A fluorescence study on swelling of hydrogels (PAAM) at various cross-linker contents

Disk-shaped acrylamide (AAm) gels were prepared from AAm with various N,N'-methylenebisacrylamide (Bis) contents as cross-linker in the presence of ammonium persulfate as an initiator by free-radical cross-linking copolymerization in water. Polyacrylamide (PAAm) gels were dried before using for swelling experiments. Steady-state fluorescence spectrometer was employed during the swelling of PAAm hydrogels in water. Pyranine was introduced as a fluorescence probe. Fluorescence intensity of pyranine from various Bis content gel samples was measured during in situ swelling process. It was observed that fluorescence intensity decreased as swelling has proceeded. Gravimetric and volumetric experiments were also performed. The Li-Tanaka equation was used to determine the swelling time constants, tc , and cooperative diffusion coefficients, Dc , from intensity, weight, and volume variations during the swelling processes. It was observed that swelling time constants, tc , increased and diffusion coefficients, Dc , decreased as the cross-linker content was increased.