Abstract
With potassium persulphate as initiator and sodium dodecyl sulphate as emulsifier, the batch emulsion copolymerization rate behaviour of styrene (S) and methyl acrylate (MA) was investigated at 50°C, varying the monomer ratio and the monomer to water ratio. The composition drift occurring during the copolymerization is determined by the reactivity ratios and by the monomer partitioning between the various phases present in the emulsion system. Monomer partitioning studies show that the monomer ratio in the latex particles is equal to the monomer ratio in the droplets, although the total monomer swellability of the (co)polymer latex particles depends upon copolymer composition and monomer droplet composition. In the absence of monomer droplets the equilibrium monomer concentration in the aqueous phase is closer to its saturation value (i.e. water solubility) than the monomer concentration in the latex particles. As a consequence of the composition drift, the kinetic behaviour differs widely from the homopolymerizations depending upon the initial monomer ratio. The copolymerization rate strongly varies during copolymerization resulting in a conversion-time plot differing in shape from the sigmoidal shape usually observed in emulsion homopolymerization. The penultimate effect in S-MA copolymerization has been proved to be responsible for this, as can be seen clearly from the conversion dependence of the copolymerization rate during batch emulsion copolymerizations starting from an MA-rich monomer feed, leading to a strong composition drift.
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