模拉过程中等规聚丙烯塑性形变及空洞化行为的研究

The microstructure of semi-crystalline polymers can be influenced by the approaches of solid processing which affects the application of the material in end products. Obviously, exploring the development of material microstructures during processing is vital for improving material properties. The purpose of the present project is to study the microstructural evolution of isotactic polypropylene (iPP) during the die drawing process using in-situ wide-, small- and ultra-small-angle X-ray scattering techniques. We aim to establish structural and processing parameters (such as molecular weight, crystallization condition, die temperature, and so on) dependencies of the plastic deformation and cavitation behavior of iPP samples during the die drawing process. It is also expected to reveal the mechanism of plastic deformation of die drawn iPP and illustrate the essence of cavitation activated around small strains. More importantly, the investigation will mainly focus on the cavitation triggered at large strains in order to propose the universal model and mechanism of this special pre-failure cavitation. Meanwhile, these studies can help to understand the macroscopic fracture of material at a microstructural level and provide new views to solve the problems encountered in the processing.

半结晶高分子材料的固态加工方式能影响材料的微观结构，进而影响材料的使用性能。探讨材料在加工过程中的微观结构变化，对改善材料在宏观上的使用性能非常重要。本项目拟采用广角、小角及超小角X射线散射技术在线研究等规聚丙烯在模拉过程中的微观结构演变行为。通过改变等规聚丙烯分子量、初始结晶条件及拉伸温度观察等规聚丙烯的形变行为，构建关于等规聚丙烯模拉过程中塑性形变和空洞化行为的多维依赖关系。揭示等规聚丙烯在模拉过程中的塑性形变机理，阐明小形变时的空洞化本质。并重点围绕等规聚丙烯大形变下的空洞化行为建立系统性模型及提出相关机理。同时，从微观上认识材料在宏观上的断裂行为，并为解决材料在加工过程中出现的一系列问题提供新思路。

本项目通过构建固态模拉加工与X射线散射表征联用技术，以等规聚丙烯为主要对象，重点阐述了模拉过程中材料的塑性形变与小形变空洞化行为，并对等规聚丙烯于拉伸过程中出现的大小形变空洞行为进行了区分，揭示了这两种行为对材料最终断裂的影响。模拉时，模具内存在力场分布，该力场由拉力，模具壁带来的压力与摩擦力组成，该力场对模拉过程中材料的微观结构演化具有决定性作用。它能促进双取向片晶结构的形成，并抑制材料成孔。其次，材料拉伸过程中出现的小形变与大形变空洞属于两种独立的过程，小形变空洞由片晶内小晶块的破碎引发，而大形变空洞则由高度取向的非晶网络结构中纤维/微纤间承受应力的系带分子的断裂引发。当非晶网络结构被大规模破坏后，材料随后出现宏观断裂，即材料的断裂与体系内纤维网络结构的失效有关。本项目的研究结果为具有双取向片晶结构，少孔，高强度的加工制件的开发提供了新思路、新方法。并且为理解材料的宏观断裂行为提供了理论基础。

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