## 3 Results and Discussion
### Characterization of PAAm, PAMPS, and PAMPS-Na Microgels
The diameter and zeta-potential of PAAm microgels were barely affected by the pH alteration (Figure 5a) due to the lack of charged moieties. The amide moieties hydrolyzed and turned into COO- in the alkaline solutions and this is indicated by hydrodynamic diameter increasing and zeta-potential reducing of PAAm microgels. From pH 9 to pH 13, the diameter of PAAm microgel increased from 233.1 to 488.0 nm and the zeta potential gradually decreased to \(-\)25.27 mV (Figure 5a). In contrast,  
Figure 2: TGA thermograms of (**a**) PAAm, (**b**) PAMPS, and (**c**) PAMPS-Na microgels. at pH from 1 to 5, both the diameter and zeta-potential of PAAm microgels were barely changed. Therefore, the charge and swelling ratios of the PAAm microgel were able to be tuned by the pH of the surrounding solutions. By comparing the data of PAMPS-Na and PAMPS (Figure 5b,c) it was found that with increasing the pH from 2 to 12, the surface charges on PAMPS increased to be more negative than PAMPS-Na due to the ionization of sulfonic acid groups of PAMPS. Also, increasing the pH reduces the surface charge on PAMPS-Na to be less negative and this can be attributed to the screen effect of NaOH on the surface charges of sodium sulfonate groups associating to charge repulsion of the sulfonate groups [41]. Moreover, it was noticed that the hydrodynamic diameters of PAMPS-Na (Figure 5b) were reduced with the increasing of pH because of the associated short-range repulsion and the large unfavorable electrostatic repulsions at high pH compared to low pH [41]. There is an alternative explanation in that the screened surface charges of the sulfonate groups with NaOH decreases the interactions of sulfonate groups with water and deswells the water from the PAMPS-Na microgels and reduce their hydrodynamic diameters [41]. It was also observed that the hydrodynamic diameters of PAMPS-Na microgels (Figure 5c) are gradually increased due to possessing higher charge density. The electrostatic repulsions introduced by sulfonate among polymer chains enhance the swelling of PAMPS-Na microgels and increases their hydrodynamic diameters. The data represented in Figure 5a-c elucidates that PAMPS (Figure 5b) has higher dispersion than PAAm and PAMPS-Na in its aqueous medium at different pHs because their zeta potential (surface charges) are more negative than 25 mV. The zeta potential value demonstrates the degree of electrostatic repulsion between particles that are adjacent, and similarly charged in a dispersion. The particles with high zeta potential do not agglomerate with each other due to the high electrostatic repulsion, and therefore form a high stable suspension. As the magnitude of zeta potential for colloids increase, the stability of the dispersed particles increases [41]. Figure 3: TEM micrographs of (**a**) PAAm, (**b**) PAMPS, and (**c**) PAMPS-Na microgels. Figure 4: The average hydrodynamic diameter of the prepared (**a**) PAAm, (**b**) PAMPS-Na, and (**c**) PAMPS microgels in deionized water. Figure 5: The average hydrodynamic diameter and zeta-potential of the microgels (**a**) PAAm, (**b**) PAMPS-Na, and (**c**) PAMPS with different pH.