Dependence of viscoelastic properties on spacer length and molecular weight for a side-chain liquid crystal polymer in a nematic solvent

Abstract

The light scattering technique was used to investigate the viscoelastic parapeters characterizing director distortions in miscible mixtures of pentacyanobiphenyl (5CB) with a side-chain liquid crystal polymer (LCP) having different spacer lengths (n=2, 3, 5, 7, 11). To separate the elastic constants from the corresponding viscosities, two approaches were attempted: (a) an AC electric field was applied to homeotropically-aligned nematic monodomains of the mixtures, and the field-dependent scattering intensities and director distortion relaxation rates were measured to obtain the twist viscosity γ1 and elastic constant K22; (b) an electric field was applied to a homogeneously aligned monodomain and the votage-dependent capacitance and the threshold voltage were measured to obtain the dielectric constants and the splay elastic constant. The remaining splay viscosity, bend viscosity and bend elastic constant were subsequently determined by fitting the angular-dependent relaxation rates in three scattering geometries which correspond principally to splay, twist, and bend modes of the director distortions ofr nematic miwtures. The additin of liquid crystal polymers cause significant decreases of the relaxation rates for all three distortion modes of 5CB which are due to small decreases in the elastic constants and large increases in the viscosity coefficients. The molecular weight dependence of the viscosities for n=3 is weak. The largest increase in viscosities is found for the LCP with shortest spacer length n=2. The dependence of viscosity on spacer length disappears when n≥5. The anisotropy in the three viscosity incrementsof the nematic mixtures also becomes smaller when n≥5. Our results indicate that, for side-chain LCPs in a nematic solvent, the backbone configurational anisotropy is enhanced by a strong coupling between the mesogenic group in the side-chain and the chain backbone.