Short time dynamics of glass‐forming liquids

Proton Spin-Lattice Relaxation in Organic Molecular Solids: Polymorphism and the Dependence on Sample Preparation

We report solid-state nuclear magnetic resonance ¹ H spin-lattice relaxation, single-crystal X-ray diffraction, powder X-ray diffraction, field emission scanning electron microscopy, and differential scanning calorimetry in solid samples of 2-ethylanthracene (EA) and 2-ethylanthraquinone (EAQ) that have been physically purified in different ways from the same commercial starting compounds. The solid-state ¹ H spin-lattice relaxation is always non-exponential at high temperatures as expected when CH₃ rotation is responsible for the relaxation. The ¹ H spin-lattice relaxation experiments are very sensitive to the "several-molecule" (clusters) structure of these van der Waals molecular solids. In the three differently prepared samples of EAQ, the relaxation also becomes very non-exponential at low temperatures. This is very unusual and the decay of the nuclear magnetization can be fitted with both a stretched exponential and a double exponential. This unusual result correlates with the powder X-ray diffractometry results and suggests that the anomalous relaxation is due to crystallites of two (or more) different polymorphs (concomitant polymorphism).

Monitoring a simple hydrolysis process in an organic solid by observing methyl group rotation

We report a variety of experiments and calculations and their interpretations regarding methyl group (CH₃) rotation in samples of pure 3-methylglutaric anhydride (1), pure 3-methylglutaric acid (2), and samples where the anhydride is slowly absorbing water from the air and converting to the acid [C₆H₈O₃(1) + H₂O → C₆H₁₀O₄(2)]. The techniques are solid state ¹H nuclear magnetic resonance (NMR) spin-lattice relaxation, single-crystal X-ray diffraction, electronic structure calculations in both isolated molecules and in clusters of molecules that mimic the crystal structure, field emission scanning electron microscopy, differential scanning calorimetry, and high resolution ¹H NMR spectroscopy. The solid state ¹H spin-lattice relaxation experiments allow us to observe the temperature dependence of the parameters that characterize methyl group rotation in both compounds and in mixtures of the two compounds. In the mixtures, both types of methyl groups (that is, molecules of 1 and 2) can be observed independently and simultaneously at low temperatures because the solid state ¹H spin-lattice relaxation is appropriately described by a double exponential. We have followed the conversion 1 → 2 over periods of two years. The solid state ¹H spin-lattice relaxation experiments in pure samples of 1 and 2 indicate that there is a distribution of NMR activation energies for methyl group rotation in 1 but not in 2 and we are able to explain this in terms of the particle sizes seen in the field emission scanning electron microscopy images.

Segmental Relaxation in Crosslinked Rubber

Studies of the local segmental relaxation in rubbery networks reveal a variety of behaviors. In experiments on networks with labeled junctions, whereby the motion of the crosslink site is specifically monitored, the segmental relaxation function broadens, accompanied by a larger activation energy, in a manner well-described by the coupling model of relaxation. The more usual experiment simply measures bulk relaxation, without discriminating among different relaxing entities. For networks, crosslinking introduces a distribution of relaxation behaviors, related to the proximity of a moiety to the junctions. The resulting inhomogeneously broadened relaxation function is difficult to analyze; nevertheless, a heightened sensitivity to temperature (larger activation energy) is exhibited, from which inferences can be made regarding the shape of the relaxation function. Finally, the segmental relaxation of highly crosslinked microgels is ostensibly homogeneous. Interestingly, however, the inverse correlation between the stretch exponent, β, and the activation energy, observed quantitatively in conventional networks, is violated by the microgels.

Dielectric Spectroscopy at High Frequencies on Glass Forming Liquids

Dielectric spectroscopy up to 950 GHz has been performed on various glass formers as glycerol, propylene-carbonate, and Salol. Special attention is given to the dielectric loss, ε″, in the crossover regime from the a-relaxation to the far-infrared (FIR) response where it can be directly compared to the dynamic susceptibilities obtained by neutron and light scattering techniques. We observe a minimum in ε″(ν) at high frequencies which cannot be explained by a simple transition from a-relaxation peak to the FIR bands but has to be attributed to additional fast processes. In all materials investigated, ε″(ν) increases significantly sublinear above the minimum. The ratio of the intensity of the α-process and the fast process as determined from our dielectric experiments is significantly higher compared to the results from the scattering experiments.