Reorientational dynamics in supercooled m-tricresyl phosphate: Its relation to main and secondary relaxation—31P nuclear magnetic resonance study of relaxation, line shape, and stimulated echo

Abstract

Supercooled m-tricresyl phosphate has been studied by different nuclear magnetic resonance (NMR) methods using the 31P spin probe (180–400 K). Combining measurements of the spin-lattice relaxation at two Larmor frequencies, of the line shape and of the stimulated echo decay, a reorientational correlation function F2(t) is probed over the entire supercooled regime. The analysis of the stimulated echo is described in detail. The long-time behavior of F2(t) (α process) does not change its shape below 260 K and is approximated by a stretched exponential, however, some deviations are observed at short times. Above 260 K a decrease of the stretching of F2(T) is found. Reorientational correlation times τrot are reported in the range 10−11 s <τrot<10 s. The times scale τrot and the one of viscous flow separate only weakly upon supercooling, if at all, whereas translational diffusion and viscous flow clearly decouple below 270 K (∼1.26 Tg). These findings are taken as an indication of a change of transport mechanism in the supercooled liquid. For T≳Tg a change of spectral density is revealed by the relaxation experiments, and it is attributed to the short-time behavior of F2(t), i.e., to the β process and to peculiarities of the α process itself. The β process has a small amplitude which decreases upon cooling.