Bending of Layered Silicates on the Nanometer Scale: Mechanism, Stored Energy, and Curvature Limits

Abstract

Bending and failure of aluminosilicate layers are common in polymer matrices although mechanical properties of curved layers and curvature limits are hardly known. We examined the mechanism of bending, the stored energy, and failure of several clay minerals. We employed molecular dynamics simulation, AFM data, and transmission electron microscopy (TEM) of montmorillonite embedded in epoxy and silk elastin polymer matrices with different weight percentage and different processing conditions. The bending energy per layer area as a function of bending radius can be converted into force constants for a given layer geometry and is similar for minerals of different cation exchange capacity (pyrophyllite, montmorillonite, mica). The bending energy increases from zero for a flat single layer to ∼10 mJ/m2 at a bending radius of 20 nm and exceeds 100 mJ/m2 at a bending radius of 6 nm. The smallest observed curvature of a bent layer is 3 nm. Failure proceeds through kink and split into two straight layers of shorter length. The mechanically stored energy per unit mass in highly bent aluminosilicate layers is close to the electrical energy stored in batteries. Molecular contributions to the bending energy include bond stretching and bending of bond angles in the mineral as well as rearrangements of alkali ions on the surface of the layers. When embedded in polymers, average radii of curvature of aluminosilicates exceed hundreds of nanometers. The small fraction of highly bent layers (<20 nm radius) can be increased by extrusion, especially in stacked layers, and by an increase in weight percentage of layered silicates above 5%. Extrusion also promotes failure and shortening of isolated layers.