Study On Modification Of Carbon Nanotubes And Properties Of Poly(Lactic Acid)and Modified Carbon Nanotubes Composites

Polylactic acid(PLA)is a new type of renewable polyester that can replace plastics.It has been approved by the FDA of USA for clinical application and has a wide range of applications.However,PLA has many advantages such as good biocompatibility,excellent mechanical strength and barrier property,as well as the disadvantages of large resistivity(>1015 ?),and it is easy to store static electricity on the material,coupled with other slow crystallization rates and poor thermal stability of faults.The disadvantages greatly limited its application in high value-added fields such as medical and electronics.Multi-Walled Carbon Nanotubes(MWCNTs)are known as one of the most hard and strong materials.They also have the advantages of good toughness,high thermal stability,excellent electrical conductivity and excellent thermal conductivity,etc.They are ideal nanofibers modified materials of filled with PLA.However,due to the weak interfacial bonding between MWCNTs and PLA matrix,a large number of agglomerations often occur,which greatly limits the wider application of MWCNTs.In this dissertation,carboxylated carbon nanotubes and grafted carbon nanotubes modified by hyperbranched polyester were prepared,and the MWCNTs before and after modification were melt-blended with PLA,respectively.It was hoped that the composites of MWCNTs/PLA would have good conductivity,heat resistance and mechanical enhancement.To provide theoretical basis and basic data for the application of MWCNTs/PLA composites in high value-added industries such as medical and electronics.The results showed that MWCNTs could be generated at the defects of MWCNTs by the covalent chemical modification of MWCNTs by concentrated nitric acid oxidation and mixed acid oxidation.The carboxyl density of MWCNTs-COOH-b(11.34%)prepared by the mixed acid oxidation method is higher than that of MWCNTs-COOH-a(3.83%)prepared by the concentrated nitric acid oxidation method.SEM showed that MWCNTs-COOH-a and MWCNTs-COOH-b were filtered and milled.The particle size was more inhomogeneous than MWCNTs,and some of the larger aggregates appeared.The defects increased and the aspect ratio decreased.Solubility tests showed that the carboxyl-modified MWCNTs-COOH-a and MWCNTs-COOH-b were significantly more hydrophilic than MWCNTs.The graft modification of MWCNTs-COOH-b and hyperbranched polyester H204 was studied.The hyperbranched polyester grafted carbon nanotubes MWCNTs-H204 were prepared and their structures were characterized.Raman spectra and TG analysis confirmed the successful grafting of hyperbranched polyester H204 onto MWCNTs-COOH-b.SEM revealed that the size of MWCNTs-H204 particles was also non-uniform,large aggregates also appeared,and the surface of MWCNTs-H204 was looser.The solubility test found that MWCNTs-H204 has excellent hydrophilicity,and can still stably disperse evenly in water after 60 days of standing.The composites of MWCNTs/PLA,the composites of MWCNTs-COOH-b/PLA and the composites of MWCNTs-H204/PLA were prepared by melt blending method.The effects of electrical properties,mechanical properties and thermal performance of composites were studied by MWCNTs,MWCNTs-COOH-b and MWCNTs-H204.adding MWCNTs,so that the overall conductivity of the composite material significantly improved,enhanced mechanical,thermal stability is enhanced and the crystallinity increased;adding MWCNTs-COOH-b,so that electrically conductive composite material is not obvious,decreased slightly mechanical,thermal stabilizers Sexual enhancement and crystallinity decreased slightly and then rose.Addition of MWCNTs-H204 made the conductivity of the composite material not increase significantly,mechanical properties decreased,thermal stability increased,and crystallinity increased first and then decreased.