Heterophase fluctuations in supercooled liquids and polymers

Abstract 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).