD, it was found that there is a difference in the crystallinity of the fibres and the powders. EVOH is a polymer with excellent properties and electropspinning of this polymer is relatively easy due to the fact that it dissolves quite easily in conductive solvents. DMF, Isopropanol/water and DMSO were all tested as suitable solvents and the morphology was compared through the use of SEM. The morphology of the fibres indicated that DMSO was the best solvent. The influence of the spinning parameters was determined for both systems of DMF and DMSO. These nanofibres were used as reinforcement in LDPE matrix and the mechanical properties of the LDPE matrix was improved with the addition of both aligned and unaligned fibres. The next step was the electrospinning of EVOH fibres containing MWCNT. TEM, FE-SEM and TGA were used to confirm the presence of the MWCNT as well as determine the distribution of the MWCNT inside the nanofibres. The nanotubes were distributed through the fibres; however agglomeration of the nanotubes did still take place. The nanofibres containing MWCNT were also used to make composites where the fibres were melted, leaving the MWCNT behind in the polymer matrix. This was done in both LDPE and EVOH matrices. The LDPE/MWCNT composites did not give positive results, on the other hand the EVOH/MWCNT composite showed an improvement in the mechanical properties compared to pure EVOH. The attachment of fluorescent dye molecules to the surface of the MWCNT was attempted and through fluorescent microscopy and the dispersion of the nanotubes inside the fibres as well as the composite could be seen. This study proved that polyolefin nanofibres could be obtained, giving rise to more applications for these versatile polymers. It also confirmed the importance of nanofibres as reinforcement and the use of nanofibres as a way to incorporate MWCNT in a polymer matrix.

AFRIKAANSE OPSOMMING: Elektrospin in ‘n oplosmiddel is ‘n tegniek wat gebruik word om polimeer mikro- en nanovesels te produseer. Die afgelope tyd word baie navorsing gedoen oor die aanwending van polimeer nanovesels wat geproduseer word op hierdie manier. Daar is nog min navorsing gepubliseer wat handel oor die elektrospin van poliolefiene uit ‘n oplosmiddel, deels oor hoe moeilik dit is om ‘n geskikte elektrospin oplosmiddel te vind vir hierdie polimere. In hierdie studie het ons mikro- en nanovesels verkry deur polipropileen kopolimere te elektrospin by kamertemperatuur. Die polimere is opgelos in ‘n oplosmiddel sisteem wat bestaan uit sikloheksaan/dimetielformamied/asetoon, by hoë temperatuur en het toegelaat dat die polimere in oplossing bly by kamertemperatuur. Hierdie diverse kopolimere het verskillende resultate gegee, sommige polimere het mikrovesels produseer, waar ander nanovesels geproduseer het. Die vessel morfologie is ondersoek deur die gebruik van Skandering Elektron Mikroskopie (SEM) en daar is gevind dat die vesels nie ‘n gladde voorkoms het nie, maar dat daar kraalvormige morfologie gesien kon word. Om dit te voorkom is sout by die oplosmiddel sisteem gevoeg. Die invloed van die verskillende parameters op die vesels se deursnit is ondersoek vir al die kopolimere. Die byvoeging van sout het gelei tot ‘n meer gladde vesel morfologie. Die effek van die gebruik van verskillende oppervlaktes om die vesels op te vang is ondersoek en die die kristalliniteit van die polimeer poeiers is vergelyk met die kristalliniteit van die polimeer vesels met die hulp van DSC en WAXD. ‘n Poging is aangewend om isotaktiese polipropileen te elektrospin uit oplossing, maar ons kon nie daarin slag om die polimeer op te los nie, al was die molekulêre gewig minder as die van die kopolimere. Dit het gelei tot die ondersoek van die smeltpunt temperatuur in oplossing deur die gebruik van oplossing kristallisasie-analise deur laser lig verstrooing (SCALLS). Al die kopolimere het min of meer dieselfde kristalliniteit en hoeveelheid komonomer bevat, tog het hulle smeltpunt temperatuur in oplossing baie verskil. Dit het gedui op die feit dat die tipe komonomeer ‘n groot rol speel in die oplosbaarheid van die kopolimeer. Die elektrospin van etileen-ko-vinielalkohol (EVOH) is ook ondersoek. DMF, Isopropanol/Water en Dimetielsulfoksied (DMSO) is getoets as geskikte oplosmiddels en die morfologie van die vesels is vergelyk deur die gebruk van SEM. Die tyd wat die polimeer in oplossing gebly het asook die morfologie van die vesels, het aangedui dat DMSO die mees geskikte oplosmiddel was. Die invloed van die elektrospin parameters was vasgestel vir beide DMF en DMSO sisteme. Hierdie nanovesels is gebruik as versterking in ‘n LDPE matriks en die meganiese eienskappe van die LDPE matriks is verbeter deur die toevoeging van beide nie-geweefde en gerigte veselsopppervlakte. Die volgende stap was die elektrospin van EVOH vesels wat multi-ommuurde koolstof nanobuisies (MWCNT) bevat. TEM, FE-SEM en TGA was gebruik om te bevestig dat die vesels wel MWCNT bevat asook om die verspreiding van MWCNT in die vesels aan te dui. Die nanobuisies was versprei deur die vesels, maar bundels nanobuisies het tog voorkom in sommige plekke. Die nanovesels wat MWCNT bevat is ook gebruik om nanosamestellings te maak, waar die vesels gesmelt is om net MWCNT agter te laat in die polimeer matriks. Hierdie was gedoen in beide LDPE en EVOH matrikse. Geen positiewe resultate is verkry vir die LDPE/MWCNT nanosamestelling nie, maar die EVOH/MWCNT nanosamestelling het aan die anderkant ‘n groot verbetering getoon in die meganiese eienskappe in vergelyking met EVOH sonder MWCNT. ‘n Poging was aangewend om fluoreseerende molekules aan die oppervlak van MWCNT te voeg en deur fluoresensie mikroskopie kon die verspreiding van die MWCNT in die vesels asook in die nanosamestellings gesien word. Hierdie studie het bewys dat poliolefien nanovesels gemaak kan word wat lei tot die aanwending van hierdie polimere in nog meer toepassings. Dit het ook die belangrikheid van die gebruik van nanovesels as versterking in nanosamestellings bevestig asook die gebruik van nanovesels as ‘n manier om MWCNT in ‘n matriks te plaas.

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