SELF-DIFFUSION OF RODLIKE POLYMERS IN ISOTROPIC SOLUTIONS

The self-diffusion coefficient, D-s, of fluorescently tagged poly(gamm a-benzyl alpha,L-glutamate) has been measured throughout the isotropic regime in pyridine. Intrinsic viscosity, phase boundary studies, epif luorescence microscopy, computer modeling, and the diffusion rates the mselves show that the labeling does not appreciably perturb the semifl exible rod structure of the polymer. It is also demonstrated that pyri dine is a good solvent for poly(gamma-benzyl alpha,L-glutamate), compa rable to N',N'-dimethylformamide. As concentration increases, two or t hree regimes are found depending on the rod length. Dilute behavior ho lds to number densities, nu, well exceeding L(-3), where L is the rod length. A somewhat better estimate of the point at which D-s begins to decrease is the classical criterion, c approximate to [eta](-1) where c is the weight/volume concentration and [eta] is the intrinsic visco sity. Over the whole range of molecular weights, the condition nu dL(2 ) approximate to 0.5-1, where d is the rod diameter, well describes th e downturn number density. This corresponds to nu/nu() approximate to 0.1-0.2, where nu() = 16/(pi dL(2)) is the Onsager critical number d ensity associated with the lyotropic liquid crystal transition. Once d iffusion begins to decrease, it does so strongly, obeying D-s similar to C(1.13+/-0.04)L-(1.8+/-0.2). The third regime of diffusion, evident only in the two longest samples, is again relatively level and lasts to the lyotropic transition. Diffusion does not cease at any concentra tion, and D-s is reasonably well scaled when: number density is normal ized by nu(), which underscores the importance of finite thickness ef fects for semidilute solutions of poly(gamma-benzyl alpha,L-glutamate) . The self-friction and mutual friction coefficients differ, with the former being lower in dilute solutions.