Soft X-ray Absorption Spectroscopy Probes OH center dot center dot center dot pi Interactions in Epoxy-Based Polymers

Using X-ray absorption spectroscopy (XAS), we investigated local chemical interactions in thermosetting epoxy resins for structural composites: N,N,N',N'-tetraglycidyl-4,4'-diaminodiphenylmethane (TGDDM) and bisphenol A diglycidyl ether (DGEBA) cured by 4,4'-diaminodiphenylsulfone (DDS). After curing, both TGDDM-DDS and DGEBA-DDS exhibited oxygen K-edge XAS spectra with prominent pre-edge peaks. Density functional theory based XAS simulations for the structures obtained by molecular dynamics revealed that the pre-edge peaks originated from OH center dot center dot center dot pi interactions between the OH groups and the adjacent aromatic rings, in addition to the OH center dot center dot center dot OH interactions. For comparison, poly(etheretherketone) (PEEK) exhibited no XAS features representing OH center dot center dot center dot pi interactions, which is consistent with the absence of the OH group in PEEK. These evaluations are supported by the radiation damage of the XAS features associated with bond enthalpies. The pre-edge peaks in XAS will be important for the study of noncovalent interactions in complex polymers to understand various polymer properties and assess the reliability of theoretical approaches.