Mechanically Responsive Crystals: Analysis of Macroscopic Strain Reveals "Hidden" Processes

Area of Science:

• Materials Science
• Chemical Engineering
• Solid-State Physics

Background:

• A general mathematical model quantitatively describes the mechanical response of single crystals to external stimuli.
• This model allows for the extraction of activation energies and kinetics of solid-state reactions by analyzing crystal bending.
• Organic actuating materials are crucial for developing advanced soft robotics.

Purpose of the Study:

• To investigate deviations from the established mathematical model in crystal mechanical response.
• To identify and characterize
• hidden
• chemical or physical processes influencing crystal deformation.
• To propose an extended model that accounts for complex mechanical behaviors.

Main Methods:

• Tracking time-dependent bending of photobendable single crystals.
• Utilizing X-ray diffraction (XRD) to confirm structural changes.
• Employing differential scanning calorimetry (DSC) to analyze thermal properties.

Main Results:

• Deviations from the model indicated "hidden" processes, including sustained structural relaxation.
• Photobendable single crystals of 4-hydroxy-2-(2-pyridinylmethylene)hydrazide exhibited irreversible E-to-Z isomerization.
• This isomerization led to amorphization and plastic deformation, causing poor correlation between transformation and strain.

Conclusions:

• The study demonstrates that deviations from established models are key indicators of complex underlying phenomena.
• Irreversible isomerization in organic crystals can lead to amorphization and plastic deformation, impacting their mechanical response.
• An extended mathematical model is proposed to better describe the intricate mechanical behavior of such materials.