First-Principles Calculations of the Protonation and Weakening of Epoxy Resin under Wet Conditions

Abstract

In this study, we investigated the protonation of the amine group in epoxy resins prepared using amine-based curing agents by theoretical methods. Density functional theory (DFT)based free-energy calculations of the corresponding deprotonation subreactions showed that the amine group of the epoxy resin is protonated at equilibrium depending on the location of the amine group when the epoxy resin is embedded in water under standard conditions. Additional DFT calculations demonstrate that the energetic barrier for breaking the ether bond of the epoxy resin is lowered by about 0.6 eV as a result of the cooperative effect of H 2 O dissociation and that the transition-state energy for breaking the amine group bond is lowered by about 0.4 eV after the protonation of the amine group. Comparing the transition-state energies, we predict that the bond breakage of the protonated amine groups is the principal process causing the weakening of epoxy resins under wet conditions.

First-Principles Simulation Study on the Weakening of Silane Coupling to Silica under Alkaline Conditions

“…To determine the deprotonation site theoretically, we need the deprotonation free energy for each −OH. To determine the deprotonation free energy by first-principles calculations, we adopted the vertical energy gap method (VEGM) − with DFT for the electronic structure calculations. VEGM has been used to predict the acidities (p K a ) of various compounds, including individual molecules ,, and substrate surfaces. , …”

“…For condensed systems where many atomic bonds coherently change their lengths along a reaction pathway, compute-intensive methods such as the nudged elastic band method using a relaxed chain of replicas of the system were successfully used to find the transition states. Considering the limited number of atomic bonds involved in the present analyses, we here used the systematic bond length variation method , to find the transition states for breaking the target bonds. We selected a given number (two to four) of chemical bonds including the target bond, the lengths of which were changed stepwise and then maintained during each calculation.…”

“…When the B3LYP hybrid functional was used for a model molecule mimicking SCA1, the transition-state energy of Si–O bond breakage increased by approximately 15%. This 10–20% increase on changing the functional to B3LYP is often observed for organic molecules, and, importantly, the order of transition-state energies is not affected . Therefore, for consistency, we used the GGA functional in DFT for deprotonation analyses, transition-state search, and dynamics simulation.…”

“…To determine the deprotonation free energy by first-principles calculations, we adopted the vertical energy gap method (VEGM) 24−28 with DFT for the electronic structure calculations. VEGM has been used to predict the acidities (pK a ) of various compounds, including individual molecules 24,25,28 and substrate surfaces. 26,27 For the VEGM calculations, AH denotes a compound containing H. We consider the virtual half-reaction of AH in water, where the proton H + is gradually transformed into a dummy atom (H d ) with zero charge.…”

“…This 10−20% increase on changing the functional to B3LYP is often observed for organic molecules, and, importantly, the order of transition-state energies is not affected. 28 Therefore, for consistency, we used the GGA functional in DFT for deprotonation analyses, transition-state search, and dynamics simulation.…”