Dipolar dynamics of low-molecular-weight organic materials in the glassy state

We have measured the dielectric relaxation of several low-molecular-weight glass-forming materials for temperatures ranging from 25 K to T ≈ T_g and for values of the loss tangent down to tanδ ≈ 10^-7. For the materials under study (salol, glycerol, N-methyl-ε-caprolactam, 2-methyltetrahydrofuran, 3-methylpentane, and o-terphenyl) a β-relaxation appearing in quenched samples can be suppressed effectively by annealing at T < T_g, thereby facilitating the study of the dipole dynamics in the absence of the secondary process. As a general behaviour of the remaining dielectric losses at f = 1 kHz we find a variation of log₁₀(ε″) = a + bT (b ≈ 0.015 K^-1) signalling the increasing constraint in the angular degree of freedom as the temperature is lowered and a further increase or peak of ε″ in the range 25 K < T < 50 K. According to a comparison with the crystalline counterparts, the latter effects are characteristic features of the disordered solid state.