ИСТИНА

In recent years, considerable interest has been focused on the development of “smart” microgels, that is, microgels whose properties change dramatically upon the application of an external stimuli especially temperature and acidity of the medium. The major advantage is that the rate of the microgel responding to external stimuli is much faster than bulky gel. Polymer microgels have a wide range of applications such as superabsorbents, surface-active stabilizing additives, carriers for drug delivery, etc. The aim of our research is the comparative study of conformational behavior of interpenetrating network (IPN) microgels based on poly(N-isopropylacrylamide) (PNIPA) and poly(acrylic acid) (PAA) with different network structures. For this purpose, PNIPA microgels were obtained by thermo-induced precipitation polymerization in the presence of conventional cross-linking agent (N,N′-methylenebis(acrylamide)) or as a result of self-crosslinking process. Then second subnetwork was synthesized by in situ polymerization of acrylic acid within these two types of PNIPA microgel particles. Dynamic and static light scattering methods were used for the comparative study of thermo- and pH-induced behavior and the internal structure of the resulting IPN microgels. It was shown that the formation of the PAA network inside the PNIPA microgels does not change the temperature of the conformational transition for PNIPA, which is 32°C, the pH of the conformational transition of the IPN-microgels lies in the range of 4.5–5.0 that is in accordance with pKa of the PAA. It was found that self-crosslinked PNIPA microgel has greater uniformity in the distribution of crosslinks and its lower density that results in the significant increase of pH-response of IPN microgel based on this type of matrix microgel. Moreover the swelling ratio of such IPN microgel is significant larger. The analysis of the asymmetry factors Rg/Rh shows that all studied microgels at temperature below LCST have a “soft sphere” structure with a somewhat denser core and a loose corona, and at temperature above LCST matrix microgels and all IPN microgels expect the sample having large corona of AA are practically hard spheres. When the IPN microgel having the corona of AA grown outside the matrix particle is in solvent, poor for the first network (PNIPA, T > TLCST) and good for the second network (PAA, pH 7.5), the PNIPA network collapses, forming a dense core, and the PAA network partially remains swollen on the surface of the particle.