Formation of polylactide stereocomplex crystallites and the electrical properties of carbon black-filled PLLA/PDLA composites

Abstract

A mixture of the enantiomers of polylactide (PLA), PLLA, and PDLA forms a stereocomplex (SC) crystal. In this study, we prepared carbon black (CB)-filled PLLA/PDLA composites with 10 and 20 wt% PDLA by melt blending the mixture at a temperature higher than the melting point of the homocrystal and lower than that of the SC crystal. We quantitatively investigated the relationship between the degree of crystallinity of the SC crystal (χc,SC) and the filler dispersion by analyzing Morisita’s index (Iδ) determined from scanning electron microscopy images and through direct current (DC) measurements and alternating current (AC) impedance measurements. Iδ as a function of the quadrat number of the CB-filled PLLA composite exhibited an aggregated pattern; however, Iδ of the CB-filled PLLA/PDLA blends followed a random or uniform pattern with increasing χc,SC. DC measurements indicated that the percolation threshold also increases with χc,SC. The equivalent electrical circuit was determined from AC impedance measurements. The resistance of the polymer matrix in the gap between the CB agglomerates increased with χc,SC, whereas its capacitance decreased with χc,SC. These results reveal that the increase in the number of SC crystals formed during melt blending improves the filler distribution to achieve a uniform pattern. Stereocomplex (SC) crystals of polylactide were formed in carbon black-filled Poly(l-lactide) (PLLA)/Poly(d-lactide) (PDLA) composite with 10 and 20 wt% of PDLA during melt blending at the temperature between the melting points of homocrystal and SC crystal. From the analysis of Morisita’s index (Iδ) determined from scanning electron microscopy images and through direct current measurements and alternating current impedance measurements, it was revealed that the increase in the number of SC crystals formed during melt blending improves the filler distribution to achieve a uniform pattern.