Temperature and Molecular Weight Dependence of the Viscometric Properties of Main-Chain Liquid Crystal Polymers in Nematic Solvents

Abstract

The twist and bend viscosities of dilute nematic solutions in 4'-(pentyloxy)-4-cyanobiphenyl (50CB) and in 4'-pentyl-4-cyanobiphenyl (5CB) of the main-chain liquid crystal polymers, TPB10 and TPB13, which have a mesogenic group, l-(4-hydroxy-4'-biphenyl)-2-(4-hydroxyphenyl)butane, separated, respectively, by flexible decamethylene and tridecamethylene spacers, were determined via dynamic light scattering analysis. A theoretical expression for the increment of twist viscosity, 71, derived by Brochard can be applied to interpret the experimental results in terms of changes in the mean square end-to-end distances parallel (R¡2) and perpendicular (R±2) to the nematic director. We find that the hydrodynamic behavior ofTPBlO oligomers moves closer to that predicted for a rigid rod as the temperature decreases. By modeling the oligomers as a mixture of configurational isomers, the hairpin probability per repeat unit and the associated activation energy can be found. The latter is approximately twice the energy barrier of a trans to gauche transformation. As the molecular weight is increased from oligomers to polymers a decrease in the exponent characterizing the contour length dependence of the intrinsic twist and bend viscosities, [71] and [m»nd], is found, emblematic of a crossover from rigid rod to biased random walk behavior. The effect of absolute temperature on the viscometric properties of TPB13 polymer (GPC determined Mw = 79100 g/mol) was studied by comparing the viscosity in high and low temperature nematic solvents at equal order parameters, viz. TPB13/5CB at T = 27.3 °C and TPB13/50CB at T = 62.0 °C. A large increase in [71] but negligible change in [tjbendl was found on decreasing temperature.

Determination of Splay and Twist Relaxation Modes in Nematic Liquid Crystals from Dynamic Light Scattering Experiments

“…The autocorrelation function measured in dynamic light scattering from a nematic liquid crystal has been shown, 1,[4][5][6][7][8][9][11][12][13][14][15][16][17] to exhibit two relaxation modes:…”

“…These are the so-called splay-bend and twist-bend modes. [10][11][12][13][14][15][16] The general expressions for these two frequencies or normal modes are given in terms of the elastic constants given in eq 5 and the viscosities The viscosities η β (q)(β)1,2) are defined as where R i refer to the five Leslie viscosity coefficients, 18,19 γ 1 is the twist viscosity, and η a , η b , and η c are Miesowicz viscosities. 20 It has been shown that, using different optical geometries, one can have access to the three pure deformations of nematic liquid crystals.…”

Orthogonal Orientations for Solvation of Polymer Molecules in Smectic Solvents

“…In this case long rodlike polymer molecules align along the nematic director to a much greater degree than the solvent molecules themselves because longer rods experience the local nematic field over much larger distances than the solvent nematogens. This striking effect, previously inferred from small angle neutron scattering [5] and viscosity measurements [6], was recently measured directly, using single molecule spectroscopy, for solutions of conjugated polymers [3,4] and colloidal virus particles [7].…”

Thermodynamics of Flexible and Rigid Rod Polymer Blends

“…In the nematic solvents, long liquid crystalline polymers align along the nematic director to give a much greater degree of translational order. These anisotropic conformations have been inferred from small-angle neutron scattering [91,92] and viscosity measurements [93] and have been directly measured using single molecular spectroscopy [94]. Figure 2.20 shows images of fluorescently labeled biopolymers in an isotopic solvent (left) and a nematic solvent (right) of fd virus [94]; Figure 2.20a-d shows, respectively, the images of actin, wormlike micelles, neurofilaments, and DNA.…”