Peng Si-Xu, Zhu Ying-Ying, Li Gang, Luo Yongkang, Han Xiaotao, Liu Shi-Yu
College of Chemistry & Chemical Engineering, Hubei University, Wuhan, 430062, Hubei, China.
Department of Electrical and Computer Engineering, University of Maryland, College Park, MD, 20742, USA.
Soft Matter. 2023 Jul 26;19(29):5575-5582. doi: 10.1039/d3sm00623a.
Intrinsic relaxation processes determine the crucial properties of glass, yet their underlying mechanisms are far from well understood. The brand-new glass-forming metal-organic frameworks (MOFs) provide desirable opportunities for looking inside glass relaxation, especially the secondary β-relaxation phenomenon and mechanism. For a representative zeolitic imidazolate framework-62 (ZIF-62) glass, reliable and fine powder mechanical spectroscopy was performed based on home-made mountings combined with a commercial dynamical mechanical analyzer. For the first time, β-relaxation was observed in a MOF glass besides the primary α-relaxation. The pronounced β-relaxation was well demonstrated by a number of characteristics including an excess wing and the full width at half maximum () of the α-relaxation peaks, which deviated from the time-temperature superposition. The stretched exponent of ZIF-62 glass is 0.71 in the supercooled region. The of ZIF-62 glass is the maximum among all known glassy materials. The structural origin of α- and β-relaxation can be attributed to an increase of density, as observed using nuclear magnetic resonance (NMR). A general linear and broad correlation of fragility and stretched exponent with of the α-relaxation peaks was established. When compared with traditional glass-formers, the resulting principles indicate a shared origin for the stretched exponent , , and β-relaxation in the case of ZIF-62 glass. The presented findings offer an effective new method to explore the glass/liquid transition of MOF glasses, which helps to obtain a deeper insight into the hierarchical relaxation dynamics of the glass transition.
本征弛豫过程决定了玻璃的关键性质，但其潜在机制仍远未被充分理解。全新的玻璃形成金属有机框架（MOF）为探究玻璃弛豫，尤其是次级β弛豫现象及机制提供了理想契机。对于具有代表性的沸石咪唑酯骨架-62（ZIF-62）玻璃，基于自制装置结合商用动态力学分析仪进行了可靠且精细的粉末力学光谱分析。首次在MOF玻璃中除了观察到初级α弛豫外，还观察到了β弛豫。显著的β弛豫通过包括α弛豫峰的过剩翼和半高宽（）等多个特征得到了很好的证明，这些特征偏离了时间-温度叠加。ZIF-62玻璃在过冷区域的伸展指数为0.71。ZIF-62玻璃的是所有已知玻璃材料中最大的。如使用核磁共振（NMR）所观察到的，α弛豫和β弛豫的结构起源可归因于密度的增加。建立了脆性和伸展指数与α弛豫峰的之间的一般线性且广泛的相关性。与传统玻璃形成剂相比，所得原理表明在ZIF-62玻璃的情况下，伸展指数、和β弛豫具有共同的起源。所呈现的研究结果提供了一种有效的新方法来探索MOF玻璃的玻璃/液体转变，这有助于更深入地了解玻璃转变的分级弛豫动力学。
