Simple versus cooperative relaxations in complex correlated systems

Abstract

A method for investigating the nature of thermally activated relaxations in terms of their cooperative character is tested in both polymer and low molecular weight crystal systems. This approach is based on analysis of the activation entropy in order to describe thermally activated relaxations. The betaine arsenate/phosphate mixed system of low molecular weight crystals was selected for investigation because pure compounds of this system show ferro-/antiferroelectric phase transitions and the mixed crystals undergo different kinds of relaxation processes involving both dipole-dipole and dipole-lattice interactions. The polymer chosen was a side chain liquid-crystalline polysiloxane, which shows the β-relaxation characteristic of disordered systems and amorphous materials. The cooperative versus local character of the relaxations is described in terms of "complex" and "simple" relaxations based on calculations of the activation entropies. The initial assumptions of the theory, as well as the resulting equations, were found to be applicable to the systems studied. © 2001 American Institute of Physics.