# Smart Poly(acrylic acid)/Poly(acrylamide) Microgels with Interpenetrating Polymer Network Structure
###### Abstract
At low temperatures, the hydrogen bonds between the PAA and PAAM monomer units are formed, which keep the polymer networks shrunk; meanwhile, upon temperature increase, these are destroyed, releasing part of the polymer chains, and enhancing the IPNs' swelling ability. Thus, the IPN's structure defines peculiar properties not attainable by copolymerization. The IPNs of PAA and PAAM have a specific structure and morphology, which are also dependent on the order of formation of the two constituent networks. Previous studies on IPNs of PAA and PAAM have focused on a case where PAAM is formed as the 1 st network followed by in situ PAA formation as the 2 nd IPN component. A recent study [14] has revealed using modeling that the PAAM network, when synthesized as the 1 st network, has good swelling ability in the aqueous solution of the AA monomer, through which PAA is formed as the 2 nd network of the PAAM/PAA IPN (we designate it as \"straight\" IPN). The hydrogen bonds between the H atoms of the AA monomers and amide group from the PAAM network were found to be stronger for the \"straight\" IPN than those formed when AAM monomer molecules enter the pre-formed 1 st PAA network, i.e., in the so-called designated \"reverse\" IPN [14]. Thus, the \"straight\" IPN is homogeneous because PAAM is synthesized first, and its swelling in the AA monomer solution is more pronounced compared to the swelling of the PAA network in the AAM monomer solution, which is the procedure used to obtain the \"reverse\" IPN. Our studies have confirmed this conclusion by revealing the nanoscale phase separation in the \"reverse\" PAA/PAAM bulk IPNs [10; 15]. The properties of the bulk PAA/PAAM IPNs have been studied [9; 16; 17; 18] and they have been exploited for biomedical applications, showing very good potential as a drug delivery system for VP, as well as a matrix for a biomineralization mimicking approach for hybrid material preparation. In contrast, micro- and nanosized particles based on the \"reversed\" PAA/PAAM IPNs have not been reported so far, and their potential as smart drug delivery systems has yet to be revealed. To our knowledge, only three papers have reported micro- and nanogels of PAAM/PAA IPNs, mainly revealing the physicochemical properties of the \"straight\" IPNs. [PERSON] et al. [19] have obtained spherical PAAM/PAA IPN nanoparticles with a diameter \(\sim\)250 nm, which exhibit unique, rapid sigmoidal swelling transition at \(\sim\)37-40 \({}^{\circ}\)C with temperature, confirming that IPNs have a much sharper swelling response upon temperature increase as compared to the copolymers. [PERSON] et al. [20] have synthesized pH- and temperature-responsive PAAM/PAA random copolymers as well as IPN nanogels. The latter exhibited a sharp swelling at \(\sim\)25 \({}^{\circ}\)C, associated with their UCST, in contrast to the former, which showed less abrupt swelling with temperature increase. The effect of pH on the swelling of nanogels prepared from either random copolymer or the IPN of PAAM and PAA was more intense than that of temperature. This was driven by the electrostatic interactions between the carboxylic groups of PAA, which are stronger as compared to the hydrogen bonds formed between the amide groups of PAAM and the carboxylic groups of PAA (which define the temperature responsiveness). The authors also studied the ZP of PAAM/PAA IPN nanogels and found that they were less negatively charged as compared to the random copolymer nanogels. This is explained by the fact that the hydrogen bonding between PAA carboxylic groups and PAAM amide groups is stronger in the IPNs due to the closer neighborhood of these groups as compared to the random copolymer. We could not find any reported data so far on micro- and nanogels of the \"reverse\" PAA/PAAM IPNs. In previous work, we have found that PAA/PAAM bulk IPNs are good candidates for the oral delivery of VP because its release is pH dependent, with minimal drug release in the stomach where pH = 1.2 and full release in the intestine where pH = 6.8.