Anisotropic molecular movements in organic crystals by mechanical stress

Nanoindents and nanoscratching on organic crystals with their anisotropic molecular packing explore far-reaching molecular movements or abrasion of material by the application of mechanical stress. Only intermolecular hydrogen bonds and van der Waals 'bonds' are broken in these experiments. Thiohydantoin (1) is a particularly suitable model owing to its well-defined layered crystal packing and its cleavability at the (10-2)-planes. Indents with a cube corner indenter on the natural (110)-face give piling-up of material by molecular movements only to one side by following the direction of the slopes of the molecular monolayers. Four orthogonal nanoscratches along and across the long crystal edge (each back and forth) on (110) give four different very distinct results: (1) molecular movements to both sides and in front with symmetric piling-up, (2) abrasion of material, (3) molecular movements to the right and (4) molecular movements to the left. The influence of the single molecular layers that stand 66degrees steep is clearly expressed by these results. No molecular movement but abrasion of material occurs upon nanoscratching on the (10-2) cleavage plane of 1, now with the molecular monolayers flat. These anisotropic molecular responses to mechanical stress are detected by atomic force microscopy. They may be of use in nanotechnology, but cannot be predicted by the more common collective mechanical properties such as hardness or Young's elasticity modulus, quantities that do not at all relate to the extreme anisotropies that are reported here for the first time. Copyright (C) 2002 John Wiley Sons, Ltd.