# Smart Poly(acrylic acid)/Poly(acrylamide) Microgels with Interpenetrating Polymer Network Structure
## 3 Results
### pH Responsiveness of PAA/PAAM IPN MGs
(2) Upon pH increase and after it became higher than 6, the carboxyl groups transformed into anions and the hydrogen bonds in which they participated were destroyed, allowing the MGs to expand more, and this expansion was additionally enhanced by the repulsion between the -COO\({}^{-}\) groups. (3) At low pH and for MGs with predominant PAAM in their composition, the same is valid as described above--hydrogen bonds formed between PAA and PAAM, as well as PAA-PAA and PAAM-PAAM. Therefore, for the low-pH region, we observed almost the same Rh for all studied MG compositions and their Rh increased linearly but also in a very similar way because the governing forces were the same (Figure S1 and Table S1, Supplementary Information). (4) With increasing pH, again, the hydrogen bonds started to disappear due to the formation of anions, but this time the swelling was controlled by the PAAM component (as it is prevailing), and since PAAM can swell significantly in water (it is superabsorbent), the relative increase in its size was higher as compared to the MGs where PAA was more abundant (Scheme 1). To illustrate the effect of the PAAM content on the Rh dependence of the pH outlined above, we created Figure 3. Here, the relative increase in the MG diameters, defined as the ratio (\(\text{R}_{\text{pH}}/\text{R}_{\text{min}}\)) between the hydrodynamic radius of IPN MGs at different pH (\(\text{R}_{\text{pH}}\)) levels and the hydrodynamic radius of the same IPN MGs at the lowest pH = 2.6 (\(\text{R}_{\text{min}}\)), is presented as a function of pH. As can be seen from Figure 3, the higher the PAAM content, the higher the relative increase in the Rh. For example, the MGs with a higher PAAM content (PC25) showed the strongest increase in their size as compared to their size at pH = 2, while on the contrary for PC75, the relative increase in MG size was not as big, which is fully in line with the explanation provided above. In all cases, the IPN MGs with 66 mol. % PAA (PC66, Table 1) did not follow the above outlined dependencies. Their different behavior could be explained by the strong complexation between PAA and PAAM when in a 1:1 stoichiometric ratio, resulting in the formation of insoluble interpolymer complexes [26]. The slight deviation from the 1:1 stoichiometry could be explained by the inverse emulsion polymerization conditions, used in the current study. Under such conditions, AAM is known to remain in the water phase, while AA can partially \"go\" to the oil phase [14]. Thus, the PC66 MGs most likely have a lower AA content, i.e., closer to the 1:1 ratio, which defines the strong complexation mentioned above. It should be mentioned here that the IPN MG compositions, including the one with 66 mol. % AA in the IPN MGs, were obtained when using the PAA and PAAM conversions derived during the formation of their bulk analogues PAA/PAAM (Table 1, Materials and Methods section). Under inverse emulsion polymerization conditions, conversions close to 100% conversion were reported for PAAm MG, while for AA monomer, it was reported to be around 60% due to the partitioning of the AA monomer between the aqueous and the oil phases [26]. In summary, due to the IPNs' interlaced structure and the strong complexation between the two PAA and PAAM components, when in equimolar ratio, PC66 MGs have a compact shrunken structure, which was additionally fixed by the chemical crosslinking. Thus, the combination of the stoichiometric interpolymer complexes with the specific IPN structure defines the different way PC66 behaves as compared to the other IPN MGs.