Effect of molecular shape on the dielectric relaxation in the glassy phase of rigid polar molecules-cis-decalin mixtures
The effect of molecular shape on the α and β relaxations in the glassy phase of rigid polar molecules bromobenzene (planar), n-butyl chloride (linear), and t-butyl chloride (nearly spherical) in cis-decalin mixtures has been studied by measuring the dielectric parameters in the frequency range 0.1 to 100 kHz and over the temperatures from 77 to 300 K. The α-relaxation peaks are pronounced in all three systems, but the β-relaxation peaks are different. β peaks are almost absent in n-butyl chloride, while in bromobenzene they are of lower magnitude than the α peaks in agreement with the earlier reported results. However, in t-butyl chloride, the β peaks appear to be of higher magnitude than the α peaks. The present studies support the model proposed by Johari that in the amorphous phase above Tg the dielectric behavior is governed by the cooperative rearrangement of molecules, while in the glassy phase below Tg the β-relaxation process arises from the hindered rotation of polar molecules encaged in the glassy matrix. The static dielectric constant has been estimated in the amorphous phase from the temperature variation of ∊' at a fixed frequency. The dipole moments have also been evaluated from the static dielectric constant in the amorphous phase as well as in the normal liquid.