Super foldable transparent paper by regulating the multi-scale structure of cellulose fibers.

Transparent cellulose papers have emerged as a promising substrate and/or functional component for next-generation sustainable flexible electronics. However, obtaining transparent paper with folding endurance comparable to that of plastic films such as polyethylene terephthalate (PET) remains a major challenge, which was addressed in this study by modulating the multiscale structure (from molecular structure, aggregation structure to fibrous morphology) of wood fibers. Compared to the natural wood fibers, the modified fibers not only retained their length and cellulose degree of polymerization (DP) to a large extent, but also had lower crystallinity and improved swelling capability, which well preserved the fiber strength and dense intertwined fiber network, and led to stronger inter-fiber interactions in the final transparent paper. The obtained paper (CM-paper) exhibits an optical transparency of 91 % and a folding endurance of 37,466 times, comparable to that of commercial PET (39,955 folds). Furthermore, the underlying mechanism for the superior foldability of the transparent paper was unveiled at three scales is explored. Finally, the potential application of CM-paper in electromagnetic induction electrodes was demonstrated, where the CM-paper electrodes exhibit satisfactory conductivity after repeated rubbing. This work offers an innovative approach to produce transparent papers with outstanding foldability, which is expected to plastic replacement.