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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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In-situ Fourier transform infrared (FTIR) measurements have been carried out at different relative pressures of water vapor to study the H(2)O diffusion in three polyimides differing in their molecular structure and fluorine substitution. Spectral data have been analyzed by difference spectroscopy, least-squares curve fitting, and two-dimensional (2D) correlation spectroscopy, which provided molecular level information on the diffusion mechanism. In particular, two distinct water species were identified corresponding, respectively, to the first and second-shell hydration layers. The spectroscopic analysis demonstrated that the relative population of these species is a function of the total water content in the system. A method has been devised to quantify the water concentration in the two hydration layers, based on a combination of spectroscopic and gravimetric data. The results have been compared with those from an earlier spectroscopic approach reported in the literature and based on the analysis of the carbonyl region.