| Due to their favorable self-supporting properties and simple components,hollow fibers are widely used in gas separation,filtration,biomedical and other industries.As the core of the extracorporeal membrane oxygenation（ECMO）technology,the localization of the preparation technology of Poly（4-methyl-1-pentene）（PMP）hollow fiber has become one of the key challenges in China.In the international environment that the COVID-19 is affecting the world,PMP hollow fibers for ECMO have become a strategic material in short supply and the market prospect in China is very broad.In this regard,it is of farreaching significance to carry out research on the preparation and properties of PMP hollow fibers.In the first part of this work,three PMP resins are selected to clarify the influence of their molecular weight and molecular weight distribution on the rheological and crystallization properties.In the second part,PMP1 resin is selected to prepare a series of hollow fibers with different melt draw ratios（MDRs）by melt stretching method.Their crystalline structure and properties are characterized by scanning electron microscope（SEM）,small angle X-ray scattering（SAXS）,differential scanning calorimetry（DSC）and other techniques.Finally,we discuss the effects of annealing conditions on the structure and properties of hollow fibers with MDR 40 in the second part.The changes in the internal structure of the fibers under different heat treatment conditions were tracked using in situ SAXS,and a suitable annealing process was determined.The experimental results and conclusions of this thesis are as follows:1)The GPC test results show that the molecular weight distributions of PMP1 and PMP2 are relatively close,but PMP2 contains a small amount of low molecular weight components.PMP3 has the widest molecular weight distribution and many low molecular weight components,which makes its high temperature stability poor.Due to the small amount of low molecular weight components,PMP2 has a higher chain mobility and lower relaxation time than PMP1.Benefiting from this,the crystallization speed of PMP2 is faster than that of PMP1,and its non-isothermal crystallization activation energy is also smaller.However,the crystal structure of PMP1 was more complete during the cooling process and its higher molecular weight than PMP2 makes the melt strength and spinnability higher than the latter.2)Based on the previous work,PMP1 resin with better spinnability is selected to prepare PMP hollow fibers（MDR 20,40,60 and 80）by melt drawing method.The SEM images reveals that the outer surface of the fiber showed a spherulite structure,while the inner surface appeared shish-kebab crystals,and the spherical to lamellar crystals transition occurred with MDR increasing.These different structures are attributed to the combined effect of the two-dimensional temperature field and stress field in the radial direction of the fiber.Using SAXS to characterize the crystal structure of the hollow fiber,it was found that the long period,crystal thickness and tilting angle of the lamella of all samples are not affected by MDR,and remained at about 26 nm,10 nm and 3.2°.The DSC and XRD results show that the crystallinity of all fibers is basically maintained a t 37~38%and the crystalline form has no change.However,the increase in MDR contributes to the lateral growth of PMP hollow fiber lamellae,and their lateral size increases from 74.1 nm to 86.5 nm.In addition,compared with other samples,the PMP hollow fiber of MDR40 exhibits better hard elastic behavior,and the elastic recovery rate of 50% strain is as high as 76.8%,indicating that the structure of the internal lamellas is perfect,which is conducive to the further study.Annealing is significant for the control of crystal morphology of hollow fibers prepared by melt drawing method.In chapter four,we used in situ SAXS to track the changes in the internal structure of PMP hollow fibers（MDR40）under different heat treatment conditions.The results show that the effect of heat treatment at different temperatures is not the same in the two dimensions of the growth for fiber lamellas: at180°C,the fiber lamella grow laterally but not thicken,which also leads to a decrease in the elastic recovery ratio,and this temperature is not suitable for the heat treatment of PMP hollow fibers;however,in the process of heat treatment at 215℃,the lateral change of the lamellas is not prominent,but the crystal thickness increased significantly,so the strength of the lamellas after heat treatment is improved,and the elastic recovery ratio of the fibers is higher.In addition,the SAXS results showed that the long period,crystal thickness and elastic recovery rate of the hollow fiber lamellas are improved after heat treatment at190℃~215℃ for 0.5h,and the improvement is most obvious after heat treatment at 215℃.At the same time,it was found that prolonging the heat treatment time at 215℃ did not improve the strength and growth of fiber lamellas.Therefore,the suitable heat treatment conditions for PMP hollow fibers are annealing at 215℃ for 0.5h. |
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