A solution birefringence study of polymer elongation in spatially periodic stretching flows

The flow birefringence of dilute polymer solutions in periodically converging/diverging channels has been employed to study the dynamics of flexible chain molecules under transient stretching stresses. The onset of periodic birefringence for chains of high molecular weight is only observed after the chains have experienced several cycles of stress, at a point deep into the channel. This slow onset indicates that the solutions possess a memory on time scales much greater than that normally associated with a relaxed flexible coil. This behavior is recorded for both aqueous and nonaqueous solutions, at concentrations both above and below the entanglement concentration. Centerline birefringence, however, which is associated with purely elongational flow, is only observed for aqueous polymer solutions. An explanation for these birefringence results is suggested, based on a configuration-dependent dumbbell model for polymer chains in dilute solution.