Tuning the carbon nanotube conductive network in PLA-based immiscible blends via stereocomplex crystallization

Abstract

Crystallization-induced segregation of nanoparticles and selective nanoparticle localization in immiscible polymer blends with cocontinuous morphology are effective strategies to control the distribution of conductive fillers to increase electrical conductivity with minimal filler content. In this research, stereocomplex crystallization and double percolation of nanoparticles are combined in 50/50 (PLLA/PDLA)/PVDF blends filled with 0.1–1.6 wt% carbon nanotubes (MWNTs) to achieve high electrical conductivity and electromagnetic shielding performance. During melt crystallization, spherulite-shaped homocrystals form in the 50/50 PLLA/PDLA phase leading to a primary segregated structure with the MWNT nanoparticles localized in the PLA phase. The generated homocrystals are then reformed into stereocomplex crystals upon annealing at elevated temperatures. Thus, a refined segregation of the MWNTs is achieved forming a conductive pathway via enhanced volume exclusion. The SC crystals formed through melting and recrystallization of homocrystals at a high temperature, where there is enough chain mobility, can effectively compact the segregated network which enhances the electrical conductivity and electromagnetic shielding performance. As a consequence, the electromagnetic shielding effectiveness of this sample is double that of samples re-crystallized at a lower crystallization temperature. The different mechanisms at play are analyzed using a range of techniques including differential scanning calorimetry, X-ray diffraction, and microscopy observations. This research demonstrates that stereocomplex crystallization via homo-recrystallization in PLA-based blends can be used as a novel approach to fabricate high-performance conductive and EM shielding composites by exploiting phase morphology and crystallization.