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Fracture surfaces created during the interfacial failure of weak interfaces between glassy polymers were investigated with high-resolution scanning electron microscopy (HR-SEM). As reported in an earlier communication (Macromol. Rapid Commun. 2007, 28, 915-921), we found that interfacial failures between immiscible polymers produced large number of nanosized fibrils on the fracture surfaces. In order to investigate the mechanism of such ultra small structure formation in the crack propagation process, welding experiments of monodispersed polystyrenes with different molecular weights were employed by the asymmetric double cantilever beam (ADCB) test. The “nano-fibrils” were produced during the fractures of the interfaces between polystyrenes with the molecular weight higher than 2000, of which thickness was comparable to the entanglement spacing. It was found that the “nano-fibrous” formation bearing in the crack propagation reflected the local entanglement structures formed at narrow interfaces comparable to the entanglement spacing.