Synthesis Of Macromolecular Coupling Agents And Their Application In Interface Modification Of Nature Fiber/Polylactide Composite

Nature fiber/polylctide (PLA) composites are gaining considerable attention due to their fully degradation and deriving entirely from renewable resources. However, there are some limitations. The most important restraint is the poor interfacial compatibility between the hydrophilic fiber and the hydrophobic polylactide matrix, which limits the performance of the final composite.In this work, the study was carried out systematically such as the synthesis of macromolecular coupling agents, surface treatment of nature fibers, static and dynamic mechanical properties of nature fiber/PLA composite, stability and biodegradibility of the composites. And the main results in this work are summed as follows:(1) Two types of coupling agent,γ-Methacryloxypropyltrimethoxysilane-graft-PLA (MPS-g-PLA) and polylactide-glycidyl methacrylate copolymer (PLA-co-PGMA), were successfully designed and synthesized. Their structure was characterized by Fourier transform infrared spectroscopy (FTIR) and Hydrogen nuclear magnetic resonance ('H-NMR). Furthermore, their synthetic procedure was optimized:in the case of MPS-g-PLA, reacting for 5 h under 110℃with the mass ratio 5:1, amounts of initiator 1wt.%; for PLA-co-PGMA, reacting for 5 h under 70℃with the molar ratio of monomers 5:1, amounts of initiator 2wt.%. Hydrolytic degradation of PLA and its copolymers showed that the degradation rates of the copolymers slightly differ from neat PLA, especially PLA-co-PGMA.(2) Macromolecular coupling agents were employed for surface modification of bacterial cellulose (BC) and sisal fiber (SF). Contact angle measurements showed that the surface energy of BC after modification was decreased due to the reduction in its polar component. Concentration of MPS-g-PLA and pH influenced surface properties of BC. Surface modification can also improve the surface hydrophobicity of SF. Scanning electron microscopy (SEM) photographs showed surface modification made SF surface rough. Interfacial thermodynamic properties were studied and the results suggested that the surface modification decreased interfacial tension of BC/PLA and were more efficient in improving the interfacial shear strength.(3) The addition of SF enhanced modulus of PLA. After fiber treatment with MPS-g-PLA. the tensile strength of PLA composite with 30% fiber can be increased by 4.8%; with an impact strength by 25.3%; a flexural strength by 3.5%, and a flexural modulus by 82.7%; while fiber treatment with PLA-co-PGMA, the tensile strength of PLA composite can be increased by 6.2%; with a impact strength by 29.4%; a flexural strength by 7.8%, and a flexural modulus by 79.3%. Plasticizer was used to decrease the processing temperature and enhance impact strength of composites; which make efficiency of surface modification obvious. At 30 wt% SF loading, tensile strength improved by 21.81%, while strain at break and impact strength dropped compared to neat plasticized PLA. Surface modification of SF had further increased tensile strength of composites and decreased its impact strength, which suggests effective stress transfer between fiber and matrix. The DMA storage modulus increased with the addition of SF. Compared to unmodified fiber, the interfacial binding strength of composites with PLA-co-PGMA and MPS-g-PLA was higher. SEM micrographs of the fracture surface of Notched Izod impact specimen indicated that the adhesion between fiber and matrix could be improved by the addition of coupling agents.(4) Stability of PLA and its composites was estimated under damp condition. The results revealed that conditioning to equilibrium at 80% relative humidity and 30℃took about 30 d and resulted in decrease of tensile strength of composites. However, surface modification of SF had a remarkable effect on the stability of SF/PLA composites and relieved decline of their tensile strength by keeping interphase damage to a minimum level. Biodegradability of PLA and its composites was evaluated by the soil-burial test. Both fiber and matrix phases of the PLA composites were biodegradable; they have a relatively high biodegradability and the presence of SF affected the weight loss. Biodegradability was restrained for treated SF composites, as the interfacial adhesion is improved due to the surface modification of the fiber.(5) Transparent nanocomposite consisting of two biocompatible materials, bacterial cellulose (BC) and PLA was prepared. Maleic anhydride was first grafted onto BC nanofibers, which made BC nanofibers well disperse into PLA matrix and the interfacial adhesion between BC and PLA was improved.Natural fiber/PLA composite is a green composite. Improvement of physico mechanical properties of the composites will expand in application field. It is of great significance for solving the current environment issues and achieving sustainable development.