On the modeling of microemulsion polymerization. Experimental validation

Abstract

A recently developed model for o/w microemulsion polymerization, tested only with styrene, is validated here by comparing its predictions with experimental data from the polymerization of three monomers with different chemical structures and water solubilities. The model reproduces well the kinetic data as well as the characteristics of the latex and polymer (particle size, average molar mass, number of polymer chains per particle, etc.) for the three monomers. The model predicts only two reaction rate intervals and that the conversion at which the maximum reaction rate occurs, depends only on the type of monomer and reaction conditions, in agreement with experimentally reported data. In the model, both micellar and homogeneous nucleation mechanisms are permitted, even for low water‐solubility monomers such as styrene, because of the large amounts of surfactant typically employed in microemulsion polymerization. The role of these two mechanisms in the polymerization process is examined here.

Microstructure, Kinetics, and Transport in Oil‐in‐Water Microemulsion Polymerizations

“…Mendizabal et al have developed a kinetic model based on population and mass balances [25,26] that captures the experimentally observed trends in microemulsion polymerization kinetics. However, the adjustment of four rate constants is required to obtain fits to data, and their equations must be solved numerically.…”

Surfactant concentration effects on the microemulsion polymerization of vinyl acetate

“…1). As detailed elsewhere [24], the conversion at which the maximum reaction rate occurs is mainly related to the volume fraction of polymer at equilibrium within the particles.…”

“…To explain the inverse dependence of reaction rate with surfactant content let us examine the well-known equation for reaction rate in emulsion and microemulsion polymerization [21,24,25]. For these processes, the reaction rate, R P , is given by k p [M] p N P ñ/N A , where k p is the propagation rate constant, [M] p is the volume fraction of monomer within the particles, ñ is the average number of radicals per particle and N A is Avogadro's number.…”

“…Table 3 reports the estimated values of ñ for the different CTAB/water ratios at conversions of 40 and 55%. The values of ñ increase with increasing CTAB/water ratio but they are much smaller than 0.5 indicating that radical desorption is a frequent event, which is typical of microemulsion polymerization due to the small size of the particles commonly encountered in this process [24]. An increase in ñ as the CTAB/water ratio is augmented should yield faster reaction rates, contrary to the experimental observations (inset in Fig.…”

Emulsion Polymerization

“…Theoretical considerations indicate that compartmentalization of radicals in polymer particles does not change the polydispersity index PDIð¼ X w =X n Þ in emulsion polymerization from its value of 2 in homogeneous polymerization when termination takes place by transfer to monomer, chain-transfer agent, or other substance [Butte et al, 2002a,b;Giannetti et al, 1988;Gilbert, 1995;Lichti et al, 1980Lichti et al, , 1982Mendizabal et al, 2000]. However, emulsion polymerization results in molecular weight broadening when termination involves bimolecular reaction between radicals.…”