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

We investigate the secondary (β-) relaxations of an asymmetric binary glass former consisting of a spirobichroman derivative (SBC; T_g = 356 K) as the high-T_g component and the low-T_g component tripropyl phosphate (TPP; T_g = 134 K). The main relaxations are studied in Paper I [B. Potzschner et al., J. Chem. Phys. 146, 164503 (2017)]. A high T_g contrast of ΔT_g = 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_TPP ≤ 60% and those from different methods of 2H and ₃₁P NMR spectroscopy. In the case of NMR, the full concentration range (10% ≤ c_TPP ≤ 100%) is covered. The neat components exhibit β-relaxation (β ₁ (SBC) and β ₂ (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 lowtemperature range, where TPP shows the typical 31PNMRecho spectra of the _α2-process, very similar spectral features are observed for the (deuterated) SBC component by 2H NMR, in addition to its "own" β ₁-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_β2 are the same. Furthermore, T₁ (T) of TPP in the temperature region of the β ₂-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_TPP ≤ 20% one finds indications that the β ₂-process starts to disintegrate. More and more TPP molecules get immobilized upon decreasing c_TPP. We conclude that the β-process is a cooperative process. [ABSTRACT FROM AUTHOR]