VISCOELASTIC PROPERTIES OF DILUTE NEMATIC MIXTURES CONTAINING CYCLIC AND HYPERBRANCHED LIQUID-CRYSTAL POLYMERS DISSOLVED IN A NEMATIC SOLVENT

The twist and bend viscosities of dilute solutions of cyclic and hyperbranched liquid crystal polymers (LCP) dissolved in low molar mass nematic solvents were determined via dynamic light scattering analysis. These results were compared to those of linear chains with similar chemical repeat structures. The nematic solvent used was 4'-pentyloxy-4-cyanobiphenyl (50CB). The cyclic LCP oligomers, Cy TPB10, have a mesogenic group, 1-(4-hydroxy-4'-biphenyl)-2-(4-hydroxyphenyl) butane, separated by flexible decamethylene spacers. The twist viscosity of the cyclic Cy-TPB10 oligomers increases with molecular weight more strongly than the linear, TPB-10, suggesting that the hydrodynamic behavior of Cy-TPB10 is closer to that of a rigid rod than TPB10. Surprisingly, the intrinsic bend viscosity [eta(bend)] of Cy-TPB10 decreases with molecular weight, in contrast to the positive dependence for linear TPB10. This may reflect the higher strain energy in the smaller ring sizes. The hyperbranched LCP, TPD-b-8, is also based on the mesogen 10-bromo-1-(4-hydroxy-4'-biphenyl)-2(4-hydroxyphenyl) decane but with octyl groups at the chain ends. We compare the viscoelastic behavior of dilute nematic solutions of TPD-b-8 in 50CB against that of a linear main-chain LCP, TPB7, with the same mesogenic group but with heptamethylene spacers. The viscometric properties of TPD-b-8/50CB and TPB7/50CB are quite different. The results suggest that each chain is prolate (i.e., R(parallel to) > R(perpendicular to) ) but that TPD-b-8 has a smaller chain anisotropy than that of TPB7. (C) 1995 John Wiley and Sons, Inc.