The strength of interfaces involving glassy polymers

The situation is more delicate when the two materials have different moduli. In this case, if the beams are of identical thickness the failure will no longer be purely mode I. In these circumstances the crack will deviate from the interface into the material with the lower deformation resistance, leading to additional energy dissipation. In these circumstances the measured values of the interfacial fracture energy will be larger than Gic- This problem can be overcome by using an asymmetrical test, in which the thicknesses of the two beams are unequal. At a particular ratio of thicknesses the measured fracture energy will be a minimum and this may be taken as the true value of G c. 

One might wonder whether it is possible to correlate the interfacial fracture energy of an incompatible polymer pair more precisely to the width of the interface. Such a correlation clearly exists at a qualitative level. For example, polystyrene is substantially less miscible with poly(2-vinyl pyridine) (PVP) than it is with PMMA. This is reflected via equation (4.2.4) in the width of the 

Brown has shown (Brown 1991b), as will be discussed in much more detail below, that the fracture energy of an interface that fails by the formation and subsequent breakdown of a craze is proportional to the square of the number of effectively entangled chains crossing the interface. Thus we would expect the interfacial fracture energy to vary like 

This weakness of the interfaces between most immiscible polymers explains why the mechanical properties of two-phase polymer mixtures are generally so poor. However, as we saw in chapter 6, we can modify the interfaces between immiscible polymers using block copolymers. In section 6 we concentrated on the role of such so-called compatibilisers in lowering the energy of the interface between the immiscible phases. However, in addition to this effect 

If this sort of experiment is supplemented by surface analysis of the fracture surfaces, in order to determine on which side of the joint the block copolymer ends up after failure, it becomes possible to identify the main possible mechanisms of failure of the interface (Kramer et al. 1994). There are essentially three possibilities, which are illustrated in figure 7.6. Which mechanism is operative 

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