原位广角X射线衍射表征茂金属聚乙烯薄膜应力松弛和疲劳过程中微观结构的演化

doi:10.19894/j.issn.1000-0518.250198

摘要/Abstract

摘要：

采用原位广角X射线衍射（WAXD）技术，系统研究了茂金属聚乙烯薄膜在2种预应变条件（0.15和0.91，分别低于和略高于屈服点0.8）下的应力松弛和疲劳过程中微观结构的演化规律。结果表明：由于拉伸至较大预应变过程中样品发生晶块滑移，晶块间耦合作用减弱，因而在疲劳过程中的能量损耗较低，相比之下，较小预应变下样品内晶块耦合作用保留较好，样品在疲劳过程中需消耗更多能量用于克服晶块间的相互作用。疲劳过程通过循环载荷减弱晶体的耦合作用而使得体系所承受的应力较松弛过程的更低。不管预应变如何，样品在疲劳过程中的结晶度、取向度等均较其在应力松弛过程中高，这是由于疲劳过程施加的循环载荷促使晶块解耦合，晶块所受的限制减弱，一方面使得分子链活动能力增加，分子链在反复拉伸-回复过程中沿应力方向排列，形成更多的有序区域作为晶核，从而促进结晶，另一方面解耦合的晶块更有利于沿着力场的方向排列使得取向增加。此外，较大预应变下疲劳对提升晶块取向的贡献不明显，这是因为较大预应变下晶块之间的耦合作用已较弱，疲劳过程无需太多能量促使晶块进一步解耦合，造成疲劳过程和应力松弛过程的取向部分含量和取向程度趋于一致。

关键词: 茂金属聚乙烯薄膜, 原位广角X射线衍射, 应力松弛, 疲劳, 晶块耦合作用

Abstract:

By employing in situ wide-angle X-ray diffraction measurements， the microstructure evolution of metallocene polyethylene film was systematically investigated during stress relaxation and fatigue processes under two pre-strain conditions （0.15 and 0.91， which are below and above the yield strain of 0.8， respectively）. The results reveal the following key findings： At higher pre-strain， the crystal blocks slip during the deformation process significantly reduced the inter-crystalline coupling effect， leading to lower energy dissipation during fatigue. In contrast， at the lower pre-strain， the coupling effect is largely remained， requiring more energy to overcome crystal interactions during fatigue. Cyclic loading during fatigue progressively diminished inter-crystalline coupling effect， resulting in lower stress levels compared to those observed during the stress relaxation process. Regardless of the pre-strain level， the crystallinity and degree of orientation are higher during fatigue than during stress relaxation. This behavior can be attributed to crystal decoupling induced by cyclic loading during fatigue. The reduced restriction on crystals due to decoupling enhances molecular mobility. During the cyclic stretching-recovery process， molecules become preferentially aligned along the stress direction， facilitating the formation of ordered domains. These domains subsequently serve as nucleation sites， thereby promoting crystallization. Additionally， decoupled crystals tend to align along the loading direction， which increases overall orientation. Notably， the contribution of fatigue to further orientation is minimal at higher pre-strain due to the initially weak crystal coupling. Since less energy is required for additional decoupling during fatigue， the crystal orientation during fatigue and stress relaxation becomes comparable.

Key words: Metallocene polyethylene film, In situ wide-angle X-ray, Stress relaxation, Fatigue, Inter-crystalline coupling effect

中图分类号:

O631

张艺怀, 廖涛, 门永锋. 原位广角X射线衍射表征茂金属聚乙烯薄膜应力松弛和疲劳过程中微观结构的演化[J]. 应用化学, 2025, 42(12): 1661-1670.

Yi-Huai ZHANG, Tao LIAO, Yong-Feng MEN. In-situ Wide-Angle X-ray Diffraction Characterization of Microstructural Evolution of Metallocene Polyethylene Film During Stress Relaxation and Fatigue Process[J]. Chinese Journal of Applied Chemistry, 2025, 42(12): 1661-1670.

图/表 11

图1 薄膜的SAXS（A）和WAXD（B）图像，牵伸方向为水平

Fig.1 SAXS （A） and WAXD （B） patterns of the film， machine direction is horizontal

图2 样品的工程应力-应变曲线。红色的点表示屈服点。选取0.15和0.91预应变进行随后的应力松弛和疲劳实验

Fig.2 Engineering stress-strain curve of the sample. The red dot represents the yield point. Two pre-strain values of 0.15 and 0.91 were selected to conduct the subsequent stress relaxation and fatigue measurements

图3 （A）样品在疲劳过程中的不同周次的应力-应变滞回曲线；（B）样品在疲劳过程中的单位体积的能量损耗

Fig.3 （A） Stress-strain hysteresis loops of the sample at selected fatigue cycles；（B） Evolution of energy dissipation per unit volume during fatigue testing

图4 样品在应力松弛和疲劳过程中的应力衰减曲线，预应变为0.15（A）和0.91（B）

Fig.4 Stress decay profiles of the sample during stress relaxation and fatigue testing with pre-strain levels of 0.15 （A） and 0.91 （B）

图5 样品在疲劳过程中的应力衰减曲线和3个时间点下的二维WAXD图案。拉伸方向为水平方向

Fig.5 Stress decay profile of the sample under cyclic fatigue loading and the corresponding two-dimensional WAXD patterns acquired at three selected time intervals. Stretching direction is horizontal

图6 样品在预应变为（A）0.15和（B）0.91时，疲劳过程中的一维WAXD曲线

Fig.6 One-dimensional WAXD curves of the sample during fatigue testing at pre-strains of （A） 0.15 and （B） 0.91

图7 样品在预应变分别为（A）0.15和（B）0.91时，疲劳和应力松弛过程结晶度的变化

Fig.7 Evolution of crystallinity of the sample during fatigue and stress relaxation process at pre-strain levels of （A） 0.15 and （B） 0.91

图8 样品在预应变分别为（A）0.15和（B）0.91时，疲劳和应力松弛过程取向度的变化

Fig.8 Evolution of the degree of orientation of the sample during fatigue and stress relaxation process at pre-strain levels of （A） 0.15 and （B） 0.91

图9 使用Halo method将WAXD图案分成取向和未取向两部分的流程

Fig.9 Separation of a WAXD pattern into oriented and unoriented components using the Halo method

图10 样品在预应变分别为（A）0.15和（B）0.91时，疲劳和应力松弛过程中取向部分的含量

Fig.10 Fraction of oriented part of the sample during fatigue and stress relaxation measurements at pre-strain levels of （A） 0.15 and （B） 0.91

图11 样品在预应变分别为（A）0.15和（B）0.91时，疲劳和应力松弛过程中取向部分的取向度

Fig.11 The degree of orientation for the oriented part of the sample during fatigue and stress relaxation process at pre-strain levels of （A） 0.15 and （B） 0.91

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