# Smart Poly(acrylic acid)/Poly(acrylamide) Microgels with Interpenetrating Polymer Network Structure
## 3 Results
### pH Responsiveness of PAA/PAAM IPN MGs  
The hydrodynamic radius (R\({}_{\text{h}}\)) of PAA/PAAM IPN MGs under varying pH, as revealed by DLS, is shown in Figure 2. The MGs of the neat PAAM (sample PC0, Table 1) showed no pH dependence, which was expected since they were neutral within the studied pH range (pK\({}_{\text{a}}\) of PAAm is \(\sim\)15 [22]) (Figure 2A). The size of PAAM MGs was constant, being around \(\sim\)200 nm for all studied pH values. In contrast, the R\({}_{\text{h}}\) of the neat PAA MGs (sample PC100, Table 1) was pH dependent (Figure 2A) due to the ionization of the AA moieties (pK\({}_{\text{a}}\) of PAA is \(\sim\)4.5-4.9 [23]). The R\({}_{\text{h}}\) values of the neat PAA MGs gradually increased from \(\sim\)200 nm at pH-2 to \(\sim\)500 nm at pH-6, where PAA carboxyl groups are known to be almost entirely transformed into carboxylates [24]. A further increase in pH did not significantly affect the neat PAA MGs and their size remains \(\sim\)500 nm up to pH = 10 (Figure 2A). The observed plateau indirectly confirms that most of the acidic groups in PAA were converted into carboxylate anions [25] as well as that the MGs reached their maximum swelling ratio, which was limited by their chemical crosslinking. All IPN MGs (Table 1) exhibited a pH-dependent size behavior similar to that of neat PAA MGs (Figure 2B). At pH \(<\) 4, the R\({}_{\text{h}}\) of the IPN MGs increased for all IPNs' compositions due to the hydrophilic nature of their constituents. When the pH approached the pK\({}_{\text{a}}\) of PAA, the size of the IPN MGs continued to gradually increase, reaching a maximum size at pH-6, similar to the neat PAA MG (Figure 2A). A further increase in pH did not seem to affect the IPN MGs' R\({}_{\text{h}}\), which remained almost constant for all IPN compositions up to pH 10, as was also observed for the neat PAA MGs. The increase in IPN MGs' size was defined by the ionization of the AA moieties, as described above--at pH-6, almost all carboxyl groups in the AA monomer units were ionized, thus strongly repulsing each other and increasing the R\({}_{\text{h}}\) of the MGs. The MG radii of the IPNs levelled off after pH-6 because all the -COOH groups were ionized and thus the further pH increase did not change the ionization of the carboxyl groups, i.e., did not further increase the number of anions. At the same time, the chemical crosslinking limited the MG swelling and defined the plateau in the pH dependence of their hydrodynamic radius. The IPNs' composition also influenced the R\({}_{\text{h}}\) of the IPN MGs (Figure 2B). The contribution of the second IPN component, PAAM, to the hydrodynamic radius of the IPN MGs can be seen by comparing the radii of the neat PAA MGs (Figure 2A) and of IPN MGs with a higher PAA content (e.g., PC75 and PC50, Table 1) (Figure 2B). The hydrodynamic radius of IPN MGs at pH \(>\) 5 was higher as compared to the neat PAA MGs, and this was due to the presence of the 2 nd network PAAM, which physically increased the volume of the MGs--a larger polymer amount built up the IPN MGs as compared to the homopolymeric  
Figure 2: Hydrodynamic radii, determined by DLS, of (**A**) neat PAAM Gd (PC0), neat PAA MGd (PC100), and (**B**) PAA/PAAM IPN MGs as a function of pH. MGs. In addition, PAAM is a hydrophilic polymer superabsorbent, which imparts an additional swelling capacity in water to the IPN MGs. Thus, two factors played a role in the observed behavior of the IPN MGs, namely the conversion of PAA into polyanion at pH \(>\) 5 and the ability of PAAM to swell but also to form hydrogen bonds among itself as well as with the carboxyl groups of PAA. Four scenarios can be described. (1) At low pH and for MGs with prevailing PAA content, the MGs shrunk due to the hydrogen bond formation between both PAA and PAAM as well as between their own macromolecules, e.g., PAA-PAA and PAAM-PAAM.