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

Abstract

In Paper I of this series of two papers we study the main relaxations of a binary glass former made of the low-T_g component tripropyl phosphate (TPP, T_g = 134 K) and of a specially synthesized (deuterated) spirobichroman derivative (SBC, T_g = 356 K) as the non-polymeric high-T_g component for the full concentration range. A large T_g contrast of the neat components is put into effect. Dielectric spectroscopy and different techniques of ²H nuclear magnetic resonance (NMR) as well as of ³¹P NMR spectroscopy allow to selectively probe the dynamics of the components. For all concentrations, two well separated liquid-like processes are identified. The faster α₂-process associated with the low-T_g component TPP shows pronounced dynamic heterogeneities reflected by quasi-logarithmic correlation functions at low TPP concentrations. The slower α₁-process involves the reorientation of the high-T_g component SBC. Its correlation function is Kohlrausch-like as in neat glass formers. The corresponding time constants and consequently their glass transition temperatures T_g1 and T_g2 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_g2 < T < T_g1). At T < T_g2 the liquid-like reorientation of TPP gets arrested too. We find indications that a fraction of the TPP molecule takes part in the slower α₁-process of the high-T_g 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.