Epoxy resins, widely used as structural adhesives and protective coatings, are susceptible to degradation under environmental and chemical stress, which limits their long-term reliability. In this study, a comprehensive theoretical investigation of the degradation mechanisms of diglycidyl ether of bisphenol A cured with 4,4'-diaminodiphenylmethane was conducted using density functional theory, focusing on the cleavage of aromatic-alkyl ether C-O bond and C-N bond at the epoxy-amine linkages through homolysis, hydrolysis, and acidic hydrolysis. Homolytic cleavage of both C-O and C-N bonds is highly energy-demanding, with bond dissociation free energies (BDFEs) of 259.7 and 296.1 kJ mol-1, respectively. In hydrolysis, the activation free energy for an ether C-O bond cleavage is calculated to be 244.1 kJ mol-1, corresponding to a 100-fold increase in the reaction rate at 400 K, thereby accelerating degradation. Protonation alone only modestly reduces bond strengths, decreasing the C-O and C-N BDFEs to 236.0 and 248.7 kJ mol-1, respectively, and thus provides limited bond weakening in isolation. Acidic hydrolysis was therefore examined to probe further the accelerated degradation pathway. Under weakly acidic conditions, both ether and amine sites are readily protonated, enhancing the bond polarization and electrophilicity at the reactive centers. Protonation reshapes the reaction landscapes by shifting the system onto distinct potential-energy surfaces, raising the surface associated with C-O bond cleavage and lowering that for C-N bond cleavage. Consequently, the activation energies for acidic hydrolysis decrease significantly to 100.0 and 139.2 kJ mol-1, leading to a pronounced acceleration of reaction rates. In this context, even though the C-N pathway is thermodynamically favored, the C-O pathway becomes more accessible once protonation occurs at the ether site. Comparative energetics demonstrates that the degradation is strongly environment-dependent, with acidic hydrolysis emerging as the most accessible route.
Shrestha et al. (Tue,) studied this question.