Temporary droplet-size hysteresis in immiscible polymer blends

The droplet size distribution during steady shearing of model polymer blend s is examined in situ by optical microscopy. The volume-average steady-stat e droplet size during shear is essentially inversely proportional to shear rate, as expected. When the shear rate is increased suddenly, the droplets break up, through a process that involves the transient formation of thread s, and rapidly establish a new steady state, comprising ellipsoidal droplet s that are extended slightly in the direction of shear. When the shear is s topped, the droplets quickly relax to a spherical shape, but virtually no c oalescence is observed, because neither Brownian nor buoyant forces are sig nificant and the volume fraction of the dispersed phase is low. Slow shear, however, induces droplet collisions that lead to coalescence. The coalesce nce process is much slower than breakup. In contrast to some predictions, h owever, there is no permanent droplet size hysteresis. The steady-state siz e produced by breakup of initially larger droplets is eventually produced a t large strain by coalescence of initially smaller droplets. The lack of pe rmanent hysteresis has implications concerning appropriate mathematical mod els of coalescence behavior. (C) 2000 Elsevier Science Ltd. All rights rese rved.