Mechanical Anisotropy in Crystalline Saccharin: Nanoindentation Studies by Kiran et.al

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The nanoindentation technique has been employed to relate the
mechanical properties of saccharin single crystals with their internal
structure. Indentations were performed on (100) and (011) faces to assess the
mechanical anisotropy. The load-displacement (P-h) curves indicate significant differences in the
nature of the plastic deformation on the two faces. The P-h curves obtained on the (011) plane are smooth, reflecting homogeneous
plasticity. However, displacement bursts (pop-ins) are observed in the P-h curves obtained on the (100) plane suggesting a discrete deformation
mechanism. Marginal differences exist in the hardness and modulus on the two
faces that may, in part, be rationalized, although one notes that saccharin has
a largely three-dimensional close-packed structure. The structural origins of
the fundamentally different deformation mechanisms on (100) and (011) are
discussed in terms of the dimensionality of the hydrogen bonding networks. Down
the (100) planes, the saccharin dimers are stacked and are stabilized by
nonspecific van der Waals interactions mostly between aromatic rings. However,
down the (011) planes, the molecules are stabilized by more directional and
cross-linked C-H...O hydrogen bonds. This anisotropy in crystal packing
and interactions is reflected in the mechanical behavior on these faces. The
displacements associated with the pop-ins were found to be integral multiples
of the molecule separation distances. Nanoindentation offers an opportunity to
compare experimentally, and in a quantitative way, the various intermolecular
interactions that are present in a molecular crystal.

for more about the paper, Please visit:

http://pubs.acs.org/doi/abs/10.1021/cg1009362

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