Dipolar correlation coefficients in the time and frequency domains for low molecular weight models of acrylate polymers

Abstract

The temperature dependence of the components of the complex relative permittivity of 2-biphenyl isobutyrate (OBPI) and bis(biphenyl-2-yl) 2,4-dimethylglutarate (OBPG), model compounds of a single repeating unit and a dimer of poly(biphenyl-2-yl acrylate), respectively, is reported in the frequency range 10^–4–10² kHz. The relaxation spectra of both compounds at 0.1 Hz exhibit glass–liquid relaxations with maxima located at –61.2 and –8.8 °C, respectively, for OBPI and OBPG. Subglass absorptions are not detected in the glassy state down to –120 °C. The free volume fraction at T_g is calculated to be 0.024 and 0.028 for OBPI and OBPG, respectively. Molecular dynamics (MD) simulations give the following values for the mean-square dipole moment of OBPI: 4.01, 2.92 and 2.80 D² at 1000, 500 and 300 K, respectively. The values of 〈µ²〉 at 500 and 300 K are in rather good agreement with the experimental result, 2.69 ± 0.13 D², at 303 K. The time dependence of the dipolar autocorrelation coefficient ϕ(t) is calculated by MD at several temperatures. Transformation of the autocorrelation coefficient from the time to the frequency domain permits the determination of the components of the complex permittivity for these compounds. At low temperatures, the mobility of the molecules is not large enough to cover the whole conformational space within a reasonable computing time. At temperatures lying in the neighbourhood of 500 K, the molecules already visit the whole conformational space, and the values of the complex permittivity in the frequency domain obtained from the ϕ(t) of OBPI and OBPG are in good agreement with the experimental results. These results suggest that the intermolecular cross-correlation terms, although present, do not alter in a significant way the time dependence of ϕ(t).