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 2H and 31P nuclear magnetic resonance spectroscopy (NMR) the component dynamics of the binary glass tripropyl phosphate (TPP)/polystyrene (PS/PS-d3) is selectively investigated for concentrations distributed over the full range. We study the secondary (β-) relaxation below Tg, which is found in all investigated samples containing TPP, but not in neat polystyrene. The dielectric spectrum of the β-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 ≅ 24 Tg. Persistence of the β-process is confirmed by 31P NMR Hahn-echo and spinlattice relaxation experiments on TPP, which identify the nature of the β-process as being highly spatially hindered as found for other (neat) glasses studied previously, or re-investigated within this work. The corresponding 2H NMR experiments on PS-d3 confirm the absence of a β-process in neat PS-d3, but reveal a clear signature of a β-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-d3 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 β-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. © 2013 AIP Publishing LLC