مکانیزم ایجاد ذرات CMC در پلیمریزاسیون امولسیون On the mechanism of particle formation above the CMC in emulsion polymerization

Introduction Emulsion polymerization is an important process for producing sub-micrometer polymer particles dispersed in water (known as latex) for a variety of applications, such as paints, coatings, sealants, additives for concrete, etc. Latexes have also found applications in the biomedical field, such as immunoassays, test for AIDS, drug delivery systems, etc. With the advent of the living-free-radical polymerization technique, there have been efforts to implement this technique in emulsion polymerization [1] as well as to produce nanocomposite materials [2, 3]. In all these applications, control of particle size distributions (PSD) is key aspects, because they have important influence on the product performance. It is well known that these characteristics of the latex are greatly influenced by the mechanism(s) of formation (nucleation), growth, and the colloidal stability of the particles. Due to the complexity of the phenomenon itself and to the experimental difficulties associated with its study, particle formation still remains a controversial and poorly understood issue [4] When the polymerizations are effected below the critical micellar concentration (CMC) of the surfactant, obviously, there are not micelles, and hence, particles are formed by homogenous nucleation [5–7]. In this mechanism, the radicals generated in the aqueous phase propagate with the small amount of monomer dissolved in the aqueous phase until the oligomers formed reach a critical degree of polymerization (jcr); at this size, they become insoluble and precipitate. Depending on the availability of surfactant, these new-born particles (also known as primary) can undergo what is called limited coagulation. Coagulation of particles implies reduction of the particles surface which in turn leads to an increase of the surface charge density, because the charges that form the parent particles are distributed in a smaller area. Coagulation occurs or it is limited up to the point, where the surface charge density is enough to impart colloidal stability to the particles, so that this phenomenon is named in the literature as ‘‘limited coagulation’’ [8].