Polymer-based piezoelectric materials are currently the object of great interest in the world of industry because they enable their use in new applications in sectors such as transport and aeronautics, amongst others.
A definition of piezoelectricity - piezo being Greek for 'subjected to pressure' - is the generation of the electrical polarisation of a material as a response to mechanical strain. This phenomenon is known as direct effect or generator effect and is applied fundamentally in the manufacture of sensors (mobile phone vibrators, lighters, etc.). In these cases piezoelectric materials, also used in actuators, undergo an inverse or motor effect, i.e. a mechanical deformation due to the application of an electrical signal. Over the last four decades perovskite-type ceramics (zirconium or lead titanate ceramics) have been mainly used as piezoelectric materials in acoustic applications, amongst other reasons because of their high elastic modularity, their high dielectric constant and their low dielectric and elastic losses. However, and although they have also been used successfully in many other applications, ceramic piezoelectric materials have some important drawbacks: limited deformation, fragility and a high mass density that limit their use in sectors such as aeronautics or electrical-electronics. These limitations can be overcome in specific applications using polymeric piezoelectric materials instead of ceramic ones.
The only piezoelectric polymer that currently exists on the market is Polyvinylidene Difluoride (PVDF). This semi-crystalline polymer is characterised by having very good piezoelectric properties, but only to 90 C. Thus the interest in synthesising new piezoelectric polymers capable of maintaining their properties at greater temperatures.
At GAIKER-IK4 we have developed amorphous piezoelectric polymers to be employed in conditions of extreme temperature where semi-crystalline polymers cannot be used. To this end, and after prior work with different materials, the use of polymides was opted for, given their excellent thermal, mechanical and dielectric properties. Various dipolar groups (-CN, -SO2-, -CF3) have been incorporated into the molecule, varying the number and position of these groups in order to fix their physical - and consequently, their piezoelectric - properties. Moreover, it has been shown that the value for the temperature of vitreous transition is fundamental for these polymides, as this determines the temperature at which piezoelectric properties are lost. Specifically, this type of polymers show piezoelectric stability up to temperatures of 150C and do not begin to degrade until above 400 C.
We have been involved in this line of research at the Plastics and Composites Area for a number of years now and, particularly as a result of the research carried out jointly with the Department of Physical Chemistry at the University of the Basque Country (UPV/EHU), we have recently applied for a patent in the Oficina Espanola de Patentes y Marcas on 'Polymides with piezoelectric properties.'