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The mechanical properties of epoxy/clay nanocomposites were investigated in relation to the particle size and shear stiffness state of the dispersed nanoplatelets. The fracture toughness and the underlying toughening mechanisms were thoroughly discussed in detail. For this study, a highly pure synthetic fluorohectorite with large lateral extensions (&3.8 lm) was used and compared to natural montmorillonite characterized by significantly smaller lateral extensions (&400 nm). Moreover, for the synthetic fluorohectorite, the subtle balance between layer charge density and the hydration enthalpy of interlayer cations allows for switching between a shear-labile and shear-stiff state, something impossible for the natural material. To ensure optimum dispersion, solution blending was followed by three roll milling for nanocomposite preparation. The addition of all three types of clay used in this study provoked a decrease in glass transition temperature, which indicated a moderate interfacial strength. The maximum increase in fracture toughness and strain energy release rate was observed for the nanocomposites prepared with the large and shear-stiff fluorohectorites at a particle content as low as 2.2 vol%. Morphological investigations by scanning electron microscopy of the fracture surfaces revealed the contribution of several micro-mechanical toughening mechanisms. In contrast to the small natural montmorillonite, the large synthetic nanoplatelets promoted additional energy dissipating mechanisms such as crack deflection and crack pinning leading to an enhanced fracture toughness. These observations are discussed in details using fracture mechanical approaches.

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The potential of platy nanofillers like clays in polymer nanocomposites is mostly determined by their aspect ratio. The degree of improvement that may be achieved in respect to reinforcement, gas-barrier properties and flame retardancy critically depends on the aspect ratio. Thus, increasing the aspect ratio is highly desirable in order to explore the full potential of the clay filler. Mechanical shear stress as generated in the grinding chamber of a stirred media mill (ball mill) induced an efficient exfoliation of highly hydrated and therefore ‘shear-labile’ synthetic Mg-fluorohectorite in aqueous dispersion. The attainable degree of exfoliation can be tuned and controlled through the shear forces applied by changing process parameters such as solid content and grinding media diameter. Characterization and evaluation of the exfoliation efficiency during milling was achieved by combining and cross-validating data obtained by powder X-ray diffraction (XRD), static light scattering (SLS), specific surface area measurements applying the Brunauer-Emmett-Teller (BET) equation, and scanning electron microscopy (SEM). This led to the identification of optimal processing parameters, allowing for control of the degree of exfoliation and, consequently, the aspect ratio of the nanoplatelets. Not surprisingly, besides exfoliation, increasing the magnitude of the shear stress also resulted in some reduction in platelet size.The clay platelets obtained showed a high average aspect ratio (>600), several times greater than that of original synthetic fluorohectorite. The increase of aspect ratio was reflected in a significant enhancement of both specific surface area and cation exchange capacity (CEC) of the external basal surfaces. This method has substantial advantages compared to microfluidizer processing with respect to feasibility, batch size and particle diameter size preservation. The exfoliated nanoplatelets obtained by milling have great potential to improve mechanical properties of polymer layered silicate nanocomposites (PLSN).