Influence of Chain Entanglement on the Failure Modes in Block Copolymer Toughened Interfaces

Abstract

We have investigated the toughness and failure mechanism of the interface between poly-(methyl methacrylate) (PMMA) and poly(phenylene oxide) (PPO) homopolymers reinforced with varying amounts of a PMMA-polystyrene (PS) block copolymer. The block copolymer was found to increase the interface fracture toughness, Gc, by up to 2 orders of magnitude. Gc increased with the areal density of block copolymer, Σ, and then showed a maximum at a copolymer layer thickness equal to about 0.75L, where L is the long period of the neat copolymer. SIMS analysis of the fracture surfaces revealed that the block copolymer was well organized at die time of fracture and that, when the surface coverage was below L/2, the block copolymer fractured in the middle. When Σ was larger than the value corresponding to L/2, patches of a lamella of pure block copolymer were formed at the interface and the locus of fracture shifted to the center of the PS lamella These results suggested that, at low coverage, the interface failed by fracture of the block copolymer chains near their junction points without any significant amount of plastic deformation. At higher Σ, but below the saturation value, the interface failed by forming a craze, the maximum thickness of which was controlled by the fracture of the block copolymer chains again near their junction points. No fracture in the brush between the block copolymer and the homopolymer was observed. When PS lamellas formed, at high coverage, the locus of fracture shifted to the center of the lamellas as the entanglement in a PS layer is significantly lower than that between PS and PPO or in a PMMA layer. © 1994, American Chemical Society. All rights reserved.