Strong, thermal-stable, flexible, and transparent films by self-assembled TEMPO-oxidized bacterial cellulose nanofibers

Research on bacterial cellulose (BC), an all-purpose biomaterial widely used in various applications, often focuses on the pellicle type. To apply BC to flexible devices such as solar cells and displays, flowability is necessary for manufacturing processes. Fabrication of a BC fiber (BCF) suspension by treating BC pellicles in water using a homogenizer is a fast and easy pathway. However, it is difficult for BCF suspensions to form a transparent film that has sufficient strength that would arise from the BC nanofibers bound by hydrogen bonds. To solve the problem, the BCF suspension was treated by 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated oxidation followed by mild mechanical treatment. The individual TEMPO-oxidized BC nanofibers (TOBCNs) with a C6-carboxylate group content of 1.1 mmol g−1 cellulose were successfully dispersed in water. The TOBCN maintained the high crystallinity of the original BC pellicle (BCP) and had two classes of fiber width: 30–60 and 4–10 nm. Interestingly, the TOBCN dispersion exhibited much more concentrated, wide and lengthy birefringence than the TEMPO-oxidized cellulose nanofibers prepared from wood cellulose, implying their different roles in liquid crystal applications. The TOBCN film prepared by cast-drying the TOBCN dispersion was flexible and transparent, showing a high Young’s modulus of 9 GPa, a high tensile strength of 163 MPa and a considerably low coefficient of thermal expansion of 3.2 ppm K−1. In addition, to discuss the effect of TEMPO-mediated oxidation on material properties, the following films were also prepared: TOBCN film with carboxylate group content of 0.7 mmol g−1 cellulose, heat-dried BCP and BCF films without oxidation. With development of mass production of BC, the TOBCN and TOBCN-based nanomaterials are promising for flexible devices, such as substrates of solar cells and displays, in the future.