The intermolecular interactions and structural features in crystals of seven halogenated N-benzylideneanilines (Schiff bases), all of which exhibit remarkable flexibility, were examined to identify the common packing features that are the raison d'être for the observed elasticity. The following two features, in part related, were identified as essential to obtain elastic organic crystals: 1) A multitude of weak and dispersive interactions, including halogen bonds, which may act as structural buffers for deformation through easy rupture and reformation during bending; and 2) corrugated packing patterns that would get interlocked and, in the process, prevent long-range sliding of molecular planes.
Distinct
macroscopic mechanical responses of the three crystals
of naphthalene diimide derivatives, 1Me, 1Et, and 1nPr, studied here are very intriguing because
their molecular structures are very similar, with the difference only
in the alkyl chain length. Among the three crystals examined, 1Me shows highly plastic bending nature, 1Et shows
elastic flexibility, and 1nPr is brittle. A detailed
investigation by nanoindentation and molecular dynamics (MD) simulations
allowed us to correlate their distinct mechanical responses with the
way the weak interactions pack in crystal structures. The elastic
modulus (E) of 1Me is nearly an order
of magnitude lower than that of 1Et, whereas hardness
(H) is less than half. The low values of E and H of 1Me indicate that
these crystals are highly compliant and offer a low resistance to
plastic flow. As the knowledge of hardness and elastic modulus of
molecular crystals alone is insufficient to capture their macroscopic
mechanical deformation nature, that is, elastic, brittle, or plastic,
we have employed three-point bending tests using the nanoindentation
technique. This allowed a quantitative evaluation of flexibility of
the three mechanically distinct semiconducting molecular crystals,
which is important for designing larger-scale applications; these
were complemented with detailed MD simulations. The elastic 1Et crystals showed remarkable flexibility even after 1000
cycles. The results emphasize that the alkyl side chains in functional
organic crystals may be exploited for tuning their self-assembly as
well as their mechanical properties. Hence, the study has broad implications,
for example, in crystal engineering of various flexible, ordered molecular
materials.
We report the design of a series of nonhalogenated and halogenated molecular crystals with specific structural features, which are essential for pronounced elasticity. These features involve (a) isotropic weak and dispersive interactions, and (b) corrugated molecular packing with interlocked structures. The effects of intermolecular interactions on the elastic properties of the crystals are ascertained using nano-scale mechanical characterization methods.
The intermolecular interactions and structural features in crystals of seven halogenated N-benzylideneanilines (Schiff bases), all of which exhibit remarkable flexibility, were examined to identify the common packing features that are the raison dÞtre for the observed elasticity. The following two features, in part related, were identified as essential to obtain elastic organic crystals: 1) A multitude of weak and dispersive interactions, including halogen bonds, which may act as structural buffers for deformation through easy rupture and reformation during bending; and 2) corrugated packing patterns that would get interlocked and, in the process, prevent long-range sliding of molecular planes.
Mechanical properties of single-walled carbon nanohorns (SWNH) and SWNH plus few-layer graphene (EG)-reinforced poly(vinyl alcohol) (PVA) matrix composites have been measured using the nanoindentation technique. The elastic modulus (E) and hardness (H) of PVA were found to improve by ∼315% and ∼135%, respectively, upon the addition of just 0.4 wt % SWNH. These properties were found to be comparable to those obtained upon the addition of 0.2 wt % single-walled nanotubes (SWNT) to PVA. Furthermore, upon binary addition of 0.2 wt % EG and 0.4 wt % SWNH to PVA, benefits in the form of ∼400% and ∼330% synergy in E and H, respectively, were observed, along with an increased resistance to viscoelastic deformation. The reasons for these improvements are discussed in terms of the dimensionality of nanocarbon, the effectiveness of nanocarbon and polymer matrix interaction, and the influence of nanocarbon on the degree of crystallinity of the polymer. The results from SWNH reinforcement in this study demonstrate the scope for a novel and, in contrast to SWNT composites, a commercially feasible opportunity for strengthening polymer matrices.
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