Diblock Copolymer Reinforced Interfaces between Amorphous Polystyrene and Semicrystalline Polyethylene

The effects of molecular architecture on the fracture properties of semicrystalline polymers were probed at diblock copolymer-reinforced interfaces between polystyrene (PS) and polyethylene (PE). The PE used for this study was a model ethylene−butene copolymer which was chosen for its compatibility with hydrogenated 1,4-polybutadiene. This compatibility allowed the use of hydrogenated poly(styrene- b -1,4-tetradeuteriobutadiene) as the block copolymer. For a series of these diblock copolymers, the areal chain density (Σ) and the molecular weight of the PE block ( M n ) were varied systematically to observe their effects on the interfacial fracture energy ( G c ). At low Σ, G c stayed relatively constant, and was roughly 1 J/m 2 . Above a critical value of Σ, the fracture energy climbed rapidly. This critical value decreased with increasing M n . The detection of deuterium on the fracture surfaces indicated that pullout of the PE block was the predominant failure mechanism when M n ≤ 30 kg/mol. Only when the molecular weight of the PE block reached 85 kg/mol was failure by chain scission observed. Since the entanglement molecular weight of PE is approximately 1 kg/mol, interfacial reinforcement does not appear to depend on the formation of entanglements for this system. The critical M n coincides instead with the point at which the root-mean-square end-to-end length of the PE block exceeds the long period of the PE crystal lamellae ( L ). The preceding observation is consistent with the decrease in G c with increasing L near the critical molecular weight.