| Poly(lactic acid)(PLA)is a representative bio-based biodegradable polyester,which can replace petroleum based non-degradable plastics in biomedical and packaging fields.The addition of boron nitride nanosheets(BNNS)to PLA can improve its thermal conductivity and expand its application areas.However,the crystallization rate of PLA/BNNS nanocomposites is low,and amorphous products are usually obtained during injection molding,resulting in difficulty in molding and poor temperature resistance.Therefore,how to improve the crystallization rate of PLA/BNNS nanocomposites is a key problem to be solved urgently.In this thesis,the crystallization rate of PLA/BNNS nanocomposites was improved by adding plasticizer poly(ethylene glycol)(PEG)and self-assembled nucleating agent N,N’,N’’-tricyclohexyl-1,3,5-phenyltricarboxylamide(BTCA).The purpose of adding PEG is to promote the crystallization of PLA jointly through its plasticization and the nucleation of BNNS;the addition of BTCA is expected that BTCA and BNNS can have a synergistic effect on the crystallization of PLA.The main research contents and results are as follows:(1)PEG/BNNS masterbatch was prepared by ultrasonic dispersion and then freeze-drying,and then it was melt-blended with PLA to obtain PLA/PEG/BNNS nanocomposites.The effect of PEG and BNNS on the crystallization behavior of PLA was researched,and heat resistance,thermal conductivity and thermal stability of nanocomposites were tested.The results of differential scanning calorimetry(DSC)show that PEG and BNNS have a synergistic effect on the isothermal and nonisothermal crystallization of PLA under static conditions.This can be attributed to PEG improving the mobility of PLA molecular chain,while BNNS plays the role of heterogeneous nucleation.It is worth mentioning that the combination of PEG and BNNS has an adverse effect on the crystallization of PLA under injection molding conditions.This may be due to the relaxation of the oriented PLA chain caused by the plasticization of PEG,which delays the flow-induced crystallization.Dynamic mechanical analysis(DMA)shows that only in the case of crystalline PLA,the existence of BNNS improves the heat resistance of PLA matrix,while the further addition of PEG has an adverse effect.The thermal conductivity results show that the simultaneous use of PEG and BNNS can improve the thermal conductivity of PLA.The results of thermogravimetric analysis(TGA)show that the effect of BNNS on the thermal stability of PLA can be ignored,and the further addition of PEG reduces the thermal stability of PLA.(2)PLA/BTCA/BNNS nanocomposites were prepared by solution blending.The effects of BTCA and BNNS on the crystallization behavior,thermal stability and heat resistance of PLA were studied.DSC results show that BTCA and BNNS have a significant synergistic effect on the crystallization of PLA when the melting temperature is 210 ~oC(or higher).The results of the polarizing microscope(POM)show that the nucleation effect of BTCA on PLA is significantly better than that of BNNS.Compared with BTCA or BNNS alone,PLA has the highest crystal density and the highest temperature at the completion of crystallization.The possible reason for the synergistic promotion of PLA crystallization by BTCA and BNNS may be that BTCA melts and then crystallizes first on the surface of BNNS during the cooling process,increasing the nucleation sites and thus significantly enhancing the crystallization rate of the PLA matrix.DMA analysis shows that the simultaneous addition of BTCA and BNNS combined with a higher mold temperature(110 ~oC)increased the minimum modulus of the PLA matrix from 6.4 MPa to 111.7 MPa during heating,which significantly improved the heat resistance of PLA.The TGA results show that the simultaneous addition of BTCA and BNNS can slightly improve the thermal stability of PLA.The work provides new methods to improve the crystallization rate and heat resistance of PLA,offering some guidance to expand its potential application areas. |
