Dielectric properties of vapor-deposited propylbenzenes

Dielectric susceptibility data of vapor-deposited films of iso-propylbenzene (IPB) and n-propylbenzene (NPB) have been recorded across a wide range of deposition temperatures, T_dep, mostly below the glass transition temperature, T_g. The results for the real and imaginary components of dielectric susceptibility are compared with recently published results for 2-methyltetrahydrofuran (MTHF). Common to all three systems are the following: (i) increased kinetic stability seen as higher onset temperature for the transformation to the liquid state for T_dep ≈ 0.85T_g; (ii) the reduction of the dielectric loss (χ″) for as-deposited glasses, a signature of increased packing density that is maximal for T_dep ≈ 0.85T_g; and (iii) a reduced level of the storage component (χ') for as-deposited glasses, an effect that is almost deposition temperature invariant for T_dep < T_g. Material specific behavior is observed when heating the as-deposited films to 1.2T_g: IPB and NPB transform directly into the ordinary liquid state if judged on the basis of dielectric susceptibility, whereas MTHF has been reported to enter an unusual liquid state prior to a liquid-liquid transition at higher temperatures. These results are discussed in the context of the curious scattering results reported by Ishii et al. for some benzene derivatives, which hint at a liquid-liquid transformation.