Investigation of Initiation and Growth of Crazes in Engineering Plastics

用实时小角X射线散射（SAXS）、电子显微镜、透射电子显微镜和透射电子显微镜研究了工程塑料在低于玻璃化转变温度的单轴拉伸过程中作为断裂前体出现的银纹的微观结构及其随应变的变化，以及小角和广角电子衍射。实时小角X射线散射测量表明：（i）在聚丙烯中桥接银纹内部的原纤结构通过其伸长而在宽度上增长，保持其厚度和间距不变，并且（ii）上述结果表明原纤通过从在银纹/本体边界中形成的活性区提供分子链而在长度上增长，并且当这种分子供应中断时，发生原纤维的分解。在银纹内部观察到的选区广角电子衍射图案表明，原纤中的微晶以这样一种方式高度取向， 关于我们 微晶中的分子链位于原纤维的纵向。从这一结果我们得出结论，断裂的起始不仅发生在非晶区或非晶/结晶边界内，而且也发生在结晶区内，并且在碎裂成小块之后，裂纹化的球晶流入原纤维中，这表明晶体内分子链的限制不是半结晶聚合物中原纤断裂的主要原因。对聚砜和聚芳酯的显微镜观察显示，在这些聚合物中形成的银纹中的原纤维通过蠕变而围绕它们的中心断裂。这种原纤维断裂的机制与许多通用的无定形聚合物中观察到的机制完全不同，在无定形聚合物中，分子链在银纹生长期间流入原纤维。用小角X射线散射法测得的原纤的厚度和间距与一般非晶态聚合物的基本相同，表明非晶态工程塑料的高强度与分子链在微观上甚至在脆性断裂时发生的韧性变形中的运动密切相关，而分子链的刚性又影响着分子链的运动。少

Microstructure of crazes appearing as precursors to fracture in engineering plastics and its variation with strain during uniaxial elongation at well below their glass transmission temperatures have been investigated by real-time small-angle X-ray scattering (SAXS), electron microscopy, and low-angle and wide-angle electron diffraction.The real-time small-angle X-ray scattering measurements has revealed that (i) the fibrillar structure bridging the interior of the crazes in polypropylene grows in width by its elongation, maintaining its thickness and spacing unaltered, and that (ii) the above result suggests that the fibril grows in length by being supplied with molecular chains from active zones formed in the craze/bulk boundaries, and that the breakdown of the fibril occurs when this molecular supply is interrupted. Selected-area wide-angle electron diffraction patterns observed for the inside of crazes indicate that the crystallites in the fibrils are highly oriented in such a way t … More hat the molecular chains in the crystallites lie in the longitudinal direction of the fibrils. From this result we are lead to the conclusion that the initiation of fracture does not occur only within the amorphous regions or amorphous/crystalline boundaries but also occurs within crystalline regions, and that on crazing spherulites flow into the fibrils after fragmenting into small pieces, suggesting that the confinement of the molecular chains within crystals is not a major reason for the fibril breakdown in semicrystalline polymers.Transmission electron microscopic observation on polysulfone and polyarylate has revealed that the fibrils in crazes formed in these polymers break around their centers by creeping. This mechanism fo fibril breakdown is completely different from that observed for many general-purpose amorphous polymers, in which molecular chains flow into the fibrils during the growth of crazes. The thickness and the spacing of the fibrils measured by SAXS are approximately the same as those for general-purpose amorphous polymers, suggesting that the high strength of amorphous engineering plastics has close reration to the mobility of molecular chains, which is affected by the regidity of the chains, in the ductile deformation that occurs microscopically even in brittle fracture. Less