【作者】 王启芬；

【导师】 王成国；

【作者基本信息】 山东大学 ， 材料学， 2010， 博士

【摘要】 高质量聚丙烯腈(PAN)纤维是制备高性能碳纤维的关键。针对PAN纤维湿法纺丝过程组织结构的演变开展基础性科学研究,深入研究了PAN纤维的结构,并掌握其结构、性能及工艺之间的相关性,对改善PAN基碳纤维的结构和提高碳纤维的性能有重要意义。本文在PAN纤维的纺丝试验线上展开一系列的试验,利用X射线衍射(XRD)、小角X射线散射(SAXS)、偏光显微镜(PM)、表面积和孔径分析仪、透射电镜(TEM)、高分辨透射电镜(HRTEM)、场发射扫描电镜(FESEM)、傅里叶变换红外光谱(FTIR)等测试分析技术,对PAN纤维凝聚态结构、形态结构和亚微观结构(包括原纤、微原纤和孔隙)及其形成过程进行了系统研究,探讨了牵伸工艺对PAN纤维结构性能的影响,并建立PAN纤维的结构模型。在PAN纤维的凝聚态结构研究方面,主要讨论了PAN纤维的晶体结构、取向结构、孔隙和微原纤结构及纺丝过程中结构的演变规律。利用XRD获得了PAN纤维的两个赤道衍射峰2θ=16.8°和2θ=29.45°和两个子午衍射峰2θ=36.1°和2θ=39.2°,进一步证实了PAN纤维中的晶体是二维有序的准晶结构,晶胞中平行于纤维轴方向的分子链呈六方排列,并在此基础上,计算出PAN的晶胞参数。通过对纺丝过程中各阶段纤维的晶体结构研究发现,随着纺丝的进行,PAN纤维中的晶体结构逐渐规整完善,赤道方向的(110)和(200)晶面间距逐渐减小,子午方向的(211)晶面间距逐渐减小,(002)晶面间距逐渐增加；结晶度和晶粒尺寸逐渐增加。系统研究了纺丝过程中PAN纤维的晶区取向和整体取向的演变。随着纺丝的进行,纤维晶区的平行度逐渐增加,晶粒长大并发生取向。纤维整体取向变化复杂,在致密化之前,整体取向不断增加,致密化和热处理阶段整体取向有所下降,蒸汽牵伸阶段纤维由于牵伸倍数较大,分子链中的氰基未能及时调整取向,此时纤维的双折射率发生了正负值的转变。利用SAXS研究了纺丝过程中PAN纤维的亚微观结构演变。结果表明,随着纺丝的进行,纤维中孔隙逐渐减少,尺寸减小；孔隙逐渐成为狭长的结构,并沿轴向排列。孔隙轴向尺寸和径向尺寸致密化之前减小幅度较小,致密化之后迅速减小。由SAXS测得的原纤半径约20nm,随着牵伸的进行其半径逐渐减小。致密化后纤维的SAXS曲线中出现散射峰,说明PAN纤维轴向具有层状结构,其周期随着牵伸的进行逐渐增大。本文分别研究了温度和牵伸对PAN结晶的影响。结果表明PAN从溶液中结晶较困难；经过热处理后PAN纤维的晶粒尺寸和结晶度小幅增加,但结晶速度逐渐减缓。增加凝固浴牵伸倍数(在2.0-4.0倍范围内)对结晶度和晶粒尺寸影响较小,晶区取向程度和整体取向程度降低,对提高力学性能贡献很小；增加沸水牵伸倍数(在1.2-1.5倍范围内),纤维结晶和取向程度同时增加,力学性能提高；增加蒸汽牵伸倍数(在2.2-2.8倍范围内),纤维的结晶度和晶粒尺寸逐渐减小,非晶区取向和整体取向程度增加,当蒸汽牵伸倍数小于2.5倍时,晶区取向随牵伸的增加而提高,大于2.5倍时,晶区取向随之减小。本文通过比较机械研磨、皮层剥离和超声蚀刻等分离原纤方法,最终选择利用超声蚀刻的方法分离原纤,这种蚀刻方法能够得到原纤的细微结构。为了验证超声蚀刻方法对PAN纤维化学物理结构是否具有破坏性,分别对比研究了超声蚀刻处理前后纤维的结构性能变化。结果发现,超声蚀刻处理后,PAN纤维的化学结构没有发生变化；未溶解的PAN分子量基本没有变化；超声蚀刻作用对PAN纤维的结晶结构影响很小,表现为结晶度和晶粒尺寸的小幅下降；经过处理后纤维强度、断裂伸长率和初始模量均减小。通过研究超声介质浓度、超声时间及超声温度对超声蚀刻效果的影响,最终得到纺丝各阶段纤维的原纤分离的最佳条件。发现纺丝各阶段纤维的耐蚀刻性逐渐增加,纤维耐蚀刻程度和它们微原纤的致密化程度一致。提出了PAN纤维超声蚀刻作用机理,解释随着超声时间的延长,纤维表面发生了周期性变化的现象。利用FESEM和HRTEM对微原纤结构进行了研究,明确了微原纤的构成,它是由垂直于纤维轴、厚度约20-40nm的片层结构和非晶夹层交替排列组成。微原纤结构存在于各阶段纤维中；在牵伸作用下,原纤结构界面和微原纤结构界面都逐渐清晰；片晶的厚度先减小后增大,其排列由平行逐渐产生一定角度。对原纤的形成进行初步探讨,发现原纤雏形产生于喷丝孔中,原纤的形成是剪切取向和凝固牵伸共同作用的结果。在对实验室自制湿纺PAN纤维A、煤化所干喷湿纺PAN纤维B和日本旭化成T300用PAN纤维C三种PAN纤维进行比较研究,发现纤维A的(110)晶面间距小,晶粒尺寸大,结晶度高；纤维B次之；纤维C的结晶程度最低,(110)晶面间距大,晶粒尺寸小,结晶度低。三种纤维间同立构序列和全同立构序列之比：纤维A>纤维B>纤维C。三种PAN纤维的整体取向程度和晶区取向均为：纤维B最大,纤维A次之,纤维C最小。三种纤维的孔隙轴向半径大小和分布大致相同,纤维B的孔隙和微原纤径向半径相对大一些。三种纤维沿纤维轴向都存在层状结构,其中纤维B的长周期最大,纤维A的长周期最小。在耐蚀刻方面干喷湿纺纤维B>自制湿纺纤维A>日本湿纺纤维C,纤维的致密程度和耐蚀刻程度一致,纤维B的微原纤结构较为均匀致密；纤维A的微原纤直径最大约100-300nm,纤维C的微原纤直径最小,约75-150nm；纤维A和B片晶的厚度大于纤维C。综合以上各因素考虑,具有高结晶度、高取向度、晶粒尺寸小、微原纤结构均匀致密的PAN纤维,是制备高性能碳纤维的最佳前驱体。通过分析归纳建立了湿法PAN纤维和干喷湿纺PAN纤维的结构模型。PAN纤维都是由原纤、微原纤、狭长的孔隙以及非晶夹层组成。平行于纤维轴、直径约300-900nm的原纤构成PAN纤维；原纤由直径为50-200nm的微原纤构成；微原纤由垂直于纤维轴、厚度约20-40nm的片层结构和非晶夹层交替排列组成。湿法纺丝制备的PAN纤维的皮层是由带状原纤组成,而干喷湿纺纤维的皮层是由微原纤直接构成,并且其原纤直径略小。更多还原

【Abstract】 The preparation of high quality polyacrylonitrile (PAN) precursor fibers is the key to obtain carbon fibers with excellent performance. The intensive study of the microstructures and their evolution during spinning process and correlation of structure, properties and techniquesis quite important, which has significance in improving PAN-based carbon fiber properties. In this work, a series of experiments were performed on a PAN fiber spinning line. X-ray diffraction (XRD), small angle X-ray scattering (SAXS), polarizing microscope (PM), surface area and pore size analyzer, high resolution transmission electron microscopy (HRTEM), field emission scanning electron microscopy (FESEM), and Fourier transform infrared spectroscopy (FTIR) are employed. Agglomerate state structure, morphology and submicroscopic struture (fibrils, microfibrils and pores) of PAN precursor fibers and their structural formation were researched. Effect of drawing technology PAN fiber structure was discussed. And PAN fiber structure models were established.The crystal structure, orientation structure, pores, microfibrils and their structural evolution during spinning process were discussed. XRD curves showed a rather strong and relatively sharp diffraction with diffraction angles 2θ=16.8°and 2θ=29.45°on the equator, and two weak reflection with angles 2θ=36.1°and 2θ=39.2°on the meridian. Furtherly, it is evidenced that PAN structure was 2D paracrystal and PAN chains arranged hexagonally in a lattice. Unit-cell parameters were obtained. Crystal structures during wet-spinning process were studied. It was found that with wet-spinning progressing d(110), d(200) and d(211) reduced gradually, but d(002) increased gradually, and crystallinity and crystal size increased.The orientations in crystalline region and in whole fibers were studied. The orientation in crystalline region increased with wet-spinning progressing while crystal grains oriented with their growth. The whole orientation change was complex. Before collapsing, the whole orientation increased. But the whole orientation decreased after collapsing. Birefringence took place from negative to positive after drawing in vapor, because CN groups in PAN chain didn’t adjust promptly with high drawing.The submicroscopic struture in PAN fibers was researched by SAXS. With wet-spinning progressing, the number of pores reduced, the pore size decreased, the pores became longer and narrower, and arranged directionally. Axial dimension and radial dimension of pores decreased slowly before collapsing, but they decreased rapidly after collapsing. Microfibrillar radii are about 20nm measured by SAXS, and their radii decreased with spinning progressing. The lamella structure appeared after collapsing and their size increased with spinning progressing.Effect of temperature and drawing on PAN crystal structure was studied. Crystallization was quite difficult from solution. The crystallinity and crystalline size increased slightly by heat treatment, and crystallization slowed down gradually. Increasing drawing multiple (2.0-4.0) in coagulation bath had a little influence on orientation in crystal region and in whole fibers and mechanics properties. Increasing drawing multiple (1.2-1.5) in boiling-water bath was effective for improving mechanical properties and was beneficial for crystallization and orientation. The crystallinity and crystalline size reduced with increasing drawing multiple (2.2-2.8) in vapor, while orientation in amorphous and in whole fiber decrease. As drawing multiple in vapor was smaller than 2.5, crystalline orientation increased with increasing of drawing multiple; as drawing multiple in vapor was bigger than 2.5, crystalline orientation decreased with increasing drawing multiple.Comparing the mechanism milling, the peel-back technique and the ultrasonic etching method for separating fibrils from PAN fiber, the ultrasonic etching method was selected, by which the fine structure of fibrils can be obtained by this method. In order to validate whether this method affected chemical and physical structure of PAN fibers, the difference of fiber structure and properties before and after ultrasonic etching treatment were studied. The experimental results indicated that ultrasonic etching had no effect on chemical structure of PAN fibers. Molecular weight of undissolved PAN did not change either. The crystallinity and crystal grains reduced slightly after ultrasonic etching. Breaking strength, breaking extension and initial modulus all decreased after ultrasonic etching.The optimum ultrasonic etching conditions of PAN fibers during wet spinning process were confirmed, which included concentration, treatment time and temperature of ultrasonic etching, It showed that resistance to ultrasonic etching of PAN fibers increased with spinning progressing, which is consistent with degree of microfibrillar compaction. Ultrasonic etching mechanism was presented, by which periodic changes of PAN fiber surface with treatment time prolonged was explained.Microfibrils were observed by FESEM and HRTEM, which are composed of periodic lamellae perpendicular to the fiber axis with thickness of 20-40nm. Fibrils were found in progressing fibers. The interface of fibrils and microfibrils are clear gradually as spinning progressing. Thickness of lamellae decreased and then increased with wet-spinning progressing. The lamellae arranged in parallel and then with an angle. The formation of fibrils was preliminarily discussed. It was considered that the embryo of fibrils formed in spinneret orifice, and the formation of fibrils was the result of shearing field in pipe and coagulation in coagulation bath.Fiber A, B and C respectively made in our lab by wet spinning, Japan by dry jet wet spinning and institute of coal chemistry by wet spinning were studied. For the crystal of fiber A, B and C, the power order of regularity and order is:A>B>C. (110) interplanar spacing is small while crystalline size and crystallinity are large in fiber A, but (110) interplanar spacing is large and crystalline size and crystallinity are small in fiber C. Syndiotactic sequences and isotactic sequences ratio was characterized by diffraction peaks integral intensity ratio (I211/I002). It shows tacticity of fiber A is highest, fiber B the second, and fiber C the third. The power order of orientation in crystal region and whole fiber of three fibers is:B>A>C. The pores axial size and distribution of three fibers are similar. But the pores and microfibrils radial size of fiber B are largest in three fibers. Lamella structure exists along fiber axis in three fibers, where long periodic of fiber B is largest and long periodic of fiber C is smallest. The power order of resistance to ultrasonic etching of three fibers is:B>A>C. The diameter of microfibrils in fiber A is about 100-300nm, which is the largest, the diameter of microfibrils in fiber C is about 75-150nm, which is the smallest, and the lamellar period of fibril B is largest among three fibers.Models of fibers prepared by wet-spinning and dry jet wet spinning are established. The PAN fiber is composed of fibrils with diameters of 300-900nm in parallel. Fibrils consist of microfibrils with about 50-200nm diameter, and microfibrils are composed of periodic lamellae perpendicular to the fiber axis with thickness of 20-40nm and amorphous interlayers arranging alternately. The main difference between wet-spinning fiber and dry jet wet-spinning fiber is that the skins of the former comprise are made up of ribbon fibrils and the skin of the latter are directly made up of microfibrils, and the diameter of fibrils in the latter is smaller than that in the former.更多还原