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

We investigate the secondary (beta-) 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 Delta T-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 cTPP >= 60% and those from different methods of H-2 and P-31 NMR spectroscopy. In the case of NMR, the full concentration range (10% <= cTPP <= 100%) is covered. The neat components exhibit a beta-relaxation (beta(1) (SBC) and beta(2) (TPP)). The latter is rediscovered by DS in the mixtures for all concentrations with unchanged time constants. NMR spectroscopy identifies the beta-relaxations as being alike to those in neat glasses. A spatially highly restricted motion with angular displacement below +/- 10 degrees encompassing all molecules is involved. In the lowtemperature range, where TPP shows the typical (31)PNMRecho spectra of the beta(2)-process, very similar spectral features are observed for the (deuterated) SBC component by H-2 NMR, in addition to its "own" beta(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 tau(beta 2) are the same. Furthermore, T-1(T) of TPP in the temperature region of the beta(2)-process is absolutely the same as in the mixture TPP/polystyrene investigated previously. It appears that the manifestations of the beta-process introduced by one component are essentially independent of the second component. Finally, at cTPP <= 20% one finds indications that the beta(2)-process starts to disintegrate. More and more TPP molecules get immobilized upon decreasing cTPP. We conclude that the beta-process is a cooperative process. Published by AIP Publishing.