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Abstract
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 To 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.