Preparation Of Polylactide-Based Nanocomposites And Research On Structure And Properties

Poly（L-lactide）（PLLA） is a kind of biodegradable eco-friendlymaterials.Whlile poor crystallization ability and fracture toughness greatlylimit the application of PLLA.Due to the excellent mechanical, electricalconductivity, thermal conductivity and biocompatibility, carbon matrixnano-fillers are widely used in PLLA modification research. For the largelength-diameter ratio and specific surface area, multi-walled carbonnanotubes （MWNTs）, graphene sheet （GS） are widely used to improvePLLA comprehensive properties. In present works, firstly, we exploredthe effect of parameters and CNT contents on morphology and propertiesof as-spun PLLA/MWNTs fibers. Polyethylene oxide （PEO） whichusually acts as a plasticizer for PLLA, Hence, we explored the effect oflow contents PEO on the morphologies and properties of as-spunPLLA/PEO/MWNTs fibers, put emphasis on whether there was asynergetic effect of PEO and MWNTs on crystallization behaviors ofPLLA. To make full understand of the feature of electrospinning, wecompared the similarities and differences of crystallization properties ofas-spun and solution-cast PLLA/MWNTs composites. In additionl, wefabricated PLLA/GS composites, investigated thoroughly the effect of GScontent on macro performance such as thermal stability and mechanical, conductive properties. According to the previous reaserch， someinteresting results have been achieved as shown in the following:（1）The optimum electrospun condition as following： PLLAconcentration of10wt%; the weight ratio of DCM and DMF was7/3;voltage of15KV; collecting distance of15cm; volumetric fow rate of0.5mL·h^-1; the drum rotate speed of500rad·min^-1.（2）The content of carbon nanotubes has effect on PLLA/MWNTs fibersmorphogies and properties. From crystallization research, which indicatedwith the increase of MWNTs contents, the temperature of coldcrystallization decreased, the rate of crystallization increased, the slightnucleation ability of MWNTs at different temperature, crystallinemodification of PLLA/MWNTs fibers is α-form, annealing temperature andMWNTs content have no obviously effect on crystallization behavior ofcomposite fibers.（3） Contrast between electrospinning and solution-casing on structureand properties of PLLA/MWNTs nanocomposites: the force duringelectrospun processing induced molecular orientation of PLLA chains, ggconformers of amorphous chains may transform to gt conformers whichdecreased conformational entropy. The temperature of cold crystallizationin as-spun fibers was lower than that in solution-casting films. Throughannealing treatment which indicate the fabricate method has no effect oncrystal form of PLLA/MWNTs composites.（4） Low contents of PEO could improve the mobility of PLLA chains,increase the fraction of mesophase and crystallization rate, However,MWNTs could not act as nucleating agent in the nano confinement spatialof PLLA/PEO/MWNTs fibers.From the crystallization of melting, therewas a synergetic effect of PEO and MWNTs on crystallization behavior ofPLLA. （5） Graphene sheet which had different sizes, surface drape, someoxygen groups.For PLLA/GS fibers, with the graphene content increasing,fiber diameter decreased at first then increased. From TEM images,graphene sheet wrapped on fiber surface which inhabited the mobility ofpolymer chains, which improved the cold crystallization temperature,lowered the crystal degree. For solution-casting PLLA/GS films, grapheneimproved thermal stability, tensile strength of films, however, decreasedthe elongnation at break, a percolation threshold as low as0.1vol%.Isothermal crystallization of films, at low content of graphene could act ascrystallization nucleating agent, at high content of graphene formednetworks, inhibited crystal growth，reduced the radius of spherical crystals.