Novel Synthesis of Macroporous Poly(N-isopropylacrylamide) Hydrogels Using Oil-in-Water Emulsions

Abstract

Porous N-isopropylacrylamide (NIPA) hydrogels having a unique structure, that is, spherelike cavities distributed randomly and a homogeneous network in the gel phase, were successfully synthesized by means of an emulsion templating method; this method involves the synthesis of NIPA gels in an oil-in-water (O/W) emulsion by free radical copolymerization with a cross-linker, followed by washing (removal) of the dispersed oil as a pore template (porogen). The synthesis conditions, O/W volume ratio, amount of added surfactant, and monomer concentration affect the internal pore structure, equilibrium swelling, and swelling/shrinking kinetics. A porous hydrogel swollen at 10 degrees C has a pore diameter distribution in the range of 1-40 microm, which was observed with a scanning electron microscope. Scanning electron micrographs and swelling degree reveal that the pore size and porosity can be adjusted by varying the O/W volume ratios and surfactant amounts. The porous hydrogels show very rapid swelling/shrinking in accordance with the temperature swing. The fast response is attributed to the convection flow of water through the macropores. In addition to a faster response gel, the emulsion templating method can yield potentially intelligent gels in which the pores function as spaces for reaction, separation, and storage.