Phase Structure Changes in a Sheared Blend of High Molecular Weight Polybutadiene and Polyisoprene Elastomers
The flow-induced phase behavior of polyisoprene—polybutadiene elastomer blends is of concern to the rubber industry because of the wide practical applications of these blends. We have been investigating examples of these blends using a Rheo-SALS instrument in conjunction with light microscopy. The subject of this work was a high molecular weight, near-critical blend (35/65 ratio) comprising polybutadiene (Mn=413, 000) and polyisoprene (Mn=177, 000) (Blend II). The results were contrasted with those for a previously studied blend (Blend I) comprising polybutadiene (Mn=164, 000) and polyisoprene (Mn=125, 000), also near the critical composition at a 40/60 ratio. Both featured the same microstructure. Each was examined under quiescence and flow conditions. For Blend I at a 24 °C quench depth, the scattering pattern was the expected spinodal ring; which, upon shearing, changed to an H-shaped pattern and then to a bright streak in the vorticity direction. Direct observation showed that these patterns corresponded to a fibrillar structure aligned in the flow direction. Similar light scattering patterns were obtained after a temperature jump (24 °C) under steady-state shearing. With Blend II at a 40 °C quench depth, the scattering pattern was again a spinodal ring, but this changed on shearing to an anisotropic pattern having the shape of an “8” or a “butterfly.” The shape of this pattern depended on shear rate, and its origin was hypothesized to be phase domains following helical paths with axes aligned in the vorticity direction. The corresponding micrographs are consistent with this supposition, as is an instability phenomena observed in the parallel-plate rheometer at higher shear rates. These purely hydrodynamic phenomena should be kept in mind when interpreting observations of “flow-induced mixing” at lower quench depths.