In situ preparation of PNIPAM biphasic hydrogels

Abstract

N-isopropylacrylamide (NIPAM) monomer undergoes phase separation in water at very high concentrations, which offers striking opportunities to engineer biphasic hydrogel systems. However, this is a metastable state and NIPAM evolves to crystallize. Besides, the concentration of NIPAM in the lower phase is very small. In this work we show that the aforementioned limitations can be overcome by the incorporation of selected components. Thus, the addition of acrylic acid enables accessing remarkably stable biphasic NIPAM-water systems, as corroborated by DSC (absence of NIPAM crystallization). The addition of a third monomer, diallyldimethylammonium chloride (DDAC), further allows modifying NIPAM partition notably. The phase-distribution of the components for different monomer mixtures was gauged by 1H NMR. Subsequent polymerization leads to the formation of biphasic hydrogels systems in a single step, whose phase morphology and temperature response (swelling response) has been characterized by SEM. This methodology also permits adjusting the crosslinker partition as a function of its nature, which can be used to tune the swelling behaviour of each phase for a certain composition.