Evaluation of Shear Piezoelectric Coefficients by a Bimorph Cantilever Technique for Extruded and Oriented Poly(l‐lactide) Films

Uniaxially stretched poly(l-lactide) (US-PLA) films display shear piezoelectric properties of interests for smart applications. However, the measurement of these properties is quite challenging and a simple technique based on bimorph cantilevers is here used. This technique is validated with commercial piezoelectric poly(vinylidene fluoride) (PVDF) films. A piezoelectric coefficient d(31) close to 19 pm/V is then obtained with a precision close to 5% and a good accuracy to expected values. For US-PLA films made by extrusion-machine-direction orientation, shear piezoelectric coefficients d(14) up to 5.9 pm/V are found. Relevant trends can be captured but a low precision on d(14), close to 10%, is highlighted. This behavior is probably related to film thickness heterogeneities arising from the extrusion processing at the laboratory scale. Piezoelectric voltage coefficients (g(14)) and electromechanical coupling coefficients could be also easily obtained with additional measurements of the dielectric permittivity and mechanical compliance. Outstanding g(14) coefficients up to 320 mV.m/N are confirmed for US-PLA films with a precision close to 20%. The bimorph cantilever technique consequently represents a simple and reliable technique to evaluate piezoelectric properties of soft materials, in particular shear piezoelectric properties of US-PLA films for future developments and optimizations at the pilot scale.