## 3 Results
### Analysis of Zpps
#### 3.2.4 Effect of Monomer Hydrophilicity  
As previously discussed, on the interfacial tension and kinetic effects on composite morphology, St was substituted with the more hydrophilic monomer, MMA, with a similar reactivity ratio of 0.58 [39]. The effect of the DVB content, i.e., 0, 1, 5, to 10% wt of total monomers, on the morphology of the ZPPs and the position of the ZnO NPs in the composite particle was investigated under TEM and SEM, as presented in Figure 6. Without DVB, the aggregates of the spherical ZPPs with OA-ZnO NPs on the surface or a polymer core/ZnO shell appeared in Figure 6A,B. This was due to the incompatibility between MMA (\(\delta\) = 11.67) and OA-ZnO NPs (\(\delta\) = 8.2). To lower interfacial tension between MMA and OA-ZnO NPs, OA-ZnO NPs tended to agglomerate and separate from MMA during polymerization. Figure 5: Proposed mechanism for the formation of Pickering-like and spherical ZPP. Once DVB was introduced, the morphologies of the resulting ZPPs were altered from non-hollow (0% DVB) to clusters of small composites (1% DVB), the mixture of small composites and hollow particles (5% DVB), and finally well-defined hollow ZPPs (10% DVB). Despite being present at a low concentration (1%), DVB strongly disturbed the formation of composites, as indicated by the aggregation of small particles (Figure 6C,D). Due to the large difference in solubility between MMA and DVB, MMA can dissolve both in an emulsion droplet and in an aqueous phase, while DVB can only reside in an emulsion droplet HD and ZnO NPs [18]. Oligoradicals react with DVB/MMA at the surface of emulsion droplets. Interestingly, the size of crosslinked ZPPs (1%, 32.0 \(\pm\) 6.8 nm) is much smaller than non-crosslinked ZPPs (150.7 \(\pm\) 16.5 nm). This might imply that the non-uniform polymerization occurs at the water and monomer droplet interface, which may cause the osmosis of water into the droplet to compensate for the depletion of monomers, especially MMA. Having an inadequate amount of crosslinker means not being able to hold the particles intact and, consequently, large composite particles burst into smaller particle sizes. In Figure 6E,F, when using 5% DVB, the mixture of hollow particles (120 \(\pm\) 25.8 nm) and aggregates of small particles, similarly found in the 1% DVB sample, were observed. It is worth noting that the size of the small particles in the aggregates was similar to the shell thickness of hollow particles, i.e., ca. 30 nm. This result supported our aforementioned assumption. Owing to the low miscibility between the relatively hydrophilic MMA and the hydrophobic DVB and the use of water-soluble initiator KPS, MMA migrated towards the water and emulsion interface to copolymerize, which led to the formation of a void or polymer-free core [40]. If crosslinked density of the shell was low, it would not have enough rigidity to maintain the whole particle. As previously reported, the type and amount of crosslinker affected the surface morphology and particle size of hollow and non-hollow polymeric particles [18; 41]. The surface roughness of HoMPs increased with the increasing the amount of DVB [18]. When using ethylene glycol dimethacrylate (EGDMA) crosslinker (10-90 mol%), pores on the surface of non-hollow PMMA particles were formed. The higher the concentration of EGDMA, the bigger the pore size. The presence of a pore was the result of a microphase separation during polymerization between MMA and EGDMA [41]. In our case, the heterogeneity should be more pronounced due to the large solubility difference between MMA and DVB. With increasing the DVB concentration to 10% (Figure 6G,H), hollow ZPPs with complete shell (thickness of 35 nm) were observed and small particles with ca. 30 nm were barely seen. Figure 6H clearly depicted the rugged surface of hollow composites, where OA-ZnO NPs were trapped in the cavity.