RHEOLOGICAL PROPERTIES AND DOMAIN-STRUCTURES OF IMMISCIBLE POLYMER BLENDS UNDER STEADY AND OSCILLATORY SHEAR FLOWS

Rheological properties of three immiscible polyisoprene (PI)/polydimet hylsiloxane (PDMS) blends (PI:PDMS = 1:9 by weight) having viscosity r atios eta(d)/eta(m) = 0.155, 0.826, and 4.02, where eta(d) and eta(m) are the viscosities of the droplet and matrix, were investigated by di rectly measuring the droplet size distributions with a video microscop e. Steady state measurements were performed by sequentially stepping u p the shear rate (gamma) over dot (step-up) and step decrease (step-do wn) 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 eta(d)/eta(m) = 0.155 and 0.826, stresses were proportional to (gamma) over dot and the droplet sizes were well regulated by the flow and inversely proportional to (gamma) over dot for (gamma) over dot greater than or equal to 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 dy namic moduli measurements, because the droplet sizes were almost uncha nged during these measurements. The results are in accord with Paliern e's model [Rheol. Acta 29, 204-214 (1990)]. For the eta(d)/eta(m) = 4. 02 blend, however, the droplet sizes were insensitive to (gamma) over dot, resulting in worse agreement between the experimental and theoret ical results than for the other blends. (C) 1997 The Society of Rheolo gy.