## 3 Results and Discussion
### Variation of the Cross-Linker Concentration On Particle Formulations  
The preparation of cross-linked NPs is a common practice in order to provide a fixed structure that is resistant to different solvents systems and more thermally stable [61] although it is a challenge to obtain monodisperse cross-linked systems. For the synthesis of cross-linked NPs, DVB, and EGDMA were used to cross-link St and MMA, respectively. The AUPDS, HD, and AIBN concentrations remained constant throughout the formulations at 2, 2 and 0.5 mol%, correspondingly, all values  
Figure 3: SEM images of (**a**) PMMA-co-AUPDS; (**b**) PMMA-co-EGDMA-co-AUPDS; (**c**) PS-co-AUPDS and (**d**) PS-co-DVB-co-AUPDS. The particles were formulated using 2 mol% AUPDS surfmer. The amount of cross-linker for (**b,d**) was 10 mol%. All scale bars represent 100 nm. relative to the monomer phase. The amount of cross-linker was varied from 2.5 to 80 mol% and the DLS results are displayed in Table 2 and Figure 4.  
Increasing cross-linker concentration above 20 mol% did not yield stable mini-emulsions when using St/DVB and, for MMA/EGDMA the particles precipitated within days and, for that reason, higher ratios of cross-linker were not further explored. This increase in cross-linker content above 20 mol% for PMMA-_co_-EGDMA-_co_-AUPDS promoted an increase in particle size (from 225 to 588 nm) and in PI from 0.29 to 0.40. The increase of EGDMA also led to a decrease in ZP values, from +15 mV to \(-\)25 mV. Therefore, it can be interpreted that an increase in the cross-linker amount from 20 mol% to up to 80 mol% leads to less AUPDS on the surface of the NPs. One of the reasons for this could be the fact that highly crosslinked PMMA-_co_-EGDMA are often porous particles [62], which would displace the surfmer from the outer surface. It was possible to produce stable and narrowly distributed NPs with both systems using a cross-linker range from 2.5 to 10 mol%. For PMMA-_co_-EGDMA-_co_-AUPDS particle sizes varied slightly from 168 to 159 nm and remained monodisperse with PI of 0.07 to 0.10, while the ZP varied from 31 to 52 mV. A similar effect was observed for PS-_co_-DVB-_co_-AUPDS particles, in which hydrodynamic \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Cross-Linker (mol\%)** & **Z-Average (nm)** & **PI** & **ZP (mV) a** \\ \hline \multicolumn{4}{c}{PMMA-_co_-EGDMA-_co_-AUPDS particles} \\ \hline
2.5 & 168.7 \(\pm\) 0.6 & 0.07 \(\pm\) 0.03 & 38.5 \(\pm\) 8.6 \\
5 & 162.7 \(\pm\) 1.0 & 0.07 \(\pm\) 0.00 & 52.2 \(\pm\) 15.3 \\
10 & 159.6 \(\pm\) 1.9 & 0.10 \(\pm\) 0.01 & 30.9 \(\pm\) 3.5 \\
20 & 225.3 \(\pm\) 5.4 & 0.29 \(\pm\) 0.04 & 14.9 \(\pm\) 4.4 \\
40 & 297.6 \(\pm\) 3.0 & 0.30 \(\pm\) 0.03 & 17.5 \(\pm\) 6.0 \\
60 & 268.8 \(\pm\) 3.6 & 0.23 \(\pm\) 0.04 & 8.7 \(\pm\) 3.2 \\
80 & 587.6 \(\pm\) 8.3 & 0.40 \(\pm\) 0.07 & \(-\)24.7 \(\pm\) 3.9 \\ \hline \multicolumn{4}{c}{PS-_co_-DVB-_co_-AUPDS particles} \\ \hline
2.5 & 120.3 \(\pm\) 1.4 & 0.04 \(\pm\) 0.02 & 26.6 \(\pm\) 0.6 \\
5 & 98.6 \(\pm\) 0.5 & 0.03 \(\pm\) 0.02 & 26.4 \(\pm\) 0.7 \\
10 & 160.4 \(\pm\) 1.2 & 0.03 \(\pm\) 0.03 & 26.7 \(\pm\) 1.0 \\
20 & 161.7 \(\pm\) 0.1 & 0.09 \(\pm\) 0.01 & 17.0 \(\pm\) 3.8 \\ \hline \hline \end{tabular}  
* Measurements were performed in water.  
\end{table} 
Table 2: Z-average hydrodynamic diameters, polydispersity index (PI) and zeta potential (ZP) values obtained for PMMA-co-EGDMA-_co_-AUPDS and PS-_co_-DVB-_co_-AUPDS particles with different amounts of EGDMA or DVB cross-linker. Values are presented in average \(\pm\) standard deviation (\(n\) = 3). Figure 4: Hydrodynamic diameter distribution curves in terms of intensity (%) obtained via DLS analysis of (**a**) PMMA-_co_-EGDMA-_co_-AUPDS and (**b**) PS-_co_-DVB-_co_-AUPDS particles formulated using with different amounts of cross-linker (in mol% in regards to the dispersed phase). diameters varied from 99 to 162 nm, PI remained monodisperse at 0.03 to 0.04, and ZP was in the same range at 26 mV.