Crystallization Behavior And Mechanical Properties Of PLLA Matrix Composites

As a renewable and biodegradable polymer, poly-L-lactic acid (PLLA) has been widely studied in years. However, PLLA is brittleness, low crystallinity, poor heat resistance and so on, its application is limited in many fields. How to solve those problems has caught the researchers’ great attention. The internal structure of materials determines its properties. This thesis investigated the PLLA crystallization behavior, and the relationship between various scale structure and its mechanical properties. On one hand, understood the formation mechanism of different crystals in the PLLA/PDLA composites system; on the other hand, studied material correspondence between the micro-scale structure and properties which would guide the PLLA material’s production processing and realize its high performance. This paper also studied the morphology of PLLA’s spherulites under different processing conditions and mechanical properties of the crystalline and amorphous regions. The effects of polylactic acid stereocomplex crystal formation conditions on material properties of modified composites were studied. The contents were as follows:(1) Polarized light microscopy (POM) was used to observe the growth of spherulites morphology of PLLA, and a ring with concentric circles or hexagons crack after quenched in ice water was seen. The appearance of cracks was effected by the size of spherulites. When the spherulite size was small, the crack was hardly observed. As the spherulite size increased, the regular hexagonal started to form, and with the spherulite size further increasing, its appearance presented the equidistant concentric ring cracks. Nanoindentation technique was used to test the microscopic mechanical properties at different locations of the spherulites and amorphous regions. The results showed that the hardness or elastic modulus of the spherulite area was higher than amorphous region.The spherulite has growth fronts, and its microscopic performance parameters were between crystal and amorphous regions.(2) The formation conditions of stereocomplex polylactic acid (SC) crystal and effects on material properties were also investigated by X-ray diffraction (XRD) differential scanning calorimetry (DSC), dynamic mechanical analyzer (DMA) and tensile testing techniques. Compared with solution casting and embossing methods, solution casting method was more conducive to form the SC crystal, and the embossing method would destroy the formed SC crystals. The cold crystallization of amorphous PLLA/PDLA composites occurred during melting process, which included not only polylactic acid homopolymers (HC) crystals, but also the SC crystals. Heat treatment can increase the crystallinity of PLLA/PDLA composites, but not on the SC crystals. The highest elongation of the annealling samples of PLLA/PDLA composites were found the annealing temperature at 90℃. It may be the phase transition occured during this annealing temperature.(3) The PLLA/BC-g-MAH composites were prepared by solution casting, then the crystal structure and performance of PLLA/BC-g-MAH composites were studied by fourier transform infrared spectroscopy (FTIR), DSC, XRD, thermal gravimetric analysis (TGA) and tensile testing. The results showed that the BC grafted MAH had greatly improved the interface compatibility between BC-g-MAH and PLLA. With increasing BC-g-MAH content, the crystallization of PLLA/BC-g-MAH composites and its mechanical properties have been significantly enhanced. Because of the more perfect crystalline structure was formed in composites, its thermal stability has also been improved.