Structural Design Of Polylactic Acid/Carbon Nanotubes Conductive Composites For High Conductivity And Low Percolation Threshold

Multi-walled carbon nanotubes have become a study spot in the field of materials science because of their excellent thermal,electrical and other properties.Carbon nanotubes as one-dimensional materials of high aspect ratio are widely used in polymer modification,giving polymer materials with excellent mechanical properties,thermal stability,conductivity,thermal conductivity,electromagnetic shielding and other properties.In recent years,compared with other carbon matrials,cost-effective carbon nanotubes are particularly widely used in the preparation of conductive polymer composites.It is well known that the key to preparing conductive polymer composites is to reduce the percolation threshold(critical filler content,above which aneffective percolation network will form).However,the formation of conductive network in conventional materials of random dispersion often requires a high amount of conductive filler addition.Therefore,the effective distribution of carbon nanotube will determine the final conductivity of the composites.From the point of morphological regulation,this paper focus on controlling the effective distribution of carbon nanotubes in the polymer matrix,and constructing efficient conductive network.In order to solve the above problems,from the perspective of crystal exclusion,phase evolution and segregation effect,this paper design three kinds of conductive carbon nanotubes polymer composites in biobased polylactide matrix,which significantly reduce the conductive percolation threshold value.The main contents include the following three aspects:(1)The crystal volume exclusion effect of polylactide on the distribution of carbon nanotubes and corresponding electrical properties are thoroughly studied by tailoring the crystal morphology of polylactide via introducing high melting point polylactide crystallites.Finally,the nanocomposites with improved conductivity,thermal resistance and mechanical properties were prepared.Polylactic acid is one kind of brittle semi-crystalline polymer material which is very difficult to crystallize.It is shown that the addition of trace of high melting point polylactic acid crystals could significantly increase the crystallization rate and crystallinity of polylactic acid matrix.Moreover,its crystallinity is almost constant regardless of the content of high melting point polylactic acid,which provides a good opportunity for studyingthe effect of matrix crystal morphology on MWCNTs distribution and corresponding electrical properties from the perspective of factor variation.The size of polylactic acid spherulitesincreases with the addition of the high melting point polylactic acid nucleating agent,but the conductivity of the composites tends to increase first and then decrease,which shows the best conductivity at medium spherulite size.Through the rheological and microstructure analysis,we discuss the change of the conductivity with the crystal size from the point of structure-performance.From the perspective of carbon nanotubes exclusion and lamella interlocking,a clear principle model is given.This provides a good idea for preparing adjustable conductive polymer composites.(2)Effective conductive network are constructed in PLLA/PCL/MWCNTs composites bytailoring the double phase morphology via constructing polylactic acid stereocomplex in PLLA phase,where the dispersed PCL phase added with carbon nanotubes were reversed and transformed into continuous phase.Therefore,a highly efficient conductive composites were prepared with small amount of MWCNTs.From the perspective of morphological structure,the content of PDLA on phase evolution and corresponding electrical properties are studiedwith constant MWCNTs.Meanwhile,the results show that the percolation threshold of PLLA/40PCL/ MWCNTs is reduced from 0.19 wt% to 0.025 wt% by introducing 20 wt% PDLA in PLLA phase.From the viewpoint of structure-performance,the mechanism of phase inversion to the formation of conductive network is deeply explored.(3)Composites with significantly decreased electrical percolation threshold were obtained by constructing segregated carbon nanotube conductive network in polylactic acid.Firstly,multi-walled carbon nanotubes were filled in amorphous low viscosity polylactic acid to prepare a coated masterbatch.Then,the surface of the segregation particles were wrapped by low viscious polylactic acid/multi-walled carbon nanotube masterbatch(lPLANTs)through controlling the processing temperature which is much lower than the melting point of the high melting point polylactic acid particles,but higher than the viscous temperature of low viscious polylactic acid.Finally,the s-PLLA/lPLANT composites with ultra-low percolation threshold were fabricated by the segregated multi-walled carbon nanotube conductive network.The glass transition temperature,crystallinity and the mechanical properties of the composites were all improved.