Effect of cross-linking on dynamic mechanical and fracture behavior of epoxy variants

Anhydride cured epoxy systems are examined to elucidate the effect of cross-linking on viscoelastic and fracture behaviour of polymers. Dynamic mechanical and quasi-static fracture tests are conducted on epoxy variants, prepared by mixing diglycidyl ether of bisphenol-A (DGEBA) and methyl tetra hydrophthalic anhydride (MTHPA) in several proportions. The molecular weight (Mc) of the epoxy system increases monotonically as its composition deviates from the stoichiometry, indicating decreasing cross-link density. Significant influence of constituents' proportion is observed on glassy, glass transition and rubbery states, however, the damping characteristics remain largely unaffected in adequately cross-linked epoxies. An inverse correlation is demonstrated between the glass transition temperature (Tg) and the molecular weight of epoxy variants. A relative change in constituents' proportion from stoichiometry monotonically increases the fracture toughness (KIc) value of the material. Fracture surface micrographs reveal distinct composition dependent toughening mechanisms. While highly cross-linked stoichiometric system provides least resistance to material fracture, crazing and plastic deformation lead to increased fracture toughness values in hardener-rich and resin-rich epoxy systems, respectively. The KIc when plotted with Mc shows increasing trend until it reaches a plateau value at higher molecular weights even if the variation distinctly differs in resin-rich and anhydride-rich cases. A model correlating Mc and KIc is proposed while addressing the effect of unreacted constituents on the fracture behaviour of epoxy system.