*3.3. NC Morphology*

The SEM micrographs of MCC and MD-MCC shown in Figure 2A reveal that MCC particles had irregular shapes with different dimensions and MD-MCC particles were swollen and porous with a rough surface. Moreover, cationic modification damaged MCC particles and eroded their surface such that the outer layer of the fibers was disrupted and cracked along the inner structure, exposing the fibril strand. These changes to the MCC granular structures facilitated the substantial increase in NC yield as discussed above.

**Figure 2.** (**A**) Scanning electron microscopy (SEM) micrographs of (**a**) MCC and (**b**) MD-MCC; (**B**) transmission electron microscopy (TEM) micrographs of (**c**) AH-NC, (**d**) HH-NC, and (**e**) US-NC.

The TEM micrographs of AH-NC, HH-NC, and US-NC shown in Figure 2B illustrate their distinctive morphologies and dispersion states. Small bundles of needle-like cellulose fibers with nanoscale diameters were observed for AH-NC and branches of smaller bundles or partly individualized nanofibers were attached to the aggregates as well (arrows in Figure 2B(c)). The formation of such aggregates significantly reduced the surface area of the AH-NC particles and thus hindered their reinforcing ability. Conversely, the HH-NC and US-NC nanofibers were well-dispersed and more individualized.

The NC dispersion state was directly associated with the surface interactions between adjacent NC particles because different surface interactions exist in aqueous solution, including attraction forces (e.g., hydrogen bonding) and repulsion forces (e.g., electrostatic repulsion). Attraction and repulsion forces are expected to compete with each other and thus determine the distinctive dispersion state of aqueous NC from different preparation methods. Therefore, the uniform dispersion of HH-NC and US-NC in aqueous solution was ascribed to profound repulsion forces whereas the AH-NC aggregates were attributed to predominant attraction forces. These theories were confirmed by the zeta potential values shown in Table 1. As expected, the order of absolute zeta potential values of NC prepared from MD-MCC was AH-NC < HH-NC < US-NC, which corresponds to the observed dispersion states. Because of the sulfate anions present during acid hydrolysis, the zeta potential value of AH-NC was not substantially changed. The dispersion states also indicate that hydrogen bonding between nanofibers was lower for HH-NC and US-NC.
