Phase inversion of polybutadiene/polyisoprene blends under quiescent and shear conditions

We investigate the effect of phase inversion on the domain morphology and t heological properties of phase-separated polybutadiene/polyisoprene blends using optical microscopy, light scattering, and rheometry. Two different bl ends, low-vinyl polybutadiene/low-vinyl polyisoprene (LPB/LPI) and low-viny l polybutadiene/high-vinyl polyisoprene (LPB/HPI), were used in this study. The LPB/LPI blend has a lower critical solution temperature (LCST) of (62 +/- 1) degreesC, while the LPB/HPI blend exhibits upper-critical-solution-t emperature (UCST) behavior with a critical temperature above the experiment ally accessible temperature window. We determine the quiescent phase-invers ion composition (phi (LPI) = 0.55 +/- 0.05) of phase-separated LPB/LPI blen ds from the discontinuity in the dynamic storage modulus and shear viscosit y. For LPB/HPI, we find that a shear-induced coexisting structure (apparent as a "walnutlike" light-scattering pattern) develops at a fixed compositio n (phi (HPI) = 0.8) and constant temperature. The coexisting morphology con sists of two different anisotropic structures; stringlike domains and small (xi < 5 mum) vorticity-aligned domains. We suggest that Shear-induced phase inversion may lie at the foundation of this effect, although more theoreti cal and experimental work is needed to verify this.