Non-polymeric asymmetric binary glass-formers. II. Secondary relaxation studied by dielectric, 2 H NMR, and 31 P NMR spectroscopy

We investigate the secondary (β-) relaxations of an asymmetric binary glass former consisting of a spirobichroman derivative (SBC; T = 356 K) as the high-T component and the low-T component tripropyl phosphate (TPP; T = 134 K). The main relaxations are studied in Paper I [B. Pötzschner et al., J. Chem. Phys. 146, 164503 (2017)]. A high T contrast of ΔT = 222 K is put into effect in a non-polymeric system. Component-selective studies are carried out by combining results from dielectric spectroscopy (DS) for mass concentrations c ≥ 60% and those from different methods of H and P NMR spectroscopy. In the case of NMR, the full concentration range (10% ≤ c ≤ 100%) is covered. The neat components exhibit a β-relaxation (β (SBC) and β (TPP)). The latter is rediscovered by DS in the mixtures for all concentrations with unchanged time constants. NMR spectroscopy identifies the β-relaxations as being alike to those in neat glasses. A spatially highly restricted motion with angular displacement below ±10° encompassing all molecules is involved. In the low temperature range, where TPP shows the typical P NMR echo spectra of the β -process, very similar spectral features are observed for the (deuterated) SBC component by H NMR, in addition to its "own" β -process observed at high temperatures. Apparently, the small TPP molecules enslave the large SBC molecules to perform a common hindered reorientation. The temperature dependence of the spin-lattice relaxation time of both components is the same and reveals an angular displacement of the SBC molecules somewhat smaller than that of TPP, though the time constants τ are the same. Furthermore, T (T) of TPP in the temperature region of the β -process is absolutely the same as in the mixture TPP/polystyrene investigated previously. It appears that the manifestations of the β-process introduced by one component are essentially independent of the second component. Finally, at c ≤ 20% one finds indications that the β -process starts to disintegrate. More and more TPP molecules get immobilized upon decreasing c . We conclude that the β-process is a cooperative process.