CLUSTER STRUCTURE, STRUCTURAL RELAXATION MECHANISM, AND RELAXATION PROPERTIES IN DISORDERED MOLECULAR-ARRANGEMENT SYSTEMS AT LOW-TEMPERATURES

Heat capacities of simple molecular compounds were measured in the liq uid, orientationally disordered crystalline, and stable crystalline st ates by using an adiabatic calorimeter, spontaneous enthalpy relaxatio n processes at a constant temperature in the liquid-glass transition r egion were tracked by a temperature jump method, and crystal nucleatio n rates were examined for 3,3'-dimethoxy-4,4';bis(2,2-diphenylvinyl)bi phenyl by using a differential scanning calorimeter. It is concluded o n the basis of the results that (1) the rearrangement of molecules in the supercooled liquid and in the crystal proceeds through a classical thermal activation process in which some activation unit surmounts a potential barrier, (2) the nonlinearity of enthalpy relaxation functio ns in the time domain is mainly attributed to a non-Arrhenius property of average relaxation times at equilibrium, (3) the disordered molecu lar-arrangement systems have cluster aggregate structures and show a s et of alpha- and beta-relaxation processes at low temperatures, and (4 ) the crystal nucleation rates are dominated by the beta-relaxation ti me but not the alpha-relaxation time and are appreciable even below th e alpha-glass transition temperature. ''Intra-cluster rearrangement'' model is proposed for the alpha-process, and the alpha-relaxation time is indicated not to diverge to infinity even at the Kauzmann temperat ure.