**3. Results and Discussion**

#### *3.1. Dispersion and Stability Analysis*

The synthesized WPU-SS emulsion was analyzed by a nano-particle size analyzer to evaluate its storage stability. The particle size and distribution of suspension waterborne polyurethane particles are shown in Figure 2a. The average particle sizes of WPU1, WPU2 and WPU3 were 71.27, 77.12 and 131.6 nm, respectively, and the PDI were 0.130, 0.084 and 0.285, respectively. The particle sizes of WPU1 and WPU2 dispersion systems are similar, and the polydispersity index shows that the distribution of WPU2 dispersion is more uniform than that of WPU1 dispersion. With the increase of soft segment content, the proportion of hydrophilic groups in WPU3 emulsion decreased, the polydispersity index increased, and the dispersion became worse.

**Figure 2.** Particle size and distribution of WPU (**a**) and their zeta potential (**b**).

Zeta potential is another important indicator for evaluating emulsion stability. As shown in Figure 2b, the Zeta potential of WPU1, WPU2 and WPU3 were −41.6, −40.7 and −43.6 mV, respectively. The high absolute value of zeta potential indicates that there is abundant electrostatic repulsion on the surface of water particles. Besides, introducing SS bonds in the WPU-SS chain had little effect on the Zeta potential value, manifesting that the addition of SS bonds had no significant effect on the stability of the emulsion. In addition, there is no deposit at the bottom of WPU-SS dispersions stored in closed glass bottles at room temperature for 2 months. The WPU emulsions were diluted into 1 wt% water dispersion and looked light blue. In summary, stable WPU emulsions can be obtained by introducing SS bond into the molecule chain.
