Durability in the life cycle of polymer composites

The effect of an interface structure and stress state on the long-term stre ngth of polymer composites is investigated and related to the adhesion betw een the polymer and the second phase. The approach specifically accounts fo r the previous life-cycle steps of the material through the evolution of th e size of the reinforcement phase and of the interfacial internal stress st ate. PET/SiOx multilayer films are used and found appropriate to tailor the interface interactions and introduce controlled compressive stresses in th e interface region. Adhesion is characterized by means of micromechanical a nd thermodynamic complementary approaches. The long-term strength of PET mu ltilayer composites aged below the glass transition temperature of the poly mer is predicted from the failure mechanism of the interface. It is shown t hat reliable predictions require the knowledge of the evolution of the inte rface shear strength, interface internal stress, and particle size through the whole life-cycle of the composite. (C) 1999 John Wiley & Sons, Inc.