Yee Song Ko
As part of the recent efforts on renewable energies, energy harvesting by exploiting environmental motions and vibrations to generate electricity is being enthusiastically developed as a novel kind of green energy source. Piezoelectric materials are a logical choice for such applications, as they can readily generate electric charges when subjected to mechanical force without the need for additional equipment. While small deformations can be easily covered by traditional piezoelectrics, new materials have to be developed to access strains in excess of af few percent. This thesis lays out steps for the fabrication of a piezoelectric elastomer for use as active sensor and energy harvesting applications containing solely organic components. To achieve this a composite approach was chosen in which organic piezoelectric materials, so-called frozen-dipole electrets, were embedded as particulate fillers in a highly elastic polydimethyl siloxane (PDMS)matrix. Nanoparticles of a frozen-dipole electret material had to be prepared and characterized first, followed by their dispersion in the PDMS matrix, and finally the characterization of the composite elastomer. Poly-2-hydroxyethyl methacrylate (PHEMA) particles were produced byminiemulsion polymerization, and Disperse Red 1 (DR1) molecules were incorporated at the same time to turn the material into a frozen-dipole electret. The DR1 was introduced either as a blend or by copolymerization with amethacrylate-functionalized DR1 derivative. The study of PHEMA particles indicates that it would be very challenging to render them piezoelectric, due to their high conductivity, the environmental influence on their properties and unfavourable components introduced with the production method. In contrast, PMMA copolymer particles could easily be polarized. Polymethylmethacrylate (PMMA) copolymers with either methacrylate-functionalized DR1 or nitroaniline was used to produce nanoparticles via the nanoprecipitationmethod. Nanoparticles with up to 70 mol% DR1 could be obtained. These materials were chosen because of their known properties as electrets in order to serve as reference materials for the proof of concept of the organic piezoelectric elastomers. They were analysed by impedance spectroscopy and the thermally stimulated depolarization current technique to assess their relaxation and polarization behaviour. PMMA particles could be blended with PDMS without any additional additives to produce composite elastomers which can be turned into piezoelectric materials when poled in an electric field at high temperatures. Depending on the chain length of the PDMSmatrix, maximum strains of up to 600% could be reached. A custom built Berlincourt measuring setup was used to quantify the longitudinal piezoelectric coefficient d33 which was found to depend strongly on the force with which the composite is loaded. The transvere piezoelectric coefficient d31 was measured with a separate setup, based on a tensile testing machine. Piezoelectricity could be measured atmacroscopic strains as high as 200%. d31 initally decreases but stabilizes after a certain time. Most importantly, composites with PMMA-DR1 copolymer particles retain their piezoelectricity after exposure to elevated temperatures for several days.
EPFL2017
