Large angle jumps of small molecules in amorphous matrices analyzed by 2D exchange NMR

Abstract

The reorientation dynamics of deuterated benzene and hexamethyl benzene as additives to the glass former oligostyrene is studied below the glass transition temperature T g. By means of 2D 2H NMR, analyzed in the frequency and in the time domain, it is shown that the dynamics of the small molecules is governed by an isotropic large angle reorientation process, which is close to the random jump model. Furthermore, the dynamics is characterized by a broad distribution of correlation times. Even 65 K below T g, a fraction of small molecules reorients on the timescale of 100 ms. In contrast, small angle reorientation dominates in the neat glass former polystyrene near T g. As a consequence of the presence of large angle jumps, the 2D spectra can be described by an additive superposition of two sub-spectra—a ridge along the diagonal and a complete exchange pattern—where the weighting factor W( t m) is directly given by the reorientational correlation function F 2( t m). Additionally, for a sample with very low benzene concentration ( c≈0.5%), the 1D spectra indicate that the same dynamic scenario is present in the single particle limit. Tentatively, we assume that the large angle reorientation of the small molecules is associated with a translational diffusion process of the small molecules within the amorphous matrix.