Structure and dynamics of nondilute polyfluorene solutions

The rotational and translational diffusion dynamics of newly synthesized polyfluorenes were studied by dynamic light scattering over a broad concentration range up to the lyotropic state. On the basis of small-angle neutron scattering and translational diffusion measurements in dilute solutions. the single polymers were described as wormlike chains with a low persistent length l = 7 +/- 0.5 nm and cross-section diameter d = 1.8 +/- 0.5 nm. In the semidilute concentration regime, the rate of increase of the cooperative diffusion and the osmotic pressure with c/c* (c* being the overlap concentration) was lower than for linear flexible chains in good solvent. The intermediate scattering function displayed a second process assigned to chain relaxation, in reasonable agreement with the longest relaxation time obtained from shear rheometry. The bimodal shape of the orientation correlation function, which is a characteristic feature of shape persistent polymers and exhibits a peculiar dependence on the scattering angle, was also observed in this model system. The fuzzy cylinder model adequately represented the concentration dependence of the osmotic modulus and the chain orientation time, conforming to the wormlike nature of these polymers.