4.2 Fabrication of flexible films from cellulose nanocrystal suspensions: properties and applications

Cellulose nanocrystals (CNC) are the most crystalline forms than the other two forms of nanocellulose, Cellulose nanofibrils (CNF) and Bacterial Cellulose (BC). CNC are unique in their iridescent behavior as forming chiral nematic self-assemblies. CNC films keep excellent transparency and mechanical strength (~ 77 MPa), making them applicable for coating, wrapping, and flexible electronics applications. The poor moisture resistance and toughness restrict its applicability for the same. Therefore, the presented work will describe processing conditions and chemical modifications to make flexible CNC films from commercial CelluForce (CF) and in-house (IH) CNC. The relationships are highlighted in different aspects as (i) surface sulfate content and its counterion, ii) process conditions, and how these affect the optical and mechanical properties of formed films, apart from the more common characterization techniques like XRD, ATR-FTIR, polarized optical microscopy (POM). In general terms, ultrasonication prior to film formation results in the greater order of crystallites and thus improved tensile properties and greater resistance to water in films. For in-house prepared CNC, ultrasonication imparts greater transparency of films, whereas films from neat Na-form CelluForce samples instead form iridescent films with a chiral nematic crystalline structure. Chemical modification of CNC by counter ions (H+, Na+, and tetraethylammonium), plasticizers (glycerol, triethanolamine), and surface grafting by dialkyl amines imparts flexibility to a rigid crystalline CNC system. The fundamental studies and findings have been used as guidelines for preparing flexible substrates (Tensile strength - 42 MPa and E% 4.5) for energy storage and electronic devices.