Rigid molecular glass-formers with no internal degrees of freedom nonetheless have a single secondary β-relaxation. For a rigid and planar molecule, 1-methylindole (1MID), although a secondary relaxation is resolved at ambient pressure, its properties do not conform to the rules established for rigid molecules reported in early studies. By applying pressure to the dielectric spectra of 1MID, we find the single secondary relaxation splits into two. The slower one is pressure sensitive showing connections to the α-relaxation as observed in other rigid molecules, while the faster one is almost pressure insensitive and dominate the loss at ambient pressure. The two secondary relaxations, identified to associate with the out-of-plane and in-plane rotations of the rigid and planar 1MID, are resolved and observed for the first time by increasing density via elevating pressure.

In a series of papers on binary glass-forming mixtures of tripropyl phosphate (TPP) with polystyrene (PS), Kahlau et al. [J. Chem. Phys. 140, 044509 (2014)] and Bock et al. [J. Chem. Phys. 139, 064508 (2013); J. Chem. Phys. 140, 094505 (2014); and J. Non-Cryst. Solids 407, 88-97 (2015)] presented the data on the dynamics of the two components studied over the entire composition range by several experimental methods. From these sets of data, obtained by multiple experimental techniques on mixtures with a large difference ΔT ≈ 200 K between the glass transition temperatures of two starting glass formers, they obtained two α-relaxations, α1 and α2. The temperature dependence of the slower α1 is Vogel-Fulcher like, but the faster α2 is Arrhenius. We have re-examined their data and show that their α2-relaxation is the Johari-Goldstein (JG) β-relaxation with Arrhenius T-dependence admixed with a true α2-relaxation having a stronger temperature dependence. In support of our interpretation of their data, we made dielectric measurements at elevated pressures P to show that the ratio of the α1 and α2 relaxation times, τ(T,P)/τ(T,P), is invariant to variations of T and P, while τ(T,P) is kept constant. This property proves unequivocally that the α2-relaxation is the JG β-relaxation, the precursor of the α1-relaxation. Subsequently, the true but unresolved α2-relaxation is recovered, and its relaxation times with much stronger temperature dependence are deduced, as expected for the α-relaxation of the TPP component. The results are fully compatible with those found in another binary mixture of methyltetrahydrofuran with tristyrene and PS with ΔT ≈ 283 K, even larger than ΔT ≈ 200 K of the mixture of TPP with PS, and in several polymer blends. The contrast between the two very different interpretations brought out in this paper is deemed beneficial for further progress in this research area.

Understanding the glass transition requires getting the picture of the dynamical processes that intervene in it. Glass-forming liquids show a characteristic decoupling of relaxation processes when they are cooled down towards the glassy state. The faster (βJG) process is still under scrutiny, and its full explanation necessitates information at the microscopic scale. To this aim, nuclear γ-resonance time-domain interferometry (TDI) has been utilized to investigate 5-methyl-2-hexanol, a hydrogen-bonded liquid with a pronounced βJG process as measured by dielectric spectroscopy. TDI probes in fact the center-of-mass, molecular dynamics at scattering-vectors corresponding to both inter- and intra-molecular distances. Our measurements demonstrate that, in the undercooled liquid phase, the βJG relaxation can be visualized as a spatially-restricted rearrangement of molecules within the cage of their closest neighbours accompanied by larger excursions which reach out at least the inter-molecular scale and are related to cage-breaking events. In-cage rattling and cage-breaking processes therefore coexist in the βJG relaxation.

We study amorphous solid dispersions (ASDs) of the chloramphenicol antibiotic in two biodegradable polylactic acid polymers, namely a commercial sample of enantiomeric pure PLLA and a home-synthesized PDLLA copolymer, to study the effect of polylactic acid in stabilizing the amorphous form of the drug and controlling its release (e.g. for antitumoral purposes). We employ broadband dielectric spectroscopy and differential scanning calorimetry to study the homogeneity, glass transition temperature and relaxation dynamics of solvent-casted ASD membranes with different drug concentrations. We observe improved physical stability of the ASDs with respect to the pure drug, as well as a plasticizing effect of the antibiotic on the polymer, well described by the Gordon-Taylor equation. We study the release of the active pharmaceutical ingredient from the films in a simulated body fluid through UV/vis spectroscopy at two different drug concentrations (5 and 20% in weight), and find that the amount of released drug is proportional to the square root of time, with proportionality constant that is almost the same in both dispersions, despite the fact that the relaxation time and thus the viscosity of the two samples at body temperature differ by four orders of magnitude. The drug release kinetics does not display a significant dependence on the drug content in the carrier, and may thus be expected to remain roughly constant during longer release times.

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