In-situ Wide-Angle X-ray Diffraction Characterization of Microstructural Evolution of Metallocene Polyethylene Film During Stress Relaxation and Fatigue Process

doi:10.19894/j.issn.1000-0518.250198

Chinese Journal of Applied Chemistry ›› 2025, Vol. 42 ›› Issue (12): 1661-1670.DOI: 10.19894/j.issn.1000-0518.250198

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In-situ Wide-Angle X-ray Diffraction Characterization of Microstructural Evolution of Metallocene Polyethylene Film During Stress Relaxation and Fatigue Process

Yi-Huai ZHANG¹^,², Tao LIAO²(), Yong-Feng MEN¹^,²()

^1.School of Applied Chemistry and Engineering，University of Science and Technology of China，Hefei 230026，China
^2.State Key Laboratory of Polymer Science and Technology，Changchun Institute of Applied Chemistry，Chinese Academy of Sciences，Changchun 130022，China

Received:2025-05-15 Accepted:2025-09-15 Published:2025-12-01 Online:2025-12-30
Contact: Tao LIAO,Yong-Feng MEN

Abstract

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

CLC Number:

O631

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.

Figures/Tables 11

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

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

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

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）

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

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

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

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

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

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

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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In-situ Wide-Angle X-ray Diffraction Characterization of Microstructural Evolution of Metallocene Polyethylene Film During Stress Relaxation and Fatigue Process

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