Vibrational Spectroscopy Evidence for the Dual Nature of Water Sorbed into Epoxy Resins

Abstract

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.

Probing the molecular interactions in the diffusion of water through epoxy and epoxy–bismaleimide networks

“…15 Vibrational spectroscopy, particularly NIR spectroscopy, is one of the best suited techniques for probing hydrogen-bonded molecular structures. 16 In a previous contribution 10 on water sorbed in TGDDM/DDS networks, there were identified, in the infrared spectrum, signals arising from mobile water localized in microvoids and from water molecules firmly bound to the network by hydrogen-bonding interactions. The stoichiometry of the interaction was determined, and the most likely proton-accepting groups along the network were identified.…”

“…8,9 Recent work has shown that the levels of both bound and free water can be estimated by Fourier transform infrared (FTIR) spectroscopy in the mid-infrared (MIR; 4000 -400 cm Ϫ1 ) and near-infrared (NIR; 8000 -4000 cm Ϫ1 ) ranges. 10 Systems comprising 4,4Ј-bismaleimide-diphenylmethane (BMI), TGDDM, and DDS have attracted considerable attention both in industrial applications 11 and in scientific research [12][13][14][15] because of their relatively low water absorption characteristics and the dual mechanism for curing reactions. A detailed spectroscopic analysis of the curing reactions has shown, in fact, that TG-DDM and BMI form individual networks without intercrosslinks.…”

“…10,[22][23][24][25][26][27] The peak at 7075 cm Ϫ1 is assigned to a combination of the as and s modes of non-hydrogenbonded water (S 0 molecules), whereas the peak at 6820 cm Ϫ1 is attributed to a combination of nb and b fundamentals in S 1 molecules (self-associated dimers). The presence of a small fraction of individual water molecules forming a single hydrogen-bonding interaction with the network cannot be entirely ruled out.…”

“…The proton-accepting groups of the network that have been identified in previ- ous work are the amino alcohol functionalities and the SO 2 groups. 10 For high-T g materials, the plasticizing efficiency of the various water species depends ultimately on whether or not they are molecularly dispersed within the polymeric network and on their molecular mobility. S 0 species, together with S 1 dimers, are expected to have low plasticizing efficiency, as they are confined into excess freevolume elements (i.e., microvoids) and are much more mobile than the hydrogen-bonded species.…”

“…A quantitative analysis of the spectra of water absorbed in the various investigated networks was performed with a curve-fitting algorithm described elsewhere. 10 The results of this analysis are shown in Figure 7(A-C), and the parameters evaluated therefrom are summarized in Table 2.…”