Shape-Memory Photonic Thermoplastics from Cellulose Nanocrystals

Responsive materials prepared using shape-memory photonic crystals have potential applications in rewritable photonic devices, security features, and optical coatings. By embedding chiral nematic cellulose nanocrystals (CNCs) in a polyacrylate matrix, a shape-memory photonic crystal thermoplastic (CNC-SMP) allows reversible capture of different colored states is reported. In this system, the temperature is used to program the shape-memory response, while pressure is used to compress the helical pitch of the CNC chiral nematic organization. By increasing the force applied ( ≈ 140–230 N), the structural color can be tuned from red to blue. Then, on-demand, the CNC-SMP can recover to its original state by heating it above the glass transition temperature. This cycle can be performed over 15 times without any loss of the shape-memory behavior or mechanical degradation of the sample. In addition, multicolor readouts can be programmed into the chiral nematic CNC-SMP by using a patterned substrate to press the sample, while the glass transition temperature of the CNC-SMP can be tuned over a 90  ° C range by altering the monomer composition used to prepare the polyacrylate matrix.