Time‐dependent changes in morphology of neat and reinforced epoxy resins part I. Neat epoxies

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Abstract
An investigation was conducted of the morphological changes in neat epoxy resins after cure, that is, during cooling and subsequent “annealing” at a temperature below the glass transition. A series of bifunctional and tetrafunctional epoxy formulations were prepared and studied. A simultaneous drive toward thermodynamic equilibrium (homogeneity) and morphological inhomogeneity was found to take place in the glassy network. A conceptual model was advanced to explain the time‐dependent development of inhomogeneous morphology in the glassy state. After a certain sub‐Tg annealing time, “morphological equilibrium” is reached while physical aging of the glassy network (decrease in enthalpy and/or free volume) continues to occur. The proposed model, based upon the variation of contractive forces throughout the epoxy network, was corroborated by experimental evidence obtained from calorimetric and viscoelastic measurements and electron microscopy.
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