The relationship between the fracture toughness (G c) and the areal density of diblock copolymer at the interface (Σ) was investigated for joints between polypropylene (PP) and polyamide-6 (PA6), two incompatible, semicrystalline polymers. Diblock copolymers were formed in situ by reaction between succinic acid groups terminally grafted onto 5% of the PP chains and the NH2 ends of the PA6 chains. Fracture toughnesses were measured using an asymmetric double cantilever beam test (ADCB). After the bulk PA6 had been completely rinsed from an adhered sample, X-ray photoelectron spectroscopy (XPS) was used to measure the areal density of copolymer at the interface. Above the melt temperature of PP, but below that of PA6, reaction at the interface was limited by diffusion of the reactive PP chains (D = 1.58 × 10-11 cm2 s-1 at 213 °C). By controlling the temperature and the time of formation, G c could be varied in the range from 5 to 100 J/m2. Dissipation during fracture was observed to occur by plastic deformation in the PP, and failure of the joint was determined to occur by chain scission of the PP part of the copolymer. The fracture toughness was found to vary as the square of the areal density of copolymer at the interface, a relationship similar to that observed and predicted for glassy polymers. This scaling behavior suggests that the stresses in the fracture are concentrated over a distance on the order of 10 nm at the head of the crack.

We present an investigation of the mechanisms of mechanical reinforcement at interfaces between polypropylene (PP) and polyamide-6 (PA6), associated with the incorporation of a small amount of maleic anhydride functionalized PP (PP-g-MA) which reacts with the NH2 groups of the PA6 to form a copolymer in situ. In a previous study we have demonstrated, for one molecular weight of PP-g-MA, that diblock copolymer molecules were indeed formed at the interface, with an areal density Σ, controlled by the reaction temperature and the reaction time, and that the measured fracture toughness of the interface scaled as G c ∝ Σ2, regardless of the reaction temperature, but for similar sample cooling conditions. We report here the behavior of the same system for a higher molecular weight PP-g-MA:  at a reaction temperature above 220 °C, very close to the melting point of the PA6, and above a given Σ, the measured G c becomes 4 times higher than that for reaction temperatures below 220 °C, where the observed G c values are identical to what has been measured for the low molecular weight PP-g-MA. G c is therefore no longer uniquely dependent on Σ. Crystallographic analysis on the PP side of the interface showed a correlation between the presence of the PP β-phase in the 20 μm near the interface and a high toughness; this crystalline phase was not present in the samples prepared at T ≤ 220 °C or with the low molecular weight PP-g-MA which always exhibited a low toughness even for samples prepared above 220 °C. It is argued that the presence of this β phase of the PP is the main factor responsible for the very high fracture toughness, first evidence of the influence of the crystallinity of a semi crystalline polymer on its adhesive properties.

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