Structure And Properties Of Polylactic Acid Composites Based On Graphite Nanosheets

Graphite is the most common carbon material for its extensive raw material resource and low cost. The basic unit to form graphite is graphene, a two dimensional crystal carbon material, which attracts much attention for its unique structure and excellent properties. Graphene is been widely studied by researchers, including the preparation methods and its composites. Although the methods to prepare graphene is various, the method of large scale production of monolayer graphene is quite rare. The existing methods mostly produce graphite nanosheets with various thichness. When filled graphene into the polymer materials, the performance of polymer matrix can be greatly improved. Polylactide (PLA) is a biodegradable aliphatic polyester with good properties, but its application has been limited because of the weakness of brittleness, easy deformation and other defects. Fill with filler is an effective strategy to improve the comprehensive properties of PLA. However, the work on PLA/graphite nanosheets composites is still in primary stage, and it’s of great instructive to produce composites meets different needs by studying the relationship between the structure and properties.In this work, graphite oxide (GO) was prepared by modified Hummers method with using natural flake graphite as a starting material. Then, PLA composites filled with different types and loadings of graphite naosheets were prepared via solution blending. The structure and properties of the composites were studied in details by the approaches of morphology, structure rheology, creep and stress relaxation, and so on. The relationship between structure and properties of PLA/graphite nanosheets composites were described by viscoelastic theory, crystallization kinetics theory, and mechanical models.(1) Graphite oxide was successfully prepared via Hummers method and the X-ray diffraction results reveal that the characteristic peak of graphite disappeared after oxidation and a new diffraction peak shows up at about 11.6° which indicates that the interlayer distance increased in the oxidation process. The characteristic diffraction peaks of different graphite nanosheets decreases with reducing the graphite nanosheets’thichness indicates that the long rang order structure destroyed. These results were proved by Raman spectroscopy.(2) Rheology study shows that the dynamic storage modulus, loss modulus and complex viscosity of the composites increased with increasing graphite nanosheets loadings. But the percolation network for PLAgCs composites don’t formed obviously even 10 wt% graphite nanosheets added into the PLA matrix because of bad graphite nanosheets dispersion in matrix. Under the circumstances of same graphite nanosheets loadings, composites filled with GO shows higher storage modulus, which due to its good affinity between GO and matrix. It can be found that the relaxation of internal molecular chain is suppressed by adding graphite nanosheets into the PLA matrix.(3) The crystal structure of PLA don’t changed after crystallization whether adding graphite nanosheets. The presence of graphite nanosheets have played as nucleation agent, promoting formation of the nuclei of PLA matrix under both isothermal and nonisothermal crystallization process. The crystallization activation energy of PLA composites increases with increasing the filler contents, reducing the thickness of graphite nanosheets because filler with larger filling volume can retards the diffusion of PLA chains and the nucleation effect domains the crystallization kinetics process lead to increasing crystallization rates.(4) Mechanical properties of PLA increases by adding graphite nanosheets. Thicker graphite nanosheets have larger filling volume that leads to better performance of PLA composites with same filler loadings. Further study on viscoelastic behavior of the composites under small stress or strain applied reveals creep and stress relaxation had dependence on temperature and stress and strain. The creep and stress relaxation behavior of PLA composites can be effectively restrained by adding graphite nanosheets. With increasing graphite nanosheets loading, creep strain decreased and the relaxation modulus increased which means graphite nanosheets can inhabit the molecular chain movements. Creep and stress relaxation behavior of PLA composites increase with the thicker thickness of graphite nanosheets by adding the same loading of graphite nanosheets, which is because thicker nanosheets have larger filling volume and better inhabit the chain movements. Viscoelastic models can be well used to describe creep behavior and stress relaxation behavior of PLA and its composits and can obtain information between viscoelastic behavior and the structure of PLA composites.