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.

Temperature dependent structure and dynamics in smectite interlayers: 23Na MAS NMR spectroscopy of Na-hectorite

“…[29][30][31][32] The temperature and pressure dependence of these interactions is complex and has been a long standing topic of discussion. [33][34][35][36][37][38] The smectite group of clay minerals has been of particular interest in geoscience, chemistry and materials science, because of their abundance and ability to intercalate a wide variety of organic and inorganic cations, along with water and other uid species. Because of their remarkable swelling capacity, they have been of longstanding interest in nuclear waste management, in composites with polymers and in 2D-nanomaterials.…”

“…However, as discussed in detail below there is growing evidence that smectite T-O-T layers are exible and dynamic, and thus that their interlayer galleries could be comparably dynamic. 11,12,[33][34][35][36][37][38]…”

Hybrid and Hierarchical Composite Materials

“…The bend energy ( E b ) of a single clay layer was calculated as (5.29) where E t is the total energy of the bent layer and the two wraps, E l is the energy of the corresponding linear clay layer without wraps, and E c is the energy due to the van der Waals interaction between the bent aluminosilicate clay layer and the carbon wraps. The bending energy evaluated for a single layer of nanoclay increases from 0 to 10 mJ/m 2 for the bending radius of 20 nm (Fu et al 2011). For smaller bending radius on the order of 10 nm, the bending energy of a single layer was calculated to be 50-100 mJ/m 2 .…”

“…The linear properties of the nanoclay composites have been studied in detail, however the properties of the bent nanoclay system are difficult to study by experimental methods. The problem has therefore only been studied by molecular simulations (Fu et al 2011). The bending of the nanoclay layers results in changes in the bond angles and lengths.…”

Simulating the ultrathin layer structure of dichloromethane on a solid substrate by means of density functional theory and molecular dynamics

“…The interatomic forces were calculated using the valence force field PCFF, Polymer Consistent Force Field approach [23], one of the highly regarded family of class II material oriented force fields for predicting the properties of many systems [38][39][40][41][42][43][44][45][46]. The strong coupling potential [47] was applied to calculate the interaction of gold atoms.…”