Toughening behavior of semicrystalline polymers was investigated using syndiotactic polystyrene (sPS)lpolyamide 6(PA-6) blends compatibilized with maleic-anhydride functionalized poly (styrene-b-(ethylene-co-butylene)-b-styrene) SEBS-MA triblock copolymer. The effect of interparticle distance and crystal microstructure near the particle/matrix interface of the blends were studied. The morphology studies revealed that the size and interparticle distance of the dispersed PA-6 particles decreased with increasing SEBS-MA concentration. sPS/PA-6 blends exhibited sharp brittle-ductile transitions at a critical interparticle distance of 0.25 mu m. With the increase of the compatibilizer concentration beyond a certain level, it was observed that the further addition resulted in decreased impact strength. This could be attributed to the formation of a separate phase in the matrix by the additional SEBS added. The TEM studies showed that when the interparticle distance is below 0.25 pm, the matrix ligaments between the inclusions will be filled with well-oriented crystalline material of reduced plastic resistance. From DSC and X-ray diffraction studies of model thin films, it was found that the fraction of small and imperfect crystallites near the particle/matrix interface increased with decreasing interparticle distance. This resulted in decreased yield stress of the whole matrix with increasing concentration of SEBS-MA accompanied by changes in the fracture mode from brittle to tough. (c) 2008 Wiley Periodicals, Inc.