LIGHT-SCATTERING IN JADEITE MELT - STRAIN RELAXATION MEASUREMENTS BY PHOTON-CORRELATION SPECTROSCOPY

Photon correlation spectroscopy has been applied to the study of longi tudinal strain relaxation of vitreous jadeite (NaAlSi2O6) in the tempe rature range 811-1014-degrees-C. The correlation function \g(1)(t)\2 i s-proportional-to exp ((-2t/tau(beta))beta) obeys a Kohlrausch type fu nction with beta=0.64+/-0.01. Individual correlation functions fit alt ogether a master relaxation curve, thus demonstrating thermorheologica l simplicity (TRS). The temperature dependence of the measured relaxat ion times shows Arrhenian behaviour with log(tau)=-21.4+/-0.3s+471.6+/ -22 kJmol-1/RT. The time scale of longitudinal strain relaxation is co nsistent with the existing data on shear relaxation derived from shear viscosity and structural relaxation calculated from calorimetric C(p) measurements. Comparison with oxygen diffusion indicates that network forming elements relax at about the same time scale as viscoelastic p roperties. On the other hand, Na+ relaxation times derived from impeda nce spectroscopy are short compared to viscoelastic relaxation times a t low temperatures. This difference is decreasing with increasing temp erature and possibly disappearing at approximately 1100-degrees-C.