RHEOLOGICAL CHARACTERIZATION OF DIRECTOR TUMBLING INDUCED IN A FLOW-ALIGNING NEMATIC SOLVENT BY DISSOLUTION OF A SIDE-CHAIN LIQUID-CRYSTAL POLYMER

We present rheological evidence which demonstrates that dissolution of a side-chain liquid-crystal polymer with a methyl methacrylate backbone (MSHMA) in a flow-aligning low molar mass nematogen (LMMN) pentylcyanobiphenyl (5CB) produces a director-tumbling response. For comparison, we also provide the rheological behavior of pure octylcyanobiphenyl (8CB), a LMMN which exhibits director tumbling. 8CB and the MSHMA/5CB mixture each show a similar pattern of shear stress oscillations, both in flow startup and flow reversal, characteristic of director tumbling, whereas pure 5CB has no oscillation response. Our results indicate that addition of a side-chain liquid-crystal polymer to a shear-aligning nematic solvent changes the sign of the Leslie viscosity coefficient alpha3 from negative (shear-aligning) to positive (director-tumbling). This is consistent with a theoretical discussion of Brochard (1979), provided that the polymer has an oblate configuration.