Birefringence measurements of nematic solutions of the poly(p-phenylene benzobisthiazole), PBT, using interference between the ordinary and extraordinary waves created when a plane polarized wave enters the crystal, are described. The measured birefringence is high, e.g., ne – no varies from 0.05 to 0.09 at 633 nm for polymer concentrations 4×10−2 < c < 10×10−2(g/cm3). These data are discussed with the implications they have on the order parameter in these solutions.

Third-order nonlinear optical properties of lyotropic liquid crystal poly(1,4-phenylene-2,6-benzobisthiazole), PBT, solutions are studied by third harmonic generation measurements. Besides the enhancement observed for this coefficient with respect to the pure PBT, coupling is observed between the mean filed nematic director n and the incident fundamental polarization B 0. Different geometries, with respect to the relative orientation of B 0 and n, provided different harmonic Maker Fringe Patterns. These data are compared using refractive index measurements, independently achieved on these materials, and necessary for Maker Fringe analysis.

Certain aspects of the physical chemistry of solutions rodlike polymers are reviewed. The range of concentrations includes infinite dilution ([ŋ]c<<l), dilute solutions ([ŋ,]c<l), moderately concentrated solutions (l<[ŋ]c<[ŋ]cNI), and concentrated solutions (c<cN,), where [ŋ] is the intrinsic viscosity and cNI. is the concentration required for a stable nematic phase. Studies of chain conformation are emphasized at infinite dilution, and rheological behavior is emphasized for more concentrated isotropic and nematic solutions. Both theoretical and experimental considerations are included.