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 Σcross2 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.