Rheological properties and domain structures of immiscible polymer blends under steady and oscillatory shear flows

Abstract

Rheological properties of three immiscible polyisoprene (PI)/polydimethylsiloxane (PDMS) blends ( PI:PDMS =1:9 by weight) having viscosity ratios η d /η m =0.155, 0.826, and 4.02, where η d and η m are the viscosities of the droplet and matrix, were investigated by directly measuring the droplet size distributions with a video microscope. Steady state measurements were performed by sequentially stepping up the shear rate γ̇ (step-up) and step decrease (step-down) of the shear rate. Dynamic frequency sweep measurements were also performed immediately following cessation of steady shear flow. In the step-up measurements for blends with η d /η m =0.155 and 0.826, stresses were proportional to γ̇ and the droplet sizes were well regulated by the flow and inversely proportional to γ̇ for γ̇⩾0.553 s −1 , in good agreement with the Doi–Ohta model [J. Chem. Phys. 95, 1242–1248 (1991)]. The results of step-down measurements were consistent with dynamic moduli measurements, because the droplet sizes were almost unchanged during these measurements. The results are in accord with Palierne’s model [Rheol. Acta 29, 204–214 (1990)]. For the η d /η m =4.02 blend, however, the droplet sizes were insensitive to γ̇, resulting in worse agreement between the experimental and theoretical results than for the other blends.