Some thermodynamic aspects in emulsion copolymerization

Emulsion polymerization proceeds in a heterogeneous medium; in copolymerization, due to discrepancy in water solubility and interactions with the resulting copolymers, monomers are partitioned in a complex way between droplet, water and particle phases. As monomer swollen polymer particles are the main loci of polymerization, it is of great interest to know the monomer feed within the particle phase to understand the behaviour of this copolymerization whatever may be the process and the experimental conditions. Two approaches are examined in this lecture to investigate emulsion copolymer and colloid properties of the latexes. A first, simple one is based on experimental partition coefficients and a second one is based on more theoretical thermodynamic considerations (volume fractions, interaction χ parameters …). The latter gives a more general insight of emulsion processes and allows to investigate influence of many parameters like monomer-monomer, monomer-polymer, monomer-emulsifier interactions, interfacial tension, particle size, surface charge density… Computer programs are derived which give predictions in rather good agreement with experiments. The thermodynamic approach allows to study crosslinking effect as well as competition between polymerization rate and monomer diffusion rate if some control by diffusion does occur. Owing to these computer programs it becomes possible to correlate many experimental parameters. Most of experimental date are relative to styrene-acrylonitrile copolymerization.