Structural rearrangements in hairy-rod polymer solutions undergoing shear

We report on a rheooptical investigation of hairy-rod poly(p-phenylene) sol utions at different concentrations and temperatures. These polymers have a reasonably high persistence length (about 28 nm) and behave as worm-like ch ains in dilute solutions, whereas they form nearly spherical fractal aggreg ates with internal anisotropy at higher concentrations. By exposing these s ystems to time-dependent simple shear and following the evolution of birefr ingence in start-up and its subsequent relaxation upon the cessation of she ar, we find a substantial broadening of the cluster size distribution, resu lting from flow-induced cluster deformation and break-up. In contrast to th e very dilute solutions, where polymers align in the flow direction, the de formed clusters main axes are aligned in the vorticity direction, presumabl y due to their strong steric local pretransitional type of ordering, with t he constituent polymers following the velocity vector. At the highest conce ntration, which corresponds to a weak gel, shear is shown to break-up the g el and the steady-state response of a broad-size aggregate suspension is ev entually recovered.