The reorientational behaviour of a nematic monodomain, in a static magnetic field, following the rotation of the mean director through an angle α ≈ π/2 to the magnetic field direction, is investigated by optical polarizing microscopy and proton NMR spectroscopy. Optical evidence supporting a reorientation process through a pure bend transient distortion of the director field is reported for poly (4,4′-dioxy-2,2′-dimethylazoxybenzene nonanediyl), a main-chain nematic thermotropic polymer. The time evolution of the distortion and flow patterns are deduced from a detailed analysis of the NMR lineshapes recorded during the reorientational process. The viscosity coefficients γ₁, α₁, α₂, η_c and the ratio K ₃/K ₁ of the bend to splay Frank elastic constants are determined and discussed in terms of the existing theories.

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.

The magnetic field reorientation of an initially aligned sample of a nematic polymer liquid crystal was followed by proton NMR. Evolution to a metastable (banded) state was considered using a Rheo-NMR technique developed previously. Late stage reorientation was studied by taking into account the dynamics of defects following the formation of splay–bend walls. NMR spectra simulation allowed us to obtain the wall density as a function of time. This result, together with a defect-controlled wall dissolution model proposed by Rey [1], was used to complement the Rheo-NMR technique of measuring the viscoelastic parameters of nematic polymer liquid crystals.

In the present work the rheological characterisation (steady-state and oscillatory data) of a non-chiral low molecular weight liquid-crystalline diol (denoted as HBP 6/6) exhibiting the nematic and smectic phases over a broad temperature range is presented. A non-Newtonian flow behaviour was found. This result, together with the progressive increase of the viscosity observed near the nematic–isotropic transition, suggests that the material behaves like a polymer, due to hydrogen bonding. The effect of an electric field applied normal to the flow direction, on the steady state in the nematic phase, was established. An electrorheological effect shows up, as an enhancement of the apparent viscosity, due to the positive dielectric anisotropy of this material. It was found that the increase of the apparent viscosity scales linearly with the applied field strength.

We study the influence of the anisotropy of the Frank elastic constants on the magnetic reorientation of the nematic phase of polymer liquid crystals. In the magnetic reorientation following a 90° director rotation with respect to an aligning magnetic field, a pattern of inversion walls develops which depends on the relative magnitude of the elastic constants and the magnetic coherence length. We show how this dependence can be experimentally studied by proton NMR. The transition from a homogeneous director reorientation to a distorted director reorientation is theoretically studied as a function of the rotation angle α. A critical angle of rotation α_c shows up, and we study its dependence on the anisotropies K ₃/K ₁ and K ₃/K ₂. Depending on these ratios and on the wavelength of the distortion, critical angles 45° < α_c < 90° are predicted for materials with positive anisotropy of the magnetic susceptibility χ_a and 0° < α_c < 45° (mod π/2) for materials with χ_a < 0. Within the frame of a phase transition analogy, a Landau-like theory predicts the transition to be second order.

The dynamics of nematic director field reorientation in non-Freedericksz geometries, after a magnetic field H is applied at an oblique angle relatively to the initial homogeneous director n 0 (H not normal to n 0), is studied considering a magnetic reorientation driven by hydrodynamic instabilities (with backflow). This study is carried out for bounded samples between two parallel plates with planar boundary conditions and with rigid anchoring. Linear stability and wave vector selection analysis predict that, when the angle of the magnetic to the initial director field is increased, for a given magnetic field intensity, two transitions from a homogeneous to a transient distorted director field reorientation can occur: a transition at a first critical angle to an aperiodic distorted director field and a transition at a second critical angle to a periodic distorted director field. It is shown that the periodic mode is cut off at a higher reduced field when the magnetic field acts away from the normal direction.