ABSTRACT
The structure of melt-quenched zeolitic imidazole framework (ZIF) glasses can provide insights into their glass-formation mechanism. We directly detected short-range disorder in ZIF glasses using ultrahigh-field zinc-67 solid-state nuclear magnetic resonance spectroscopy. Two distinct Zn sites characteristic of the parent crystals transformed upon melting into a single tetrahedral site with a broad distribution of structural parameters. Moreover, the ligand chemistry in ZIFs appeared to have no controlling effect on the short-range disorder, although the former affected their phase-transition behavior. These findings reveal structure-property relations and could help design metal-organic framework glasses.
ABSTRACT
The dynamics of the phosphate chains and the attendant shear relaxation in a short-chain silver phosphate glass-forming liquid with the composition 51.5%Ag2O-48.5% P2O5 are studied using a combination of high-temperature 31P NMR spectroscopy and parallel plate rheometry. The temperature-dependent evolution of the 31P NMR spectral line shapes indicates that the constituent PO4 tetrahedral chains in this liquid undergo rapid rotational reorientation. The time scale of this dynamics is in complete agreement with that of shear relaxation and, thus, must be responsible for the viscous flow of this liquid. These results demonstrate for the first time that, although the shear relaxation of the network oxide glass-forming liquids is typically controlled by the scission and renewal of bonds between the network-forming cations and oxygen atoms, such a scenario may not always be tenable for liquids with low-dimensional structures consisting of chains.
