Superelasticity — a form of elasticity that involves a phase transition — has been observed for the first time in a pure organic crystal. The material could find applications in microfluidics.
Since its discovery1 in 1932, in a gold–cadmium alloy, the property of superelasticity has never been observed in organic crystals — until now. Writing in Angewandte Chemie, Takamizawa and Miyamoto2 report the discovery of this phenomenon in a single crystal of a simple organic molecule, terephthalamide.
In metallic alloys and ceramic materials, the individual components are strongly bound to one another, forming hard crystals. Under an applied stress, some of these materials can undergo a phase transformation, which can lead to macroscopic deformation3. On removal of the stress, the new phase becomes unstable and the initial phase reappears, and with it the original shape. A typical class of such superelastic material is shape-memory alloys3, which can deform to up to 10% of their original size but return to their pre-deformed shape. Titanium–nickel alloys are the main type of shape-memory materials, and have applications in devices such as medical stents and spectacle frames4.
Takamizawa and Miyamoto examined a soft crystal of terephthalamide about 150 micrometres thick and 59 μm wide. The crystal was initially in what is called the α phase (mother phase) and was pushed with a metal blade, 25 μm wide, against one crystal surface at a speed of 500 μm per minute. The authors found that, when the stress applied by the blade reached a constant value, the crystal underwent a phase transformation into a daughter phase (β phase) at the contacting area between the blade and the surface. Interestingly, the daughter phase grew first along the pushing direction of the blade, but when this phase hit the bottom of the crystal it grew at a right angle to the pushing direction (Fig. 1).
As the daughter-phase region grew and propagated, the crystal bent at the interface between the two phases. When the authors pulled the blade back, the area of the daughter phase started to decrease and the crystal underwent the reverse phase transformation, eventually reverting to its original shape. The researchers repeated this transformation cycle 100 times, and showed that the crystal deformed by up to 11.34% of its original shape. The stress necessary to induce the transformation from the mother to the daughter phase is roughly 1,000 times smaller than that for the equivalent transformation in the typical titanium–nickel alloy.
In the terephthalamide crystal, molecules associate to form sheets that are held together by a network of hydrogen bonds. Because each terephthalamide molecule has four sites with which to form hydrogen bonds, owing to the presence of an amide group (CONH2) at each end of the molecule's benzene ring, the network contains end-to-end double hydrogen bonds along the long axis of the molecule and side-to-side double hydrogen bonds along its short axis. These two-dimensional structures stack together to form the three-dimensional crystal. Takamizawa and Miyamoto found that in the β phase the terephthalamide molecules are more densely packed than in the α phase, but that the hydrogen-bond network is maintained. And this latter feature turns out to be the key to superelasticity.
The intermolecular forces that hold organic crystals together are usually much weaker than the interatomic covalent forces that bind together alloys and ceramics. However, in the present system, the collective hydrogen-bond network along the long and short axes of terephthalamide strengthens the otherwise weak intermolecular forces enough to prevent the crystal from fracturing on application of stress. More generally, the authors' study demonstrates the importance of hydrogen bonds in the supramolecular architectures of soft materials5. Because hydrogen bonds are much weaker than covalent bonds, supramolecular structures based on hydrogen bonds are more flexible against applied perturbations such as mechanical force, heat and light. This flexibility means that dissociation and association of the components that make up the supramolecular structure take place easily on application of such external stimuli, dissipating the applied perturbation smoothly6.
Soft superelastic materials could find several applications. For example, in microfluidic devices, the pressure of the fluid that flows in microchannels needs to be maintained below a crucial level to avoid damage to the channels. Generally, external pumps or internal valves control such pressure, but independent sensors are used to measure it. A superelastic organic crystal such as that presented here could be used to make internal valves that both sense and control the pressure in these devices. Such superelastic materials could also act as fillers in shock absorbers designed to dampen shock and vibration.