Hysteresis in domain size of immiscible polymer blends under shear flow and related viscosity behavior

We discuss hysteresis in shear rate dependence of domain size for immiscible polymer blends observed in break-up process of relatively large domains after stepwise increase of shear rates and in coalescence process of relatively small domains after stepwise decrease of shear rates based on the applicability of Doi-Ohta theory. The sample was 7:3 blend of polydimethylsiloxane and a hydrocarbon-formaldehyde resin. Below the critical shear rate, where the theory is not applicable, domain size in the break-up process is not well controlled by the flow resulting in the broad size distribution and poor reproducibility of viscosity. However, shear rate dependence of average domain size is practically the same as that observed above the critical shear rate. On the other hand, the domain size observed in the coalescence process is well controlled by the flow and the viscosity is reproducible but the shear rate dependence of domain size are lower than those observed above the critical shear rate. It is concluded that when the break-up process is first carried out for blend samples containing relatively large domains started from a shear rate lower than the lower critical shear rate and then the coalescence process is carried out, domain size hysteresis is observed.