## 3 Results
### Analysis of Zpps
#### 3.2.3 Effect of Crosslinking Agent  
To further validate the contribution of kinetic parameters on the morphology of ZPPs, the influence of the crosslinking agent, DVB, was investigated. Having two reactive sites, the crosslinking agent accelerates the polymerization rate and viscosity in the growing monomer-swollen micelles [33; 34]. The TEM images in Figure 4A-C reveal the morphologies of the ZPPs containing ZnO NPs in PS made from 1, 5, and 10% DVB. When the amount of DVB increased, the resulting ZPPs changed from ZnO NPs coated on PS core or Pickering-like morphology (1% DVB) to ZnO NPs embedded in the PS particle (5% DVB) and, finally, to the PS particle inhomogeneously decorated with ZnO NPs on the surface. At the same time, average particle sizes varied from 182 \(\pm\) 49.9 to 340.0 \(\pm\) 48.3 and 199.0 \(\pm\) 67.2 nm. At 1% DVB, the morphology of ZPPs was relatively similar to the non-crosslinked ZPPs (Figure 2C), i.e., Pickering-like morphology, but the protrusion of the ZnO NPs seemed to be lower. This is because the presence of DVB promotes the formation of primary particles at the beginning of polymerization [34; 35]. The larger particle size and the entrapment of ZnO NPs in ZPP when using 5% DVB were due to the fact that DVB raises the internal viscosity and the crosslinked density in growing particles, which could then trap ZnO NPs before exerting to the surface of the ZPP. However, some ZnO-free particles (see arrow) were found and their sizes (256.4 \(\pm\) 6.7 nm) were smaller than that of the composites (340.0 \(\pm\) 48.3 nm) because of the inhomogeneous distribution of ZnO NPs and the non-uniformity during polymerization. At 5%, DVB might not be completely miscible with St, since solubility of DVB (\(\delta\) = 8.5 (cal/cm\({}^{3}\))\({}^{1/2}\)) is lower than that of St (\(\delta\) = 9.85 (cal/cm\({}^{3}\))\({}^{1/2}\)) [19]. In addition, OA-ZnO NPs (\(\delta\) = 8.2 (cal/cm\({}^{3}\))\({}^{1/2}\)) might have higher affinity to DVB or the DVB-rich phase than St. High crosslinker, at 10% DVB, causes surface shrinkage, which subsequently yields smaller particle size [36]. The increased DVB could cooperate with the HD (\(\delta\) = 8.0 (cal/cm\({}^{3}\))\({}^{1/2}\)) hydrophobe to suppress the Ostwald ripening process, which also results in the formation of small particles. Consequently, the  
Figure 3: TEM and SEM images of spherical ZPPs synthesized by using 0.5 (**A**,**B**), 1.0 (**C**,**D**), and 2.0% (**E**,**F**) of KPS initiator. ZnO NPs could be protruded to the surface of the particle. ZnO-free particles were also found, as in the case of 5% DVB. Zeta potential and % weight residue of ZPPs prepared at different DVB percentages are presented in Table 1. All crosslinked ZPPs had lower absolute zeta potential values compared to the non-crosslinked ones [37]. Due to the higher polymerization rate, DVB residing inside the monomer-swollen micelles might pull the diffused oligomer and OA-ZnO NPs from the surface of miniemulsion to polymerize, and the carboxylic end of OA modified ZnO surface is then trapped in the inner part of the emulsion. The composition of ZPPs was confirmed by TGA thermograms in Figure 4D. The weight residues of crosslinked ZPPs were 26.9, 19.5, and 21.9 for 1, 5, and 10% DVB, respectively, and were significantly higher than that of the non-crosslinked ZPPs (12.8%). The higher ZnO content are attributed to the higher polymerization rate caused by DVB (reactivity ratios of DVB = 1.3 and of St = 0.55) [38] and the miscibility between OA-ZnO and DVB, as already mentioned. Figure 5 presents the proposed formation mechanism of ZPPs. OA-ZnO NPs are included in the SDS-stabilized monomer droplets. Aside from St (and DVB), a small amount of AA monomer existed in the reaction system and the polymerization of St/AA was initiated in an aqueous phase [18] by KPS to form radical species, which then continued polymerizing in the nanodroplet. Due to the low number of radical species compared with the amount of initiator, the polymerization rate in the emulsion droplet was low.