ORAL REFERENCE: xxxxx FRACTURE TOUGHNESS OF POLYMER INTERFACES: FROM THE MOLECULAR TO THE CONTINUUM SCALE C. Creton1, E.J. Kramer2, H.R. Brown3 and C.Y. Hui4 1 Laboratoire PCSM, E.S.P.C.I. 10, Rue Vauquelin, 75231 Paris Cédex 05, FRANCE 2 Department of Materials Engineering, University of California, Santa Barbara, CA 93106, USA 3 BHP Steel Institute, University of Wollongong, Wollongong, NSW 2522, AUSTRALIA 4 Department of Theoretical and Applied Mechanics, Cornell University, Ithaca, NY 14853, USA ABSTRACT In order to be mechanically strong, an interface must be able to transfer stress across it. For very narrow interfaces between polymers, this capability is weak and a significant reinforcement can be achieved by the use of suitably chosen connector molecules (block copolymers) or by a broadening of the interface (random copolymers). In both of these cases the stress is transferred by entanglements between polymer chains. We review the main molecular characteristics which are necessary for this reinforcement effect to take place. Furthermore, recent theoretical advances on the relationship between interfacial stress and fracture toughness are discussed and the essential role of plastic deformation in the immediate vicinity of the interface is specifically addressed. KEYWORDS Fracture, polymer interfaces, adhesion, crazing, block copolymer, random copolymer INTRODUCTION Controlling the level of adhesion between two polymers is essential for many applications such as multiphase polymeric alloys, multilayer structures or adhesives. In all of these cases adhesion is caused by molecular interactions at the interface, but the level of adhesion is typically measured with a destructive test. Ideally one would like to be able to relate the macroscopic measurement of fracture toughness, which typically gives a critical energy release rate Gc, with the underlying surface chemistry (chemical bonds, specific interactions or simply entanglements). Recent experimental advances on the micromechanics of cracking at interfaces combined with easier access to polymers with well-defined chemical structures and surface analysis techniques have made it possible, at least for glassy polymers, to bridge the gap between the molecular and the continuum scale[1]. Interfaces between polymers which are immiscible are typically very narrow, with an interpenetration distance which is significantly smaller than the average distance between entanglement points in the bulk polymers. This lack of entanglements is responsible for the very low level of adhesion (typically a few J/m2) which is measured. One strategy to increase the level of adhesion of such interfaces is to replace the natural entanglement network at the interface with connecting molecules such as block copolymers which will be able to entangle with the bulk polymers on either side of the interface acting as molecular stitches. An
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