Nanomechanics and Dispersion of Layered Silicates in Polymer Matrices: Moduli, Cleavage Energy, and Free Energy of Exfoliation
Mechanical properties of layered silicates on the nanometer scale have been discussed controversially in spite of extensive application in polymeric materials, thin films, cosmetics, and drilling liquids. We employed mutiscale simulation and experimental data to quantify tensile moduli, shear moduli, and potential failure mechanisms for a range of minerals (mica, montmorillonite, pyrophyllite) up to extreme stress. Anisotropy and stress-dependence are closely related to the presence of rigid layers and interlayer spaces of variable cation density. In-plane tensile moduli (xx and yy) are ~160 GPa independent of cation density and stress.[p]A thermodynamic model for exfoliation of layered minerals in polymer matrices further reveals the importance of the cleavage energy. Cleavage energies remain difficult to measure in experiment and the analysis of cleavage energy and surface reconstruction of 50 different alkylammonium montmorillonites using molecular dynamics simulation shows a wide range of values between 25 and 200 mJ/m2 that suggests how to lower the free energy of exfoliation without highly functionalized surfactants and polymers.