Covalent Functionalization Of Hexagonal Boron Nitride Nanosheets And The Properties Of Polymeric Composites

Due to its exceptional physical and chemical properties,hexagonal boron nitride(h-BN)with a graphene-like layered structure is widely used in the field of polymeric composites.To improve the solubility of boron nitride nanosheets(BNNSs)in various solvents and the compatibility with polymeric matrix,BNNSs need to be covalently functionalized.In this thesis,BNNSs were covalently functionalized by various groups such as hydroxyl,amino and polyethylene glycol via ball milling and chemical reduction reaction.Meanwhile,polymer-based(polyethylene glycol and polyvinylidene fluoride as polymer matrices respectively)composites filled with functionalized BNNSs were fabricated.We studied the effect of functionalized BNNSs on the physical(thermal and crystalline)properties of polyethylene glycol(PEG)and polyvinylidene fluoride(PVDF)based composites.The main research contents are as follows:1.Hydroxyl-functionalized boron nitride nanosheets(OH-BNNSs)and amino-functionalized boron nitride nanosheets(NH₂-BNNSs)were prepared via ball milling.Fourier transform infrared spectroscopy(FTIR)and X-ray photoelectron spectroscopy(XPS)analysis showed that hydroxyl and amino groups were successfully covalently grafted onto BNNSs.X-ray diffraction(XRD),transmission electron microscopy(TEM)and Raman spectroscopy were used to characterize the degree of exfoliation and morphology of functionalized BNNSs.Functionalized BNNSs are thin and free of structural damages.2.BNNSs were negatively charged in a sodium-naphthalene solution by chemical reduction,and then bromine-terminated polyethylene glycol(PEG)was added to the reductive system to obtain PEG-functionalized BNNSs.A solution blending method was used to prepare polyethylene glycol based composites filled with PEG-functionalized BNNSs(PEG-BNNSs).FTIR and thermogravimetric analysis(TGA)results showed that PEG chains were successfully grafted onto BNNSs and the grafting rate was 9.2%.XRD and TEM were used to characterize the crystal structure,degree of exfoliation and structural integrity of PEG-BNNSs.Ultraviolet visible(UV-vis)absorption spectroscopy was used to study the dispersibility of PEG-BNNSs in trichloromethane.PEG-BNNSs were well dispersed in various solvents including water.Differential scanning calorimeter(DSC)and TGA results showed that PEG-functionalization can improve thermal stability of polymeric composites.3.OH-BNNSs and NH₂-BNNSs were blended with PVDF via a solution casting method to form PVDF/OH-BNNSs and PVDF/NH₂-BNNSs nanocomposites.XRD and FTIR were used to study the effects of functionalized BNNSs on PVDF nucleation and crystal transformation.It is found that the addition of OH-BNNSs has little influence on the crystal structure of PVDF,and non-polar?-crystals are mainly formed in the PVDF/OH-BNNSs nanocomposites,whereas the incorporation of a small amount of NH₂-BNNSs can cause exclusive formation of polar?-and?-phases.XPS and FTIR were used to compare the interaction between PVDF and functionalized BNNSs.There exists stronger hydrogen bond interaction between PVDF and NH₂-BNNSs than that between PVDF and OH-BNNSs in both solid and melt states.DSC and polarized optical microscopy(POM)results showed that NH₂-BNNSs behaving as nucleating agents promoted the crystallization process of PVDF.