title: Relaxation behavior of pure and binary glass formers – studied by Dielectric and NMR spectroscopy
Creators: name: Körber, Thomas
Publisher: name: University of Bayreuth
Year: 2022
Subjects: subject: glass transition
subject: generic streching
subject: binary mixtures
subject: dielectric spectroscopy
subject: nmr spectroscopy
subject: 530

description: Dielectric (DS) and nuclear magnetic resonance (NMR) spectroscopy are applied to investigate the component-specific relaxation behaviour of binary low-molecular glass formers. The systems are characterized by a more or less high Tg-contrast, i.e. a more or less high glass transition temperature (Tg) difference of the neat components. In cooperation with the department of Macromolecular Chemistry I (group of H.-W. Schmidt), several high-Tg systems are synthesized and characterized by DS regarding the interplay of main (α) and secondary (β) relaxation (Publication 1). Re-addressing relaxation features of neat glass formers, systematic differences in the relaxation stretching probed by DS vs. NMR and photon correlation spectroscopy (PCS) are discovered (Publication 2). Frequency-temperature-superposition (FTS) appears to be valid and a generic stretching (β_K≈0.58±0.06) is found in the case of PCS, NMR, and DS provided the systems are not polar. For highly polar systems, deviations of FTS and the stretching are found for DS in contrast to PCS and NMR. This difference is explained by assuming collective (coherent) dynamics measured by DS vs. incoherent dynamics probed by 2H and 31P NMR and PCS in accordance with recent studies. Thereby, a method analysing the NMR spin-lattice relaxation rate as function of the spin-spin relaxation rate is proposed, which directly provides a kind of susceptibility function (and thus the relaxation stretching) in addition to the NMR coupling constant. Starting with two binary mixtures with a moderate Tg difference (ΔTg=63 K and 89 K), especially the distribution of correlation times G(ln⁡(τ_α1)), specifying the dynamics of the polar high-Tg component, is addressed by DS (Publication 3). In contrast to neat glass formers, the spectra show a broadening and failure of FTS with increasing additive (low-Tg component) concentration. This is confirmed by stimulated echo NMR experiments probing directly the orientational correlation function. Considering isodynamic points in the ns regime, a dynamical decoupling of the two components as well as strong dynamic heterogeneities are found concerning the low-Tg component. Choosing the most suitable high-Tg glass former m-TPTS (Tg=350 K) from the series of newly synthesized systems concerning the glass stability in the mixture with tripropyl phosphate (TPP, Tg=134 K), DS and DSC investigations are performed in the concentration range w_TPP=4% up to 90% at first (Publication 4). Anticipating the NMR results, which yield two isotropic processes, two main relaxations α1 (high-Tg component) and α2 (low-Tg component) with two Tg values Tg1(w_TPP) and Tg2(w_TPP) are found up to highest concentrations. For Tg2(w_TPP), a maximum is found at intermediate concentrations. To cope with the extraordinary spectral broadening, a spectral model consisting of a generalized Cole–Davidson (α1) and a Havriliak–Negami function with a low frequency truncation (α2) is introduced. The total relaxation strength Δε strongly deviates from the expected behaviour for ideal mixing. Therefore, care must be taken interpreting the corresponding Δε_αi as representation of component populations. The actual populations are accessed via component-selective NMR experiments (Publication 5). Spectra and stimulated echo decays of deuterated m-TPTS-d4 (2H) and TPP (31P) are analysed to quantitatively understand the dielectric spectra. The high-Tg component shows relaxation like that of neat systems, yet with some broadening upon mixing. This correlates with a high-frequency broadening found in the dielectric spectra. The low-Tg component exhibits highly stretched relaxations and strong dynamic heterogeneities indicated by “two-phase” NMR spectra, reflecting varying fractions of fast and slow liquid-like reorienting molecules down to lowest concentrations (4%). This indicates, that isotropic reorientation prevails in the rigid high-Tg matrix stretching from close to Tg_TPP to Tg1 (w_TPP), correlating with a low-frequency broadening of the dielectric spectra. The maximum behaviour of Tg2(w_TPP) is recovered also in the NMR experiments, signaling extreme separation of the component dynamics at low w_TPP. Scrutinizing NMR relaxation data of several (polymeric) mixtures investigated previously and in this work as a function of concentration, a generic scenario is found for systems with high Tg-contrast (ΔTg≥170 K). Differences are recognized for systems with lower ΔTg. To understand the observed maximum in Tg2(w_add), a crossover from “single glass” to “double glass” scenario is suggested, which is revealed by recent molecular dynamics simulations of hard spheres. Furthermore, a second population of TPP molecules is revealed in the analysis, which is associated with the dynamics of the high-Tg component. However, the fractions are lower than suggested by the dielectric spectra, which is discussed by the role of collective dynamics probed by dielectric but not by NMR spectroscopy.