Structure And Properties Of Poly(lactic Acid)/cellulose Nanofiber Composites

In recent years,with the strengthening of environmental protection awareness and the deepening of the concept of sustainable development,the research on biodegradable polymer materials has gradually increased.Among them,poly(lactic acid)(PLA)is the most representative and most promising one,which has good biocompatibility and degradability.However,PLA has disadvantages such as low mechanical strength,poor toughness,low degree of crystallinity,and poor heat resistance,which limits its application prospects.Cellulose Nanofibers(CNF)is a green,high-strength nanoscale reinforcement and does not destroy the biodegradability of PLA.In this thesis,CNF was used to the modify PLA.In order to increase the dispersion of CNF in PLA matrix and achieve better enhancement effect,two difrerent methods were used to modity CNF.Poly(butylene succinate)(PBS)was selected as the toughening material of PLA.PBS is rubbery at normal temperature and has good toughness,which can make up for the deficiency of PLA.And adding CNF as a reinforcing material,while improving toughness without significantly reduce mechanical strength.To increase the compatibility of PLA and PBS phases,a reactive compatibilizer,di-tert-butylperoxyisopropylbenzene(BIBP),was used to conipatibilize the system.At the same time,poor heat resistance limits the application of PLA.Blending poly(L-lactic acid)(PLLA)with poly(D-lactic acid)(PDLA)can form stereocomplexed poly(lactic acid)(sc-PLA).Sc-PLAThe melting point of sc-PLA is about 50℃ higher than that of homopolymer,which can effectively enhance the heat resistance of PLA.This thesis focuses on the preparation and performance of PLA-based biodegradable composites.The details are as follows:(1)PLA/CNF nanocomposites were prepared by solution blending method.The dispersion state,rheological properties and mechanical properties of CNF in PLA matrix were studied.The results show that the coupled modified CNF(cNF)and acetylated modified CNF(aNF)significantly enhance the affinity of CNF and PLA matrix.Compared to the coupling reaction,surface acetylation better increases the compatibility due to the formation of hydrogen bonds between PLA and aNF.CNF has a good enhancement effect on PLA whether modified or not,and the acetylated CNF has the best compatibility with PLA,so the enhancement effect is best.The modulus of the nanocomposite can be well fitted by the H-T model.(2)Morphology observation and interfacial tension test indicate that the PLA and PBS blends are two-phase incompatible systems,and PBS is dispersed in PLA matrix with the island structure The addition of PBS significantly increases the impact strength of PLA,and the toughness is improved while the strength and modulus are lowered.The addition of a reactive compatibilizer can cause a crosslinking reaction between PLA and PBS,resulting in a reduction in the size of the dispersed phase and increases the compatibility of the two phases.After adding BIBP and CNF,the strength and modulus of PLA did not decrease obviously,and the impact toughness improved significantly.(3)PLLA/PDLA composites with different blending ratios were prepared by solution blending and melt blending.FT-IR,XRD,DSC and other tests showed that sc-PLA was formed in the blend The formation of sc-PLA was not affect by the change of annealing temperature.The modulus of the PLLA/PDLA blend in the low frequency region is significantly higher than that of pure PLA,showing a solid-like behavior,indicating that sc-PLA increases the melt strength of the PLA.The blends prepared by melt blending tend to for:m sc-PLA.The Vicat softening point test showed that the heat resistance of PLA was improved after annealing,and the heat resistance of melt blended samples was better than that of solution blending samples.The mechanical strength of PLLA/PDLA blend was decreased compared with that of pure PLA.When 5 wt%of CNF was added as reinforcing material,the mechanical strength of PLA could be maintained.