Mechanistic insights into mixed-solvent-induced swelling of amine-cured epoxy networks: a combined MD and DFT study

The permanent three-dimensional network of epoxy resins (EP) confers outstanding performance but also renders them difficult to be recycled. Although solvent pretreatment has been shown experimentally to facilitate polymer degradation, the lack of molecular-level understanding renders solvent selection largely empirical. In particular, how mixed solvents modulate solvent-polymer interactions and promote swelling of cross-linked networks remains unclear. Here, molecular dynamics (MD) simulations combined with density functional theory (DFT) calculations were employed to investigate the swelling behavior of amine-cured, diglycidyl ether of bisphenol A (DGEBA)-based epoxy networks in organic solvent-water mixtures. Three cross-linked epoxy models cured with 4,4'-diaminodiphenylmethane (DDM), bis(4-aminocyclohexyl)methane (PACM), and m-phenylenediamine (MPDA) were examined in mixed solvents composed of tetrahydrofuran (THF), dichloromethane (DCM), acetonitrile (ACN), or N,N-dimethylformamide (DMF) and water. The results show that dispersive interactions dominate the association of THF with the phenyl and ether groups of DGEBA-DDM in THF, while in water-rich THF/H₂O mixtures, cooperative THF-H₂O-polymer hydrogen-bonding networks disrupt intramolecular noncovalent interactions (e.g., π-π stacking, hydrogen bonding, and CH⋯π interaction), thereby promoting polymer swelling. Compared with PACM-and MPDA-based networks, the DDM-based epoxy exhibits stronger solvent-polymer interactions and enhanced swelling, attributed to its higher phenyl density and more flexible CH₂ linkages. Solvents with strong dispersive interaction capability and favorable polarity matching, such as THF and DCM, promote effective swelling, with DCM showing the highest swelling efficiency due to its small molecular size and strong van der Waals affinity toward hydroxyl, ether, and amino groups of amine-cured EP. These results offer molecular-level insight into mixed-solvent effects on epoxy swelling and inform the rational design of pretreatment strategies for degrading waste epoxy and carbon fiber-reinforced polymer matrix composites (CFRPs).