EFFECTS OF THE FORMATION OF COPOLYMER ON THE INTERFACIAL ADHESION BETWEEN SEMICRYSTALLINE POLYMERS

The relationship between the fracture toughness (G(c)) and the areal d ensity of diblock copolymer at the interface (Sigma) was investigated for joints between polypropylene (PP) and polyamide-6 (PA6), two incom patible, semicrystalline polymers. Diblock copolymers were formed in s itu by reaction between succinic acid groups terminally grafted onto 5 % of the PP chains and the NH2 ends of the PA6 chains. Fracture toughn esses were measured using an asymmetric double cantilever beam test (A DCB). After the bulk PA6 had been completely rinsed from an adhered sa mple, X-ray photoelectron spectroscopy (XPS) was used to measure the a real density of copolymer at the interface. Above the melt temperature of PP, but below that of PA6, reaction at the interface was limited b y diffusion of the reactive PP chains (D = 1.58 x 10(-11) cm(2) s(-1) at 213 degrees C). By controlling the temperature and the time of form ation, G(c) could be varied in the range from 5 to 100 J/m(2). Dissipa tion during fracture was observed to occur by plastic deformation in t he PP, and failure of the joint was determined to occur by chain sciss ion 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 interfa ce, a relationship similar to that observed and predicted for glassy p olymers. This scaling behavior suggests that the stresses in the fract ure are concentrated over a distance on the order of 10 nm at the head of the crack.