The dynamics of two fragile glass forming liquids was studied as a function of temperature and pressure using dynamic light scattering. On the basis of measured data we evaluated the pressure and ...temperature scaling of the alpha relaxation. All the isotherms can be superimposed and form a master curve when the reduced relaxation times are plotted versus reduced pressure. The Kohlrausch-Williams-Watts stretching parameter beta(KWW) is increasing with increasing temperature and decreasing pressure and plotted versus log<tau(KWW)> follows a master curve at all temperatures and pressures studied.
Glass-forming liquids and polymers consist of `solid-like' and `fluid-like' clusters of a finite lifetime in a dynamic equilibrium. The experimental observations leading to the heterophase ...fluctuation model are reviewed. They are: (i) long range density fluctuations as measured by static light scattering and ultra small angle X-ray scattering, (ii) ultra-slow hydrodynamic modes of density fluctuations as observed by photon correlation spectroscopy, (iii) temperature depending structural changes as studied by wide angle X-ray scattering. In order to explain these observations a new order parameter
n
s
(
x
,t)
is proposed, which describes the local mass fraction of energetically preferred structures (`aperiodic solid states'). The thermodynamic potentials can be analyzed in terms of the temperature dependence of 〈
n
s(
T)〉. The α-relaxation process of supercooled liquids is directly related to the local fluctuations of
n
s (
x,t).
