Fracture toughness of interfaces between glassy polymers in a trilayer geometry

We studied the fracture behavior of trilayer A/B/A assemblies based on poly styrene (PS) and poly(methylmethacrylate) (PMMA) where the central layer of the A polymer was confined (0.5-200 mu m) between two thick plates of the B polymer (1- 3 mm). Diblock and random P(S-MMA) copolymers were used to pr ovide a good stress transfer across the interfaces. Fracture experiments we re performed with the double-cantilever beam method and the fracture mechan isms were observed by optical microscopy on microtomed slices of the: damag ed zone. The measured G(c) of the A/B interface fractured during the test w as dependent on the molecular structure at the interface (random copolymer, diblock copolymer or no copolymer), on the crazing stress of the hulk mate rials and on the interfacial sheer stresses. When the phase angle of the lo ading was even slightly positive, oblique crazes were observed in the PS in creasing greatly G(c). If PS was the central layer, this resulted in a very marked dependence of G(c) on the thickness of the central layer for a thic kness range 10-200 mu m which was not observed when the PMMA was the centra l layer. Thermal treatments modifying the interfacial shear stresses were a lso found to have a very strong effect on G(c). (C) 2000 Elsevier Science L td. All rights reserved.