FRACTURE-BEHAVIOR OF LIQUID-CRYSTAL EPOXY-RESIN SYSTEMS BASED ON THE DIGLYCIDYL ETHER OF 4,4'-DIHYDROXY-ALPHA-METHYLSTILBENE AND SULFANILAMIDE .1. EFFECTS OF CURING VARIATIONS

The fracture behaviours of the pour-cast, unoriented diglycidyl ether of 4,4'-dihydroxy-alpha-methylstilbene/sulphanilamide liquid crystalli ne epoxies (LCE) cured at various temperature steps are investigated. It is found that, depending on how the LCE is cured, the liquid crysta lline (LC) domain size varies dramatically. These, in turn, affect how the LCEs fracture. The operative toughening mechanisms in the toughes t LCE are studied in detail and found to include the formation of nume rous segmented, unlinked microcracks in front of the main crack. When the crack opens up, the matrix material between the segmented microcra cks acts as a bridge between the opening crack planes. Furthermore, cr ack bifurcation appears to take place when the segmented cracks are ev entually linked with the main crack. This entire fracture process acco unts for the high fracture toughness (G(IC) = 580 J m(-2)) of this par ticular LCE with respect to conventional epoxies (G(IC) = 180 J m(-2)) . The relationship between the LCE morphology and the corresponding fr acture mechanisms is discussed.