Nucleation Of Latex Particles Research Articles

Kinetics of emulsifier adsorption and nucleation of latex particles

AbstractEvidence suggesting that the ultimate latex particles formed in the emulsion polymerisation of styrene are formed by the coalescence of unstable primary particles appears incompatible with the usual assumption that latex particles are predominantly nucleated from monomer‐swollen micelles in this system. However, this is not necessarily the case if the rate of emulsifier adsorption is not always sufficient to maintain a saturated monolayer of adsorbed emulsifier at the surface of growing latex particles so long as emulsifier micelles remain in the system. This appears to be the situation particularly when non‐ionic emulsifiers are used. Even with ionic emulsifiers abnormally high emulsifier/monomer ratios are needed to prevent the coalescence of primary latex particles. Some very high molecular weight polymer (controlled by the transfer constant to monomer) would be expected in the initial stage of styrene emulsion polymerisation if the micellar nucleation of latex particles is dominant.

Mechanism of nucleation of copolymer latex particles

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.