Molecular Dynamics Simulation of Thermomechanical Properties of Montmorillonite Crystal. II. Hydrated Montmorillonite Crystal

We present the results of molecular dynamics simulation of the thermomechanical behavior of Wyoming-type Na+-montmorillonite (MMT) with water intercalates. Montmorillonite is commonly used as a filler in polymer-clay nanocomposites, and calculation of the elastic properties of the composite requires accurate knowledge of the elastic moduli and thermal properties of the components. To calculate the properties of the filler, we used a computational cell containing two MMT lamellae [Si248Al8] [Al112Mg16]O-640[OH](128) and periodic boundary conditions in all three directions. The galleries between each pair of MMT lamella were filled with 24 Na+ counterions and either one or two water layers (100 and 200 mg/g clay, respectively). The results obtained for the interlayer distance and the number density profiles of water molecules and Na+ ions in galleries are in good agreement with experimental data and results of other computer simulations. The thermal properties were analyzed over the range of 300-400 K; the isothermal linear compressibility and all of the components of the elasticity tensor were calculated. It turns out that the elasticity tensor possesses orthotropic symmetry and changes very weakly with temperature in the range 300-350 K. The calculated in-plane Young's moduli of the hydrated MMT are equal at 180 GPa for the case of an intercalated monolayer of water and 150 GPa for that of an intercalated bilayer of water. The shear moduli parallel to the lamellae decrease from 20 GPa for the monolayer case to 2-4 GPa for the bilayer case. The water interlayer significantly alters the linear coefficient of thermal expansion and Young's modulus perpendicular to the clay lamellae in the hydrated crystal. In the monolayer case, the linear coefficient of thermal expansion K-T,K-Z was only slightly larger than that for pyrophyllite but increased noticeably in the bilayer case.