Effect of length on the diffusion of a rodlike polymer at concentrations spanning the isotropic-lyotropic transition

The optical tracer self-diffusion of rodlike poly(benzyl-alpha,l-glutamate) (PBLG) is measured in pyridine solutions spanning the isotropic-liquid crystalline phase boundary for two different molecular weights, M = 134 500 (contour length L = 921 A, axial ratio x = 58) and M = 24 600 (L = 168 A, x = 10.5). The results are compared to previously published data for PBLG with M = 232 000 (L = 1590 A, x = 99). Under the conditions of the measurements, the two longer rodlike polymers align in the liquid crystalline phase with their cholesteric screw axis in the vertical direction. The diffusion is measured in a particular direction normal to this over distances that are long compared to the polymer lengths. Except for the shortest polymer, whose cholesteric screw axis does not assume the vertical alignment, the diffusion suddenly increases as the liquid crystalline phase is entered, then declines, signaling the disappearance of topological constraints in the isotropic phase having a vertical component. The solutions also contain a small, fluorescent component whose mobility was not at all affected by the isotropic-liquid crystalline phase transition.