## 3 Results
### Analysis of Zpps  
#### 3.2.1 Effect of Surfactant Concentration  
Aside from the time and amplitude of the sonication, the amount of surfactant usually influences the equilibrium rate, size, and stability of nanodroplets, or miniemulsion during ultrasonicating, as well as particle morphology [24; 25]. Higher surfactant concentrations result in lower surface tension and, hence, the smaller latex particle size [26]. The effect of the concentration of SDS on the size of the ZPPs, made from P(St/AA) synthesized with 0.5%KPS, was studied. Table 1 shows that the average particle size decreased from 200.5 \(\pm\) 24.8 to 139.8 \(\pm\) 41.1 and 64.5 \(\pm\) 4.3 nm, when increasing SDS from 0.5 (below the CMC), 1.5 (at the CMC) to 3.0% (above the CMC). This agreed well with the previous works reporting that, at above the CMC, there were enough SDS molecules for stabilizing a large number of small monomer droplets formed after shearing [26]. The SDS concentration also affected the morphology of the composite particle. Figure 2 shows the TEM and SEM images of the spherical ZPPs prepared from 0.5 (Figure 2A,B), 1.5 (Figure 2C,D), and 3.0% (Figure 2E,F) of SDS using PS as the matrix. The polydispersed spherical ZPPs of P(St/AA), containing a high amount of ZnO NPs, were observed at all conditions. At below or equal to the CMC of SDS, Figure 2A-D revealed the Pickering-like morphology, i.e., small OA-ZnO NPs (dark sphere), located on the surface of the bigger PS core (lighter sphere). The OA-ZnO NPs, having higher hydrophilicity than PS, were separated and pushed towards the aqueous phase and played the role as another stabilizer of composite particles. If there was enough available space at the particle surface, i.e., at low SDS in Figure 2A,B, ZnO NPs could embed more at the PS and water interface. More SDS at its CMC, in Figure 2C,D, might repulse non-polar OA, resulting in the protrusion of ZnO NPs from the PS phase. At above the CMC, the appearance of ZnO NPs as a Pickering stabilizer on the small-sized ZPPs core was not clearly observed in Figure 2E,F. This might be due to the higher surface area of smaller PS core particle, which allowed for the faster trapping of oligoradicals and, hence, the higher polymerization rate, especially at the surface of miniemulsion. This rationale agrees well with the following Equation (1):Figure 1: TEM images (**A**–**D**), TGA thermogram (**E**) of OA-ZnO NPs using the added OA content of 0.01, 0.04, 0.10, and 0.4 mL, and (**F**) FTIR spectra of bare ZnO and OA-ZnO NPs (4.8 wt% OA). \[r_{p}\ =\ \frac{K_{p\cdot}[M]_{p\cdot}\overline{n}.N_{p}}{N_{AV}} \tag{1}\]  
where \(r_{p}\) is the rate of polymerization, \(K_{p}\) is propagation rate coefficient and \([M]_{p}\) is the monomer concentration in the particle, while \(\overline{n}\), \(N_{p}\), and \(N_{AV}\) are the average number of the radical, the number of the particle, and Avogadro's number, respectively [27]. Consequently, the high viscosity in each nanodroplet impeded the migration of OA-ZnO NPs towards the particle surface, whereas a large amount of the SDS molecules rapidly moved to cover the particle surface [28]. This effect is more pronounced in the case of the small-sized and oligomer-rich monomer droplet. The complete encapsulation of OA-ZnO NPs within the PS matrix in Figure 2E,F depicts a kinetically controlled morphology [25]. The existence of OA-ZnO NPs on the ZPPs surface decreased the absolute zeta potentials, i.e., from \(-54.7\pm 3.3\) mV to \(-26.1\pm 1.1\), \(-24.8\pm 0.6\), and \(-32.3\pm 2.3\) mV when using 0.5, 1.5, and 3.0% SDS, respectively [29]. This implied that the combined electrostatic repulsion, from ionic groups of SDS/KPS/AA, and the steric repulsion, from hydrocarbon chains of OA on ZnO, were responsible for the high stability of ZPPs at all SDS concentrations. The FTIR spectra in Figure S3A show the characteristic peaks of the benzene ring in the St at 3027 cm\({}^{-1}\) for C=C-H stretching, 1584 and 1493 cm\({}^{-1}\) for C=C stretching, 757 and 697 cm\({}^{-1}\) for C-H bending [13].