Tuning the cellulose nanocrystal alignments for supramolecular assembly of chiral nematic films with highly efficient UVB shielding capability†
The exploration of clean and sustainable cellulose based films with ultraviolet (UV) shielding capabilities has gained considerable attention due to the current situation of ozone layer damage and potential risk of skin cancer, mostly caused by radiation from the UVB region (wavelength: 280–320 nm). In the present work, cellulose nanocrystal (CNC) films with a characterized chiral nematic structure were used for the first time to fabricate UV shielding materials by facile and convenient esterification with salicylic acid (SA) and p-aminobenzoic acid (PABA). The prepared films present perfect UVB-shielding properties of 99.7% and an ordered cholesteric organization showing birefringence after evaporation induced self-assembly. Xylose (Xyl) that is eco-friendly and can be simply derived from lignocellulose was introduced for tuning the supramolecular alignments of the CNC composite films. The mechanical strength of the CNSP-X5 film was improved up to 70.5 MPa together with higher transparency and flexibility with an increased amount of Xyl. Interestingly, satisfying UVB-shielding performances were well retained (∼99%) after Xyl adjustments, which can be explained by the stabilized supramolecular structure due to the dual intermolecular hydrogen-bonding linkages between Xyl and both SP and CNC. Meanwhile, the interactions also caused aggregation of nanoparticles during evaporation, gradually generating more isotropic orientation for dried CNC films and weakening the iridescence observed under polarized light. In the present work, the CNC/SA/PABA composite films were obtained with improved multi-functions via Xyl tuning, thus showing high potential for producing applicable UV protective and monitoring sensors, anti-counterfeiting devices as well as surface coating composites for vehicles.