Study Of Relationship Between The Mechanical Evolution And Its Polymeric Structure Of Poly-(3-Hydroxybutyrate-CO-3-Hydroxyvalerate) As-Spun Fiber

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate)(PHBV)copolyester is a thermoplastic bio-polymer synthesized by microbial fermentation,presenting the advantages of good biodegradability,histocompatibility and antithrombogenicity.It has a wide range of applications in industry,agriculture,packaging,biomedicine and other fields. It has a rich source in nature and no need of complex polymerization equipment. Besides,it is an alternative to general resins.For all these reasons,it has attracted many domestic and foreign researchers a great deal of attention in recent years. However,PHBV suffers from its poor thermal stability,narrow melt processing window,and especially its embrittlement stored at room temperature,which has,to a large extent,limited its industrial processing and application.In particular,there is an obvious mechanical evolution process for PHBV as-spun fiber during storage.The mechanical behavior of PHBV as-spun fiber shows a significant change from elasticity to plasticity,and finally brittleness.Such an evolution has a serious impact on the follow-up processing.Even being drawn,the fiber would also become brittle as a result of the secondary crystallization at room temperature.Therefore,studying the law of the mechanical evolution of PHBV as-spun fiber and the changes in its polymeric structure,and then exploring the relationship between the two,will lay the foundation for the formation of desired fiber structure and performance.Specific activities include the following:1.In this paper,the mechanical properties,crystallization and orientation behaviors of PHBV as-spun fibers changing with time were synchronously tracked by means of fiber mechanical testing machine,wide-angle X-ray diffraction(WAXD), density meter and sound velocimeter in order to find out the relationship between the polymeric structure and mechanical properties.The results showed that the stress-strain curve patterns had a significant change in the storage process,from entropic elasticity to plastic behavior and then to brittle characteristics.During this process,breaking elongation had a rapid decrease in 20 minutes from more than 600 percent to below 200%,and kept at very low level(about 10%or less) after 250 minutes.Meanwhile,breaking strength increased faster within 125 minutes, and stabilized after 1500 minutes.While the mechanical properties had witnessed tremendous changes,density(crystallinity) and sound velocity(orientation) values also greatly increased.The density raised rapidly in 125 minutes and remained stable after 1500 minutes while the sound velocity values grew quickly in 500 minutes and stabilized after 1500 minutes.WXRD showed the crystal structure didn't change in the entire process and the crystallinity had reached 50%in 20 minutes.The change of grain size had a similar trend to the change of crystallinity.2.The link between the mechanical evolution and polymeric structural changes of PHBV as-spun fibers was preliminarily studied.The stress-strain curve evolution of PHBV was due to its continuous rise in crystallinity.There were two stages for PHBV crystallization,primary crystallization and secondary crystallization.The primary crystallization had completed in 20 minutes.The rapid increase in degree of crystallinity within primary crystallization stage was the main reason for its breaking elongation decrease.But the embrittlement was caused by secondary crystallization.The increase in fiber crystallinity had a major effect on change of breaking strength.3.The mechanical evolution and polymeric structural changes of PHBV/SiO₂ fiber were also studied.By adding nano-SiO₂ nucleating agent,we inspected the impact of nucleators on the mechanical evolution and the polymeric structural changes of PHBV fiber.The results showed that by adding nano-SiO₂,the mechanical evolution didn't change significantly.But because of the role of the nucleating agent,the secondary crystallization was delayed and the grain size was reduced. Therefore,the final elongation was increased.4.PHBV with different molecular weight and molecular weight distribution was chosen and the mechanical evolution and polymeric structural changes of its as-spun fiber were studied.By doing so,we investigated the effect of molecular weight on the mechanical evolution and the polymeric structural changes.As for the PHBV as-spun fiber of high molecular weight and narrow molecular weight distribution,the progress of its mechanical evolution was slower,and the breaking elongation maintained a relatively high level finally.