Deformation Measurements on Thin Clay Tactoids

Abstract

Many modern composite materials contain clay platelets as functional compounds. The clay filler determines properties like barrier activity, [1] fire retardancy, [2] electrical properties, [3] and mechanical reinforcement. [4] Recent work has brought the mechanical properties of polymer-layered silicate nanocomposites (PLSNs) even closer to ultrastrong and stiff natural materials like nacre. [5][6][7] For most of these applications, it is crucial to maximize the aspect ratio (lateral extension divided by height) of the clay platelets, so-called tactoids, which are quasi-crystals consisting of between two and up to several thousands of individual silicate lamellae stacked in a parallel mode along c à (Scheme 1). At the same time the stiffness of the tactoids has of course to be preserved. This issue is not trivial because increasing the aspect ratio by exfoliation (definition according to Lagaly et al. [8] ) necessarily will produce thinner and thinner tactoids and eventually they will be of nanoscale thickness.Up to now, very little information has been available about the elastic properties of individual (natural) clay tactoids. The edges of natural clay tactoids are badly fringed and the lateral extension of natural clay platelets is limited to less than 300 nm, [9,10] rendering measurements on individual tactoids (typical thickness 10 nm) nearly impossible with conventional procedures. Therefore, bulk values are usually extrapolated to nanoscale dimensions, which might be critical since elastic constants can show size dependencies. [11] Even reliable bulk values of natural swelling clays (e.g., smectites like montmorillonite) are inherently difficult to obtain: it is very tedious to separate auxiliary minerals without affecting the clay structure at the same time and the charge density of these natural clay minerals is inhomogeneous, leading to interstratified materials, that is, neighboring interlayer galleries within one tactoid exhibit different compositions, for example, different hydration states. [12] Consequently, reliable information about elastic constants of ''bulk clay'' is scarce. In an overview, Chen et al. [13] discussed the mechanical properties of clays obtained via experimental or theoretical approaches: the latter include computer simulations on montmorillonite. [14,15] Experimental results were, inter alia, achieved by extrapolation of acoustic measurements (dickite, kaolinite, montmorillonite) [16,17] or by extrapolation of data of epoxy-clay hybrids [18] or via compressibility measurements. [19][20][21][22] The most reliable experimental bulk elastic constants of 2:1 layered silicates were obtained via ultrasonic pulse method, [23] inelastic neutron scattering, [24] and Brillouin scattering [25,26] of micas. The values obtained were partially crosschecked by computer simulations. [27] Micas are nonhydrated and well-crystalline relatives of smectites within the family of 2:1 layered silicates. Micas, however, carry a two-to threefold higher charge density as compared to smectites. Please no...

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

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Elastic Properties of Freely Suspended MoS2 Nanosheets

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Mechanical properties of freely suspended atomically thin dielectric layers of mica

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