Reorientational dynamics in highly asymmetric binary low-molecular mixtures-A quantitative comparison of dielectric and NMR spectroscopy results

Previously, we scrutinized the dielectric spectra of a binary glass former made by a low-molecular high-T-g component 2-(m-tertbutylphenyl)-2'-tertbutyl-9,9'-spirobi[9H]fluorene (m-TPTS; T-g = 350 K) and low-T-g tripropyl phosphate (TPP; T-g = 134 K) [Korber et al., Phys. Chem. Chem. Phys. 23, 7200 (2021)]. Here, we analyze nuclear magnetic resonance (NMR) spectra and stimulated echo decays of deuterated m-TPTS-d(4) (H-2) and TPP (P-31) and attempt to understand the dielectric spectra in terms of component specific dynamics. The high-T-g component (alpha(1)) shows relaxation similar to that of neat systems, yet with some broadening upon mixing. This correlates with high-frequency broadening of the dielectric spectra. The low-T-g component (alpha(2)) exhibits highly stretched relaxations and strong dynamic heterogeneities indicated by two-phase spectra, reflecting varying fractions of fast and slow liquid-like reorienting molecules. Missing for the high-T-g component, such two-phase spectra are identified down to w(TPP) = 0.04, indicating that isotropic reorientation prevails in the rigid high-T-g matrix stretching from close to T-g(TPP) to T-g1(w(TPP)). This correlates with low-frequency broadening of the dielectric spectra. Two T-g values are defined: T-g1(w(TPP)) displays a plasticizer effect, whereas T-g2(w(TPP)) passes through a maximum, signaling extreme separation of the component dynamics at low w(TPP). We suggest understanding the latter counter-intuitive feature by referring to a crossover from single glass to double glass scenario revealed by recent MD simulations. Analyses reveal that a second population of TPP molecules exists, which is associated with the dynamics of the high-T-g component. However, the fractions are lower than suggested by the dielectric spectra. We discuss this discrepancy considering the role of collective dynamics probed by dielectric but not by NMR spectroscopy. Published under an exclusive license by AIP Publishing.

Automated summary

The dielectric spectra and nuclear magnetic resonance spectra of a binary glass former composed of high-T-g and low-T-g components were analyzed. The high-T-g component showed similar relaxation to neat systems while the low-T-g component exhibited highly stretched relaxations and strong dynamic heterogeneities. The presence of two T-g values and extreme separation of component dynamics were observed, providing insight into the behavior of the system at different temperatures.