## 3 Results
### Coatings by Polymer Particles
#### 3.2.1 Surface Observation  
Surface observation using SEM and AFM gives qualitative information about the coating of polymer particles on WTBs [19]. We observed the surfaces of WTBs to confirm the influence of physical modification with polymer particles, by using SEM. The surfaces of bare WTBs were then observed (Figure 4a). The microfibrillar structures of cellulose I were observed. The surface roughness of the bare WTBs indicated \(R_{\text{rms}}\) = 114 nm, which is in agreement with the literature [19].  
First, the effect of the volume fraction of polymer particles on the surface structure of WTBs was examined. The surfaces of WTBs immersed in PS\({}_{800}\) were compared by using \(\phi\) = 0.020 and 0.005. The surface obtained in the case of \(\phi\) = 0.020 involved steps like 1-1\({}^{\prime}\), 2-2\({}^{\prime}\), and 3-3\({}^{\prime}\), and four layers (four terraces) between the steps were observed, which was similar to the formation of a multilayered deposition of latex particles (polystyrene) [29; 30]. In contrast, the surface obtained from immersion in PS\({}_{800}\) at \(\phi\) = 0.005 gave no definite terrace on the surface (Figure 4c). Therefore, the volume fraction of the polymer particles made an impact on the surface. Alternatively, regarding the effect of the mean diameter of the polymer particles, the surface states of WTBs were compared with \(\phi\) = 0.020. The surface in the case of PS\({}_{30}\) (Figure 4e) gave no definite terrace structure different from PS\({}_{800}\) (steps 4-4\({}^{\prime}\) and 5-5\({}^{\prime}\) in Figure 4d). The formation of terraces appeared to depend on the size and volume fraction of the polymer particles.  
Figure 4: (**a**–**e**) SEM and (**f**–**h**) AFM images of bare WTB and WTBs coated by polystyrene particles with different diameters at two volume fractions (\(\phi\) = 0.005 and 0.020). (**f**–**h**\({}^{\prime}\)) Both side and top views of each AFM image. White and black arrows mean the step on the surface. Lines 1–1\({}^{\prime}\), 2–2\({}^{\prime}\), \(\dots\) and 7–7\({}^{\prime}\) mean the step.  
For further investigation, AFM observation was performed. The surfaces of WTBs immersed in PS\({}_{800}\) at \(\phi\) = 0.020 indicated a conspicuous undulating structure of \(R_{\rm rms}\) = 83.5 nm (Figure 4f). This surface morphology might result from the terrace structure as illustrated in Figure 4f. Decreasing \(\phi\) to 0.005, the AFM image showed a clear undulating structure of \(R_{\rm rms}\) = 531 nm (Figure 4g), which possibly resulted from the exposure of bare WTBs. It was likely that PS\({}_{800}\) could not considerably coat the WTB surfaces (Figure 4g'). Furthermore, in contrast to PS\({}_{800}\) at \(\phi\) = 0.020, the size of the PS decreased to 30 nm. Thereby, the AFM image of the surfaces of WTBs immersed in PS\({}_{30}\) at \(\phi\) = 0.020 indicated a terrace structure at the nanoscale of \(R_{\rm rms}\) = 18.1 nm (steps 6-6' and 7-7' in Figure 4h,h'), although no definite terrace structure at the microscale was observed (Figure 4e). It is noted that the \(R_{\rm rms}\) value strongly depends on the bare WTB surface before adhesion.