The droplet-size evolution in polymer blends, due to flow-driven coale
scence, is investigated both experimentally and theoretically. In the
experiments, preshearing at a high rate is applied to generate a fine
morphology in the blend; subsequently, the shear rate is suddenly decr
eased to induce coalescence, and the resulting growth in droplet size
is measured. It is demonstrated that rheological as well las microscop
ic techniques can be used for an in-situ determination of the size evo
lution of the droplets. Effects of shear rate, step-down ratio, and bl
end concentration were studied systematically. A master curve for the
droplet growth at each step ratio can be obtained by plotting the rela
tive increase in droplet diameter us. the strain after step-down in sh
ear rate, multiplied by the fraction of the disperse-phase squared. To
model the droplet-size evolution the strategy of Chesters is followed
. It is implemented here for semiconcentrated blends, resulting in a o
ne-parameter model equation that correctly describes the experimental
observations.