Possible evidence for the existence of a hairy layer at the surface of polymer latex particles

Abstract

In this work, an attempt is made to verify the existence of a hairy layer at the surface of polymer latex particles, and determine its thickness under different conditions. Two systems have been studied. In one system, sodium dodecyl sulphate (SDS) and sodium dodecanoate (laurate-SL) were used, separately, as emulsifiers in ab initio (unseeded) emulsion polymerization of styrene carried out at different ionic strengths. At low levels of electrolyte (NaCl in the range 0.00- 0.10M), the hydrodynamic radius (dH) was found to decrease gradually, reaching a minimum value at 0.10M, about 7-9nm smaller than the original value in the absence of electrolyte. At electrolyte levels higher than 0.10M, dH increased monotonically. In the first observation, the results are interpreted in terms of a hairy layer model, which suggests that the thickness of this layer is reduced by the presence of an electrolyte because of charge shielding, while the increased dH at high ionic strength is interpreted in terms of coalescence between latex particles. The extent of the effect depends mainly on the concentration of emulsifier. In the second system, the effect on a seeded emulsion polymerization of styrene was investigated. In this system, a seed latex containing about 10% solids was prepared with SDS as emulsifier. The monomer was then further polymerized at different ionic strengths with no more added emulsifier. The effect on dH was very similar to that of the first system. However, in a previous investigation on this system (Polymer International, 40 (1996) 307) the rate of seeded polymerization was not affected in the electrolyte range 0.0-0.1M, while it was found to increase with increasing electrolyte level in the range 0.10-0.20M. Post-addition of electrolyte (0.0-0.1M) to the seed latex results in a 4-6nm size contraction due to an increase in the hydrophobicity of the surface, leading to a backward extension away from the aqueous phase.