ABSTRACT
Critical analysis of the results of studies of molecular rotational dynamics in liquid crystalline substances with the aid of the dielectric spectroscopy (DS) and nuclear magnetic resonance (NMR) is given. Both methods are known to be sensitive to different aspects of molecular rotations: the polarization vector and the relaxation time τ(DS) in the case of DS, a tensor describing a nuclear interaction and the correlation time τ(NMR) for NMR method. Furthermore, both methods provide correlation functions with different rank values. A common basis for the comparison between τ(DS) and τ(NMR) is postulated. Several examples of the temperature dependence of the correlation times coming from the two spectroscopic methods are presented. Qualitative agreements of the correlation times were achieved in most cases.
ABSTRACT
Fluorinated calamitic thermotropic liquid crystals represent an important class of materials for high-tech applications, especially in the field of liquid crystal displays. The investigation of orientational ordering in these systems is fundamental owing to the dependence of their applications on the anisotropic nature of macroscopic optical, dielectric, and visco-elastic properties. NMR spectroscopy is the most powerful technique for studying orientational order in liquid crystalline systems at a molecular level thanks to the possibility of exploiting different anisotropic observables (chemical shift, dipolar couplings, and quadrupolar coupling) and nuclei ((2)H, (13)C, and (19)F). In this paper, the basic theory and NMR experiments useful for the investigation of orientational order on fluorinated calamitic liquid crystals are reported, and a review of the literature published on this subject is given. Finally, orientational order parameters determined by NMR data are discussed in comparison to those obtained by optical and dielectric anisotropy measurements.
ABSTRACT
Orientational order properties of two nematogens containing a fluoro- and isothiocyanate-substituted biphenyl moiety have been investigated by means of (13)C NMR spectroscopy. (13)C NMR spectra acquired on static samples under high-power (1)H-decoupling allowed both (13)C chemical shift anisotropies and (13)C-(19)F couplings to be measured. These data were used to determine the local principal order parameter and biaxiality for the different rigid fragments of the molecules. To this aim, advanced DFT methods for the calculation of geometrical parameters and chemical shift tensors were used. The orientational order parameters obtained by NMR have been critically compared with those obtained by dielectric spectroscopy. Trends of order parameters with temperature have been analyzed in terms of both mean field theory and the empirical Haller equation.
ABSTRACT
To confirm the molten state of the alkyl chain in soft crystalline phase, smectic E (SmE) phase, thermodynamic and spectroscopic analyses were performed on 4-n-alkyl-4'-isothiocyanatobiphenyl (nTCB, n: the number of carbon atoms in the alkyl group). DSC results of 11TCB and 12TCB, having extra smectic A phase besides smectic E phase, show that their chain-length dependence of entropies of transition (Δ(trs)S) from the ordered crystalline (OC) phase to the SmE phase matches the trend found for nTCB (n = 4-10), while no chain-length dependence is observed in Δ(trs)S at the SmE-to-SmA and SmA-to-isotropic liquid (IL) phase transitions in 11TCB and 12TCB. Temperature dependences of FT-IR spectra of six compounds (n = 2, 3, 5, 8, 10, and 12) were recorded. The CH stretching modes of the chain exhibited more pronounced change at the transition from the OC to the SmE phase than at the transition from the SmE phase to the IL or SmA phase. These results indicate that the alkyl chain is molten in the SmE phase as in IL. The disordering process of nTCB molecules from the OC to IL via anisotropic mesophases is discussed in terms of entropy.
ABSTRACT
Molecular rotational dynamics in p,p'-di-n-heptyl-azoxybenzene was studied by means of quasielastic neutron scattering (QENS) and 13C cross-polarization magic-angle-spinning (CPMAS) NMR. Fast reorientation of the hydrogen nuclei was observed by QENS in the two liquid crystalline (LC) phases nematic and smectic A, as well as in the crystalline phase. The latter could not be restricted to the -CH3 rotations alone, and a clear indication was found of some other reorientation motions persisting in the crystal. Two Lorentz-type components convoluted with the resolution function gave an excellent fit to the QENS spectra in both LC phases. The narrow (slow) component was attributed to the reorientation of the whole molecule around the long axis. The corresponding characteristic time of approximately 130 ps agreed well with the values obtained in recent dielectric relaxation and 2H NMR studies. The full width at half maximum of the broader (fast) component shows a quadratic Q dependence (Q is the momentum transfer). Hence the corresponding motions could be described by a stretched exponential correlation function and were interpreted as various "crankshaft-type" motions within the alkyl tails. The 13C CPMAS experiments fully corroborated the QENS results, sometimes considered ambiguous in complex systems.
ABSTRACT
The low frequency relaxation times tau//, which characterize the flip-flop molecular motions in liquid crystalline phases, recently determined in high-pressure experiments for eight liquid crystalline substances, were reanalyzed considering a relation proposed for the glass-forming liquids [C. Dreyfus, Phys. Rev. E 68, 011204 (2003); R. Casalini and C. M. Roland, Phys. Rev. E 69, 062501 (2004)]. The data, measured at constant pressure, constant temperature, and constant molar volume, could be rescaled onto a master line in the ln tau// vs 1/(T V(gamma)m) plot, with gamma as an adjustable parameter (Vm=1/rho is the specific volume). The obtained gamma values are in good agreement with other estimations; here, the value of gamma parameter was determined for the crystal-like smectic- E phase.
