Preparation and characterization of fast response macroporous poly(N-isopropylacrylamide) hydrogels

Macroporous temperature-sensitive poly(N-isopropylacrylamide) (PNIPAAm) hydrogels have been successfully synthesized by using poly(ethylene glycol) (PEG) as the pore-forming agent. Scanning electron microscope graphs reveal that the macroporous network structure of the hydrogels can be adjusted by applying different molecular weights of PEG during the polymerization reaction. The surface roughness of the hydrogels is also investigated using atomic force microscopy, and the results indicate that the surface of the PEG-modified gel is much rougher compared to that of the conventional PNIPAAm gel. The newly invented macroporous hydrogels exhibit much better properties as temperature-sensitive intelligent polymers. For instance, at a temperature below the lower critical solution temperature (LCST), they absorb larger amounts of water and show obviously higher equilibrated swelling ratios in the aqueous medium. Particularly, due to their unique macroporous structure, the PEG-modified hydrogels show a tremendously faster response to the external temperature changes during deswelling and reswelling processes as the temperature cycles across the LCST. They can also shrink and lose water with dramatically rapid rates at temperatures above the LCST. The macroporous PNIPAAm gel has potential applications in controlled release of macromolecular active agents.