*3.1. Morphology of Bamboo Fibers and Bamboo-Derived Nanocellulose*

Figure 1 displays an optical image and electron microscope image of BF and its derived cellulose nanofibrils (CNFs). The single microfiber consists of many elementary fibrils in bamboo fibers, and rough and flat oval-shaped fibers can be observed. The rough surface of these bamboo fibers is known to cause the mechanical reinforcement effect of FRP by increasing the interfacial active bonding sites between the fiber surface and the polymer matrix. However, the effect is insignificant due to the roughness on the micrometer scale and the extreme hydrophilicity difference between the matrix (hydrophobic polymer) and filler (hydrophilic fiber) [29]. On the other hand, bamboo nanofibers are dispersed in water and have viscosity like a sticky glue substance. In addition, it can be seen from the transmission electron microscope image that it is composed of nanofibers with a diameter of about 10–20 nm. The viscous flow of CNFs means that the aqueous CNFs can act as a general adhesive binder and penetrate the fiber surface [17,30]. Additionally, the CNF's long aspect ratio with nanofibrillar morphology is expected to enable the CNFs to impart a nano brush structure to the BF fiber surface in the BF/CNF system and enable bonding between the BF fibers.

**Figure 1.** Morphology of bamboo fibers (BFs) and cellulose nanofibrils (CNFs). (**a**) Optical and (**b**) FE-SEM images of BFs. (**c**) Optical and (**d**) FE-TEM images of CNFs.
