Probing the Molecular Interactions in the Diffusion of Water through Epoxy and Epoxy-Bismaleimide Networks

Fourier transform infrared spectroscopy in the near-infrared (NIR) frequency range was used to investigate the molecular interactions occurring between absorbed water molecules and network based on tetrafunctional epoxy resin. One of these networks was a typical formulation containing 4,4’-diamino diphenylsulfone as a hardner, and the other was a modified resin containing 4,4’-bismaleimide-diphenyl methane (BMI) as a creative monomer. Molecular spectroscopy analysis confirmed the existence of mobile water localized into network defects (microvoids) that did not interact with the networks and water molecules bond to the networks through hydrogen bond interactions. In the BMI-containig system, the fraction of bound water decreased significantly with respect to the unmodified epoxy resin. This was a relevant result because the bound water was primarily responsible for the plasticization of the network and for the consequent worsening of mechanical performance. Water diffusion was investigated with gravimetric sorption measurements and time-resolved Fourier transform NIR spectroscopy measurements. These showed that the presence of BMI decreased the water uptake at equilibrium, enhanced the diffusivity, and reduced the activation energy of diffusion. A dual-mode model for diffusion was found to be suitable for accurately describing the mass transport process in both investigated systems. The results of the model simulations allowed us to estimate the ratio of free and bound water, which was in good agreement with that obtained from the spectroscopic analysis.