Microemulsion Polymerization. 2. Influence of Monomer Partitioning, Termination, and Diffusion Limitations on Polymerization Kinetics (original) (raw)
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Polymerization in nonaqueous microemulsions
Colloid & Polymer Science, 1996
Nonaqueous microemulsions containing formamide, the anionic surfactant AOT (bis(2-ethylhexyl)sulfosuccinate sodium salt), sodium bromide (NaBr), and either of the monomers hexyl methacrylate or styrene are polymerized at 60 ~ For both monomers, the final product is a stable, bluish dispersion of particles of ca. 80 nm diameter. Based on phase behavior studies as a function of NaBr concentration, we describe how a systematic variation of composition and monomer causes changes in reaction rates and latex characteristics. The monomer solubility in the continuous phase plays an important role in determining the final polymer characteristics. Decreasing monomer solubility shifts the mechanism from one similar to solution polymerization to one closer to traditional emulsion polymerization.
2011
Here, we present the oil/water (O/W) microemulsion polymerization in three-component microemulsions of n-butyl acrylate, ethyl acrylate, and methyl acrylate, monomers with similar chemical structures but different water solubilities using the cationic surfactant dodecyl trimethyl ammonium bromide. The effects of monomer water solubility, initiator type and initial monomer concentration on the polymerization kinetics were studied. Reaction rates were high with final conversions between 70 and 98% depending on the monomer and reaction conditions. The final latexes were bluish, with a particle size ranging between 20 and 50 nm and polymer with molar masses in the order of 10 6 g mol À1 . Increasing monomer water solubility resulted in a slower reaction rate, larger particles and a lower number density of particles. A higher reaction rate, larger average particle size and higher particle number density were obtained by increasing the monomer concentration.
Transition from microemulsion to emulsion polymerization: Mechanism and final properties
Journal of Polymer Science Part A: Polymer Chemistry, 2004
Microemulsion and emulsion polymerization can have some similarities in starting conditions and polymerization mechanisms, but the resulting latices are unalike in particle size and molecular weight. Here we show that polymerizations can be formulated that display the characteristics often separately associated with microemulsion or emulsion polymerization. Kinetic modeling and particle size measurements show that emulsion polymerizations with initial concentrations close to the microemulsion-emulsion phase boundary demonstrate relatively fast consumption of monomer droplets and produce smaller particles. Because of their high surfactant concentrations, none of the emulsion polymerizations examined demonstrate the classical Smith-Ewart kinetics usually associated with emulsion polymerization. Instead these emulsion polymerizations have a long period of particle nucleation that subsides only after the disappearance of monomer droplets.
Particle Nucleation during Microemulsion Polymerization of Methyl Methacrylate
Macromolecules, 1994
The evolution of the reaction conversion and particle size distribution during the microemulsion polymerization of methyl methacrylate (MMA) is used to determine the particle nucleation mechanisms. A pseudo-3-component oil-in-water microemulsion is formed with water, MMA, and a mixture of dodecyltrimethylammonium bromide (DTAB) and diodecyldimethylammonium bromide (DDAB) in a 3:l weight ratio as surfactant. Polymerization is initiated with either an oil-soluble or a water-soluble initiator and conversion followed either by measurement of the unpolymerized monomer concentration in samples taken during the reaction or by direct on-line densimetry. A two-stage process is observed. The first stage, described by a very slow increase in conversion, is attributed mainly to homogeneous nucleation, and the second stage, characterized by a much higher rate of conversion, involves continuous nucleation and is governed mainly by a micellar-entry mechanism.
Colloid and Polymer Science, 2014
Vinyl acetate (VAc) was polymerized in microemulsion using sodium bis(2-ethylhexyl) sulfosuccinate (AOT) as surfactant and n-butanol (n-BuOH) as the cosurfactant (1:1, wt/wt) with potassium persulfate (KPS) as initiator. The effects of monomer, surfactant, and initiator concentration, as well as the temperature on the kinetics, particle size, and molar masses were studied. It was observed that polymerization rate increases as monomer, initiator, and temperature increases; however, the opposite was observed as surfactant concentration increases. Average particle diameters (Dp)< 50 nm and polymers with weight average molar masses (Mw) between 2.7×10 5 and 7.5×10 5 g/mol were obtained. For low amounts of stabilizing agents (AOT and n-BuOH), a bimodal molar mass distribution (MMD) was obtained. As nbutanol concentration increased, Mw decreases and a monomodal MMD was observed, which can be explained because chain transfer events are promoted by the presence of n-BuOH. A thermodynamic model was implemented to simulate the partitioning of VAc in the presence of n-BuOH between the phases throughout polymerization. It was found from simulations that microemulsion droplets are depleted fast at 5 % conversion and, due to the high water solubility of VAc, most of the monomer (70 %) and n-BuOH (97 %) reside in the aqueous phase at the beginning of polymerization, which indicates that both homogeneous and micellar nucleation are present.
Journal of Colloid and Interface Science, 2006
The polymerization of n-hexyl methacrylate (n-HMA) in three-component microemulsion stabilized with dodecyltrimethylammonium bromide (DTAB) is reported as a function of monomer and initiator concentrations and temperature. The obtained latices were bluish, transparent, and translucent. Particle sizes and molar masses were on the order of 20 nm and 3 × 10 6 g/mol, respectively. In all cases, high reaction rates and final conversions of 98% were obtained. Polymerization temperature has a strong effect on reaction rate and conversion.
Elucidation of the miniemulsion stabilization mechanism and polymerization kinetics
Journal of Applied Polymer Science, 2003
Styrene/hexadecane miniemulsions were polymerized at 50°C using a redox initiator. The miniemulsions and their corresponding latexes were characterized in terms of size, polymerization rate, and surface properties. The resulting data were analyzed to elucidate the miniemulsion stabilization and polymerization mechanisms. It was found that the free surfactant concentration exceeded the critical micelle concentration when large amounts of surfactant (60 mM sodium lauryl sulfate) were used, resulting in simultaneous micellar and droplet nucleation. Most surfactant was on the surface of the droplets (85%) or particles (95%). The fractional surface coverage was proportional to the surfactant concentration to the 0.55 power. Using a particle diameter equation, the number of particles was calculated to be proportional to the surfactant concentration to the 1.35 power. Through direct particle size measurements, a power of 1.38 was confirmed. The rate of polymerization was determined by reaction calorimetry to be proportional to the number of particles to the 0.59 power, in contrast to classical Smith-Ewart kinetics for conventional emulsions (1.0 power). The average number of radicals per particle was estimated from the rate and number data, and varied with the particle diameter to the 0.97 power. The observed kinetic dependencies were validated through an extension of Smith-Ewart theory.
Synthesis and characterization of poly(n-hexyl methacrylate) in three-component microemulsions
European Polymer Journal, 2001
The polymerization of n-hexyl methacrylate in three-component microemulsions stabilized with dodecyltrimethylammonium bromide is examined here as a function of the concentration of a water-soluble (V-50) and an oil-soluble (2, 2-azobisisobutyronitrile) initiators, of monomer concentration in the parent microemulsions and temperature. At high temperatures and high initiator concentrations, only two reaction rate intervals are observed; however, at low temperatures and low