Preparation of poly(styrene-co -divinylbenzene) microspheres by membrane emulsification

The paper discusses the method of preparation poly(styrene-co -divinylbenzene) microspheres by means of membrane emulsification (ME). Two-step process was evaluated: ME of monomers followed by suspension polymerization. The effects of changing some emulsification parameters as well as polymerization conditions were investigated in relation to microspheres size. It was observed that for membrane of nominal pore size 5 mm used for monomer emulsification, the parameters of polymerization, such as temperature, time, kind of used initiator, have also affected the diameter and distribution of sphere size. It was also noted that the amount of organic phase has a marginal effect on particles diameter and distribution even if high quantity of dispersion phase was applied. Addition of poly(vinyl alcohol) modified the size distribution of obtained spheres; however, the effect was not so pronounced.

The ME can be used to obtain microspheres with small diameter and narrow droplet size distribution. The prepared emulsions are stable enough to withstand the suspension polymerization but, for most cases, the average diameter of obtained microspheres were larger than emulsion droplets size. It is not recommended to use 2,2?azobis(isobutyronitrile) as the polymerization initiator. It is profitable to use 0.12 wt.% solution of poly(vinyl alcohol) with 31 kDa molecular weight to stabilize emulsion. Emulsion size can be reduced by addition of simple salts also. The mean emulsion size was 5 Á 6 times larger than the mean pore size of membrane. When emulsion is polymerized it is reasonable to increase temperature slowly. Polymerization should also be carried out with vigorous mixing that reduces droplets coalescence and preparation of large spheres.