【作者】 付诚杰；

【导师】 邵正中；

【作者基本信息】 复旦大学 ， 高分子化学与物理， 2010， 博士

【摘要】 动物丝(主要指蜘蛛丝和蚕丝)由于其优异的力学性能,近半个世纪来一直受到研究人员的关注。经过科学家的不懈努力,人们已经掌握了部分动物丝(如Nephila clavipes蜘蛛主腺体丝和桑蚕丝等)的丝蛋白序列结构,初步探讨了某些基序(motif)与固态动物丝中二级结构的对应关系。然而,这些二级结构(结构域)是以何种方式组装成更高级的凝聚态结构并使动物丝表现出相应的力学性能仍然是个未解之谜。尽管科学家已经通过各种精巧的实验描绘出了蜘蛛和蚕等的丝腺体与纺器的生理构造,并揭示了纺丝液在其中经历的流体作用、pH值和离子浓度变化等,但当人们尝试人工制造动物丝时仍然遇到很多困难。即使直接使用丝腺体中的纺丝液,所得到的再生丝性能往往远不及天然丝。这说明我们在模拟制备动物丝,特别是构建动物丝所特有的多级结构方面还有很长的道路要走。全面地掌握动物丝结构和力学性能间的关系无疑将为整个探索过程提供可靠的研究思路。目前对动物丝的研究主要集中于蜘蛛主腺体丝和桑蚕丝。虽然蜘蛛主腺体丝力学性能非常优异,但是由于蜘蛛的地盘意识强,容易相互残杀而无法大规模集体饲养,因此蜘蛛丝的产量有限；另一方面,桑蚕丝在几千年前就被人类大规模地利用,在全球的年产量达到几十万吨。相比之下,尽管柞蚕丝的全球年产量达万吨级,但对这种材料的研究却十分有限,而且主要是基于脱胶柞蚕丝。由于在缫丝和脱胶过程中引入的机械拉伸和化学试剂处理都可能导致天然丝的结构发生变化,因此,若直接测定脱胶柞蚕丝的力学性能,其数据的可靠度不高,进而无法将柞蚕丝的力学性能和结构相互关联。此外,柞蚕丝在动物丝家族中具有十分特殊的地位,即柞蚕丝的天然用途类似于桑蚕丝,用于织茧(保护蚕蛹)；而柞蚕丝素蛋白的序列结构却与蜘蛛主腺体丝蛋白的序列结构非常相似,特别是其形成β-折叠结构的聚丙氨酸序列。因此,获得可靠的柞蚕丝力学性能数据、研究其与柞蚕丝(丝蛋白)各级结构的关系也将有助于我们了解整个动物丝家族一系列基本特性。在本论文中,我们通过强(迫)拉丝的方法从野生柞蚕的纺器中获取几乎没有宏观缺陷的强拉柞蚕丝。在研究强拉柞蚕丝在常温常湿条件下的力学性能、水及湿气对强拉柞蚕丝结构和力学性能的影响以及强拉柞蚕丝在低温下的力学性能等的基础上,对柞蚕丝(动物丝)的断裂模式进行了深入的讨论。首先,我们从野生柞蚕的纺器以一定速率获取强拉柞蚕丝,通过观察其表面形貌和截面形状发现：强拉丝的方法可以获得几乎没有宏观缺陷的丝纤维,这为后续的力学性能研究提供了可靠的原材料；我们还发现,即使拉丝速率不变,强拉丝的截面形状在较长距离内也有显著的变化,提示柞蚕在被强拉丝的过程会改变其纺器口模的形状,进而在一定程度上影响丝纤维的性能。接着,我们通过拉伸试验研究了强拉柞蚕丝在常温常湿条件下的力学性能,包括断裂强度、断裂应变、模量以及回弹性等,并与蜘蛛主腺体丝和强拉桑蚕丝进行对比。结果表明,柞蚕丝的断裂强度和断裂韧性与蜘蛛主腺体丝和强拉桑蚕丝处于同一量级,但回弹性介于蜘蛛主腺体丝和强拉桑蚕丝之间。此外,通过分析人为引入缺陷的强拉柞蚕丝断面形貌,提出了柞蚕丝在常温下的断裂机理。还利用Instron模拟商业缫丝过程探讨了商业生产中导致蚕丝力学性能发生改变的原因。在研究水对强拉柞蚕丝的结构和力学性能的影响时,我们借鉴了前人研究蜘蛛丝的方法。将强拉柞蚕丝经过水汽或水处理后,利用拉曼光谱和X-射线衍射法检测其结构的变化,并跟踪其力学性能发生的改变,此外还探讨了能否利用超收缩和湿态条件下预拉伸处理来提高柞蚕丝的力学性能。我们发现,一定温度下当环境湿度超过某一临界值时,强拉柞蚕丝将发生超收缩现象,其最大超收缩能力介于蜘蛛主腺体丝和强拉桑蚕丝之间；同时,在这个临界湿度附近,超收缩柞蚕丝的初始模量突降,表明非晶区丝蛋白链段发生了玻璃化转变,也证明了超收缩现象是熵驱动的分子链解取向过程。通过在不同温度下测定超收缩柞蚕丝发生玻璃化转变的临界湿度,我们发现升高温度和增大湿度(即增大柞蚕丝中的水含量)对超收缩柞蚕丝的力学性能的影响具有等效性。对桑蚕丝和柞蚕丝进行湿态拉伸试验证明,湿态预拉伸会导致纤维本身的模量升高、屈服应力提高、断裂应力(工程应力)增大、断裂伸长率降低而总断裂能降低。而且,湿态预拉伸对上述性能的影响与干态预拉伸相比,差别并不大。从工程应用的角度来说,模量提高和屈服应力提高的确意味着提高了蚕丝作为结构材料的使用价值。结构表征实验显示,强拉柞蚕丝中的α-螺旋结构在水或水汽处理过程中逐步消失,此构象转变所需的湿度在80%RH(相对湿度,25℃)以上,略高于强拉柞蚕丝发生超收缩所需的湿度(约75%RH,25℃)。经过水处理后,强拉柞蚕丝的模量下降,屈服应力降低且断裂伸长率增高。此外,我们将超收缩后多种动物丝的干/湿(态)模量比作为力学性能参数,与由丝蛋白一级结构所指的动物丝内在有序度进行比对,发现这一性能/结构关系可以用D. Porter的基团作用模型(Group Interaction Model)很好地模拟。实验测量发现,即便在液氮温度下,强拉柞蚕丝仍然是韧性材料,其断裂伸长率几乎与室温下的一致。这不仅表明“动物丝的低温韧性源于-70℃附近的脂肪侧基松弛过程”的早期研究结果的片面性,而且还促使我们通过采用一系列方法进一步探讨了强拉柞蚕丝在低温下仍具有优异韧性的原因。我们发现,当强拉柞蚕丝未经脱胶处理并且不够干燥时或者人为地在强拉柞蚕丝表面引入微米级缺陷时,都可以使其在低温下的断裂模式由韧性转变为半韧性(或脆性)；结合强拉柞蚕丝断面分析,并与高性能合成尼龙纤维和聚酯纤维的早期研究结果进行更多还原

【Abstract】 Animal silks have drawn the attention of biologists, chemists, and material scientists for more than a century. Yet their applications by human beings can be traced back millennia. The recent resurgence in scientific interest in such materials, which have evolved independently in spiders as well as a wide range of insects, is due to the silk’s marvelous combination of modulus, strength and extensibility. After intensive study, the scientists have gained considerable knowledges about the primary sequences (although often only partial) of a wide range of natural silk proteins, and about the secondary structure of certain motifs. However, it is still less understood about the details of how the secondary structures arrange and organize into the condensed state structure at a higher level in the natural silks, and how these secondary structures contribute to the remarkable mechanical properties of the natural silks. Moreover, scientists have uncovered considerable details on natural spinneret construction and the gradients of proton and metallic ions along the duct, as well as their effects on the in vivo formation of silks. However, it seems to be a long way from producing artificial silk fibers comparable to those masterpieces produced by the spiders or silkworms themselves, whether the regenerated silk protein solutions or recombinant ones are used. Therefore in-depth understanding of the structure-property relationship of animal silks will provide valuable guideline for further research.Most of previous studies of animal silks are focused on major ampullate silks of spider and mulberry silk. The commercial production of spider silks is limited because of the cannibalistic character of most spiders. On the other hand, mulberry silk has already been used by human beings for several thousand years with a global production scaling up to hundreds of thousands tons per year. Compared to these two widely studied silks, A pernyi silk is produced at the scale of tens of thousands tons per year with limited research works, most of which were based on the degummed A. pernyi silk. Nevertheless, the degumming process may damage the silk structures on various levels, and then affect the results obtained. This may arise problems when someone wants to link the mechanical properties of the silks with their structure. Moreover, A. pernyi silk is quite special in the family of animal silks. Like mulberry (B. mori) silk, A. pernyi silk is used to construct cocoon. However, examination of the primary structure, especially the motifs, of A. pernyi fibroin reveals that it is more like major ampullate spidroins than B. mori fibroin. Therefore, obtaining reliable mechanical properties data of A. pernyi silk and figuring out its relationship with the structure of A. pernyi silk will provide important insight into the structure-property relationship of the whole family of animal silks.In this work, we first obtained forcibly reeled A. pernyi silk with few macro defects. Then the mechanical properties of such silk was studied in normal and moist environment as well as at low temperatures. Also, the effects of water on the structure and properties of forcibly reeled A. pernyi silk was discussed. Furthermore, the failure behavior of it was investigated at both ambient temperature and cryogenic temperature.The first part of our work (Chapter two) tells how to obtain the forcibly reeled A. pernyi silk directly from the spineret of silkworm, their mechanical performance at normal environment, and their morphology and shape of cross section. It was found that forcibly silking can provide silk fibers with few macro defects, which can be used for subsequent delicate studies. However, even reeled under constant rate, the forcibly reeled A. pernyi silk fiber would vary its cross sectional shape over long reeling time. This finding implies that silkworm may be able to change the shape of their spineret die. Then, the mechanical properties of forcibly reeled A. pernyi silk fiber was tested at normal environment, and the results were compared with those from major ampullate silk of spider and forcibly reeled B. mori silk. The results show that the A. pernyi silk has tensile strength and toughness of the same magnitude as those of spider silk and B. mori silk, while the elasticity of the A. pernyi silk lies between those of spider silk and B. mori silk. Moreover, the fracture mechanism of A. pernyi silk was discussed based on fractography of artificially notched silk. The force on the silk fiber during filature process was monitored with Instron machine, which revealed that over-stretching during the filature process may be the reason for the poor properties of commercial cocoon silkworm silks. The Chapter three focuses on the effect of water on the structure and mechanical properties of forcibly reeled A. pernyi silk. It is found that the A. pernyi silk fiber will supercontract when the environmental humidity reaches a critical value, and its supcontraction force is between the major ampullate silk of spider and forcibly reeled B. mori silk. Near the critical humidity, the modulus of supercontracted A. pernyi silk fiber plummets, which suggests the glass transition of the noncrystalline regions of silk protein (fibroin) takes place. The coincidence between the onset humidities of supercontraction and glass transition supports the assumption that the supercontraction of animal silks is a process of entropy-driven disorientation of molecular chains when the intra-or intermolecular frictional force is not strong enough to hold these chains in their elongated conformations. By determining the onset humidities of glass transition of supercontracted A. pernyi silk fiber at different temperatures, we found that increasing humidity is equivalent to raising temperature in affecting the mechanical properties of A. pernyi silk. By prestretching in water, both B. mori silk and A. pernyi silk showed improved modulus, yield stress and breaking stress, as well as decreased breaking elongation and breaking energy. Prestretching silk fiber in air and in water have similar effects on the mechanical properties of silk. Raman spectroscopy and X-ray diffraction were used to monitor the structure change of A. pernyi silk during water or moisture treatment. The result shows that forcibly reeled A. pernyi silk will gradually lose its a-helix structure when the humidity goes beyond 80%RH (relative humidity,25℃) which is a bit higher than the glass transition humidity (ca.75%RH,25℃). After supercontraction in water, the modulus and yield stress of forcibly reeled A. pernyi silk drop, while breaking elongation increases. Moreover, the ratio between dry and wet modulus of water-contracted silk was correlated with the ordered fraction of silk proteins, and the plot was compared with the simulated curve delivered by Dr. D. Porter. From these results, we proposed a mechanism how the primary structure determines the mechanical properties of animal silks.In Chapter four, dynamic mechanical thermal analysis (DMTA), tensile test in liquid nitrogen and fractography were used to explore the mechanism, by which forcibly reeled A. pernyi silk fiber remains tough at low temperatures. The silk breaks in a ductile way even at the temperature of liquid nitrogen, i.e.-196℃, and its breaking elongation doesn’t differ from that at room temperature. The result implies that previous attribution of animal silk’s low temperature toughness to the relaxation at-70℃is unsatisfactory. Furthermore, the failure mode of forcibly reeled A. pernyi silk fiber in liquid nitrogen was found to depend on sericin coating, moisture content, artificial notch and so on. For example, the silk fiber breaks in semi-ductile (or brittle) way in liquid nitrogen if it is undegummed and wet or it is artificially notched. According to fractography of forcibly reeled A. pernyi silk fiber and the previous observation on high tenacity nylon and PET fibers, we speculate that the well-defined nano-fibrillar structure in natural animal silks is the origin of their low temperature toughness.更多还原