Dynamical processes in organic glassforming van der Waals liquids

Abstract

This report presents measurements of the frequency dependent dielectric loss ε″(ω) and quasielastic light scattering of thermal density fluctuations of simple organic glassforming van der Waals liquids BCDE (bis-phenol-C-dimethylether) and BKDE (bis-kresol-C-dimethylether) having glass transition temperatures of Tg=240 K and Tg=261 K, respectively. The dielectric measurements of BCDE were carried out in a temperature range between 250 and 316 K, those of BKDE between 270 and 384 K covering a frequency range from 10−1 to ∼109 Hz in each case. Quasielastic light scattering data were taken between the glass transition temperatures and T≂Tg+30 K in a correlation time window of 10−6 s up to 102 s. As main results we have found the following: The relaxation process in the BCDE melt can be described using a Kohlrausch–Williams–Watts (KWW) function with a temperature independent β parameter. Its value was found to be identical either from the light scattering result or from the stretched exponential fit to the Fourier transform of the dielectric loss data. The temperature dependence of the mean relaxation times of the BCDE melt was fitted with a Williams–Landel–Ferry (WLF) equation, or Vogel–Fulcher–Tammann equation, leading to WLF constants close to the universal ones. The BKDE system, however, behaves differently, where the dynamic response in the bulk amorphous state could only be described using a temperature dependent β parameter. This finding was confirmed by either the light scattering method as well as the dielectric relaxation method. Additionally we were able to measure a second dielectrically active process in BCDE only, arising in the high frequency region with an Arrhenius type of activation energy. In the BKDE melt, however, the superposition of different processes causes a temperature dependence of the β parameter when the data is forcefitted with a single KWW function. To correlate the fast process of BCDE (β process) with a possible molecular model of motion we have performed additional 2H-NMR solid echo measurements on both materials around and well below Tg. Thereby a 180° phenyl ring flip motion in BCDE was found in contrast to BKDE. This assignment of the secondary relaxation process is discussed together with the results from other methods.