We propose a simple NMR technique, and the underlying theory, to evaluate, with high accuracy, four out of the five viscosity coefficients and the ratio $\frac{k_{33}}{k_{11}}$ of the bend to splay Frank elastic constants, in nematic polymers. Quantitative data on the viscosities ${\gamma }_1$, ${\alpha }_1$, ${\alpha }_2$, and ${\eta }_c$ and the ratio $\frac{k_{33}}{k_{11}}$ are given for the first time for a thermotropic nematic polymer. The material studied is poly (4,4′-dioxy-2,2′-dimethyl azoxybenzenedodecanediyl), a linear polyester usually labeled DDA9.

We have measured the twist viscosity Y′(T) by the rotating field method, over the entire nematic range of the 4,4′-di-n-alkoxyazoxybenznes (mOAB) from m = 2 to m = 7. The results of these measurements are presented here together with a new development of our previous theory of γ₁(T) which accounts for the free volume variation with temperature and gives the following main result: where S is the order parameter, ϵS is the height of the Maier-Saupe potential, T_O is the temperature of motional “freezing” of the nematic director, and g and ø are nearly constant parameters which may be either computed from other physical data or extracted from the data discussed here. This the oretical result fits very well to the experimental data over the entire nematic range of all the above mentioned materials.

A theoretical analysis of the order parameter and temperature dependence of the complete set of five independent viscosities of incompressible nematic liquid crystals is presented. It is an extension of a previous theory by the authors on the thermal dependence of the twist viscosity in nematics (Port. Phys. 9 (1975) 129 and Mol. Cryst. Liq. Cryst. 66 (1981) 133) and develops from the concept of free volume and the theory of rate pro-cesses. Theoretical expressions are derived for each of the five viscosities in terms of the order parameter S(T), the current temperature T, eight parameters which depend on molecular properties, and some fixed temperature T0 below which viscous molecular motion is not possible due to the lack of enough free volume. These expressions are contrasted with the experimental data available for M.B.B.A. and all of them are found to be in good agreement with the data, over the full nematic range of this material

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.

Simulations of nematic and smectic mesophases based on a dissipative particle dynamics approach are discussed. Mesogenic units are built in the form of standard semirigid bead-spring chains. It is shown that nematic phases can be formed for chains containing at least eight beads, provided that the conservative soft-repulsive potential between nonconnected beads is sufficiently strong. Smectic phases are observed only by modifying the repulsive interaction between the main-chain and terminal beads. The simulations indicate that the smectic-nematic and smectic-isotropic phase transitions take place through the buckling of the smectic layering in the system.