The miscibility and viscoelastic behavior of liquid crystal polymers in nematic solvents

Qualitative phase diagrams were constructed using the contact method for binary mixtures of several low molar mass nematogens (LMMN) with a main-chain liquid crystal polymer (LCP), TPBx, which has a mesogenic group, 1-(4-hydroxy-4′-biphenyl)-2-(4-hydroxy phenyl)-butane, separated by flexible alkyl spacers of variable length, x. TPBx exhibits an odd-even oscillation in miscibility with 4′-pentyl-4-cyanobiphenyl (5CB) in the nematic state (2n + 1 = miscible, 2n = immiscible), but not with 4′-pentyloxy-4-cyanobiphenyl (5OCB) in which all polymers are completely miscible. These observations are qualitatively consistent with a thermodynamic model by Brochard, Jouffroy and Levinson.

The twist and bend viscosities of dilute nematic solutions of TPB10 in 5OCB were determined over a wide range of molecular weight via dynamic light scattering analysis. As the molecular weight is increased from oligomers to high polymer a decrease in the exponent α in [γ₁] or [η_bend] ∝ M^α is found, emblematic of a crossover from rigid rod to biased random walk behavior. The persistence length, the configurational anisotropies and the probability of a hairpin turn per repeat unit of the polymer chains were also calculated. We further studied the effect of absolute temperature on the viscoelastic properties of TPB10 and TPB13 using high and low temperature nematic solvents at equal order parameters. The hydrodynamic behavior of TPB10 oligomers moves closer to that predicted for a rigid rod as the temperature decreases. The activation energy of the hairpin probability is approximately twice that of a trans to gauche transformation. For TPB13/5CB at T = 27.3° C and TPB13/5OCB at T = 62.0° C, a large increase in [γ₁] but negligible change in [η_bend] occurs on decreasing temperature.

An odd-even variation in viscoelastic properties with spacer length of TPBx is found. TPB2n/5OCB mixtures have substantially lower splay and twist decay rates and slightly higher bend decay rate than TPB2n+1/5OCB mixtures. These differences are primarily due to changes in the associated viscosity coefficients, suggesting that the TPB2n has a more extended chain configuration along the director.

Finally, the viscoelastic properties of dilute nematic solutions in 5OCB of the cyclic liquid crystal oligomer Cy-TPB10 and a hyperbranched LCP, TPD-b-8, based on 10-bromo-1-(4-hydroxy-4′-biphenylyl) -2(4-hydroxyphenyl) decane with octyl groups at the chain ends, were determined. The hydrodynamic behavior of Cy-TPB10 oligomers suggests greater rigidity than that of TPB10. A negative exponent in [η_bend] ∝ M^α is found for Cy-TPB10 oligomers, while a positive case is true for TPB10 oligomers. The viscometric properties of TPD-b-8/5OCB and TPB7/5OCB are quite different. The results suggest that each chain is prolate (i.e., R_// > R_⊥) and that TPD-b-8 has a smaller chain anisotropy than that of TPB7.