We investigated the dielectric relaxation in blends of 4-n-pentyl-4′-cyanobiphenyl (5CB) and polystyrene (PS) in order to examine the size effect of low mass molecules on the cooperativity of local dynamics. To obtain the dielectric data in the isotropic state, the phase diagram was produced. The temperature (T) dependence of the dielectric loss factor ε″ of PS/5CB blends containing 20 to 30 wt% of 5CB exhibited a bimodal peak near the calorimetric glass transition temperature T g. The bimodal peak was resolved into two peaks termed α and β. The α process locates above Tg and can be assigned to cooperative motions of the PS segments and the 5CB molecules. The β process is responsible to independent reorientation of the 5CB molecules in the glassy PS matrix. The α and β loss peaks tend to merge with increasing T. This behavior contrasts to the dielectric behavior of the PS/toluene system which exhibits loss peaks due to segmental motions of PS and rotation of the toluene molecules in well separated temperature regions. The toluene molecule has a size of ca. 1/3 of the 5CB molecule and does not move cooperatively with PS but the 5CB molecule does. From the intensity of the α and β peaks, the fraction of the 5CB molecules moving cooperatively with PS has been estimated to be 0.25 and 0.63 for the blends containing 30 and 20 wt% of 5CB, respectively.
All Science Journal Classification (ASJC) codes
- Polymers and Plastics
- Materials Chemistry