Designing the disorder: the kinetics of nonisothermal crystallization of the orientationally disordered crystalline phase in a nematic mesogen

Exploring Crystal-Phase Molecular Dynamics of the Low-Viscous Mesogen 6CHBT: A Combined FFC and High-Field NMR Relaxometry Investigation

The molecular dynamics study of thermotropic mesogens exhibiting the crystal phases is valuable in unraveling the complex global (collective) and local (noncollective) motions executed by liquid crystal molecules, which would further advance the existing knowledge on orientationally disordered crystalline (ODIC) phases. Toward the fulfillment of such a task, a combined nuclear magnetic resonance (NMR) relaxometry approach employing the fast field cycling (FFC) NMR (10 kHz-30 MHz) and high-field pulsed NMR (400 MHz) techniques is utilized to sample the broad frequency range offered by molecular motions in the crystal phase of 4-(trans-4'-n-hexylcyclohexyl)-isothiocyanatobenzene (6CHBT). The validity of the observed relaxation data is tested and interpreted by the Bloembergen-Purcell-Pound (BPP) model involving the superposition of four mutually independent Lorentzian spectral densities, reflecting molecular dynamical processes on different time scales. The salient feature of the detailed analysis reveals that the lengthening of temporal dynamics in the crystal phase due to molecular rotations by jumps, which are of intermolecular origin, is evident and further supports the presence of collective-like local dynamics. The analysis does permit decoupling of the molecular reorientations about their short axes (∼100 ns) as well as long axes (∼50 ns) and methyl group rotations (∼0.5 ns) on distinct time scales. The activation energies for reorientations about the short axes and methyl group rotations are found to be 27.3 ± 2.7 and 15.8 ± 1.1 kJ/mol, respectively. The fast methyl rotations in the crystal phase of 6CHBT obtained from FFC NMR are further well complemented by high-field NMR, where 1H NMR line shapes are relatively narrow when compared to those of the nematic phase.

Design of Crystal Growth Dimensionality in Synthetic Wax: The Kinetics of Nonisothermal Crystallization Processes

The demand for the development of multifunctional materials in emerging technologies has stimulated intensive research on the control of crystallization processes in numerous scientific and engineering fields. In this article, we examine the kinetics of nonisothermal melt crystallization in synthetic wax using differential scanning calorimetry (DSC) supported by polarized optical microscopy (POM) to describe crystallization modes in a multicomponent molecular system. We detected the macroscopic growth of three crystal phases and the formation of two crystal phases as a transformation from a disordered crystal mesophase into an ordered crystal. To characterize individual crystal phase formation, we examine the activation energy evaluated by isoconversional analysis and utilize the Ozawa and Mo methods to determine the kinetic details of the crystal growth from the isotropic phase. Our investigation reveals the possibility of the design of crystal growth dimensionality as three-dimensional spherulitic-like, two-dimensional rodlike, and one-dimensional needle-shaped crystal forms of shorter n-alkanes by controlling the solidification pathway of long-chain n-alkanes and the interplay of the thermodynamic and kinetic mechanisms of crystallization.

On relaxation and vibrational dynamics in the thermodynamic states of a chiral smectogenic glass-former

This article shows the full characteristics (i.e., the phase situation as well as the relaxation and vibrational dynamics) of the (S)-4'-(1-methyloctyloxycarbonyl)biphenyl-4-yl 4-[5-(2,2,3,3,4,4,4-heptafluorobutoxy)pentyl-1-oxy]-benzoate chiral liquid crystal. Besides two enantiotropic chiral smectic phases (SmC* and ), the compound under study also forms the hexatic smectic phase and two crystal phases (Cr1 and Cr2). The XRD patterns imply a similar structure of both crystal phases. The sample crystallizes upon slow cooling, while the phase undergoes a glass transition during fast cooling. Upon subsequent heating, cold crystallization is observed. Our research reveals the complex relaxation dynamics in the identified thermodynamic states, e.g., two relaxations up to the beginning of cold crystallization, three modes in the crystal phases and seven processes in all smectic phases. The results from the scaling of the dielectric response indicate that the origin of the dynamics and behavior of the dielectric permittivity is the same for all phases, regardless of the change in temperature and/or external biasing field. The high value of the fragility index (m_f ≈ 146) indicates that the compound under study is a fragile glass-forming system. The region of the -COO- and -COC- group stretching vibrations is primarily sensitive to the structural changes occurring during phase transitions.

Halogen Bonding in Dithiane/Iodofluorobenzene Mixtures: A New Class of Hydrophobic Deep Eutectic Solvents

While research into deep eutectic solvents (DESs) has expanded over the previous two decades, the focus has remained on the utilization of hydrogen bond donors in these systems. Additionally, the majority of the known DESs rely on at least one ionic component. Through the combination of 1,3-dithiane and 1,2-diiodo-3,4,5,6-tetrafluorobenzene (1,2-F₄ DIB), we report the first known DES based on halogen bonding. This mixture remains a liquid, with a eutectic melting temperature of 13.7 °C over a range of 1,3-dithiane mole fraction (0.35 to 0.75). Additionally, cocrystals of 1,3- and 1,4-dithiane with 1,2-, 1,3-, and 1,4-F₄ DIB, as well as 1,3,5-trifluoro-2,4,6-triiodobenzene were studied via single-crystal X-ray diffraction. These data reveal a wide range of halogen bonding strengths (0.85<R_XB <0.99; R_XB =normalized halogen bond distance parameter) and geometries about the sulfur atom. By including intermolecular interactions beyond hydrogen bonding, the scope of possible DES systems can be greatly expanded.