Effects of Cross-Link Density on Structures and Properties of Dual-Sensitive Semi-Interpenetrating Polymer Networks Hydrogel Microspheres

Semi-interpenetrating polymer network (semi-IPN) strategy is employed to fabricate uniform microspheres with temperature and pH dual-responsive behavior, which is composed of poly(N-isopropylacrylamide) and poly(acrylic acid) in the presence of N,N′-methylenebisacrylamide (MBA) as the cross-linker. The influences of MBA amount (Mm) on the structures and properties of the microspheres are investigated in terms of particle size, surface morphology, pH sensitivity, and thermo-sensitivity with low critical solution temperature in the range Mm = 2.5–15%. Additionally, functional group distributions in the microspheres are probed by titration and employing the Henderson–Hasselbalch equation. The results show that all the properties strongly change depending on Mm. Based on transmission electron microscopy and confocal laser scanning microscopy measurements, and these properties, the Mm-induced changes in the structures of the semi-IPN microspheres are discussed.