Microwave-Assisted Sintering to Rapidly Construct a Segregated Structure in Low-Melt-Viscosity Poly(Lactic Acid) for Electromagnetic Interference Shielding

Abstract

Formation of a segregated structure in conductive polymer composites is one of the most effective strategies for achieving good electrical conductivity and electromagnetic interference (EMI) shielding performance. Nevertheless, for low-melt-viscosity poly(lactic acid) (PLA), intense molecular motion occurs at the molding temperature, which is detrimental to the fixation of the conductive networks. In this study, a novel molding technique assisted by microwave heating was proposed to construct a segregated structure in a PLA/carbon nanotube (CNT) composite. The coating layer of CNTs acted as the microwave absorber and caused intense localized heating of PLA surfaces upon microwave irradiation. The surface temperature of the PLA granule was precisely regulated by adjusting the coated CNT content, microwave power, and irradiation time. Thus, the coated granules were softened and fused at an optimal sintering zone, which effectively hindered the excessive migration of CNT strips into the interior of PLA phases, and a majority retained the original CNT network in the molded composite. Meanwhile, benefiting from microwave sintering, sufficient chain diffusion and entanglement occurred in the interfacial regions, enhancing the adhesion strength among the neighboring PLA phases. The prepared PLA/CNT composite with only 5.0 wt % CNTs exhibited a high electrical conductivity of 16.3 S/m and an excellent EMI shielding effectiveness (EMI SE) of 36.7 dB at a frequency of 10.0 GHz. The results indicate that microwave-assisted sintering might be a promising alternative for constructing a segregated structure in low-melt-viscosity polymers.