Lack of local structural orientation in oxide glasses quenched during flow: NMR results

Despite of importance of viscous flow of silicate and oxide melts in industry and nature, and the many theoretical treatments of the subject, spectroscopic observations pertinent to the mechanism of flow are few. Birefringence in sheared glasses and liquids has often been interpreted in terms of the alignment of small-scale structural units, as have Raman spectroscopic data that reveal changes in polarizability. NMR spectroscopy is very sensitive to the orientation of many types of non-spherical local structural units, such as planar BO3 groups. By using 11B, 27Al, and 29Si NMR, we have examined a variety of borate and silicate glasses (e-glass and Pyrex fibers, B2O3, and Na2Si2O5) quenched during rapid shearing of liquids. Despite the fact the computer simulations of NMR lineshapes show that a few percent of local alignment could be detected, none whatsoever is seen. These results indicate that alternate explanations of anisotropy in oxide glasses should be sought, and may place important constraints on flow mechanisms in the liquid.