polymer chains chemically grafted at the interface followed well equation 4[16]. Furthermore, in the crazing regime, the measured value of Gc was found to directly increase with 1/ σcraze as predicted by equation 4. Figure 5 shows experiments performed on two systems where the molecular structure at the interface is identical (giving therefore the same value of σint) but the crazing stress (in this case rather the yield stress in hydrostatic tension) varies by a factor of 5. Accordingly the Gc values for the system with the lower yield stress are five times higher than for the harder system[17]. 6 8 10 2 4 6 8 100 2 4 G c (J/m2) 0.01 2 3 4 5 6 7 8 9 0.1 Σ (chains/nm2) Figure 5: Gc vs. Σ for an interface between polypropylene (PP) () or a blend of PP with ethylene-propylene rubber (PP/EPDM)(z) and polyamide-6 (PA6), reinforced by end-grafted PP chains. The yield stresses of the PP and PP/EPDM blends are 21 and 4 MPa respectively. Data from [17]. Optical and electron microscopy observations of the plastically deformed zone ahead of the crack tip show that in both systems the dissipation is localized in a strip analogous to a Dugdale plastic zone near the interface. This is not however a very general case. If the softer polymer for example, is able to nucleate diffuse plasticity in the hard matrix, far away from the crack tip, greatly increasing the dissipated energy associated with the propagation of the crack, the correlation between interfacial structure and Gc can be much more complicated and is no longer described by Brown's model[18]. CONCLUSIONS We have shown that the fracture toughness of interfaces between polymers is dependent on the molecular structure at the interface as well as on the bulk properties of the polymers on either side of the interface. This relationship is now relatively well established for interfaces between glassy polymers and the main results have been summarized here. Two important points must be emphasized: • In order to obtain a high value of fracture toughness, the interface must be able to transfer a stress which is at least as high as the crazing stress of one of the bulk polymers on either side of the interface. • If this condition is met, Gc will depend on the interfacial stress σint and on the bulk crazing stress σcraze. If all the plastic deformation is confined in a localized craze near the interface, Gc is well predicted by equations 3 and 4. REFERENCES 1. Creton, C.,Kramer, E. J.,Brown, H. R.and Hui, C. Y. to appear in Adv. Polym. Sci. 2. Brown, H. R. (1990) J.Mat.Sci. 25, 2791.
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