|dc.description.abstract||Since a vast array of devices, instruments, and equipment in modern society is powered by electrical energy, it has been a very important research topic to develop and fabricate a material that effectively conducts electricity. Typically, metals have been widely used in a variety of electric applications due to their excellent electrical conductivity and mechanical strength. Nevertheless, the development of an electrically conductive polymer composite has been highly desirable because of its numerous advantages, including excellent chemical stability and high corrosion resistance, light weight, great processability and low production cost. The conductive polymer composites can be employed in a number of applications, such as electromagnetic interference (EMI) shielding and electrostatic discharge (ESD) for electronic devices, transducers in chemical sensors, electrostatic painting, and electrodes for energy storage and conversion systems.
Since its discovery in 2004, graphene has been extensively utilized to produce polymer nano-composites due to its exceptional thermal, mechanical, and electrical properties. In this thesis, graphene is incorporated into polymer matrices via two distinct fabrication approaches: (I) infiltration of a three-dimensional filler matrix with elastomer mixture and (II) compounding by twin screw extruder, followed by injection moulding. In the first approach, graphene oxide solution is synthesized by applying Hummer’s method, and as-prepared graphene oxide solution is directly used to construct a three-dimensional graphene architecture by means of freeze casting. This filler matrix is then subsequently infiltrated with elastomer mixture. This unique approach significantly improves the state of filler dispersion, achieving a high electrical conductivity with low percolation threshold. This study finds that the use of graphene with large diameter significantly improves electrical conductivity. In a follow-up study, graphene nano-ribbons (GNRs) with a high aspect ratio are used to prepare the filler matrix, and the variation of electrical conductivity under uniaxial elongation is also investigated. It is revealed that the incorporation of high aspect fillers considerably improves the consistency of conductivity under a uniaxial tensile strain.
In the second approach, conductive thermoplastic composites were produced by means of twin screw extrusion followed by injection moulding. The compounding process by twin screw extrusion is still widely used in industry due to its ease of processing, efficiency, and low cost. Having clearly seen from the first part of a study that a large diameter is beneficial for obtaining elevated electrical conductivity with low filler contents, we have used the graphene nano-platelets (GnPs) with largest diameter size available to incorporate into polypropylene (PP). Due to a large diameter of GnPs employed in this study, a high electrical conductivity is realized at a low content of GnPs fillers. Nevertheless, mechanical properties of PP/GnP composites show inferior improvements. This is owing to the compromised filler morphology and the lack of efficient bonding between polymer melt and the GnPs.
In a follow-up study, we have extended the inclusion of GnPs of various grades with different dimensions. The diameter of GnPs included in this study spans from a few microns to hundreds of microns. This follow-up study finds that thermal stability and tensile property are considerably improved with decreasing GnPs’ sheet size. This is largely due to the improved dispersion with less agglomeration of fillers with retained morphology that maximized the filler effects. The improved tensile strength with the use of small-sized GnPs can additionally be attributed to the enhanced load transfer between GnPs and PP matrix with greater surface area and prolonged crack propagation length. The incorporation of GnPs shows a minor induction effect for β-crystals, and this effect is intensified with increasing GnPs’ diameter. The degree of crystallinity is not significantly varied by the addition of GnPs, although GnPs raise the crystallization temperature of PP by serving as seeds for heterogeneous nucleation. As expected from previous studies, the lowest percolation threshold is observed when the largest sized GnPs are employed. This study clearly shows that the physical dimensions of GnPs have a significant influence on a range of different properties of final composites, and suggests that the GnPs dimensions should carefully be tailored to meet the particular requirement of final composites for each application.||en