doi:10.1038/nindia.2012.155 Published online 19 October 2012
Researchers have discovered a new kind of elastic and bendable organic crystal using caffeine, chloro-nitrobenzoic acid and methanol. When force is applied, these crystals bend quickly but regain their original shape without any deformation when force is withdrawn.
Such properties will be very handy in making high-performance flexible materials for optoelectronic devices.
Self-healing materials are a boon for electronic devices. Metal-organic materials display high structural flexibility during lattice expansion or compression. However, achieving high flexibility in fully organic materials has been challenging.
The researchers prepared a caffeine co-crystal mixing caffeine (CAF) with chloro-nitrobenzoic acid (CNB) in methanol solution using a slow evaporation method. The CAF and CNB molecules formed comb-like two-dimensional sheets with channels formed by disordered methanol molecules.
For flexibility testing, the researchers held a crystal of about 0.1 mm thickness and 5 mm length with a pair of forceps and pushed it with a metal pin. This bent its shape without breaking. When the push was withdrawn, the crystal quickly regained its original shape without any cracks.
The crystal finally broke when it was pushed beyond a certain radius. However, the two broken halves underwent excellent shape recovery. A crystal is considered truly elastic if all its constituent molecules regain the original thermodynamic positions on withdrawal of mechanical stress. To clarify this, a bent crystal mounted on a nylon loop was examined. The results confirmed the exceptional elastic nature of the crystal with recovery of molecular positions at the microscopic level.
"Such flexible organic crystals will find use as efficient charge transporters in electronics, mechanical actuators and artificial muscles," says lead researcher C. Malla Reddy.