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 (M-c) 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 crosslinked epoxies. An inverse correlation is demonstrated between the glass transition temperature (T-g) and the molecular weight of epoxy variants. A relative change in constituents' proportion from stoichiometry monotonically increases the fracture toughness (K-Ic) 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 K-Ic when plotted with M-c 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 M-c and K-Ic is proposed while addressing the effect of unreacted constituents on the fracture behaviour of epoxy system. (C) 2015 Elsevier Ltd. All rights reserved.