Effect of carboxylic acid monomer type on particle nucleation and growth in emulsifier-free emulsion copolymerization of styrene-carboxylic acid monomer

Carboxylated polystyrene latexes were prepared by emulsifier-free emulsion copolymerization of styrene with various carboxylic acid monomers (AA, MAA and IA). DLS analysis and SEM observations were used to investigate the effect of various carboxylic acid monomers on the particle formation and growth processes. It was found that number of particles per unit volume of aqueous phase (N-p) increases with increasing hydrophobicity of carboxylic acid monomers in order of IA < AA < MAA. There was significant difference in polymerization rate per particle (Rp/Np) in all the experiments also. The results revealed that both particle nucleation and growth processes are dependent on the hydrophilic nature of carboxylic acid monomers. SEM studies showed that N-p is almost constant in the particle growth stage (conversion above 10%). Through some calculations by data obtained from DLS technique, average diameter of monomer swollen polymer particles (d(pswol)) of all the carboxylated polystyrene latexes at the same conversion of 0.4 was obtained to be 325.24, 199.30 and 661.02 nm for AA, MAA and IA respectively. Attempt was made to calculate the average number of propagating radicals per particle and to determine its relevance to the kinetics of particle growth by various carboxylic acid monomers.