It should be noted that since the
measurement of viscosity of a glass is dicult and can be
erroneous, this value is derived from an extrapolation.
When the temperature of the glass is increased to Tg,
this material changes to rubbery state with a viscosity
ranging from 106 ÿ108 Pa s (Downton et al., 1982). The
mechanical properties of a material such as relaxation
time and viscous properties above its glass transition
temperature (T> Tg, e.g. Tg + 100 K) are observed to
follow William±Landel±Ferry (WLF) kinetics (