| Polylactide(PLA) is a kind of biodegradable polyester derived from renewable source(mainly starch and sugar). PLA possesses favorable biocompatibility and processability, and has the similar mechanical properties with petroleum-based plastics, such as PET and PS. However, its abroad applications(foaming, injection molding, extrusion, blow molding, etc.) are currently limited by several main drawbacks, such as low melt strength, low crystallizaty, and low thermal deformation temperature. In this thesis, an ultraviolet-induced reactive extrusion is presented to prepare the long-chain branching of PLA to overcome the drawbacks mentioned above. The structures and properties of degradation and long-chain branching products are evaluated and characterized in detail. The specific contents and conclusions are as follows:(1) The effects of feeding rate, screw speed, and processing temperature on PLA degradation under the UV irradiation were investigated. FTIR measurements showed that the degraded samples supported the Norrish II mechanism during this UV-induced reactive extrusion process of PLA. The results of the MI, GPC, and SAOS experiments indicated that compared with solid irradiation of PLA, this UV-induced molten PLA during extrusion had a high level of degradation efficiency. The thickness of PLA in screw channel could be controlled by changing the feeding rates, so that the UV degradation of PLA was controlled as well. Raising the screw rates could shorten the UV irradiation time. As photo-degradation reaction was fast, degradation of PLA would command by mechanical degradation. When the processing temperatures ranged in 185~200℃, the degree of PLA degradation was similar. When it comes to 230℃, the degradation was dramatical. Mechanics and DSC experiments showed that the higher degree of PLA degradation the less tensile strength and break elongation it possessed, as well as the cold crystallization process would become more complex.(2) Long-chain branching(LCB) structure of PLA was obtained when a multifunctional chemical agent, trimethylolpropane triacrylate(TMPTA), was added into the PLA matrix during extrusion, which induced free radical reactions by UV. By analysing the results of GPC and SAOS experiments, it could conclude that the addition of TMPTA in PLA during UV induced extrusion could produce LCB PLA with high molecular weight and melt strength. LCB PLAs had showed the thermorheological complexity, and their activation energies were in dependence on the molecular structure. Mechanics and TGA measurements showed that long-chain branching structure had no effect on thermostability for PLA, but it could improve the ensile strength and break elongation.(3) The crystallization property of LCB PLAs and the effects of nucleating agent on LCB PLAs’ crystallization property were investigated by using DSC, XRD, and POM. The results showed that the crystallization rate and crystallinity of LCB PLAs was significantly improved and the sphaerocrystal was smaller, because of the heterogeneous nucleating effect of LCB structures. As the existence of LCB structures and nucleating in PLA-T1.5-talc sample, its crystallization rate and crystallinity was less than PLA-talc. But its crystallization rate was faster than PLA-T1.5 and crystallinity was similar. Because of amides nucleating agent and molten PLA could form intermolecular hydrogen bonds, PLA-TMC and PLA-T1.5-TMC showed better crystallization properties. Nucleating agent could make the spherulite of PLA smaller than LCB structures. The samples with talc had appeared needle-like crystal, but it did not happened with the TMC system,which indicated that TMC was better than talc for PLA crystallization. Furthermore, the crystal form of PLA could not be transited by LCB structures, talc, or LCB structures. |
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