SIZE DEPENDENCE OF POLYMER COMPOSITION IN THE SURFACE-LAYER OF POLY(ACRYLAMIDE-CO-ACRYLIC ACID) HYDROGEL MICROSPHERES

Poly(acrylamide-co-acrylic acid) hydrogel microspheres of four differe nt average sizes were prepared using a membrane emulsification techniq ue. These four kinds of microspheres had average diameters of 1.35, 2. 33, 3.09, and 7.43 mu m in distilled water and were all fairly monodis perse. Their surface properties were studied by measuring the electrop horetic mobility of the microspheres. The mobility values were negativ e at pH 7.4 with the ionic strengths between 0.005 and 0.154 at 25 deg rees C. More negative mobility values were obtained with smaller micro spheres than the larger ones. By analyzing the data with an electrokin etic theory for ''soft'' surfaces, it was found that smaller microsphe res have higher surface charge density than the larger ones, although all kinds of microspheres were prepared from monomer solutions with th e same monomer composition. The observed size dependence of the electr ophoretic mobility suggests that copolymerization of acrylamide monome rs and acrylic acid monomers does not proceed homogeneously within a m icrosphere. That is, charged acrylic acid monomers have a tendency to be localized in the microsphere core region, whereby the surface regio n of microspheres becomes poor in charges, reducing the mobility of la rger microspheres. This is not the case for smaller microspheres, so t hat their mobility becomes more negative. (C) 1998 Academic Press.