Semi-continuous emulsion co-polymerisation of methylmethacrylate and butylacrylate using zwitterionic surfactants as emulsifiers: Evidence of coagulative nucleation above the critical micelle concentration

Zwitterionic surfactants based on phosphorylcholine and sulfobetaine have been used as emulsifiers to prepare stable, monodisperse copolymer latices of methyl methacrylate and butyl acrylate at high volume fractions using seeded semi-continuous emulsion copolymerisation. The amounts of these surfactants required when semi-continuously fed into the preparation are more than an order of magnitude less than is conventionally used. The low amounts of emulsifier used in successful polymerisations allowed subtle features of the mechanism of polymerisation for this system to be revealed. At specific surfactant concentrations in the polymerisations, the results suggested that the rate of incorporation of surfactant into the growing primary polymer particle was greater than the rate of addition. This led to primary particle instability and a coagulative mechanism being the dominant nucleation mechanism. This occurred at points at or just above the critical micelle concentration (CMC). The total number of particles formed was proportional to the concentration of surfactant to the 0.87-1.48 power and is consistent with Gilberts analysis on the coagulative nucleation mechanism. This suggests that at other surfactant concentrations above and below the CMC for each zwitterionic surfactant used, the presence of a classical homogenous or micellar nucleation mechanism, each coupled with a coagulative process could be present, although the experimental data presented are not conclusive. The surfactants were found to be crucial to the stability of the growing latex particle, but their mechanism of action is still unclear. The final polymer latex particles were electrostatically stabilised, at least in part, by the persulphate initiator fragments.