The physical aging of an epoxy resin based on diglycidyl ether of bisphenol‐A cured by a hardener derived from phthalic anhydride has been studied by differential scanning calorimetry. The isothermal curing of the epoxy resin was carried out in one step at 130°C for 8 h, obtaining a fully cured resin whose glass transition was at 98.9°C. Samples were aged at temperatures between 50 and 100°C for periods of time from 15 min to a maximum of 1680 h. The extent of physical aging has been measured by the area of the endothermic peak which appears below and within the glass transition region. The enthalpy relaxation was found to increase gradually with aging time to a limiting value where structural equilibrium is reached. However, this structural equilibrium was reached experimentally only at an aging temperature of Tg‐10°C. The kinetics of enthalpy relaxation was analysed in terms of the effective relaxation time τeff. The rate of relaxation of the system given by 1/τeff decreases as the system approaches equilibrium, as the enthalpy relaxation tends to its limiting value. Single phenomenological approaches were applied to enthalpy relaxation data. Assuming a separate dependence of temperature and structure on τ, three characteristic parameters of the enthalpic relaxation process were obtained (In A = −333, EH = 1020 kJ/mol, C = 2.1 g/J). Comparisons with experimental data show some discrepancies at aging temperatures of 50 and 60°C, where sub‐Tg peaks appears. These discrepancies probably arise from the fact that the model assumes a single relaxation time. A better fit to aging data was obtained when a Williams‐Watts function was applied. The values of the nonexponential parameter β were slightly dependent on temperature, and the characteristic time was found to decrease with temperature. © 1994 John Wiley & Sons, Inc.

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ABSTRACT:The cure reaction kinetics of epoxy resin, with organically modified montmorillonite loadings of up to 20 wt % and with stoichiometric conditions, has been studied by differential scanning calorimetry with a view to understanding further the fabrication of epoxy-based polymer layered silicate nanocomposites. The kinetic analysis of isothermal and nonisothermal cure shows that the autocatalytic model is the more appropriate to describe the kinetics of these reactions, and it is observed that a dominant effect of the montmorillonite is to catalyze the curing reaction. However, it was not possible to model the reactions over the whole range of degrees of conversion, in particular for nonisothermal cure. This attributed to the complexity of the reactions, and especially to the occurrence of etherification by cationic homopolymerization catalyzed by the onium ion of the organically modified montmorillonite. The homopolymerization reaction results in an excess of diamine in the system, and hence in practice the reaction is off stoichiometric, which leads to a reduction in both the heat of cure and the glass transition temperature as the montmorillonite content increases. Small angle X-ray scattering of the cured nanocomposites shows that an exfoliated nanostructure is obtained in nonisothermal cure at slow heating rates, whereas for nonisothermal cure at faster heating rates, as well as for isothermal cure at 708C and 1008C, a certain amount of exfoliation is accompanied by the growth of dspacings of 1.4 nm and 1.8 nm for dynamic and isothermal cure, respectively, smaller than the d-spacings of the modified clay before intercalation of the resin. A similar nanostructure, consisting of extensive exfoliation accompanied by a strong scattering at distances less than the d-spacing of the modified clay, is also found for resin/clay mixtures, before the addition of any crosslinking agent, which have been preconditioned by storage for long times at room temperature. The development of these nanostructures is attributed to the presence of clay agglomerations in the original resin/clay mixtures and highlights the importance of the quality of the dispersion of the clay in the resin in respect of achieving a homogeneous exfoliated nanostructure in the cured nanocomposite.