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-Tg component 2-(m-tertbutylphenyl)-2'-tertbutyl-9,9'-spirobi[9H]fluorene (m-TPTS; Tg = 350 K) and low-Tg tripropyl phosphate (TPP; Tg = 134 K) [Körber 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-d4 (2H) and TPP (31P) and attempt to understand the dielectric spectra in terms of component specific dynamics. The high-Tg component (α1) 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-Tg component (α2) 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-Tg component, such two-phase spectra are identified down to wTPP = 0.04, indicating that isotropic reorientation prevails in the rigid high-Tg matrix stretching from close to Tg TPP to Tg1 wTPP. This correlates with low-frequency broadening of the dielectric spectra. Two Tg values are defined: Tg1 (wTPP) displays a plasticizer effect, whereas Tg2 (wTPP) passes through a maximum, signaling extreme separation of the component dynamics at low wTPP. 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-Tg 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.

Reorientational dynamics of highly asymmetric binary non-polymeric mixtures - a dielectric spectroscopy study.

We present an analysis of dielectric spectra measured for a specially designed non-polymeric asymmetric binary glass former characterized by a large difference of the component's Tg (ΔTg = 216 K). We cover the whole additive concentration range from 4% up to 90% (by mass). Two main relaxations α1 and α2 are identified, which are characterized by well separated time scales and are attributed to the dynamics associated with the high-Tg component (α1) and the low-Tg component (α2). Frequency-temperature superposition does not apply. To cope with the extraordinary spectral broadening, we introduce a model consisting of a generalized Cole-Davidson (α1) and a Havriliak-Negami function with a low frequency truncation (α2). Whereas the α1-relaxation reflects essentially homogeneous dynamics and its spectra mainly broaden on the high-frequency flank of the relaxation peak, the α2-relaxation becomes broader on the low-frequency side reflecting pronounced dynamic heterogeneity in a more or less arrested matrix of high-Tg molecules. From the extracted time constants, two glass transition temperatures Tg1 and Tg2 can be derived, showing a non-trivial concentration dependence for Tg2. Supplementary, we find a ß-relaxation. The total relaxation strength Δε strongly deviates from ideal mixing, and therefore care has to be taken interpreting the corresponding Δεαi as representation of molecular populations.

Main and secondary relaxations of non-polymeric high-Tg glass formers as revealed by dielectric spectroscopy.

A series of high-Tg glass formers with Tg values varying between 347 and 390 K and molar masses in the range of 341 and 504 g mol-1 are investigated by dielectric spectroscopy. They are compared to paradigmatic reference systems. Differently polar side groups are attached to a rigid non-polar core unit at different positions. Thereby, the dielectric relaxation strength varies over more than two decades. All the relaxation features typical of molecular glass formers are rediscovered, i.e. stretching of the main (α-) relaxation, a more or less pronounced secondary (ß-) process, and a fragility index quite similar to that of other molecular systems. The position of the polar nitrile side group influences the manifestation of the ß-relaxation. The α-relaxation stretching displays the trend to become less with higher relaxation strength Δεα, confirming recent reports. Typical for a generic ß-process is the increase of its amplitude above Tg, which is found to follow a power-law behaviour as a function of the ratio τα/τß with a universal exponent; yet, its relative amplitude to that of the α-relaxation varies as does the temporal separation of both processes. The mean activation energy of the ß-process as well as the width of the energy distribution gß(E) increases more or less systematically with Tg. The latter is determined from the dielectric spectra subjected to a scaling procedure assuming a thermally activated process. Plotting gß(E) as a function of the reduced energy scale E/Tg, the distributions are centred between 19-35 and their widths differ by a factor 2-3.

Organic Glasses of High Glass Transition Temperatures Due To Substitution with Nitrile Groups.

The glass transition temperature (Tg) of a molecular glass depends on its molar mass. However, the nature of intermolecular interactions also plays a major role in both the glass transition temperature and its glass-forming ability. In this context, we report on novel molecular glasses containing nitrile groups and investigate the influence of this highly polar group on Tg and the glass-forming ability. As reference compounds, we studied the thermal properties of synthesized molecular glasses with C-C-bonded phenyl rings. The molar mass of the studied compounds ranges from 341 to 568 g/mol. Despite their relatively low molar mass, glass transition temperatures from 347 K (74 °C) to 471 K (198 °C) were observed. Most of the compounds possess high Tg/Tmratios between 0.7 and 0.8. By introducing highly interacting nitrile groups, the dependence of the molar mass on Tg could be increased by a factor of 2-3.