Self-diffusion of a semiflexible polymer measured across the lyotropic liquid-crystalline-phase boundary

The self-diffusion of fluorescently tagged poly(gamma-benzyl-alpha,L-glutam ate), a helical, semiflexible synthetic homopolypeptide, has been measured in isotropic and cholesteric liquid-crystalline solutions by pattern fluore scence photobleaching recovery. On the isotropic side of the sharp isotropi c-liquid-crystalline (ISO-LC) phase boundary, the rodlike polymers assume a ll possible orientations in a three-dimensional space, becoming enmeshed. I n liquid-crystalline solutions, as first shown by Robinson [Trans. Faraday Soc. 52, 571 (1956)], spontaneous alignment of the cholesteric screw axis p arallel to the optical (z) axis of the instrument produces small monodomain s in which parallel rodlike polymers are organized into planes. Each horizo ntal plane is twisted slightly compared to its neighbors. Over the thicknes s of the sample, the rodlike polymers assume all possible orientations in t his two-dimensional space. Despite the small size of the monodomains, it wa s possible to determine the self-diffusion coefficient of the semiflexible rods, orientationally averaged in two dimensions. Crossing the sharp ISO-LC phase boundary corresponds to the sudden release of any putative topologic al constraints active in the isotropic phase, and produced a modest but sig nificant increase in diffusion. A relationship developed by Hess, Frenkel, and Allen [Mol. Phys. 74, 765 (1991)] is used to show that diffusion perpen dicular to the rod axis is about ten times slower than diffusion parallel t o the rod axis in the liquid-crystalline phase. In dilute solution, the com parable number would be 2. The perpendicular diffusion had decreased to abo ut 8% of its initial value in dilute, isotropic solution. The parallel diff usion decreased to about 40% of its initial value. These results were obtai ned by neglecting the uncertain effects of semiflexibility. Likewise, the e ffects of modest polydispersity have not been treated explicitly. (C) 1999 American Institute of Physics. [S0021-9606(99)50328-0].