EFFECTS OF COALESCENCE AND BREAKUP ON THE STEADY-STATE MORPHOLOGY OF AN IMMISCIBLE POLYMER BLEND IN SHEAR-FLOW

The steady-state morphology of an immiscible polymer blend in shear fl ow has been investigated by optical microscopy techniques. The blend i s composed by poly-isobutylene (PIB) and poly-dimethylsiloxane (PDMS) of comparable viscosity. Experiments were performed by means of a home -made transparent parallel plate device. The two plates can be indepen dently counter-rotated, so that sheared droplets of the dispersed phas e can be kept fixed with respect to the microscope point of view, and observed for long times. The distribution of drops and their average s ize were measured directly during flow at different shear rates and fo r different blend compositions. It was found that the average drop siz e in steady-state conditions is a decreasing function of the applied s hear rate, and does not depend on blend composition for volume fractio ns up to 10%. Experiments have proved that, in the shear rate range wh ich could be investigated, the stationary morphology is controlled onl y by coalescence phenomena, droplet breakup playing no role in determi ning the size of the dispersed phase. More generally, it has been show n that the steady-state morphology is a function not only of the physi cal parameters of the blend and of the shear rate, but also of the ini tial conditions applied to the blend. The steady-state results reporte d in this paper constitute the first direct experimental confirmation of theoretical models which describe the mechanisms of shear-induced d rop coalescence.