Study of secondary relaxation in disordered plastic crystals of isocyanocyclohexane, cyanocyclohexane, and 1-cyanoadamantane

Non-Debye to Debye spectral shift in solid solutions of orientationally disordered crystals

Debye relaxation is a simple and unique physical mechanism by which a macroscopic orientational polarization decays monoexponentially with time. However, the very existence of the Debye process in complex systems such as water, aqueous solutions, and monohydroxyl alcohols, among others, is puzzling to date and their microscopic origin is still ambiguously explained. In order to shed light on some of these aspects, orientational dynamics of an orientationally disordered dipolar crystal with an identically structured nonpolar matrix has been studied in the form of solid solutions. A crossover from non-Debye to Debye-type spectral behavior has been observed with increasing concentration of the nonpolar matrix in the solid solutions. Analysis of the dynamic response shows that the evolution of cooperativity and spatial heterogeneity with concentration of nonpolar matrix is responsible for the observed trends. The results not only authenticate a possible mechanism of the Debye process as originating from localized orientational fluctuations due to molecular dipoles but also shed light on the evolution of non-Debye characteristics in these systems.

Dynamics in the plastic crystalline phase of cyanocyclohexane and isocyanocyclohexane probed by 1H field cycling NMR relaxometry

Proton Field-Cycling (FC) nuclear magnetic resonance (NMR) relaxometry is applied over a wide frequency and temperature range to get insight into the dynamic processes occurring in the plastically crystalline phase of the two isomers cyanocyclohexane (CNCH) and isocyanocyclohexane. The spin-lattice relaxation rate, R₁(ω), is measured in the 0.01-30 MHz frequency range and transformed into the susceptibility representation χ_NMR ^″ω=ωR₁ω. Three relaxation processes are identified, namely, a main (α-) relaxation, a fast secondary (β-) relaxation, and a slow relaxation; they are very similar for the two isomers. Exploiting frequency-temperature superposition, master curves of χ_NMR ^″ωτ are constructed and analyzed for different processes. The α-relaxation displays a pronounced non-Lorentzian susceptibility with a temperature independent width parameter, and the correlation times display a non-Arrhenius temperature dependence-features indicating cooperative dynamics of the overall reorientation of the molecules. The β-relaxation shows high similarity with secondary relaxations in structural glasses. The extracted correlation times well agree with those reported by other techniques. A direct comparison of FC NMR and dielectric master curves for CNCH yields pronounced difference regarding the non-Lorentzian spectral shape as well as the relative relaxation strength of α- and β-relaxation. The correlation times of the slow relaxation follow an Arrhenius temperature dependence with a comparatively high activation energy. As the α-process involves liquid-like isotropic molecular reorientation, the slow process has to be attributed to vacancy diffusion, which modulates intermolecular dipole-dipole interactions, possibly accompanied by chair-chair interconversion of the cyclohexane ring. However, the low frequency relaxation features characteristic of vacancy diffusion cannot be detected due to experimental limitations.

Voltammetric monitoring of a solid-liquid phase transition in N,N,N′,N′-tetraoctyl-2,6-diamino-9,10-anthraquinone (TODAQ)

Exploratory experiments on effects from a phase transition are reported for a low-melting microcrystalline anthraquinone (N,N,N′,N′-tetraoctyl-2,6-diamino-9,10-anthraquinone or TODAQ). Data for the solid-liquid phase transition are obtained by differential scanning calorimetry and then compared to data obtained by voltammetry. In preliminary electrochemical measurements, microcrystal deposits on a basal plane pyrolytic graphite electrode are shown to undergo a solid-state 2-electron 2-proton reduction in contact to aqueous 0.1 M HClO4 with a midpoint potential Emid,solid = − 0.24 V vs. SCE. The reduction mechanism is proposed to be limited mainly by the triple phase boundary line and some transport of TODAQ molecules towards the electrode surface for both solid and melt. A change in the apparent activation energy for this reduction is observed at 69 °C, leading to an enhanced increase in reduction current with midpoint potential Emid,liquid = − 0.36 V vs. SCE. A change of TODAQ transport along the crystal surface for solid microcrystalline material (for the solid) to diffusion within molten microdroplets (for the liquid) is proposed. Upon cooling, a transition at 60 °C back to a higher apparent activation energy is seen consistent with re-solidification of the molten phase at the electrode surface. Differential scanning calorimetry data for solid TODAQ dry and for TODAQ in contact to aqueous 0.1 M HClO4 confirm these transitions.

Cooperativity in plastic crystals

A statistical mechanical model previously adopted for the analysis of the α-relaxation in structural glass formers is rederived within a general theoretical framework originally developed for systems approaching the ideal glassy state. The interplay between nonexponentiality and cooperativity is reconsidered in the light of energy landscape concepts. The method is used to estimate the cooperativity in orientationally disordered crystals, either from the analysis of literature data on linear dielectric response or from the enthalpy relaxation function obtained by temperature-modulated calorimetry. Knowledge of the specific heat step due to the freezing of the configurational or conformational modes at the glass transition is needed in order to properly account for the extent to which the relaxing system deviates from equilibrium during the rearrangement processes. A number of plastic crystals have been analyzed, and relatively higher cooperativities are found in the presence of hydrogen bonding interaction.

Anhydrosaccharides-A new class of the fragile plastic crystals

In this paper, 1,6-anhydro-β-D-glucopyranose (anhGLU), 1,6-anhydro-β-D-mannopyranose (anhMAN), and 1,6-anhydro-β-D-galactopyranose (anhGAL), three new materials that form the Orientationally Disordered Crystal (ODIC) phase, have been thoroughly investigated using various experimental techniques. All measurements clearly indicated that these compounds possess a series of very interesting physical properties that are considerably different than those reported for ordinary plastic crystals. X-Ray diffraction investigations have revealed enormously long-range static correlations between molecules, reaching even 120 Å. Moreover, dielectric studies showed that besides Freon 113, the investigated anhydrosaccharides are the most fragile systems that form the ODIC phase. Further analysis of Fourier transform infrared spectra indicated that such peculiar behavior of anhydrosaccharides might be closely related to multidirectional H-bonds of various strengths that most likely affect the number of available conformations, density states, and the potential barriers in the energy landscape of these compounds. This is consistent with the results from previous reports [L. C. Pardo, J. Chem. Phys. 124, 124911 (2006) and Th. Bauer et al., J Chem. Phys. 133, 144509 (2010)] showing that the higher fragility of Freon 112 as well as a mixture of 60% succinonitrile and 40% glutaronitrile (60SN-40GN) can be closely related to the enhanced conformational ability and additional disorder introduced by various substituents, which further make energy landscape more complex. Finally, by studying the properties of 2,3,4-tri-O-acetyl-1,6-anhydro-β-D-glucopyranose (ac-anhGLU) it was found that besides the shape of the molecules, H-bonds or generally strong intermolecular interactions are extremely important parameters contributing to the ability to form the plastic phase. This is in line with current observations that in most cases the ODIC phase is created in highly interacting compounds.

Phase Behavior and Dynamics of the Liquid Crystal 4'-butyl-4-(2-methylbutoxy)azoxybenzene (4ABO5*)

The results of dielectric relaxation spectroscopy and polarizing microscope observation of the 4'-butyl-4-(2-methylbutoxy)azoxybenzene (abbreviated as 4ABO5*) are presented. Numerical analysis of the dielectric spectra results points to complex dynamics of 4ABO5* molecules in isotropic, cholesteric, and crystalline phases. Two well-separated maxima on the imaginary part of dielectric permittivity and the third low frequency relaxation process, hidden in the conductivity region, were detected and described in cholesteric and crystalline phases. Temperature dependence of mean relaxation times characterizing flip-flop motions and rotation around long axes, observed in all phases, is of the Arrhenius type.

Primary and secondary relaxation process in plastically crystalline cyanocyclohexane studied by 2H nuclear magnetic resonance. I

We study the main (α-) and secondary (β-) relaxation in the plastically crystalline (PC) phase of cyanocyclohexane by various 2H nuclear magnetic resonance (NMR) methods (line-shape, spin-lattice relaxation, stimulated echo, and two-dimensional spectra) above and below the glass transition temperature T(g) = 134 K. Our results regarding the α-process demonstrate that molecular motion is not governed by the symmetry of the lattice. Rather it is similar to the one reported for structural glass formers and can be modeled by a reorientation proceeding via a distribution of small and large angular jumps. A solid-echo line-shape analysis regarding the β-process below T(g) yields again very similar results when compared to those of the structural glass formers ethanol and toluene. Hence we cannot confirm an intramolecular origin for the β-process in cyanocyclohexane. The fast β-process in the PC phase allows for the first time a detailed 2H NMR study of the process also at T > T(g): an additional minimum in the spin-lattice relaxation time reflecting the β-process is found. Furthermore the solid-echo spectra show a distinct deviation from the rigid limit Pake pattern, which allows a direct determination of the temperature dependent spatial restriction of the process. In Part II of this work, a quantitative analysis is carried out, where we demonstrate that within the model of a "wobbling in a cone" the mean cone angle increases above T(g) and the corresponding relaxation strength is compared to dielectric results.

Broadband dielectric spectroscopic, calorimetric, and FTIR-ATR investigations of D-arabinose aqueous solutions

The dielectric relaxation behavior of D-arabinose aqueous solutions at different water concentrations is examined by broadband dielectric spectroscopy in the frequency range of 10(-2) -10(7) Hz and in the temperature range of 120-300 K. Differential scanning calorimetry is also performed to find the glass transition temperatures (T(g)). In addition, the same solutions are analyzed by Fourier transform infrared (FTIR) spectroscopy using the attenuated total reflectance (ATR) method at the same temperature interval and in the frequency range of 3800-2800 cm(-1). The temperature dependence of the relaxation times is examined for the different weight fractions (x(w)) of water along with the temperature dependence of dielectric strength. Two relaxation processes are observed in the aqueous solutions for all concentrations of water. The slower process, the so-called primary relaxation process (process-I), is responsible for the T(g) whereas the faster one (designated as process-II) is due to the reorientational motion of the water molecules. As for other hydrophilic water solutions, dielectric data for process-II indicate the existence of a critical water concentration above which water mobility is less restricted. Accordingly, FTIR-ATR measurements on aqueous solutions show an increment in the intensity (area) of the O-H stretching sub-band close to 3200 cm(-1) as the water concentration increases.