Thermoplastic graphene composites for mechanical, thermal stability and conductive applications

The aim of this research was to prepare graphenes, to reactively modify their surface to enhance exfoliation and to facilitate bonding to chosen matrix polymers, to characterise the prepared materials and determine their physical and mechanical properties. This investigation was motivated by the exceptional performance characteristics of graphene to find new ways of creating and dispersing graphene in polymers, so polymer-graphene composites were fabricated. Composites enhanced by the properties of graphene were prepared inspired by the unique nanostructure of graphene that imparts high strength, stiffness and resistance to deformation.<br><br>Graphenes were prepared using thermal expansion in an oxidising atmosphere (air), an inert atmosphere (nitrogen N2) and a reducing atmosphere (hydrogen H2 or carbon monoxide CO). The reduction of graphene using thermal expansion with simultaneous CO reduction was granted a provisional patent (Appendix A).<br><br>The graphenes were characterised using thermogravimetry (TGA), Raman spectroscopy, electrical resistance and surface energy measurements. All graphenes showed a mass loss in TGA which was attributed to oxygen‑containing functional groups present on the graphene surface. The mass loss was lowest for the inert and reducing atmospheres. Raman spectroscopy (using the D/G peak ratio) showed that graphene had the fewest defects in the order CO<br>The graphenes were then used to produce four different polymer composites including a thermoplastic elastomer (poly(styrene-b-butadiene-b-styrene); SBS at two compositions), a semi-crystalline thermoplastic (polyethylene terephthalate; PET at two compositions) and two high-performance amorphous polymers (polycarbonate (PC) and poly(ether sulfone) (PES)). The composites were prepared by dispersion of the graphene into polymer solutions using ultrasonication and high torque melt dispersion.<br><br>The SBS-graphene (1 %∙w/w) composites were produced using graphenes prepared in air, N2 and H2 and using solvent dispersion (SD). The greatest changes occurred using the H2 reduced graphene which showed increased stored energy (storage modulus), energy absorption (loss modulus) and damping (tan delta) in SBS. The damping effect (move to a more liquid state and greater free volume) was largest at low temperatures as a result of the large size of the graphene sheets used. Functionalising the graphene with Fe3O4 before combining it with SBS resulted in a composite that displayed magnetic properties.<br><br>The SBS-graphene (0-20 %∙w/w) composites were produced using GT-CO reduced graphenes and using solvent dispersion. Stress-strain measurements showed a progressive decrease in deformation and increase in damping as the graphene content increased suggesting uniform dispersion in the SBS. The presence of aromatic interactions and hydrogen bonding between the SBS and graphene was supported by density functional theory calculations. Some scrolling of graphene was observed in this SBS composite.<br><br>The PET-graphene (1 %∙w/w) composites produced using the GT-H2 reduced graphene were prepared using solvent dispersion (with ultrasonication) and melt dispersion (without ultrasonication). Nucleation of PET did not occur using the low defect graphene although oxygen permeation of the composite increased which was attributed to an increased free volume. The results suggested that using a combination of ultrasonication and melt dispersion of graphene to produce the composite would increase the exfoliation and dispersion further.<br><br>The PET-graphene (1 %∙w/w) composites using GT-CO reduced graphenes were prepared using melt dispersion (MD) alone or combined with ultrasonication. When using ultrasonication graphene agglomeration in PET was diminished, and reduction of the graphene could be seen by a darkening of the colour of the composite. PET deformation (ductility) increased with ultrasonication and melt dispersion of graphene.<br><br>The PC-graphene (0.1 %∙w/w) composites using GT-H2 or GT-CO reduced graphene with low filler content were prepared using melt dispersion with ultrasonication. The storage modulus of PC-graphene composites was greater than PC alone indicating stronger interfacial interactions existed with graphene. The time-dependent loss of energy (loss modulus) and damping (greater liquid properties) increased when graphene was added to PC. The increase in damping suggested an increase in free volume. DFT calculations indicated that the interactions between graphene and PC were due to a combination of π-π and CH/π bonding. Despite being relatively weak, the interaction of the aromatic rings and the H atoms on the methyl groups, in the monomer, both play a significant role in the attraction with graphene.<br><br>The PES-graphene (1 %∙w/w) composites using GT-H2 or GT-CO reduced graphenes were prepared using solvent dispersion with ultrasonication. GT-CO reduced graphene showed evidence of rolling or scrolling in PES which increases cross-sectional area. Interactions between PES GT-CO increased indicating a move to a more solid state and an increase in elasticity. The results demonstrated that PES binds well with graphene using only non-covalent bonding.<br><br>In each case, the polymer-graphene composites demonstrated good dispersion which establishes that π-interactions and hydrogen bonding are an effective way to disperse graphene. Where similar or comparable data was found for covalent bonding, it demonstrated that non-covalent bonding gave similar results. By using only non-covalent bonding, the pristine nature of the graphene was maintained creating low defect polymer-graphene composites. Low defect graphenes have advantages such as improved electrical and thermal conductivity, fewer contaminants, greater biocompatibility and the benefit of a less dangerous processing method. The optimal processing method combined ultrasonication and melt dispersion which are synergistic reducing (signalled by darker colour) and exfoliating graphene further. The scrolling of low defect GT-CO reduced graphene is of particular interest because the increased cross-sectional area gives it the potential to improve thermal and electrical conductivity.