Effect of preparation temperature on salt-induced deswelling and pattern formation in poly(N-isopropylacrylamide) hydrogels

Abstract

An ever-increasing demand for novel hydrogels for smarter applications necessitates careful investigation of their swelling–deswelling behavior. In the present work, poly(N-isopropylacrylamide) (PNIPA) hydrogels were prepared at different temperatures. Hydrogels prepared below 25 °C were transparent and homogeneous whereas those prepared above 25 °C were slightly hazy to opaque. Scanning electron microscopy (SEM) studies showed variation in microstructures of the PNIPA hydrogels. In addition to the swelling studies, the deswelling kinetics of the swollen gels were studied in presence of different concentration of additives e.g. NaCl, NaOH at room temperature. The hydrogels prepared at higher temperatures were found to swell more and faster in water. Interestingly, during the salt-induced deswelling process, formation of several patterns was observed. The pattern formation of the PNIPA hydrogels was not only dependent on the concentration of additives in solution, but also on the hydrogel preparation temperature. The microphase separated inhomogeneous nature of the structure for the hydrogels prepared at intermediate temperatures was possibly responsible for the observed pattern formation.