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The physical interactions occurring between absorbed water molecules and the network of a tetrafunctional epoxy resin were studied by mid-infrared (MIR) and near-infrared (NIR) Fourier transform spectroscopy. The spectrum of the sorbed water molecules was isolated in both frequency ranges and the assignments of the various components of the spectrum were proposed accordingly. The results of the vibrational analysis reaffirm the existence of two populations of penetrant molecules, a mobile variety residing within microvoids and those strongly bound to the polymeric network through hydrogen bonding interactions. A comparative analysis of the NIR spectra makes it also possible to deduce the most likely structures of these adducts and their stoichiometry. Finally a method was identified for the quantitative estimation of the bound water responsible for the plasticization of the matrix.

The diffusion of water into polyimide films was studied by in situ FTIR spectroscopy using several methods of spectral data analysis, namely, difference spectroscopy, least-squares curve fitting, 2D correlation spectroscopy, and normal coordinate analysis. The results gave an insight into the molecular mechanism of diffusion in terms of number and population of penetrant species present in the system and with respect to the nature of the molecular aggregates. In particular, two water species were identified and quantified, i.e., H2O molecules interacting with the carbonyl groups of the polyimide and self-associated water. An enthalpy of formation of −0.9 kcal mol-1 was estimated for the H2O−polyimide interaction, which points to a relatively weak H-bonding tendency of the imide carbonyls. Finally, the infrared spectrum of the H2O−imide aggregate was calculated by a quantum mechanic (QM) model chemistry to rationalize the effects observed in the spectrum of the water saturated films. The results of the computation were in good agreement with the experiment, confirming the predictive capabilities of the chosen QM method and supporting the proposed molecular structure of the H-bonding aggregate.

ABSTRACT:Fourier transform infrared spectroscopy in the near-infrared (NIR) frequency range was used to investigate the molecular interactions occurring between absorbed water molecules and networks based on a tetrafunctional epoxy resin. One of these networks was a typical formulation containing 4,4Ј-diamino diphenylsulfone as a hardener, and the other was a modified resin containing 4,4Ј-bismaleimide-diphenylmethane (BMI) as a coreactive 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 bound to the networks through hydrogen-bonding interactions. In the BMI-containing 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 for 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.

ABSTRACT:The kinetics and mechanism of the curing process of a thermosetting blend formed by tetraglycidyl-4,4 -diaminodiphenyl methane and N,N-bismaleimido-4,4 -diphenyl methane (BMI) cured in the presence of 4,4-diaminodiphenyl sulfone, was investigated in detail by Fourier transform infrared spectroscopy. Information on the molecular structure of the network formed upon curing was derived. Dynamic-mechanical measurements on dry samples indicated an interpenetrated polymer network-like structure. Sorption measurements at 70ЊC showed a reduction of the water uptake at equilibrium in the presence of substantial amounts of BMI in the system (43.5% body weight). Finally, the dynamic-mechanical analysis of wet samples demonstrated a reduction of the plasticizing efficiency of the absorbed water in the presence of BMI.

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