Orientational dynamics of the glass forming liquid, dibutylphthalate: Time domain experiments and comparison to mode coupling theory

Abstract

Orientation dynamics of the glass forming liquid, dibutylphthalate (DBP), were studied using optical heterodyne detected optical Kerr effect (OHD-OKE) techniques. A combination of experimental methods permitted acquisition of data over 6 decades of time and 5 decades in signal amplitude. Data collected from several hundred fs to several hundred ns, cover the full range of orientational dynamics. The data are compared to the predictions of ideal mode-coupling theory (MCT). Two of the MCT scaling law predictions yield an average value of 227 K for the MCT critical temperature, Tc. Measurements were made at temperatures below Tc. The data were found to agree well with some of the predictions of MCT. The long time scale structural relaxation (α relaxation) obeys the MCT scaling law, and the slowest time scale power law (the von Schweidler power law) is also in accord with MCT predictions. However, the “master curves” predicted by MCT fit the data well only at higher temperatures. The master curves show increasingly significant deviations at shorter times (<100 ps) as the temperature is decreased, in contrast to previous OHD-OKE studies on salol [G. Hinze, D. D. Brace, S. D. Gottke, and M. D. Fayer, J. Chem. Phys. 113, 3723 (2000)] and ortho-terphenyl [S. D. Gottke, G. Hinze, D. D. Brace, and M. D. Fayer, J. Phys. Chem. B 105, 238 (2000)] in which the agreement with ideal MCT is far superior. The DBP data show no discontinuity in the trends as the temperature is reduced below Tc, but an increasingly prominent peak appears in the data at ∼2.5 ps. The peak may be related to the boson peak.