The crazing and fracture behaviors of glassy-glassy block copolymers were investigated for polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymers that had similar overall molecular weights but different poly (methyl methacrylate) (PMMA) molar fractions. A liquid chromatography technique was applied to separate as-synthesized PS-b-PMMA [(1) weight-average molecular weight (M-w) = 94,000 g/mol and PMMA molar fraction = 0.35 and (2) M-w = 65,000 g/ mol and PMMA molar fraction = 0.28] into three fractions with different chemical compositions. With a copper-grid technique, the fracture behaviors of 0.5-mu m-thick PS-b-PMMA films were studied as a function of the applied strain. For the higher M-w PS-b-PMAU samples, the median strains at crazing and fibril breakdown increased with an increase in the PMMA molar fraction from 0.24 to 0.46, corresponding to an increase in the chain entanglements in the PMMA domains. In contrast, for the lower Mw samples, the two values were not significantly changed even when the PMMA molar fraction was varied from 0.16 to 0.35. Mw of the minor component in PS-b-PMMA played a critical role in controlling the fracture behaviors of the block copolymers. Specifically, M-w/M-e of the minor component (where Me is the molecular weight between entanglements) had to be roughly larger than 2 for the block copolymers to sustain sufficient strains before fracture. (c) 2006 Wiley Periodicals, Inc.