Experimental study of the director pattern dynamics in the vicinity of the Fréedericksz twist geometry

We present an experimental study of the transient periodic structures appearing in the nematic director field in the magnetically induced reorientation of the director in the vicinity of the twist Fréedericksz geometry. Thin nematic samples (50?µm thick) were exposed to magnetic fields of variable intensity and orientation relative to the surface aligning direction of the sample. The director reorientation was induced by a rapid rotation of the sample in the static magnetic field producing a misalignment between the director and the magnetic field. The director field was optically monitored during the reorientation process and the transient periodic structures were characterized. Two types of periodic structures could be identified, namely bands and walls. Walls grow from bands close to the twist Fréedericksz geometry. The time dependence of the wave length and inclination of the periodic structures was obtained as a function of the magnetic field intensity and orientation relative to the surface aligning direction of the sample. The results for the bands are compared with the predictions of a model that we specifically developed to account for the non-orthogonal field orientations. It is seen that our model can account rather well for the experimental results considering that it uses only the field rotation time as adjustable parameter. All other model parameters are known.     

On the Influence of Elastic Modes on the N.M.R. Lineshapes of Polymer Liquid Crystals

Abstract

We have studied the influence of thermally excited orientational fluctuations on the N.M.R. lineshape of a nematic monodomain. The influence is characterized by a static order parameter S _stat whose theoretical expression in terms of viscoelastic parameters is derived. This model is applied to the proton N.M.R. spectrum of a thermotropic main chain polymer. The values of S _stat are deduced from the study of the changes of the N.M.R. lineshape as a function of the angle between the static magnetic field and the nematic director. Good agreement is obtained with theoretical values calculated using estimated values of the Leslie viscosity coefficients and the Frank elastic constants. The importance of an accurate knowledge of S _stat for a number of problems is stressed.     

Viscoelastic Parameters of a Side-Chain Nematic Polymer: II—Analysis of NMR Results in Terms of the Anisotropic Dumbbell Model

The two viscoelastic parameters of a siloxane side-chain nematic polymer—twist viscosity coefficient, and static order parameter—deduced from the NMR study of Reference 1, are discussed in terms of the anisotropic dumbbell model of Brochard using structural information obtained by small angle neutron scattering. It is found that this model, originally developed for dilute solutions of flexible polymers in low molecular mass nematics, describes satisfactorily the data, supporting the idea that the backbone polymer chains are not entangled in the nematic melt. An average elastic constant is deduced. This constant is found to increase sharply as the nematic-smectic A phase is approached. The twist elastic constant is calculated using splay and bend elastic constant values of a very similar polymer. Some (possibly only apparent) difficulties associated with the dumbbell model are pointed out.     

Viscoelastic Parameters of a Side-Chain Nematic Polymer: I—Results of a Proton NMR Study

The reorientational behaviour in a static magnetic field of a nematic siloxane side-chain polymer is investigated by proton NMR. At variance with what occurs in main-chain and lyotropic nematic polymers, the return to equilibrium of a tnonodomain whose mean director has been rotated at an angle α to the field, is found to be homogeneous for all angles a between 0 and π/2. The usual nematic order parameter and the static (on the proton NMR time scale) order parameter are deduced from analysis of the equilibrium lineshapes. The twist viscosity γ₁ is deduced from the study of time dependent lineshapes following the rotations α. The temperature dependence of γ₁ is discussed in terms of the Vogel equation γ₁ = B S ^β exp(E _β/R(T - T _o}). It is not possible to discriminate between β = 1 or 2, but in both cases, the freezing temperature of the director T_o is found to be non-zero. A nematic to smectic pretransitional effect on γ₁ is observed. The information concerning the viscoelastic parameters contained in the value of the static order parameter is also discussed.     

On the effect of an applied magnetic field on the shear-induced pattern formation in nematic polymer liquid crystals

The effect of an applied magnetic field and of the boundaries on the stability of the shear flow of nematic polymer liquid crystals of the tumbling type with respect to the formation of the transient spatially periodic patterns that appears after the start-up of shearing is studied. The unidirectional shear of a nematic director initially uniformly oriented orthogonally to the sample plane and with strong anchoring is considered. The magnetic field is applied orthogonally to the sample plane or in the same direction of the flow. For a given value of the Ericksen number of the flow a critical value of the magnetic-to-viscous energy ratio shows up above which the uniform flow is stable. When the influence of the boundaries may be neglected, an internal Ericksen number that stays constant in a given material is considered such that the wavelength of the periodic pattern is a self-adjusted length. An internal generalized Ericksen number is defined such that it is a universal function of the magnetic-to-viscous energy ratio.To the memory of my mother     

On the Influence of Elastic Modes on the N.M.R. Lineshapes of Polymer Liquid Crystals

We have studied the influence of thermally excited orientational fluctuations on the N.M.R. lineshape of a nematic monodomain. The influence is characterized by a static order parameter S_stat whose theoretical expression in terms of viscoelastic parameters is derived. This model is applied to the proton N.M.R. spectrum of a thermotropic main chain polymer. The values of S_stat are deduced from the study of the changes of the N.M.R. lineshape as a function of the angle between the static magnetic field and the nematic director. Good agreement is obtained with theoretical values calculated using estimated values of the Leslie viscosity coefficients and the Frank elastic constants. The importance of an accurate knowledge of S_stat for a number of problems is stressed.     

Fréedericksz transition thresholds in pretilted nematic slabs

The appearance of periodically distorted director structures in pretilted nematic slabs with rigid boundary conditions above the Fréedericksz transition is determined as a function of the Frank elastic constant ratios K(2)/K(1), K(3)/K(1), and the pretilt angle. It is found that the periodically distorted state can be reached either directly from the undistorted state on increasing the magnetic field or indirectly through the homogeneously distorted state. A new threshold field for the transition undistorted state, homogeneously distorted state was found for specific ranges of the Frank elastic constant ratios K(2)/K(1), K(3)/K(1), and the pretilt angle.