Unveiling the Amorphous Phase Contribution to Shear Piezoelectric Properties of Polylactide

Polylactide (PLA) represents an interesting biobased alternative to ferroelectric fluoropolymers with shear piezoelectric coefficients d₁₄ of up to 10 pC/N solely activated by uniaxial orientation. A significant contribution of the oriented amorphous phase is suspected, and this work consequently proposes a detailed analysis of the piezoelectric activity for a fully amorphous polylactide grade (aPLA). Oriented aPLA films were produced by machine-direction orientation (MDO), and the amorphous phase orientation (F_am) was precisely measured along with piezoelectric responses. The absence of mesophase/cavitation phenomena is confirmed, and F_am increases with the draw ratio, in agreement with deformation theories of rubber networks. Oriented aPLA films display d₁₄ values up to 2 pC/N with perfect linear coupling to F_am. The significant contribution of the oriented amorphous phase is validated, and for the first time, the intrinsic piezoelectricity of the amorphous phase was extrapolated to 7.2 pC/N. Additional experiments at various draw rates indicate that relaxation modes of aPLA are only active below 0.3 s^–1. Relaxation phenomena are consequently inactive in MDO conditions at an elevated draw rate. The effect of the optical purity was finally discussed. The intrinsic piezoelectricity of the amorphous phase can be considered constant for PLA grades with low D-content. This study helps in refining the current models for future detailed analyses on oriented semicrystalline PLA films.