The fracture toughness, G, of the interface between a nitrogen plasma-treated poly(ethylene terephthalate) (PET) film and a poly(styrene-co-maleic anhydride) (PSMA) substrate was measured by using asymmetric double cantilever beam method. The effects of plasma treatment condition on PET films and post-plasma bonding treatment of the bi-material on the adhesion and the failure mechanism were investigated. For a given plasma pressure and energy, the amount of incorporated nitrogen on the PET surface as determined from X-ray photoelectron spectrometry (XPS) increased with increasing plasma treatment time and reached a plateau value of 7.7 at.%. XPS measurement showed that the incorporated nitrogen was primarily in the form of amine and amide. For bonding temperatures between 130 degrees C and 160 degrees C, the fracture toughness increased with increasing nitrogen incorporation on PET surface and reached a saturation G. which significantly depended on the bonding temperature. The saturation G(c) increased from 10 J/m(2) at 130 degrees C to 40 J/m(2) at 140 degrees C, reached a maximum of 120 J/m(2) at 150 degrees C, and then decreased to 60 J/m(2) at 160 degrees C. The location of failure also changed drastically with the bonding temperature. SEM and XPS measurements showed that for bonding temperature < 140 degrees C, failure occurred at the PET/PSMA interface. For bonding temperature = 150 degrees C, the interfacial adhesion exceeded that of the cohesive strength of PET film and failure occurred within the PET film. At the bonding temperature of 160 degrees C, failure occurred within PSMA bulk material. XPS measurement was used to measure the areal joint density, Sigma(cross) of PSMA chains pinned on the functionalized PET film surface. A transition in areal joint density below which G(c) scales linear with Sigma(cross) and above which G(c) scales with Sigma(2)(cross) transition was identified as the transition from the pure chain scission of in situ formed copolymers to plastic deformation of the interface. (c) 2006 Elsevier Ltd. All rights reserved.