Effects Of Cellulose Nanocrystals On The Crystallization And Mechanical Properties Of Polylactic Acid

Since the synthesis of the first polymer material,polymer materials have penetrated into almost every aspect of national economy and human life with their rich varieties,excellent properties,appropriate price and other convenient applications.Recently,with the shortage of resources and the continuous emergence of environmental problems,people realize the importance of protecting the environment,and the academia also pays more attention to the research of environmental problems,especially vigorously studying and developing biodegradable materials has become an urgent need to solve problems such as white pollution.Polylactic acid（PLA）is a bio-based polymeric material that has attracted much attention due to its good biocompatibility,degradable properties as well as mechanical strength.However,the application of PLA is limited due to its poor crystallinity,heat resistance and brittleness.Cellulose nanocrystals（CNC）,as an environmentally benign material,exhibit a good enhancement effect on the mechanical properties of PLA,while diphenyl dihydrazide（TMC-300）,an exclusive nucleator of PLA,can greatly enhance the crystalline properties of PLA.Therefore,in this paper,CNC and TMC-300 were selected as the reinforcement and nucleating agents of poly（L-lactic acid）（PLLA）to explore the effects of CNC and TMC-300 on the properties of PLLA.Firstly,the effect of CNC on the performance of PLLA was investigated.The results showed that the addition of CNC increased the nucleation density of PLLA and improved the thermal stability and mechanical properties of PLLA.However,due to the introduction of CNC,the mobility of PLLA chain decreased,the cold crystallization temperature of PLLA increased,and the cold crystallization enthalpy and crystallization rate decreased,indicating that the crystallization property decreased.Secondly,in order to solve the problems of CNC agglomeration,acetylated cellulose nanocrystals（ACNC）were prepared by acetylation modification.The appearance of carbonyl peak in fourier transform infrared spectroscopy（FTIR）,the change of diffraction peak position and intensity of CNC crystal structure in X-ray diffraction analysis（XRD）,and the improvement of thermal stability studied by thermogravimetric analysis（TGA）indicate the successful synthesis of ACNC.When studying the effect of ACNC on the properties of PLLA,it was found that after adding ACNC,the nucleation density of PLLA increased,the cold crystallization temperature decreased,the crystallization rate increased,and the tensile strength and elongation at break increased,indicating that ACNC can improve the crystallization and mechanical properties of PLLA.Thirdly,the effect of TMC-300 on the performance of PLLA was investigated.The results showed that the addition of TMC-300 significantly promoted the crystal nucleation of PLLA and increased the crystallization rate of PLLA.At 2wt%TMC-300 content,TMC-300 self-assembled to form fibrillar crystals,which induce the epiphytic crystallization of PLLA,but a small amount of fibrillar crystals lead to the poor uniformity and the lower mechanical properties of PLLA sample.With the further increase of TMC-300 content,TMC-300 formed a network of crystal structure and the mechanical properties of PLLA are promoted again.Finally,different contents of CNC（or ACNC）were added to PLLA containing 2wt%TMC-300,and the effect of the two additives on the PLLA performance was studied.In contrast to the binary CNC（or ACNC）/ PLLA composites,with the addition of TMC-300,the crystallization peak of PLLA during cooling process can be observed,the cold crystallization peak disappeared during heating process,the nucleation density increased,and the crystallization rate increased.Compared with 2wt%TMC-300 / PLLA composites,the addition of CNC（or ACNC）reduced the crystallization enthalpy of the composites.The above results indicate that the addition of TMC-300 improved the crystallization properties of CNC（or ACNC）/ PLLA composites,while CNC（or ACNC）would reduce the promoting effect of TMC-300 on the nucleation of PLLA.For the mechanical properties of ternary composites,when the content of CNC（or ACNC）was low（1 ～3wt%）,the mechanical properties of composites decreased compared with the 2wt%TMC-300 /PLLA binary composites,and when the content of CNC（or ACNC）was high（4 ～ 5wt%）,the composites still had a rather high tensile strength and elongation at break.