In situ deuteron NMR investigations of sheared liquid crystalline polymers

The flow behavior of nematic liquid crystalline polysiloxanes of the side-chain type is studied by in situ H-2 NMR spectroscopy on samples under shear in a cone-and-plate cell. The director orientation as a function of applied shear rate is determined from the quadrupole splitting of the spectra. The data analysis yields the two Leslie viscosity coefficients alpha(2) and alpha(3) and the flow-alignment parameter lambda = -(alpha(3) + alpha(2))/(alpha(3) - alpha(2)). The values of X were determined for several homopolymers with only one type of side chain and random copolymers containing two different side chains. The results show that the flow behavior is related to the phase structure of the polymers, which varies with their composition. Only polymers with large amounts of smectic clusters in the nematic state show the tumbling instability (\lambda\ < 1); other polymers are flow aligning (\alpha\ greater than or equal to 1). For some polymers, a transition from tumbling at low temperature to flow aligning at high temperatures was observed. (C) 2002 Elsevier Science (USA).