Investigation of Initiation and Growth of Crazes in Engineering Plastics

用实时小角X射线散射(SAXS)、电子显微镜和小角、广角电子衍射仪研究了工程塑料中作为断裂前兆的银纹的微观结构及其在远低于其玻璃传输温度的单轴拉伸过程中随应变的变化。实时小角X射线散射测量表明：(1)连接聚丙烯银纹内部的纤维结构通过伸长而变宽，保持其厚度和间距不变，(Ii)上述结果表明，纤维的长度是通过从银纹/块体边界形成的活化区提供分子链来实现的，并且当这种分子供应中断时，纤维就会破裂。银纹内部的选区广角电子衍射图表明，纤丝中的微晶以…的方式高度取向。微晶中的分子链更多地位于纤维的纵向。由此得出结论：断裂不仅发生在非晶区或非晶界面上，而且也发生在结晶区内，银纹化球晶在碎裂成小片后流入原纤维，这表明晶体内分子链的限制不是半结晶聚合物原纤维断裂的主要原因。对聚砜和聚芳酸酯的透射电子显微镜观察表明，在这些聚合物中形成的银纹中的原纤维是通过蠕变断裂的。这种纤维断裂的机制与许多通用无定形聚合物完全不同，在银纹生长过程中，分子链流入纤维中。SAXS测得的原纤层厚度和间距与通用非晶态聚合物基本相同，表明非晶态工程塑料的高强度与分子链的流动性密切相关，而分子链的刚性是影响其延性变形的重要因素，即使在脆性断裂的情况下也会发生这种延性变形。较少

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