Abstract: Microdomain morphology in styrene-butadiene-styrene triblock copolymer(SBS) and polystyrene(PS)blends was investigated as a function of PS concentration and molecular weight.The triblock copolymer containing 28% of PS resembles cylindrical morphology. Two PS with molecular weight of 500 and 12,000,respectively,were blended with the triblock copolymer up to a concentration of 25%.It was found that the microdomain morphology did not change with the addition of low molecular weight PS up to the concentration of 25%,while the cylinder-lamellar transition was observed for the blends containing the high molecular weight PS.The difference between the two systems is attributed to the addition of the homopolymer,which not only changes the volume fraction of one component,but also varies the values of)ON(where χ is the interaction parameter and N is the polymerization index of copolymer).Thus the microdomain morphology in blends of block copolymer and homopolymer is determined by the composition and the values of χN,and the latter determines whether the blend is in the strong segregation limit or the weak segregation limit.In the strong segregation limit,the microdomain morphology depends strongly on the composition; while in the weak segregation limit,it is weakly dependent on the composition.This is in good agreement with the predictions of mean-field theory.