Cellulose-based photonic crystal nanofilms with stretching-responsive circularly polarized luminescence and high mechanical robustness.

This work applies cellulose-based nanofilm as the platform to design a kind of stretching-responsive circularly polarized luminescence (CPL) material. Hierarchical structure photonic crystal material is prepared through the co-assembly of cellulose nanocrystals (CNCs) and poly(vinyl alcohol) (PVA) additive. By optimizing the amounts of PVA, the composite film presents high tensile strength (61.83 MPa) and superior modulus (656.67 MPa). Next, the lanthanide chromophore that wraps in the PVA matrix is doped into the CNC organization to produce the CPL. Uniform chromophore distribution and the chiral induction effect make the nanofilm with a large quantum yield and considerable luminescent dissymmetry factor (g). Then, salting-out treatment is applied to induce the formation of PVA nanocrystals in the hierarchical organization, which efficiently enhances the film's toughness and mechanical flexibility. Two-dimensional X-ray diffraction analysis demonstrates that stretching the film can unwind the chiral spiral organization and vary the film's inner arrangement, which causes the attenuation of both the chirality and CPL. Finally, this work applies the thermal dehydration treatment to reconstruct the interior hydrogen bond interactions of the film, to enhance its waterproofing performance significantly. This water-resistant CNC film can be applied to sense the Ca concentration and is expected in the wearable area.