Cleavage Energy of Alkylammonium-Modified Montmorillonite and Relation to Exfoliation in Nanocomposites: Influence of Cation Density, Head Group Structure, and Chain Length

Abstract

Cohesion between layers of organically modified aluminosilicates creates a barrier to exfoliation in polymer matrices and can be thermodynamically described by the cleavage energy. While the measurement of cleavage energies at the nanometer scale is experimentally challenging, molecular simulation indicates a tunable range between 25 and 210 mJ/m2 upon variation of the cation exchange capacity (CECs of 91 meq/100 g and 143 meq/100 g), headgroup chemistry (primary and quaternary ammonium), and chain length (C2 to C22) in a series of 44 alkylammonium-modified montmorillonites. Designed organoclays of low cleavage energy can support exfoliation when the surface tension, which relates to reconstructed surfaces after cleavage, is comparatively higher. The difference between high and low cleavage energies is associated with Coulomb interactions depending on the position of the positively charged surfactant head groups in the interlayer with respect to the two clay layers at equilibrium separation and further wi...