Measurement of the fracture toughness of polymer-non-polymer interfaces

Abstract

One of the primary factors limiting the development of a better understanding of polymer-non-polymer adhesion is the lack of a good testing method for the measurement of the strength of the interface. In this paper polymer-non-polymer adhesion is evaluated in terms of the fracture toughness of the interface using an asymmetric double cantilever beam testing geometry. The test is applied to the measurement of polystyrene (PS)-glass and PS-silicon (native oxide) interfaces modified by PS-poly(2 vinylpyridine) (PVP) and PS-poly methyl methacrylate (PMMA) diblock copolymers. The importance of mixed mode crack propagation is demonstrated and it is shown that through an appropriate choice of sample geometry, the crack-tip trajectory can be controlled so that the crack is forced to propagate along the interface. The PS-glass test, in particular, is shown to overcome many of the traditional problems of adhesion measurements, such as failure, away from the interface and effects of far-field deformation in the polymer. The interfacial fracture toughness of the PS-glass and PS-silicon interfaces without copolymer modification are approximately the same and weak with values of 1 J m⁻². The addition of the block copolymers results in significant (>40-fold) increases in the interfacial fracture energies. The increase in fracture toughness is dependent on the quantity and degree of organization of the block copolymer at the interface.