Stress Development in Solutions of Polystyrene Blends

The development of shearing stress after onset of steady shear flow has been measured for concentrated solutions in chlorinated biphenyl of monodisperse polystyrenes (M=2.75×105 and 1.53×106) and their blends. The effect of molecular weight distribution on the transient behavior has been discussed on the basis of experimental results for solutions of blend and the components.The parameter discussed here is the total strain κ tms at the time tms, when the shear stress takes its maximum. κ is the imposed rate of shear. Rate of shear dependence curves of κ tms measured at various temperatures can be superposed into a master curve. The shift factor is the same as that obtained from the time-temperature superposition of linear viscoelastic functions.For the solutions of monodisperse polystyrene, κ tms is a constant of 2.4, independent of κ, at relatively low rates of shear, and increases monotonically with further increase of κ. On the other hand, the κ dependence curve of κ tms for a blend solution gives a constant value of 3.8 at low rates of shear, and shows the two-step increase with increasing κ. This higher value of κ tms is believed to be related to the higher value of steady-state compliance of the blend sample. It has also been found that each of the above two steps corresponds to each of the components. In the certain range of κ, two overshoot peaks have been observed for stress development curve of a binary blend, whose components have very different molecular weights from each other.