Strengthening interfaces between biaxial oriented PET and PSMA: Effects of nitrogen plasma and bonding treatments

The fracture toughness, Gc, 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 °C and 160 °C, the fracture toughness increased with increasing nitrogen incorporation on PET surface and reached a saturation Gc which significantly depended on the bonding temperature. The saturation Gc increased from 10 J/m2 at 130 °C to 40 J/m2 at 140 °C, reached a maximum of 120 J/m2 at 150 °C, and then decreased to 60 J/m2 at 160 °C. The location of failure also changed drastically with the bonding temperature. SEM and XPS measurements showed that for bonding temperature < 140 °C, failure occurred at the PET/PSMA interface. For bonding temperature = 150 °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 °C, failure occurred within PSMA bulk material. XPS measurement was used to measure the areal joint density, Σcross of PSMA chains pinned on the functionalized PET film surface. A transition in areal joint density below which Gc scales linear with Σcross and above which Gc scales with Σ cross 2 was found. The transition was identified as the transition from the pure chain scission of in situ formed copolymers to plastic deformation of the interface.