Fracture behaviour of liquid crystal epoxy resin systems based on the diglycidyl ether of 4,4'-dihydroxy-alpha-methylstilbene and sulphanilamide .1. Effects of curing variations

The fracture behaviours of the pour-cast, unoriented diglycidyl ether of 4,4'-dihydroxy-alpha-methylstilbene/sulphanilamide liquid crystalline epoxies (LCE) cured at various temperature steps are investigated. It is found that, depending on how the LCE is cured, the liquid crystalline (LC) domain size varies dramatically. These, in turn, affect how the LCEs fracture. The operative toughening mechanisms in the toughest LCE are studied in detail and found to include the formation of numerous segmented, unlinked microcracks in front of the main crack. When the crack opens up, the matrix material between the segmented microcracks acts as a bridge between the opening crack planes. Furthermore, crack bifurcation appears to take place when the segmented cracks are eventually linked with the main crack. This entire fracture process accounts for the high fracture toughness (G(IC) = 580 J m(-2)) of this particular LCE with respect to conventional epoxies (G(IC) = 180 J m(-2)). The relationship between the LCE morphology and the corresponding fracture mechanisms is discussed.