QUASIELASTIC LIGHT SCATTERING MEASUREMENTS OF THE ANISOTROPIC MECHANICAL PROPERTIES OF A POLYMER NEMATIC LIQUID CRYSTAL.

摘要

An experimental investigation of macroscopic mechanical properties of a polymer nematic liquid crystal was conducted on nematic solutions formed from rigid or semiflexible main chain polymers that are fully extended in a solution. The particular system under investigation is a poly-(gamma)-benzyl glutamate (PBG) which belongs to a class of synthetic polypeptides.;The splay and bend elastic constants were found to be very similar in magnitude; both showed a linear dependence on concentration. The twist elastic constant was much smaller than either splay or bend and showed only a weak concentration dependence.;Four out of five independent Leslie viscosities were measured (except the elongational flow viscosity (alpha)(,1)). The viscosities were found to exhibit very large anisotropies. Different types of viscosities showed distinctly different patterns of concentration dependence. (gamma)(,1) and (eta)(,c) were quadratic in concentration, (eta)(,a) was linear in concentration, and (eta)(,b) did not show any strong concentration dependence. The viscosity data indicate that even at a moderate length to diameter ratio studied (L/D - 30), the PBG approaches a behavior predicted for a system composed of infinitely long molecular chains.;The concentration dependence data for the elastic constants and the anisotropic viscosities is consistent with the theoretical models for nematic solutions of partially flexible chains rather than those of rigid rods.;Quasielastic Rayleigh light scattering on well aligned nematic single crystals of PBG was used to measure the elastic constants and the anisotropic viscosities as a function of concentration. The elastic constants are measured from the intensity of the scattered light which is determined by the amplitude of thermally excited director fluctuations. That amplitude depends on the energy of the fluctuation which is proportional to the appropriate elastic constant. The viscosities associated with the elastic deformation modes are computed from the characteristic relaxation times of the fluctuations by measuring their temporal autocorrelation function.