Experimental evidence of ß-relaxation and its structural origin in ZIF-62 glass.

ABSTRACT

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 ß-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, ß-relaxation was observed in a MOF glass besides the primary α-relaxation. The pronounced ß-relaxation was well demonstrated by a number of characteristics including an excess wing and the full width at half maximum (W) of the α-relaxation peaks, which deviated from the time-temperature superposition. The stretched exponent ß of ZIF-62 glass is 0.71 in the supercooled region. The W of ZIF-62 glass is the maximum among all known glassy materials. The structural origin of α- and ß-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 ß with W of the α-relaxation peaks was established. When compared with traditional glass-formers, the resulting principles indicate a shared origin for the stretched exponent ß, W, and ß-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.