  Knowledge of relaxation processes is fundamental in glass science and
technology because relaxation is intrinsically related to vitrification,
tempering as well as to annealing and sev-eral applications of glasses.
However, there are conflicting reports -- summarized here for different glasses
-- on whether the structural relaxation time of glass can be calculated using
the Maxwell equation, which relates relaxation time with shear viscosity and
shear modulus. Hence, this study aimed to verify whether these two relaxation
times are comparable. The structural relaxation kinetics of a lead metasilicate
glass were studied by measuring the re-fractive index variation over time at
temperatures between 5 and 25 K below the fictive temperature, which was
initially set 5 K below the glass transition temperature. Equilibrium shear
viscosity was measured above and below the glass transition range, expanding
the current knowledge by one order of magnitude. The Kohlrausch equation
described very well the experimental structural relaxation kinetics throughout
the investigated temperature range and the Kohlrausch exponent increased with
temperature, in agreement with studies on other glasses. The experimental
average structural relaxation times were much longer than the values computed
from isostructural viscosity, as expected. Still, they were less than one order
of magnitude higher than the average relaxation time computed through the
Maxwell equation, which relies on equilibrium shear viscosity. Thus, these
results demon-strate that the structural relaxation process is not controlled
by isostructural viscosity, and that equilibrium shear viscosity only provides
a lower boundary for structural relaxation kinetics.
