The Combined Effects of Optical Purity, Chain Orientation, Crystallinity, and Dynamic Mechanical Activation as Means to Obtain Highly Piezoelectric Polylactide Materials

The shear piezoelectricity of polylactide (PLA) depends on the chain orientation, crystallinity, and optical purity, but their combined influence on the piezoelectric properties has never been studied. This work examines the effect of optical purity on the piezoelectric chain morphology induced by cold-drawing and heat treatment. Moreover, the influence of force and frequency during dynamic mechanical activation on the piezoelectric response of PLA was studied. The results of this work present for the first time a piezoelectric coefficient of a purely amorphous as-drawn PLA film (d14 = 2.3 ± 0.1 pC N–1), exhibiting high optical purity and high chain orientation but without crystallites, leading to a ductile behavior. Postdrawn annealing increased the piezoelectric coefficient by 157% due to induced crystallinity and high crystal orientation, resulting in a brittle behavior. Upon increasing the optical purity from 88 to 99% l-isomer content, the piezoelectric coefficient (d14) was shown to be improved due to the increased degree of crystallinity, chain orientation, and lamellar thickness. Dynamic mechanical analysis revealed for the first time a linear strain rate dependence of the piezoelectric response of PLA. This led to a further increase of the piezoelectric coefficient of the amorphous as-drawn film by 470% by increasing the frequency of dynamic mechanical activation to 1 kHz. These results unfold the potential for the fabrication of piezoelectric amorphous PLA films with improved ductility for final biomedical applications.