Preparation and structural characterization of hydrogel microspheres

Monodisperse hydrogel microspheres (microgels) were prepared by precipitation polymerization from acrylamide (AAm), methylenebisacrylamide (MBAAm), and methacrylic acid (MAc) in isopropanol. MAc was found to be essential for the preparation of fine spherical particles. The structural analysis of microgels at each step of conversion suggests that the precipitation polymerization proceeds in two steps of (1) the formation of star-shaped nuclei mainly by crosslinking agents and the coupling of the nuclei to constitute a network structure, and (2) the incorporation of remaining acrylamide onto the microgel surface and the further crosslinking with free double bonds in microgels. The particle size was found to depend on various polymerization conditions, including the monomer composition, the amount of initiator and the total monomer concentration. The internal structure of microgels was observed by small-angle X-ray scattering (SAXS). The scattering profile was analyzed by Fourier-transformation to yield the distance distribution function p(r), and by fitting an assumed scattering function to evaluate the correlation length ξ and the fractal dimension D.