## 3 Results
### Analysis of Zpps
#### 3.2.2 Effect of Initiator Content  
Initiator also exerts an influence on the particle size and morphology of composite particles generated by the mini-EP. Table 1 shows the average size of ZPPs, which increased from \(139.8\pm 41.1\), \(170.6\pm 21.0\), to \(174.3\pm 41.2\) nm, when increasing the amount of water soluble initiator, KPS, from 0.5, 1.0, to 2.0%, respectively. When the higher concentration  
Figure 2: TEM and SEM images of spherical ZPPs prepared from P(St/AA) with various SDS concentrations of 0.5 (below CMC) (**A**,**B**), 1.5 (at CMC) (**C**,**D**), and 3.0% (above CMC) (**E**,**F**). of KPS was added, the larger number of short chain oligoradicals, rapidly formed, would stay in water for a longer period due to their higher hydrophilicity. Until reaching its critical chain length, these oligoradicals were adsorbed on the surface of nanodroplets and continued polymerization [18]. This period allowed the coalescence of the small miniemulsion droplets and, hence, the formation of larger composite particles. The process of an increase in primary radical concentration, which promotes the particle coagulation, could be expressed by the modified Von Smoluchowski Equation (2) [31; 32]:  
\[B(i,j)=f(\gamma)N(i)N(j)\big{[}d_{p}(i)+d_{p}(j)\big{]}^{3} \tag{2}\]  
where \(B(i,j)\) are the number of collisions between particles class \(i\) and \(j\), and \(f(\gamma)\) is constant. \(N(i)\) and \(N(j)\) are the number of particles in class \(i\) and \(j\), whereas \(d_{p}(i)\) and \(d_{p}(j)\) are particle diameter of class \(i\) and \(j\), respectively. \(B(i,j)\) and, consequently, the extent of the coagulation directly increased with the increasing the number of primary particles of St at the initiation step, leading to the agglomeration between primary particles and monomer-swollen micelles. The number of primary particles and miniemulsion droplets (\(N(i)\) and \(N(j)\)) were depleted during polymerization, whereas the size of monomer-swollen latex particles increased. A similar observation was reported in the emulsion polymerization system of St when methanol was used as co-solvent, i.e., the final size of the PS latex particles increased with the amount of KPS, due to the particle coalescence during polymerization [32]. Increasing the amount of initiator enhanced the extent of the particle coagulation. In addition, as a water soluble initiator and inorganic salt, dissolution and decomposition of KPS enhanced the ionic strength of the aqueous phase and, consequently, suppressed the charge of existing species, e.g., primary particles and monomer-swollen micelles, leading to the particle coagulation [32]. The amount of KPS also affected the morphology of ZPPs as presented in Figure 3. The TEM images of ZPPs using 0.5 and 1% KPS in Figure 3A,C reveal a spherical PS core surrounded with small OA-ZnO NPs in Pickering-like or raspberry-like structure, which correlated well with their SEM images in Figure 3B,D. As OA on ZnO NPs have a higher affinity with oligoradicals, as compared to St monomer and PS, OA-ZnO NPs moved outwards on the surface of the monomer-swollen micelles [18]. In the case of 2% KPS, both images of ZPPs in Figure 3E,F show a relatively smooth surface with few portions of attached ZnO NPs. The decrease of the protrusion of ZnO NPs on the ZPP surface might be attributed to the high polymerization rate. The large amount of St oligoradicals rapidly polymerized, and consequently, the polymer impeded the migration of OA-ZnO NPs from the PS matrix as already mentioned. This confirmed that the morphology transitioning from the non-Pickering (at 2% KPS) to Pickering-like (at 0.5-1% KPS) was kinetically controlled. The negative zeta potentials of all ZPPs derived from the sulfate groups of SDS, KPS, and carboxyl groups of OA-ZnO NPs/AA, which confirm the good dispersion of ZPPs in an aqueous solution. The FTIR spectra of ZPPs prepared using different concentrations of KPS are shown in Figure S4A.