A brief survey is given of the methods which can be used to produce polymer latices with a narrow size distribution. The factors which control both the particle size and the nature of the chemical groupings on the latex particle surface are discussed for latices prepared without the addition of an emulsifier. In particular, it is demonstrated experimentally that the ionic strength of the aqueous phase plays a major role in controlling particle size and that the free radicals produced by thermal decomposition of the initiator become incorporated as end-groups on the polymer chains and hence form the surface groups on the particle. A theoretical explanation is given which indicates the reasons for the importance of ionic strength in controlling particle size.

Control of particle size in the formation of polymer latices

Electrophoretic studies on polystyrene latices

The adsorption of cationic surface active agents on polystyrene surfaces

Characterization of surface charge on polystyrene latices

Solute electrolyte and adsorbed emulsifier can be removed by ion exchange from polystyrene latices prepared with persulphate initiator. Rigorous purification of the ion-exchange resins is necessary to avoid contamination by leached polyelectrolytes. These ion-exchanged latices are stabilised by strong acid surface groups - sulphate end-groups of the polymer molecules. The number of surface groups as determined by conductometric titration of the latex, plus the number buried within the particle as determined by conductometric titration of the polymer in solution (or by X-ray fluorescence), is less than the expected value of two per polymer molecule. The other end-groups are shown to be hydroxyl groups by oxidation to the carboxyl form followed by ion exchange and titration. The total number of end-groups thus determined is close to two per molecule. As the pH of the polymerisation medium is decreased, the proportion of hydroxyl end-groups increases and that of the sulphate end-groups decreases. No carboxyl groups are detected in ion-exchanged latices prepared in a nitrogen atmosphere, independent of the nature of the emulsifier. The dye partition method gives end-group concentrations that are far lower than those determined by titration or X-ray fluorescence, probably because the formation of the dye end-group ion-pair is not complete. Finally, for latices prepared at relatively low emulsifier concentrations, the contribution of the sulphate end-groups to the surface charge density is significant and cannot be overlooked, as has been done in the past.

Inferences on the mechanism of emulsion polymerisation of styrene from characterisation of the polymer end-groups†

Infrared spectroscopy was empolyed to study the nature of the structural changes which occurred through oxidation during the emulsion polymerization of styrene. Aliphatic carboxylic, amino, and phosphate emulsifiers and hydrogen peroxide and potassium persulfate initiators were empolyed for polymer preparation. In addition, a polystyrene dispersion prepared in the absence of any emulsifier or stabilizer was examined. Irrespective of the nature of the initiator–emulsifier combination employed, all of the polymer spectra revealed bands at 1705 and 1770 cm.−1. The band at 1705 cm.−1 was assigned in part to the carbonyl stretching mode of dimertic carboxylic acid, formed by oxidation, in the polystyrene chains. Absorption at 1770 cm.−1, which was very weak, was tentatively attributed to the carbonyl stretching mode of the monomeric form of this acid. The structure of the acid endgroup was not established, but the results obtained suggest that it was possibly a phenylacetic acid residue or a residue with a similar structure.

M. C. Marshall

Papers

Infrared spectroscopic studies of polystyrene emulsion polymers: Effect of oxidation during polymerization