Non-polymeric asymmetric binary glass-formers. I. Main relaxations studied by dielectric, 2 H NMR, and 31 P NMR spectroscopy

In Paper I of this series of two papers we study the main relaxations of a binary glass former made of the low-T component tripropyl phosphate (TPP, T = 134 K) and of a specially synthesized (deuterated) spirobichroman derivative (SBC, T = 356 K) as the non-polymeric high-T component for the full concentration range. A large T contrast of the neat components is put into effect. Dielectric spectroscopy and different techniques of H nuclear magnetic resonance (NMR) as well as of P NMR spectroscopy allow to selectively probe the dynamics of the components. For all concentrations, two well separated liquid-like processes are identified. The faster α -process associated with the low-T component TPP shows pronounced dynamic heterogeneities reflected by quasi-logarithmic correlation functions at low TPP concentrations. The slower α -process involves the reorientation of the high-T component SBC. Its correlation function is Kohlrausch-like as in neat glass formers. The corresponding time constants and consequently their glass transition temperatures T and T differ more the lower the TPP concentration is. Plasticizer and anti-plasticizer effect, respectively, is observed. At low temperatures a situation arises that the TPP molecules isotropically reorient in an arrested SBC matrix (T < T < T ). At T < T the liquid-like reorientation of TPP gets arrested too. We find indications that a fraction of the TPP molecule takes part in the slower α -process of the high-T component. All the features known from polymer-plasticizer systems are rediscovered in this non-polymeric highly asymmetric binary mixture. In Paper II [B. Pötzschner et al., J. Chem. Phys. 146, 164504 (2017)] we study the secondary (β-) relaxations of the mixtures.