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...
We examined the free surface as well as the substrate−film interface of thermally equilibrated block copolymer films forming stacks of glassy−rubbery lamellae and having quantized film thickness due to terrace formation. Upon reversing onto a new substrate, ∼100 nm thick floated films deform without loss of the film integrity, so that the characteristic macroscale topographic pattern is transmitted to the newly formed free surface. The adhesion-driven deformation is attributed to a local shear of the step regions in order to adjust the surface relief structures to the flat substrate geometry. Further, the polystyrene sheet at the film− substrate interface exhibits a heterogeneous phase structure which we assign to a partial autophobic dewetting. Stepwise erosion and reconstruction of the inner structure of the film disclosed a precise connection of the alternating wetting conditions at the substrate with the surface topography of the top layer. Apart from unveiling an adaptive mechanical behavior of nanostructured polymer films under confinement, observations reported here can be used for designing superimposed topographic structures by controlling the wetting at the substrate as well as allowing better prediction of possible mechanisms of structure formation and pattern transfer on chemically patterned surfaces.
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