Effect of a trace amount of deep eutectic solvents on the structure and optical properties of cellulose nanocrystal films

Cellulose nanocrystal (CNC) suspensions can form chiral spiral structures because of their unique self-assembly characteristics, and their structures can be maintained in dried films. This makes CNC films possess iridescent and liquid crystal properties at the same time. However, neat CNC films are so fragile that they are not suitable for wide use. We first explored the influence of a new type of green plasticizer-deep eutectic solvent (formed by urea and choline chloride) on CNC composite films under different molar ratios and proportions. Deep eutectic solvents (DES) have attracted extensive attention in recent years due to their low cost, easy fabrication and no need for purification. Urea/choline chloride, as the original DES, can improve the apparent uniformity and smoothness of the film only by adding 1‱ of DES, maintaining the optical properties and structural stability of the CNC composite film while effectively reducing the brittleness of the film (the molar ratio of the CNC/DES film with the best performance is 2:1). When the amount of DES in the CNC aqueous suspension reaches 10‱, it will destroy the structure of the CNC, resulting in loss of the optical properties of the CNC/DES films. Different from the common small molecule plasticizers previously reported, the addition of DES not only increases the number of hydrogen bonds but also introduces charge force, which forms a network structure that synergistically improves the flexibility of CNC films.Graphical abstract