An Organosuperelastic Mechanism with Bending Molecular Chain Bundles

Abstract

Specific kinds of alloys, so-called shape-memory alloys (SMAs), show superelasticity. Their mechanism has been known as slight atomic rearrangement by keeping relative positional relationships. Contrary to this, not only molecular rearrangement but also orientational and conformational changes are involved in superelasticity in molecular single crystals. Herein, we investigate superelasticity correlated with molecular bending in a crystal formed by a one-dimensional hydrogen-bonding supramolecular chain of pimelic acid. Because of the structural analogy of the supramolecular chain and polyethylene as one of the linear covalent polymers, the bending mechanism has a possibility of being applied to the similar deformability in the crystalline polymer solids. According to single-crystal X-ray structural analyses, the superelasticity based on mechanical generation and spontaneous dissipation of a thermodynamically unstable phase is induced by bending of the molecular chains along with a normal stress effect based on conformation changes via precession movement.

Phase-Diagram Rationalization of Thermally and Mechanically Triggered Multiple Mechanical Responses of an Organic Crystal

“…The rotation angle was measured graphically from the single-crystal X-ray structure. The molecular movement is completely different from that in superelastic behavior: i.e., bending of PA by conformational change of the alkyl group causing a reversible polymorphic phase transition between thermally stable modification III and metastable modification II at ambient temperature . There are no strong hydrogen bonds across the twinning interface.…”

“…In Figures 1e,f bending of PA by conformational change of the alkyl group causing a reversible polymorphic phase transition between thermally stable modification III and metastable modification II at ambient temperature. 42 There are no strong hydrogen bonds across the twinning interface. A molecular electrostatic potential map of PA (Figure S4) confirmed that only relatively weak intermolecular interactions (very weak hydrogen bonding and dispersion forces) are possible across the twin plane.…”

Mechanical twinning in organic crystals

“…Ferroelastic behaviors are also caused by mechanically induced polymorphic transitions (pFE) at a specific temperature range, leading to another type of SME based on pFE. 24,25,29,86…”

“…The relationship is achieved not by an actual molecular rotation of 180°but by small molecular movements of a (super)molecule having a pseudo-twofold rotation symmetry element. The movements are an orientational rotation of a rigid molecule, 70,71 flipping of phenyl rings 62 or cyclohexyl rings, 63 chain rotation of an alkyl chain, 29,65 or rotation of functional groups 68 in conformationally flexible molecules, and rotation of guest molecules in a host framework. [15][16][17] The movements can be reduced in a crystal having two or more crystallographically independent molecules (Z′ >1) (Fig.…”

“…Such diffusionless orientational transition is referred to as a kind of martensitic transition. 20 As a side note, there is another kind of mechanically induced martensitic transition: a polymorphic transition, which can drastically change the shapes [21][22][23][24][25][26][27][28][29] and/or functions of crystals [30][31][32][33][34][35] and has been reviewed recently. 20,36 In addition to morphological and functional aspects, strain-independent deformation stress in mechanical twinning due to cooperative molecular movements is also noteworthy.…”