Rheological characterization of director tumbling induced in a flow-aligning nematic solvent by dissolution of a side-chain liquid-crystal polymer

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Abstract
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 α3 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.

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