The Preparation Of The Graphene Based Nanocomposites And The Thermal Transition Signals And Crystal Transformation Of PLA

In this thesis,by taking advantage of supercritical CO2,we prepared and studied graphene-based composites.The thesis is divided into three sections.In the first section,we synthesized graphite oxide(GO)by Hummers method with some modification,and graphite oxide was reduced to graphene through thermal expansion.By the use of the high penetrating ability and solubility of supercritical carbon dioxide,tin oxide(SnO2)nanoparticles were successfully deposited on the surface of graphene sheets and the reultant nanocomposite was characterized by the means of FT-IR,XRD,TEM,XPS.The results showed that tin dioxide was uniformly distributed on the surface of graphene,forming functional nanocomposites.In the second section,the water-soluble polymer PEG6000 was successfully decorated on the surface of GO by a simple supercritical CO2 anti-solvent method.The formation of GO/PEG6000 hybrid was confirmed by IR,XRD and TEM.PEG6000 was uniformly deposited on the surface of the GO in the form of crystalline nanoparticles,changing the surface properties of GO.X-ray diffraction(XRD).The last section deals with the effect of supercritical carbon dioxide CO2 treatment on the thermal behavior of poly(L-lactic acid-co-D-lactic acid)(D<4%)(PLLA)filled with graphite oxide(GO).Compared to pure PLLA sample,the PLLA/GO samples displayed less thermal transition signals during differential scanning calorimetry(DSC)heating,which includes enthalpy relaxation,endothermic annealing,melting-recrystallization process and cold crystallization.The addition of GO led to more perfect crystal form of the semi-crystalline polymer PLLA.We found that,withoui the aid of compressed CO2,?"-crystal form was observed by using solution casting fabrication method at room temperature and the presence of GO resulted in the transition from ?" to ?'-form while the crystallization temperature dependence of the diffraction peak position(2??16°)and crystallinity for the CO2-treated PLLA indicatesd that the crystal modification changes continuously from the disorder ?(?")to a forms not through the ?' one with increasing temperature..Meanwhile,different dimensionality and the surface properties of the fillers(carbon nanotubes(CNTs),SiO2,graphene)would significantly influence the formation of crystal type for the semi-crystalline polymer possessing polymorphism of PLLA.