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SynopsisThe stress-strain properties of poly(2,6-dimethyl-l,4-phenylene oxide)/polystyrene composites containing crosslinked polystyrene microspheres have been measured at strain rates of 0.167, 1.67, and 16.7 min-'. It is found that Young's modulus almost has no increase with the filler content. The elongation at break and tensile strength decrease with the volume fraction of the filler, but both tend to flatten out at the volume fraction 0, > 0.25 at the strain rate of 1.67 min-'. The two ultimate tensile properties also have maximum values in the relationship with strain rate at the same filler concentration and strain rate conditions. Considering that elongation can be brought about by both matrix and filler, the well-known equation of elongation at break becomes with c = 1 -(a, -l)/cg, a, is the relative elongation at break of the filler. SEM indicates a characteristic brittle fracture of the material. Optical microscopy shows that dewetting takes place at both poles of the microsphere in the drawing direction.