Transient flow experiments in a model immiscible polymer blend

The rheological and morphological changes that are induced in emulsions or simple polymer blends by simple flow histories are relatively well understo od. Here these results are extended to more complex Bow histories, which no t only approach more closely real processing conditions but also provide mo re critical tests in model assessment. For this purpose, a semiconcentrated , incompatible, model blend is subjected to a stepwise increase in shear ra te followed by a flow reversal. The blend morphology consists of droplets i mmersed in a matrix. Two different types of behavior are identified, depend ing on the instant at which the flow is reversed. When this occurs after th e droplets are broken up, the stress transients resemble those of the corre sponding step-up experiment. When flow is reversed prior to the droplet bre ak-up, the stress transients are much longer than those in simple step-up e xperiments, the normal stress curves also become more complex. A model is p resented for flow reversal prior to droplet break-up. A satisfactory agreem ent, albeit only qualitative, is obtained assuming that the drops deform af finely during the entire transient. As expected, tumbling of the droplets d oes not seem to provide an adequate description of the observed transients. The experimental results show that a residual interfacial stress is always present, thus indicating a residual mean deformation of the droplets in th e flow direction during the whole transient. Polydispersity of the droplet population is proposed as a possible explanation for this phenomenon. Light scattering and conservative dichroism measurements, performed on a similar blend undergoing the same type of Bow history, confirm indeed that a resid ual mean stretching of the drops persists throughout the flow reversal. (C) 1999 The Society of Rheology. [S0148-6055(99)01903-3].