Statistical design strategies to optimize properties in emulsion copolymerization of methyl methacrylate and acrylonitrile

Emulsion copolymerization was performed by using (NH₄)₂S₂O₈ as an initiator in the presence of an emulsifier (sodium lauryl sulfate). The effect of changing the process variables {initiator concentration, emulsion concentration, feed ratio of monomer [acrylonitrile (AN)/methyl methacrylate (MMA)], and temperature} on the properties of the copolymer was monitored. The polymerization rate, particle size, molecular weight, and polydispersity index (of the polymer chain length) were determined for the polymerization conditions designed by the fractional factorial design method. Estimation of the effects on the polymerization rate through 2^4-1 fractional factorial design reveals that it depends on the AN/MMA ratio, surfactant concentration, and temperature. The surfactant concentration mainly influences the particle size and molecular weight, while the surfactant concentration and temperature are the key variables in influencing the polydispersity index. A regression equation relating the surfactant concentration, temperature, and polydispersity index is deduced, and the contour plot is made using that. A condition for obtaining the minimum molecular weight distribution was arrived statistically and verified by trial experiments. The minimum molecular weight polydispersity (<3.0) can be obtained at a surfactant concentration of 0.393 g/500 mL of solution and a reaction temperature of 54.6 °C.