Surface characteristics of layered silicates:: Influence on the properties of clay/polymer nanocomposites

A sodium montmorillonite and six organophilic montmorillonites coated with different surfactants were characterized in various ways in the study. Particle and surface characteristics were determined by nitrogen adsorption and inverse gas chromatography, respectively. The gallery structure of organophilic montmorillonite, the orientation of surfactants in the galleries, and surface coverage were estimated by X-ray diffraction measurements and model calculations. The effect of organophilization on the properties of polypropylene/ clay composites was determined by the measurement of tensile properties. The results showed that the surface energy of uncoated layered silicates is large; thus, the forces keeping the layers together are very strong. The long chain surfactants used for the coating of montmorillonite orientate more or less parallel to the surface and usually cover the platelets in a single layer in commercial silicates. Surplus surfactant is not located in the galleries, but among the particles, and might influence the properties of composites negatively. Organophilization leads to the drastic decrease of surface free energy. Surface tension of all coated clays is practically the same, irrespective of the type of the surfactant used for treatment. Low surface energy leads to weaker forces between the layers, which might facilitate exfoliation. This effect can be further enhanced by the use of surfactants with two long aliphatic chains, one of which orientates vertically to the surface, leading to larger gallery distance. Polymer/silicate interaction also decreases as an effect of decreasing surface tension proved by the decrease of tensile yield stress of polypropylene/ montmorillonite composites. Besides surface tension, the exfoliation of layered silicates is influenced by several other factors as well, like gallery distance, mutual solubility of the components, competitive adsorption, or possible chemical reactions.