Heterogeneities during the formation of poly(sodium acrylate) hydrogels

In situ fluorescence and light scattering techniques were used to study the formation of heterogeneities during the free-radical crosslinking polymeri zation of sodium acrylate (NaAc) in the presence of N,N'-methylene(bisacryl amide) (BAAm) as a crosslinker. Pyranine was used as a fluorescence probe i n the experiments. The reactions were carried out at room temperature and a t a monomer concentration of 20.6 w/v% in water. It was found that the gel forming system became turbid at the start of the reaction; continuing polym erization and crosslinking reactions turned the opaque gels into slightly o paque and finally transparent gels. Two peaks were observed both in the sca ttered and emitted light intensities versus the reaction lime plots. The fi rst peak corresponds to the formation and growth of the phase separated dom ains whereas the second peak reflects the macrogelation point in the reacti on system.