Preparation of monodisperse poly(acrylamide-co-acrylic acid) hydrogel microspheres by a membrane emulsification technique and their size-dependent surface properties

Abstract

Monodisperse poly(acrylamide-co-acrylic acid) hydrogel microspheres with different polymer compositions (0–10 mol% acrylic acid and 90–100 mol% acrylamide) and with different sizes have been prepared from w/o emulsion using microporous glass membranes with average pore sizes of 0.33, 0.73, 1.15, and 1.70 μm. The broader size distribution was observed in the microspheres composed of hydrogel containing more acrylic acid. The size distribution was not affected by the membrane pore size. The microsphere size on the other hand was dependent on both the membrane pore size and the polymer composition. As the membrane with a larger pore size was used, larger microspheres were obtained. When the membrane with a pore size of 1.70 μm was used, the microsphere size clearly increased as the acrylic acid concentration in the monomer mixture increased, while it decreased as the acrylic acid concentration increased when the membranes with smaller pore sizes were used. The surface properties of the microspheres have been studied from their electrophoretic mobility values. The electrophoretic mobility values of poly(acrylamide-co-acrylic acid) microspheres were negative at pH 7.4 with ionic strengths between 0.005 and 0.154 at 25°C, although those of poly(acrylamide) microspheres were almost zero. The electrophoretic mobility of the microspheres became more negative as the acrylic acid concentration increased, with two exceptions. Only two types of poly(acrylamide-co-acrylic acid) microspheres prepared with the membrane of the largest pore size (1.70 μm) showed the same electrophoretic mobility values at all ionic strengths, although they were composed of hydrogels of different acrylic acid concentrations (5 and 10 mol%). The electrophoretic mobility values of the microspheres were also size dependent. More negative mobility values were obtained with smaller microspheres than larger ones. By analyzing the data with an electrokinetic theory for “soft” surfaces, it was found that the microsphere surface becomes softer by the addition of acrylic acid. It was also suggested that the microspheres with smaller sizes have higher surface charge density than those with larger sizes, although the microspheres were prepared from solutions with the same monomer composition. From this, it is found that copolymerization of acrylamide monomers and acrylic acid monomers does not proceed homogeneously within a single microsphere. The density of polymer networks composed of both monomers is higher in the core region than in the surface layer of the microspheres. The accumulation of polymers in the core region of a microsphere explains the decrease in microsphere size with the increase in acrylic acid concentration observed in smaller microspheres. When microspheres are relatively large, enough volume is available for 10 mol% acrylic acid inside a microsphere and the polymer networks are looser than in smaller microspheres, which causes the increase in microsphere size depending on the acrylic acid concentration. This volume increase explains the same electrophoretic mobility values observed in two types of large microspheres containing different concentrations of acrylic acid (5 and 10 mol%).