Block copolymers, termed compatibilizers, are often added to immiscible homopolymer blends to improve blending. The aim of this research is to quantify the effect of compatibilizer on the rheological properties of immiscible polymer blends having a droplet-matrix morphology with the droplet volume fractions approaching 50%. We studied steady shear viscosity, creep recovery after cessation of shear, and dynamic oscillatory behavior in poly(isobutylene) (PIB) / poly(dimethylsiloxane) (PDMS) blends with volume fractions ranging from 30% to 70% of PIB. Small amounts (0.01% to 0.5% by weight) of a PIB-PDMS diblock copolymer were added as compatibilizer. The blends were sheared at various stress levels in a stress-controlled rheometer and their viscosities were measured. Shearing was then halted and the strain recovery was measured. Finally, we performed dynamic oscillatory measurements after shearing at each stress level to probe the blend morphology.

Previous results on blends with less than 30% dispersed phase volume fraction show that the blend viscosity and recovery increase slightly with small amounts (less than 1%) of added compatibilizer. Also, at low compatibilizer loadings, dynamic oscillatory measurements showed a very slow relaxation process which was absent in blends without compatibilizer; this additional relaxation has been attributed to interfacial elasticity induced by the compatibilizer. All of these results are seen in higher dispersed phase volume fraction blends; however, the magnitude of these effects is far larger. Thus the viscosity and recovery are affected much more by the compatibilizer when the drop volume approaches 50%. For example, the relative viscosity of the compatibilized blends can be nearly a factor of two higher and the recovery can be as much as 50% higher than uncompatibilized blends, even with as little as 0.5% compatibilizer.

Furthermore, the block copolymer used was able to suppress droplet coalescence if PIB was the continuous phase. Therefore, the droplets in the 30% and 40% PIB (PIB droplets in PDMS) blends were able to coalesce in stress step down experiments, whereas the droplets in the 60% and 70% PIB (PDMS droplets in PIB) blends could not. This asymmetry in coalescence suppression led to an asymmetry in the rheological behavior. For example, the viscosities of the PIB in PDMS blends are slightly higher than those of the PDMS in PIB blends at the same dispersed phase volume fraction and compatibilizer loading. Furthermore, the ultimate recoveries are much more sensitive to stress level for the PIB in PDMS blends.

In summary, we find that even small amounts of compatibilizer can have large rheological consequences which are of considerable relevance to polymer processing flows.