Chemical and Biomolecular Engineering - Theses
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Item3D printing of flexible and efficient polymeric piezoelectric energy conversion materials( 2020)The unique capability of piezoelectric materials to convert between mechanical and electri-cal energy holds tremendous potential in enabling a range of emerging applications. Pol-ymers, as soft and biocompatible materials, are excellent candidates for the use in power-ing wearable and implantable electronics, as well as for the primary sensing mechanism in soft robotic interfaces. However, piezoelectric polymers are sparsely utilised due to their chemical and structural complexity, and the tremendous energetic cost to maximise their energy conversion efficiency. Fluoropolymers have piezoelectric figures of merit rivalling those of the widely used ceramics and are therefore promising to investigate. The common processing techniques for fluoropolymers revolve around solution casting from toxic, hazardous, and/or high boiling point solvents, which require lengthy solvent evaporation times and arduous post-processing by electrical poling, applying high electric fields to align the dipoles. Recent advances in three-dimensional (3D) printing show promise in order to process fluoropolymers into piezoelectric devices, inducing shear forces on the polymer chains during extrusion toward greater alignment and tailored architectures. In this work, pathways to improving the piezoelectric output of a fluoropolymer, poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE) were thoroughly investigat-ed. The solvent evaporation-assisted (SEA) 3D printing technique was adapted to printing fluoropolymers, investigating the effects of layer-by-layer deposition on the optical, pol-ymorphic and electromechanical properties. In combination with 3D printing, two classes of nanoscale additives were further investigated, single-walled carbon nanotubes (SWCNTs) and transition metal carbides (MXenes), to elucidate their role in the evolution and alignment of the piezoelectric polarisation. The first part of the thesis focused on the development and optimisation of 3D printing capabilities for fluoropolymers. A binary solvent mixture was optimised by Hansen sol-ubility parameters and the rheological properties were thoroughly probed to optimise the polymer concentration. The effects of printing parameters were further investigated in or-der to minimise spreading of the resultant ink post-printing. The polymers, 3D printed up to 19 layers were transparent and exhibited piezoelectricity, with minimal changes in the electroactive phase fraction and without electrical poling. These results confirmed that shear stresses impart partial polarisation on the extruded materials, and provided a strong foundation for the further studies investigated in this thesis. The second study of this thesis critically investigated the effects of the incorporation of SWCNTs, as a nanoscale additive, into the PVDF-TrFE coupled with the developed 3D printing process. The composites were printed as single-layer films, found to be transpar-ent at carbon nanotube loadings up to 0.05 wt%, with low haze. The piezoelectric proper-ties were investigated through two techniques, piezoresponse force microscopy (PFM) and bulk electromechanical characterisation, finding the greatest enhancement in piezoelec-tric properties at a 0.02 wt% loading of the SWCNTs from both techniques. Molecular dynamics (MD) simulations of the carbon nanotube interface with the polymer confirmed a polarisation enhancement effect, providing the first report of polarisation in the absence of electrical poling. Furthermore, the composites were found to be recyclable in pure ace-tone, a green and low boiling point solvent, allowing the printed piezoelectric polymers to be reprinted, with minimal changes in the chemical, physical and electroactive properties. The final part of this thesis utilised two-dimensional (2D) MXene nanosheet additives as a model non-piezoelectric system to deduce and provide the first report on the mechanism of physical polarisation locking in the PVDF-TrFE, building on the knowledge of the first two studies. The composites were printed directly from acetone as a physical gel, allowing for a faster solvent evaporation rate and therefore improved crystallisation kinetics. MD simulations found a suppressed electroactive phase fraction of the polymer directly adja-cent the surface of the additive, confirmed experimentally by Raman microscopy and dif-ferential scanning calorimetry. Furthermore, the MD simulations found the polarisation vector direction was locked perpendicular to the basal plane of the MXene, which was governed by electrostatic interactions. PFM results confirmed the dipole locking phenom-enon, whereby the polarisation magnitude increased logarithmically with an increase in the MXene loading, while demonstrating the transition metal carbide had no discernible out of plane polarisation. Direct piezoelectric effect measurements via macroscale electromechan-ical testing showed that the composite material exhibited a larger piezoelectric coefficient relative electrically poled polymer, implying that physical poling and polarisation locking from a nanomaterial template can impart greater polarisation than standard electrical poling techniques. In summary, this thesis has developed and applied a fundamental understanding of the origins of piezoelectricity in fluoropolymers and how this phenomenon can be controlled at the nanoscale. The implications of this research are far-reaching, enabling commercial viability of piezoelectric materials in a multitude of emerging applications.