Mechanism of nucleation of copolymer latex particles

Abstract

Coalescence of latex particles has generally been assumed to be negligible in emulsion polymerisations although limited coalescence has been shown to determine the number of latex particles formed in the emulsion polymerisation of the more water-soluble monomers and also of non-polar monomers below the critical micelle concentration of any emulsifier present. It now appears that constancy of latex particle numbers during the major part of the course of emulsion polymerisations is not an essential characteristic but a coincidence consequent on the choice of emulsifier concentrations not much above the minimum concentration required to stabilise the latex. The strongest evidence for the nucleation of latex particles from monomer solubilised in micelles is the sharp increase in the number of latex particles formed which is observed at the critical micelle concentration of the emulsifier with the less water-soluble monomers: however, an alternative explanation of this may be that this represents a transition from fast coalescence through slow coalescence to complete kinetic stability of particles nucleated by oligomeric precipitation. The introduction of a polar comonomer ultimately suppresses this transition most probably because of increasing the rate of homogeneous nucleation. However, there is also evidence that latex particles can be nucleated from emulsion droplets when these are small enough and that the rate of polymerisation of a non-polar monomer during Interval I is proportional to the amount solubilised in the emulsifier micelles.