Molecular Dynamics simulations were used to investigate the effects of absorbed water on interfacial adhesion in an epoxy glass composite. The composite comprised SiO 2 fibres,-Aminopropyltriethoxysilane (AMPTES) fibre sizing, and a diglycidyl ether bisphenol A (DGEBA)/isophorone diamine (IPD) epoxy matrix. An average solubility limit of 2.07 wt.% water was found in the simulated system, equivalent to 0.99 wt.% water in a full composite, while an experimental value of 1.15 wt.% was determined. The work of adhesion was calculated for simulated composites and compared to experimental tensile strength in unidirectional composites. Results for both dry and saturated conditions indicated SiO 2-sizing as the critical interface for failure. Scanning electron microscopy of failed composites suggests more cohesive failures in the epoxy for the dry samples, while relatively clean fibre surfaces in the saturated samples indicate adhesive failure at the interface. This study shows how molecular simulation can be applied to sized fibre-reinforced composites.

Abstract. This study presents molecular dynamics (MD) simulation methods for determining the solubility limit of water in a crosslinked epoxy network. Procedures are first presented for dynamically crosslinking an epoxy network consisting of diglycidyl ether bisphenol A (DGEBA) and isophorone diamine (IPD). Water molecules are then introduced into the crosslinked DGEBA-IPD structure. The excess chemical potential for the absorbed water was determined through combining thermodynamic integration and Widom's test particle insertion methods. The limiting moisture uptake of the epoxy structure was determined through comparing the reduced chemical potential of the water held within the epoxy to that of pure water. The DGEBA-IPD epoxy system was found to have a moisture solubility of 3.50-3.75 wt.% when immersed in water at 300 K.

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A model is proposed which relates externally applied tensile stresses to changes in absorption capacity as well as diffusion rate. The model postulates that changes seen in the diffusion process are the result of stress-dependent changes in the free volume of the epoxy resin. The free volume changes of the resin are calculated through laminate plate theory, which itself becomes a function of fiber angle as well as a host of elastic properties of the constituents. Consequently, according to the proposed model, changes in diffusion parameters are dependent upon the magnitude of applied stress, the loading angle, as well as elastic properties of the constituents. Additionally, a finite element model is presented. The proposed finite element model establishes an analogy between thermal and mass diffusion for use in solving the moisture diffusion problems, both in free and stressed states. Input parameters for the FE model are found through use of the previously established mathematical diffusion model. In order to experimentally verify the proposed models, a series of epoxy glass laminate samples were manufactured at varying fiber angles and immersed in a moist environment while subjected to varying levels of tensile loading. Weight gain measurements were recorded throughout the diffusion process until full saturation was achieved. The experimental values exhibited excellent agreement with both the suggested theoretical model and the finite element model.