Thesis of Mélanie Girardot

Defense of thesis Mélanie Girardot - laboratory UMET

Abstract :

Piezoelectric devices are constituted of materials capable of converting mechanical energy into electrical energy (direct piezoelectric effect) or electrical energy into mechanical energy (inverse piezoelectric effect). These materials can be single crystals, ceramics, or polymers. Over the past decades, poly(vinylidene fluoride) or PVDF has garnered increasing interest due to its excellent mechanical properties (flexibility) and remarkable ferroelectric and piezoelectric properties, making it a prime candidate for use in sensors and actuators.
PVDF is a semi-crystalline polymorphic polymer, in which the most polar crystal phase is classically obtained by mechanical stretching or solution crystallization from the most stable non-polar phase. To avoid the post-processing step, a fluorinated co-monomer such as trifluoroethylene (TrFE) can be added to the VDF monomer during polymerization to synthesize the P(VDF-co-TrFE) copolymer and to obtain the direct formation of the polar crystal phase and to achieve a higher piezoelectric response.
A primary objective of this thesis is to study the relationships between the structure and the physical, mainly piezoelectric, properties of two P(VDF-co-TrFE) copolymers with VDF/TrFE molar ratios of 80/20 and 55/45. Initially, a detailed analysis of the copolymer structure was conducted. An in-depth study of the crystal phases was carried out using wide-angle X-ray scattering (WAXS) in-situ during heating or stretching and ex-situ for various poled film. The mobility of the amorphous phase was probed by dynamic dielectric spectroscopy. The presence of secondary crystals was evidenced by differential scanning calorimetry (DSC).
Subsequently, the piezoelectric response of the copolymer films was measured using the piezoelectric coefficient d33. A thorough analysis of the evolution of d33 as a function of the P(VDF-co-TrFE) copolymers and for different poling parameters was conducted. Our results show that P(VDF-co-TrFE) 55/45 exhibits significantly higher piezoelectric properties compared to P(VDF-co-TrFE) 80/20 due to a higher fraction of defective ferroelectric phases.
To optimize the piezoelectric properties of P(VDF-co-TrFE), electroactive BTO (barium titanate) particles can be incorporated, forming a flexible composite with improved piezoelectric properties. However, BTO nanoparticles and the P(VDF-co-TrFE) matrix are known to have low physico-chemical affinity, which can lead to the formation of cavities at the interface and then disrupt the electroactive properties of the composite. To enhance the ceramic/polymer interface, coupling agents such as dopamine derivatives can be used. These agents can, from one side, strongly interact with BTO ceramics via their catechol functions and, from the other side, form weak bonds with the P(VDF-co-TrFE) matrix.
Thus, a secondary objective of this thesis focuses on the impact of different types of coupling agents on the structural and electroactive properties of the composites, as well as the comparison of their respective efficiencies. Our results demonstrate that the use of coupling agents improves the dispersion of BTO particles and that the polarity of these agents can significantly influence the electroactive properties of the composite.

Keywords : Polymer,Piezoelectric,Composite,Crystalline phase,X-rays diffraction,Electroactivity