The synthesis and characterization of temperature and pH sensitive poly(acrylic acid) poly-(N-isopropylacrylamide) IPN hydrogel

The aqueous solution of Poly(N-isopropylacrylamide) (PNIPA) exhibits a lower critical solution temperature (LCST, 32 degrees C). Its hydrogel undergoes phase separation as the temperature is raised above LCST. This temperature-sensitive hydrogel has been widely used in many fields, for instance, controlled drug release, enzyme immobilization and release of solutes. However, response to pH of hydrogel may be required at the same time for some applications. Here, a new intelligent hydrogel which can respond to temperature and/or pH changes is described. The new hydrogel was prepared by using interpenetrating polymer networks (IPN) technique. Poly(acrylic acid)/Poly(N-isopropylacrylamide) (PAAc/PNIPA) IPN hydrogel was synthesized. The temperature and pH sensitivity of this hydrogel was studied. The Swelling Ratio(SR) of this hydrogel in weak alkaline solution is much greater than that in acidic solution. Experimental results revealed that its SR rises as the temperature is raised in acidic solution. On the other hand, in weak alkaline solution, when the temperature is below the LCST of PNIPA (32 degrees C), the SR of the hydrogel also rises as the temperature is raised. However, when he temperature reaches the LCST or higher, its SR slumps suddenly and drops quickly. This IPN hydrogel may be useful in the field of controlled drug delivery.