The effect of stoichiometry on curing and properties of epoxy-clay nanocomposites

Epoxy-clay nanocomposites have been prepared with an organically modified montmorillonite. The epoxy network was based on diglycidyl ether of bisphenol A (DGEBA) cured with diaminodiphenylmethane (DDM). The stoichiometry DGEBA-DDM was varied, the molar ratio of amine hydrogen/epoxy groups, r, ranged from 0.85 to 1.15. The influence of stoichiometry on curing and properties of the nanocomposites was studied using differential scanning calorimetry, dynamic mechanical thermal analysis and X-ray diffraction. All nanocomposites had intercalated clay structures. The clays accelerated the curing reaction whose rate was also increased when increasing r. The heat of reaction, -Delta H (J/g epoxy), increased as r increased, reaching a constant value for r >= 1. In the presence of clays -Delta H was lower than in the neat DGEBA-DDM. The glass transition temperature (T (g)) of the neat epoxy thermosets reached a maximum at r = 1; however, the nanocomposites showed the T (g) maximum at 0.9 < r < 1. The presence of clay lowered the T (g) for r > 0.94 and raised T (g) for r <= sign 0.85. The elastic modulus of neat epoxy thermosets reached a maximum in the rubber state and a minimum in the glassy state at r = 1. The nanocomposites showed similar behavior, but the maximum and the minimum values of the elastic modulus were reached at stoichiometry r < 1. The comparison of the properties of neat epoxy with those of the nanocomposites varying the stoichiometry indicates that the clay itself induces stoichiometric changes in the system.