The nucleation mechanism of styrene emulsion polymerization in the presence of the nonionic surfactant Emulphogene BC‐840 was investigated. It was found that the conversion‐time curves of this system show two rate regions. The change in rate occurs at ca. 40% conversion. Second‐stage nucleation leads to a faster rate and consequently to a bimodally dispersed latex. The particle size distribution is relatively narrow before 40% conversion. The bimodally dispersed latex shows a bimodal molecular weight distribution in gel permeation chromatography and the results from fractional “creaming” of the latex indicate that the small‐size particles contain the higher‐molecular‐weight polymer.
Poly@-methylstyrene)-graft-pol y(oxyethy1ene) copolymers were prepared from p-methylstyrene and a-hydroxy-w-methyl-poly(oxyethy1ene) using a 'grafting on to' technique. A total of 10 copolymers were synthesized varying in the hydrophobic backbone chain lengths, hydrophilic branch lengths and frequency of branches. The stabilizing efficiency of these amphipathic copolymers was studied in the emulsion polymerization of styrene. Using the number, size and size distribution of the particles as the criterion of stability, it was found that the polyoxyethylene (PEO) chain length is not as crucial for stability as the availability of the backbone for anchoring. A change in the backbone chain length (from &?, = lo00 to 24000) with the same percent PEO had no effect on the outcome of the reaction: the latices stabilized to the same number of particles of the same size.
Four electrostatic surfactants, CmH2m+iO(CH2CH20)nS03Na, with m averaging between 12 and 15 and n, the ethylene oxide chain length, being 3,7,9, or 15, were used to investigate the steric chain length effect in the stabilization of model latices and in emulsion polymerization of styrene. It was found that during polymerization, the number of particles stabilized and the rate of reaction decreased with increasing ethylene oxide chain length in the surfactant. The addition of an electrolyte to the polymerization system caused a decrease in the rate of polymerization. This electrolyte effect decreased with increasing ethylene oxide chain length. Coagulation tests of latices stabilized by these four surfactants indicated a switch in stabilization mechanism from primarily electrostatic to steric stabilization between 9 and 15 ethylene oxide units.
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