Epoxy-clay nanocomposites : morphology, moisture absorption behavior and thermo-mechanical properties

Nanocomposites based on polymer matrix are of great industrial as well as scientific interest. In a nanocomposite, one of the phases is dispersed on a nanometer (10-9m) scale, which is a new scale available intermediate between molecular- and micro-scale. The potential nano-effect for unique material properties is apparent. The major interest in the development of nanocomposites lies in the dramatically improved properties. The improvements include mechanical properties, thermal properties, barrier properties, electrical properties, chemical stability, etc. The objective of this thesis is to study of the epoxy-based nanocomposite with different curing systems, clay types, loadings and additional ultrasonication process to obtain desired material properties, as well as some insights into the mechanisms of how nanocomposites exhibit property enhancement and how nanocomposites sustain damage, such as under impact condition. Three different types of organoclay, namely the octadecylamine modified montmorillonite (I30P) and the quaternary alkylamine modified montmorillonite (KH-MT and Cloisite), and four types of curing agents, namely D230, E007B, F206 and mPDA, were used. The morphology of synthesized nanocomposites were examined by XRD and directly observed by TEM. The effects of clay type and curing agent on the mechanical properties, the effects of clay type and curing agent on the thermal and thermomechanical properties, the effect of morphology of nanocomposites on the moisture barrier properties and moisture-induced thermomechanical property changes, as well as the dielectric properties of nanocomposites were studied. XRD diagrams demonstrate that a longer mixing time would slightly benefit the extent of intercalation. Higher clay content will limit the dispersion level of silicate layers into epoxy. XRD also indicates the formation of intercalated nanocomposite for Cloisite and KH-MT filled epoxy system regardless of curing agents. TEM photographs confirm this conclusion. Intercalated/ordered-exfoliated nanocomposites are formed for I30P filled epoxy system regardless of curing agents. The morphology of nanocomposites mainly depends on the clay modification. Primary alkylammonium ions show better efficiency in silicate layers separation. Both intercalated and intercalated/ordered-exfoliated nanocomposites are mainly in the form of aggregate and retained the initial stacking arrangement. The ultrasonication process can effectively separate the primary particle from the aggregates of the initial clays, and lead to a better dispersion. The observations showed that the properties enhancements of nanocomposites strongly depended on the micromorphology of the silicate layers of clay in nanocomposites. Considerable increase of flexural modulus was observed for the I30P reinforced composites, and less pronounced increase for Cloisite reinforced composites. The modulus increments were at the expense of concomitant drops in flexural strengths, but it can be optimized by ultrasonic treatment. Ultrasonic treatment can further increase flexural modulus, and diminish the magnitude of flexural strength decrease. A considerable reduction of CTE was observed for intercalated/ordered-exfoliated nanocomposites, but a negligible reduction for intercalated nanocomposites. The decomposition temperature of nanocomposites increased substantially with increasing clay content for both the Cloisite and I30P filled systems. The well dispersed clay particles in the I30P system provided a better moisture barrier property than the KH-MT system or neat epoxy: the moisture absorption rate of the former system was much slower than the latter two systems. The equilibrium moisture gain increased with increasing KH-MT clay content, whereas the reverse is true for the I30P system. The Fickian equation was used to predict the moisture diffusion through the KH-MT and neat epoxy systems, whereas the non-linear Fickian equation based on the 1D Langmuirian solution should be used to fit the experimental data for the I30P system. The diffusivity thereby determined decreased with increasing clay content, the reduction being more pronounced for the I30P system than the KH-MT system. The normalized moisture permeability showed a systematic decrease with increasing clay content, which agrees well with the prediction based on the tortuous path model. Keywords: epoxy-clay nanocomposite, intercalation, exfoliation, flexural strength/modulus, fracture toughness, CTE, Tg, permeability, diffusivit