Near-infrared characterization of water in epoxy networks for modeling property changes

Date of Completion

January 2003


Chemistry, Polymer




Epoxy films were prepared from the stoichiometric reaction of diglycidyl ether of 1,4-butanediol (DGEB) either with 4,4-diaminodiphenyl methane (DDM) or with 4,4-diaminodiphenyl sulfone (DDS). The films were characterized during sorption and desorption experiments using near-infrared, fluorescence, gravimetric measurements, differential scanning calorimetry and dynamic mechanical analysis. There were three water molecules per repeat unit in DGEB-DDM and four water molecules per repeat unit in DGEB-DDS films containing a saturated water content. Fluorescence provided evidence that water hydrogen bonds to the sulfone group and interacts with the tertiary nitrogen groups. These interactions accounted for two-thirds of the water in DGEB-DDM and three-fourths of the water DGEB-DDS. The remaining water hydrogen bonds to the hydroxyl and ether groups. No more than one out of every four-hydroxyl groups per repeat unit in either film formed a secondary cross-link to the tertiary nitrogen, sulfone or ether group based on shifts in the NIR peaks of the free and hydrogen bonded hydroxyl groups in films heated above the Tg. The isolated NIR spectrum for water was deconvoluted to identify three spectroscopically distinct species in which zero, one or two of the hydrogen atoms were hydrogen bonded. The three species designated S0 , S1 and S2 were present in a ratio of approximately 3:9:5 respectively in both saturated polymers. Results were combined to develop a two-dimensional static model of the molecular interactions. The glass transition temperature (Tg) and DMA properties were measured simultaneously to model property changes as a function of the bound water content. Two glass transition temperatures were identified in samples containing a heterogeneous concentration of water, while a single Tg was identified in samples containing a homogeneous concentration of water. Water within the epoxy networks formed more hydrogen bonds with the polymer network than the number of secondary cross-links that were broken. A mathematical model was derived and tested based on the relative mole fraction of the three distinct species of water to predict the Tg of the wet polymer. The mathematical model has the potential to be applied to any polymer that undergoes changes in secondary bonding forces, if no covalent bonds are broken. ^