Melt rheology of lower critical solution temperature polybutadiene/polyisoprene blends

The viscoelastic properties of near critical entangled polybutadiene (PB)/p olyisoprene (PI) blends were investigated in oscillatory shear above and be low the lower critical solution temperature (LCST). The terminal loss modul us of a near critical PB/PI blend above and below the LCST is well describe d by means of a log additive mixing rule. A single master curve in the loss modulus of the critical blend exhibiting WLF behavior was obtained above a nd below the LCST by using the empirical time-temperature superposition (tT S) principle. However, the storage modulus above the LCST deviates from bot h the tTS principle and the log additive mixing rule. The phase-separated P B/PI blends within the linear viscoelastic regime display higher than expec ted values of storage modulus at low frequencies, due to the interfacial te nsion between the two phases of the blend. This increase causes a discontin uity in the temperature dependence of the storage modulus at low frequencie s. The discontinuity occurs at a frequency-dependent temperature that extra polates in the limit of zero frequency to the cloud point measured under qu iescent conditions by optical microscopy. The evaluations of interfacial te nsion from low-frequency linear viscoelasticity are compared with the expec tations of Helfand-Tagami theory. The rheological determinations of interfa cial tension are within a factor of 2 of the expected values for all compos itions except those near the phase-inversion point.