Dipole correlation and relaxation behavior of flexible bulky low molecular weight esters

The isochrones representing the dielectric relaxation spectra of pentaerythritol tribenzoate and pentaerythritol tetrabenzoate exhibit a poorly defined subglass absorption followed in increasing order of temperature by an ostensible glass-liquid relaxation or α absorption. The α relaxation peaks at 1 Hz are centered at 10 and 17 °C for pentaerythritol tribenzoate and pentaerythritol tetrabenzoate, respectively. The subglass absorption of each compound is deconvoluted into three peaks whose activation energies slightly increase as the locations of the peaks shift to higher temperature. The mean-square dipole moment obtained for isolated molecules suggests a strong correlation between dipoles in pentaerythritol tetrabenzoate whereas the dipoles of pentaerythritol tribenzoate are uncorrelated. The interpretation of the relaxation spectra of both compounds in terms of the Fröhlich theory leads to conclude that more than 90% of the mean-square dipole moment relaxes through the α process. From the temperature dependence of the relaxation times of pentaerythritol tribenzoate and pentaerythritol tetrabenzoate in the vicinity of the respective glass transition temperatures, a fragile parameter close to the conventional value that separates fragile from strong glasses is obtained.