VISCOELASTIC RELAXATION MEASUREMENTS IN THE SYSTEM SIO2-NAALSIO4 BY PHOTON-CORRELATION SPECTROSCOPY

Relaxation times of longitudinal strain for five supercooled liquids a long the join SiO2 (qz)-NaAlSiO4 (ne) including the compositions of al bite (ab), ab(50)jd(50), jadeite (jd), jd(66)ne(33), and jd(33)ne(66) were measured by photon correlation spectroscopy within the temperatur e and relaxation time ranges 711-1116 degrees C and 35-2 x 10(-4) s, r espectively. The measured time correlation functions are fitted to the Kohlrausch-Williams-Watts equation, Phi(1)(t) = exp(-t/tau(P,T)(1))(b eta), yielding isothermal relaxation functions for longitudinal strain at constant stress. The temperature dependence of the longitudinal st rain relaxation times obeys an Arrhenian type equation. The relaxation of the longitudinal strain cannot be described by a single relaxation time but by a relaxation time distribution. The width of the relaxati on time distribution, which is represented by the parameter beta, is i ndependent over a large temperature range, indicating thermorheologica l simplicity. The decrease of the parameter beta corresponds to an inc rease of the excess (configurational) heat capacity with increasing (N a+Al) content, together indicating an increasing fragility of the supe rcooled liquids toward the composition of NaAlSiO4. Increasing fragili ty is also shown from the activation energy obtained from longitudinal strain relaxation and shear viscosity measurements, which passes a mi nimum near 40 mol% NaAlSiO4 at low temperatures but decreases toward N aAlSiO4 composition at higher temperatures. A comparison between strai n, stress, and enthalpy relaxation times shows clearly that the relaxa tion process resulting from a thermal pertubation is slower than from a mechanical perturbation.