Superelastic Organic Crystals

Abstract

Superelastic materials (crystal-to-crystal transformation pseudo elasticity) that consist of organic components have not been observed since superelasticity was discovered in a Au-Cd alloy in 1932. Superelastic materials have been exclusively developed in metallic or inorganic covalent solids, as represented by Ti-Ni alloys. Organosuperelasticity is now revealed in a pure organic crystal of terephthalamide, which precisely produces a large motion with high repetition and high energy storage efficiency. This process is driven by a small shear stress owing to the low density of strain energy related to the low lattice energy.A phase transition has been observed that is maintained in a single solid state induced by heat and pressure [1,2] and by light. [3] In that transition, the solid is affected by the structural distortion that occurs during transition. In an ideal case with the conservation of one-to-one correspondence of the atoms, the solid transformation is regarded as martensitic. [4] In the special case of martensitic transformation, the material acquires superelastic characteristics (transformation pseudo elasticity), which is important from a practical viewpoint and, in fact, is expected for the wide applications to structural materials and devices. Superelastic materials are primarily developed from metallic solids such as Ti-Ni alloys, and recently from ceramics. [5,6] Superelastic material made of organic components, which satisfies the definition of superelasticity, [7] has not been obvious over eight decades since superelasticity was first discovered in a Au-Cd alloy in 1932. [8] Based on the objective state of the current research trends related to superelastic materials, it might have been considered that covalent solids, which have a high degree of interatomic interaction, only fulfill the two essential requirements when being loaded with stress on the body: 1) Conservation of the one-to-one correspondence among neighboring atoms; and 2) storage of the elastic strain, which drives the spontaneous restoring motion back to the original state. In a comparison with the covalently interatomic interaction in alloys and ceramics, the relatively weak intermolecular interaction between organic components can be unreliable for generating superelasticity in molecular solids in which, furthermore, the molecular components vary in geometry and size. In investigating an organic superelastic system, we thought that the science of superelasticity could markedly advance by material diversification and functionalization, as chemical design can be effectively applied to superelastic materials. Thus, we were looking for a pure organic superelastic body as strict and simple as possible, and found the terephthalamide crystal as the first candidate for such a superelastic material. Its superelasticity was characterized by microscopy, stress tests, and X-ray diffraction analysis.Well-formed single crystals of terephthalamide (1) were obtained by recrystallization of a reagent-grade terephthalamide from hot water and used...

Photo-triggered phase transition of a crystal

“…Additionally, enthalpy at the transition β → γ is 10-200 times larger than the energy densities of superelastic and ferroelastic molecular crystals, which can deform with the creation of a new crystalline phase or twin domains by manually applying shear stress [36][37][38][39] . This suggests that it will be difficult to create the γphase by shear stress in the enol-(S)-1 crystal at the β-phase.…”

Chirality-controlled spontaneous twisting of crystals due to thermal topochemical reaction

“…There are two pathways of relaxation leading to different kinds of mechanical responses: (i) slow and continuous relaxation of strain, as and when generated, by mechanical reconfiguration of the crystal leading to bending (2,, twisting (6,10,14,32,33), coiling (16,34), shape change (35), or size change (7,25,31,(36)(37)(38) or (ii) sudden relaxation leading to stimuli-salient effects (2,9,12,(39)(40)(41)(42)(43)(44)(45)(46)(47)(48)(49)(50) such as jumping, hopping, exploding, displacement, and so on, after accumulation of stress over an induction period, to a critical level. All of the known mechanical responses in crystals are due to (i) photoisomerizations (16, 24-31, 34, 43), (ii) photochemical reactions (2, 5-23, 32, 33, 35-37, 39-42), (iii ) pressure-induced change in molecular packing (44,(51)(52)(53)(54)(55)(56)(57), or (iv) heat-induced polymorphic transitions (4,38,(44)(45)…”

A superelastochromic crystal

“…The σ f and E S values are ca. 3 and 9 times, respectively, those of phase transition-based organosuperelasticity in a terephthalamide crystal 15 are ca. 22 and 47 times, respectively, which are those of twinning-based organosuperelasticity in a 3,5-difluorobenzoic acid crystal 23 (Supplementary Table 7).…”

“…In this context, the focus of this work is organosuperelasticity, which we first discovered in 2014 15 . Elasticity is a common physical property in the spontaneous shape recoverability of materials.…”

“…Recently, research on the manner of elastic deformation of organic crystals has been intensive [16][17][18][19][20] despite a general perception of their brittleness. In contrast, superelasticity or more specifically plastic deformation with spontaneous shape recoverability is a minor and unusual physical property, except in special kinds of metallic solids called superelastic alloys and shapememory alloys 21,22 , and research is still in its infancy especially in organic crystals 15,[23][24][25][26][27][28][29][30] . In the elastic deformation, the densitygradient distribution of components causes strain to accumulate, whereas the superelastic deformation can accommodate strain through orientation changes of domains upon phase transition or twinning; thus, superelastic deformation has a potential ability to abruptly change physical properties.…”