Structure and Microdeformation of (iPP/iPP-g-MA)-PA6 Reaction Bonded Interfaces Christopher J. G. Plummer* and Hans-Henning Kausch Laboratoire de Polyme ` res, E Ä cole Polytechnique Fe ´ de ´ rale de Lausanne, Lausanne CH-1015, Switzerland Costantino Creton Laboratoire de Physico-Chimie Structurale et Macromole ´ culaire, ESPCI, rue Vauquelin, 7231 Paris Cedex 05, France Fre ´ de ´ rique Kalb and Liliane Le ´ ger Laboratoire de Physique de la Matie ` re Condense ´ e, URA CNRS 792, College de France, 11, place Marthelin-Berthelot, 75231 Paris Cedex 05, France Received December 23, 1997; Revised Manuscript Received May 13, 1998 ABSTRACT: Transmission electron microscopy has been used in conjunction with RuO4 staining to investigate the structure of (iPP/iPP-g-MA)-PA6 reaction bonded interfaces and microdeformation at interfaces which had failed by crack propagation along the interface. In relatively weak interfaces, the crack tip was preceded by a single, well-defined crazelike fibrillar deformation zone on the iPP/iPP-g-MA side of the interface, with fibril diameters of between 10 and 15 nm. The crack tip itself also propagated along the interface, providing some justification for the extension of established models for interfacial failure in glassy polymers to the present case of two semicrystalline polymers. In stronger interfaces, however, considerable diffuse deformation was observed to accompany crack propagation. This was accounted for by the deeper penetration of the damage zone into the iPP/iPP-g-MA, into regions where the microstructure was less well ordered than immediately adjacent to the interface. This transition from a well-defined to a diffuse damage zone may modify the relationship between the local interfacial strength and the global fracture toughness. Introduction The present report describes work intended to clarify the nature of microdeformation ahead of and around the crack tip during crack propagation along reaction bonded interfaces between isotactic polypropylene (iPP)/ maleic anhydride functionalized isotactic polypropylene (iPP-g-MA) blends and polyamide 6 (PA6). There exists a considerable body of recent experimental work con- cerning fracture of interfaces between immiscible amor- phous glassy polymers. 1-10 Much of this work has been successfully interpreted in terms of recent theoretical advances on the microdeformation mechanisms occur- ring at the crack tip 11-14 and on the effects of mixed- mode crack propagation at an interface. 15,16 However, the mechanical behavior of interfaces between two immiscible semicrystalline polymers is less well char- acterized. It was therefore hoped that direct observa- tion by transmission electron microscopy (TEM) of local structure and crack tip deformation mechanisms as- sociated with such an interface would be useful in assessing the extent to which the aforementioned models for glassy polymers might be used to interpret measurements of the macroscopic fracture toughness (quantified here in terms of the critical strain energy release rate, G c , for mode I crack propagation along the interface). The report is organized as follows. In the remainder of this introduction a brief description is given of those aspects of cohesive and interfacial failure in glassy polymers which are pertinent to the subsequent discus- sion. This is followed by a summary of the results of previous investigations into the fracture behavior of reaction bonded (iPP/iPP-g-MA)-PA6 interfaces as a function of sample preparation conditions. 17,18 A micro- scopic investigation of the microstructure and micro- deformation behavior at (iPP/iPP-g-MA)-PA6 interfaces is then presented, and the earlier results are reas- sessed in terms of the possible relevance of established models for bonded interfaces in amorphous glassy polymers. Cohesive and Interfacial Failure in Glassy Poly- mers. In constrained geometries, such as at crack tips in relatively thick samples, flexible, un-cross-linked or lightly cross-linked amorphous glassy polymers gener- ally deform by crazing. Craze microstructures have been intensively studied for many years by TEM and X-ray scattering. 19-24 It has been established, for example, that the characteristic dimensions of the internal structure of crazes obtained by deforming films thin enough to be electron transparent in a conventional TEM are consistent with those inferred from small- angle X-ray scattering of crazes in much thicker samples. 22,24 It is with some justification, therefore, that the results of TEM and low angle electron diffraction (LAED) studies of thin film crazes are invoked when considering the behavior of crack tip crazes in bulk tests. A particularly important observation in the present context is that of numerous cross-tie fibrils running perpendicular to and linking the main fibrils. 25-28 These cross-tie fibrils permit stress transfer within the craze in directions parallel to the craze faces. This can in turn give rise to a local stress concentration at the * To whom correspondence should addressed. 6164 Macromolecules 1998, 31, 6164-6176 S0024-9297(97)01852-4 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/21/1998