Shear Piezoelectricity of Poly(l-lactide) Films Manufactured by Extrusion–Orientation: An Insight on Process–Structure–Property Relationships

Shear piezoelectric properties of uniaxially stretched poly(l-lactide) (US-PLA) films manufactured by an industrially relevant technique (i.e., extrusion–orientation without poling) are investigated, and specific insight on the process–structure–properties relationships is provided. Two commercially available PLA grades with d-isomer content between 2 and 4% are selected. The shear piezoelectric coefficient d14 of US-PLA films tends to increase with the draw ratio applied during the orientation stage, and a maximal d14 of 5.9 pC/N is reported. However, a dramatic degradation of piezoelectric properties could be observed at elevated draw ratios, in particular for PLA grades with low d-isomer content. Structures induced by the orientation stage are subsequently explored, and relations with shear piezoelectric properties are discussed. The mesophase is detected by DSC/WAXS up to draw ratio 4 after being replaced by strain-induced crystallization at higher draw ratios. The orientation state of the amorphous phase, mesophase, and α′-crystals is assessed by 2D-WAXS and polarized FTIR. Piezoelectric properties obtained at a moderate draw ratio are supported by the amount/orientation of each phase (partly oriented amorphous phase and fully oriented mesophase/α′-crystal phase). A model is proposed to evaluate the contribution of each phase. However, other structural parameters deserve careful attention at elevated draw ratios, in particular mechanical damage and formation of voids/cavities. 2D-SAXS analysis coupled with complementary characterizations indicates that the amount of voids/cavities controls the deterioration of shear piezoelectric performances at high draw ratios. This phenomenon is critical for highly crystalline PLA grades. A final discussion is dedicated to the quality of α′-crystals formed by current orientation conditions, a factor that could also limit the d14 coefficient of US-PLA films. This work consequently demonstrates that environmentally friendly piezoelectric films could be manufactured by a straightforward process of the plastic industry and opens up several scientific/technological perspectives for their future implementation into practical applications.