Research On The Structure Control And Mechanism Of Enhanced Dielectric Properties Of Polylactic Acid Nanocomposites

In view of the serious environmental problems caused by the rapid growth of e-waste,it is an effective method to prepare environmentally friendly high dielectric composites using biodegradable polylactic acid(PLA)as the polymer matrix,and how to improve the mechanical and dielectric properties of PLA becomes the focus of current researches.In this paper,the mechanical properties of PLA are improved by the blending of ethylene-vinyl acetate(EVA)and polybutylene adipate terephthalate(PBAT),while the carbon nanotubes(CNTs),Barium titanate(BT),Boron nitride(BN)and Mica were added into the matrix to improve the dielectric properties of PLA.The research content of the thesis is mainly divided into the following three parts:(1)PLA/EVA/CNTs composites were prepared by a melt blending method,and the effects of CNTs content on crystallization behavior,aggregated structure,mechanical properties and dielectric properties of the composites were studied.According to the result,EVA elastomer improves the mechanical properties of PLA.CNTs promoted the nucleation of PLA,but hindered the movement of the PLA segments,which ultimately led to a decrease in the crystallinity of PLA.The microcapacitors formed by CNTs in composites can improve the dielectric properties of PLA.The PLA/EVA/1.0CNTs composites exhibited relatively high?'(11.08)and low tan?(0.084),while its tensile strength and elongation at break were 42.7 MPa and 70.5%,respectively.The composites possess excellent mechanical properties and dielectric properties.(2)PLA/PBAT composites containing different functionalized CNTs were prepared by a melt blending method,and the effects of different functionalized CNTs and their contents on chain extension reaction,crystallization behavior,rheological behavior,aggregated structure,mechanical and dielectric properties of the composites were studied.CNTs-OH and CNTs-COOH will compete with the reaction between polymers and the chain extender,resulting in a decrease in the efficiency of the chain extension reaction.As a result,the interfacial adhesion and mechanical properties of PLA/PBAT/CNTs-OH and PLA/PBAT/CNTs-COOH composites were significantly reduced.Different functionalized CNTs hindered the segment movement of PLA,resulting in a decrease in the crystallinity of PLA in PLA/PBAT composites.The microcapacitors formed by different functionalized CNTs in the composites can improve the dielectric properties of PLA.The PLA/PBAT/4CNTs-OH composites exhibited relatively high?'(24.9)and low tan?(0.23),while its tensile strength and elongation at break were 38.2 MPa and 255.4%,respectively.(3)The sandwich structure dielectric composites were prepared by the layer-by-layer coating method.By means of structural design,Mica or BN with high breakdown strength was introduced into the outer layer and BT with high dielectric constant was introduced in the inner layer to increase the dielectric constant and breakdown of the composites,synergistically.At the same time,all the inorganic fillers were coated with polydopamine(PDA)to improve the compatibility of filler with PLA matrix,and the dielectric difference between inorganic filler and PLA matrix was reduced.The PLA/40PDA@BT/8PDA@Mica composites exhibited an energy density of 1.72 J/cm~3and an energy storage efficiency of 83%,because the large-size Mica could effectively suppress the growth of electrical tree branches and the generation of leakage current as an insulation barrier.