Non-polymeric asymmetric binary glass-formers. II. Secondary relaxation studied by dielectric, 2H NMR, and 31P NMR spectroscopy.

We investigate the secondary (ß-) relaxations of an asymmetric binary glass former consisting of a spirobichroman derivative (SBC; Tg = 356 K) as the high-Tg component and the low-Tg component tripropyl phosphate (TPP; Tg = 134 K). The main relaxations are studied in Paper I [B. Pötzschner et al., J. Chem. Phys. 146, 164503 (2017)]. A high Tg contrast of ΔTg = 222 K is put into effect in a non-polymeric system. Component-selective studies are carried out by combining results from dielectric spectroscopy (DS) for mass concentrations cTPP ≥ 60% and those from different methods of 2H and 31P NMR spectroscopy. In the case of NMR, the full concentration range (10% ≤ cTPP ≤ 100%) is covered. The neat components exhibit a ß-relaxation (ß1 (SBC) and ß2 (TPP)). The latter is rediscovered by DS in the mixtures for all concentrations with unchanged time constants. NMR spectroscopy identifies the ß-relaxations as being alike to those in neat glasses. A spatially highly restricted motion with angular displacement below ±10° encompassing all molecules is involved. In the low temperature range, where TPP shows the typical 31P NMR echo spectra of the ß2-process, very similar spectral features are observed for the (deuterated) SBC component by 2H NMR, in addition to its "own" ß1-process observed at high temperatures. Apparently, the small TPP molecules enslave the large SBC molecules to perform a common hindered reorientation. The temperature dependence of the spin-lattice relaxation time of both components is the same and reveals an angular displacement of the SBC molecules somewhat smaller than that of TPP, though the time constants τß2 are the same. Furthermore, T1(T) of TPP in the temperature region of the ß2-process is absolutely the same as in the mixture TPP/polystyrene investigated previously. It appears that the manifestations of the ß-process introduced by one component are essentially independent of the second component. Finally, at cTPP ≤ 20% one finds indications that the ß2-process starts to disintegrate. More and more TPP molecules get immobilized upon decreasing cTPP. We conclude that the ß-process is a cooperative process.

Non-polymeric asymmetric binary glass-formers. I. Main relaxations studied by dielectric, 2H NMR, and 31P 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-Tg component tripropyl phosphate (TPP, Tg = 134 K) and of a specially synthesized (deuterated) spirobichroman derivative (SBC, Tg = 356 K) as the non-polymeric high-Tg component for the full concentration range. A large Tg 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-Tg 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-Tg component SBC. Its correlation function is Kohlrausch-like as in neat glass formers. The corresponding time constants and consequently their glass transition temperatures Tg1 and Tg2 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 (Tg2 < T < Tg1). At T < Tg2 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-Tg 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 (ß-) relaxations of the mixtures.

Corticolimbic brain reactivity to social signals of threat before and after sertraline treatment in generalized social phobia.

BACKGROUND: Generalized social phobia (gSP), also known as generalized social anxiety disorder, is characterized by excessive fear of scrutiny by others and pervasive avoidance of social interactions. Pathophysiologic models of gSP implicate exaggerated reactivity of the amygdala and insula in response to social evaluative threat, making them plausible targets for treatment. Although selective serotonin reuptake inhibitor (SSRI) treatment is known to be an effective treatment, little is known about the mechanism through which these agents exert their anxiolytic effects at a brain level in gSP. METHODS: We acquired functional magnetic resonance imaging data of brain response to social signals of threat (fearful/angry faces) in 21 gSP patients before and after they completed 12 weeks of open-label treatment with the SSRI sertraline. For comparison, 19 healthy control (HC) subjects also underwent two functional magnetic resonance imaging scans, 12 weeks apart. RESULTS: Whole-brain voxelwise analysis of variance revealed significant Group×Time interactions in the amygdala and the ventral medial prefrontal cortex. Follow-up analyses showed that treatment in gSP subjects reduced amygdala reactivity to fearful faces (which was exaggerated relative to HCs before treatment) and enhanced ventral medial prefrontal cortex activation to angry faces (which was attenuated relative to HCs before treatment). However, these brain changes were not significantly related to social anxiety symptom improvement. CONCLUSIONS: SSRI treatment response in gSP is associated with changes in a discrete limbic-paralimbic brain network, representing a neural mechanism through which SSRIs may exert their actions.