Dynamical phase transition in simple supercooled liquids and polymers - an NMR approach

Abstract

We present NMR investigations on m-tricresyl phosphate (m-TCP; 31P NMR) and on polybutadiene (PB; 2H NMR). Relaxation studies have been combined with the analysis of the stimulated echo, and a reorientational correlation function is probed over the entire supercooled regime providing correlation times in the range 10 -11 s-10 s. Furthermore, we have performed Raman scattering (RS) experiments on m-TCP and glycerol. Testing predictions of mode coupling theory (MCT), the scaling behaviour observed by RS on m-TCP is well described by MCT for T > T c with T c ∼ 260 K. The idea of a change of dynamics at T ∼ T c is further supported by phenomena which - although not yet emerging from MCT - are revealed by combining results from NMR, RS and neutron scattering. (i) The time scale of reorientational and translational motion separates below T c (m-TCP). (ii) The stretching parameter of the α-relaxation changes significantly near T c (m-TCP). (iii) NMR and dielectric relaxation probe a third process which exists only below T c. This process has all properties of the secondary relaxation discussed by Johari, however, it is well distinguished from the fast β-process analyzed within MCT and observed by RS. Thus, above T c the dynamics are described by a two-step correlation function with all the features predicted by MCT, whereas below T c a three-step function holds. As a consequence, a second order parameter appears, and the third process is described in terms of an activated process with a distribution of local environments.