Theo Rasing
Radboud University, Netherlands
Title: Jumping crystals: Controlled giant thermoelastic deformation of an organic molecular crystal
Biography
Biography: Theo Rasing
Abstract
Thermosalient molecular crystals are characterized by thermally induced single crystal to single crystal phase transitions that are accompanied by sudden anisotropic lattice expansion, giving rise to huge mechanical responses to external stimuli: upon heating or cooling they jump distances many times their own size. This makes these materials invaluable for the design of a new generation of switchable smart materials which are central to, e.g., soft robotics, artificial muscles and microfluidic valves. The abrupt and strong macroscopic shape changes are connected to a structural phase transition inside such crystals. However, the detailed mechanisms of these phase transitions are unknown. In addition, the large changes in crystal shape and size is difficult to accommodate for the often brittle organic crystals and the mechanical effects are usually accompanied by crystal cracking, splitting or even explosion. Here, we report on a layered crystal structure of the fluorenone derivative 4-DBpFO, clearly showing a strong and reproducible shear deformation when it undergoes a structural phase transition upon heating. Moreover, this shear deformation can be observed along two orthogonal crystal directions, which appears to be connected to its crystal structure. The deformation of the single crystal can be controlled by heating/cooling cycles without destroying it. Modelling shows that the shear deformation that accompanies the in-plane anisotropic lattice expansion results from in-plane molecular rotations during the phase transition and follows a nucleation and growth path. We believe that 4-DBpFO could serve as a model structure to guide the development of new types of robust thermosalient organic crystals.
Microscopic images showing the shear deformation of a planar quadrangular single crystal (left) to a diamond shape (right) during the α- to β-phase transformation of 4-DBpFO. The arrow in the middle picture indicates the movement of the phase boundary.
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