Solvent Evaporation-Assisted Three-Dimensional Printing of Piezoelectric Sensors from Polyvinylidene Fluoride and its Nanocomposites

RESUME Les materiaux piezoelectriques sont connus pour generer des charges electriques lors de leur deformation. Leur capacite a transformer lineairement l'energie mecanique en energie electrique, et vice versa, est utilisee dans la detection, l'actionnement, la recuperation et le stockage d'energie. Ces appareils trouvent des applications dans les domaines de l'aerospatiale, de la biomedecine, des systemes micro-electromecaniques, de la robotique et des sports, pour n'en nommer que quelques-uns. On retrouve la propriete de piezoelectricite dans certaines ceramiques, roches, monocristaux et quelques polymeres. Le poly(fluorure de vinylidene) (PVDF) est un polymere piezoelectrique qui presente un coefficient piezoelectrique tres eleve par rapport aux ceramiques, ce qui laisse presager des applications de detection et de recuperation d'energie. La facilite de fabrication, la flexibilite et la biocompatibilite du PVDF sont autant de qualite qui en font un tres bon candidat pour ces applications. Les dispositifs actuels a base de PVDF commercial sont disponibles en films plats ou en fibres unidimensionnelles (1D). L'impression tridimensionnelle (3D) du PVDF peut amener a des sensibilites, souplesses et capacites de fabrication accrues des capteurs embarques en cas d'impression multi-materiaux. Le PVDF est un polymere semi-cristallin possedant cinq polymorphes, dont la phase β polaire qui presente les meilleures proprietes piezoelectriques. Malheureusement, le PVDF, provenant de la fusion ou de la dissolution, cristallise en une phase α non polaire thermodynamiquement stable. Diverses transformations physiques telles que le recuit, l'addition de charge, l'etirement ou le polissage sont effectuees pour transformer la phase α en phase β. En raison de la cristallisation inherente du PVDF dans la phase α, il y a eu tres peu de tentatives de fabrication de structures 3D a partir du PVDF. L'electrofilage en champ proche et la Deposition de Filament Fondu ont permis de fabriquer certaines structures 3D couche par couche avec du PVDF, soit avec l'application de hautes tensions electriques, soit avec la fusion a haute temperature du polymere. Et les deux necessitent un traitement de polarisation pour conferer la piezoelectricite aux structures imprimes. Pour fabriquer des capteurs incorpores ou conformes, sur des substrats donnes, il est essentiel de ne pas avoir d'effets negatifs sur les materiaux adjacents a cause de la polarisation pendant le processus d'impression. Ainsi, dans ce travail, nous avons developpe un procede d'impression 3D qui cree des structures PVDF principalement en phase β, a temperature ambiante et sans application de tension de polarisation. ----------ABSTRACT Piezoelectric materials are known to generate electric charges upon deformation. Their ability to linearly transform mechanical energy into electrical energy and vice versa, is utilized in sensing, actuation, transducing, energy harvesting and storage. These devices find applications in aerospace, biomedicine, micro electromechanical systems, robotics and sports, to name a few. Piezoelectricity is found in some ceramics, rocks, single crystals and a few polymers. Polyvinylidene fluoride (PVDF) is a piezoelectric polymer that exhibits a very high piezoelectric stress coefficient as compared to the ceramics, making it the forerunner for sensing and energy harvesting applications. PVDF’s formability, flexibility and biocompatibility, further reinforce its candidature. Present commercial PVDF-based devices come in flat films or one-dimensional (1D) fibers. Three-dimensional (3D) printing of PVDF leads to higher sensitivity, better compliance, and ability to print embedded sensors in case of multi-material printing. PVDF is a semi-crystalline polymer possessing five polymorphs, of which the polar β-phase exhibits highest piezoelectric properties. Unfortunately, PVDF from melt or solution crystallizes into a thermodynamically stable non-polar α-phase. Various physical transformations like annealing, filler addition, stretching or poling are carried out to transform the α-phase into β-phase. Due to the inherent crystallization of PVDF into α-phase, there have been very few attempts in fabricating 3D structures from PVDF. Near-field electrospinning and fused deposition modelling have demonstrated some layer-by-layer 3D structures with PVDF, either with application of high electric voltages or high temperature melting of the polymer, respectively. Also, both these techniques require a poling treatment to impart the desired piezoelectricity to the printed features. To fabricate embedded or conformal sensors on given substrates, it is essential to not have any adverse effects on the adjacent or substrate materials due to poling during the printing process. Thus, in this work, we develop a 3D printing process, that creates PVDF structures that inherently crystallize in the piezoelectric oriented β-phase at room temperature without any applied voltages. Solvent-evaporation assisted 3D printing is employed to print 3D piezoelectric structures of PVDF based solutions. In this process, the polymer solution is filled into a syringe which is inserted into a pneumatic dispenser. The pneumatic dispenser is mounted on a robotic arm that is controlled via a computer program.