On the cooperative nature of the β-process in neat and binary glasses: A dielectric and nuclear magnetic resonance spectroscopy study

By means of dielectric as well as H-2 and P-31 nuclear magnetic resonance spectroscopy (NMR) the component dynamics of the binary glass tripropyl phosphate (TPP)/polystyrene (PS/PS-d(3)) is selectively investigated for concentrations distributed over the full range. We study the secondary (beta-) relaxation below T-g, which is found in all investigated samples containing TPP, but not in neat polystyrene. The dielectric spectrum of the beta-process is described by an asymmetric distribution of activation energies, essentially not changing in the entire concentration regime; its most probable value is E/k congruent to 24 T-g. Persistence of the beta-process is confirmed by P-31 NMR Hahn-echo and spin-lattice relaxation experiments on TPP, which identify the nature of the beta-process as being highly spatially hindered as found for other (neat) glasses studied previously, or re-investigated within this work. The corresponding H-2 NMR experiments on PS-d(3) confirm the absence of a beta-process in neat PS-d(3), but reveal a clear signature of a beta-process in the mixture, i.e., polystyrene monomers perform essentially the same type of secondary relaxation as the TPP molecules. Yet, there are indications that some fractions of PS-d(3) as well as TPP molecules become immobilized in the mixture in contrast to the case of neat glasses. We conclude that in a binary glass the beta-process introduced by one component induces a highly similar motion in the second component, and this may be taken as an indication of its cooperative nature. (C) 2013 AIP Publishing LLC.