Piezoelectric poly(lactide) stereocomplexes with a cholinium organic ionic plastic crystal

Abstract

This paper describes a novel method to produce poly(lactic acid) (PLA) stereocomplex (SC) crystallites directly from high molecular weight PLLA by incorporation of small quantities of choline dihydrogen phosphate ([Ch][DHP]), at room temperature. Combined results of X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) confirm the formation of the PLA SC crystals. ¹H NMR spectroscopy indicates a modification of the short range structural environment of choline in the blend with PLA in good agreement with the major long-range structural changes suggested by XRD, both suggesting a direct role of the choline in the PLA stereocomplexation. The electrical conductivity of these membranes measured using impedance spectroscopy is in the 6 × 10⁻⁷–4 × 10⁻⁶ S cm⁻¹ range, which increases with increasing choline content and with increasing relative humidity (RH). The dependence on RH is weaker than for pure PLA, thus suggesting that the SC enhances the hydrophobic character of PLA, in agreement with contact angle measurements. Atomic force microscopy (AFM) and piezoresponse force microscopy (PFM) reveal a new phase with piezoelectric properties at the microsphere interfaces, with d₃₃ corresponding to ∼60% of d₃₃ of a periodically poled lithium niobate reference, which is directly related to the formation of SC crystalline domains. This first report of intrinsic piezoelectricity in PLA stereocomplexes opens the possibility for the development of new types of bio-based piezoactive polymers.