Microemulsion Polymerization of Alkyl (Meth)acrylates Initiated by UV Light

Abstract

ABSTRACT:Radical polymerizations of polar unsaturated monomers ethyl acrylate, methyl methacrylate and nonyl methacrylate in an o/w microemulsion photoinitiated by a UV light were investigated. Polymerizations were conducted at low temperature to follow the effects of a varied solubility of monomer (coemulsifier) and/or diffusional degradation of monomer droplets on the kinetic and colloidal parameters. The rate versus conversion curve showed four regions with two rate maxima ca. at 5% to 20% and 50 to 70% conversion. The first rate maximum results from abrupt decrease of the monomer concentration at microemulsion loci and the second one from the increased accumulation of radicals in the latex particles. The higher polymerization rate of ethyl acrylate compared to that for methyl methacrylate or nonyl methacrylate was attributed to higher monomer concentration at reaction loci and propagation rate constant and lower quenching rate. The rate of polymerization was found to increase with incident light intensity and was most pronounced with nonyl methacrylate. The reaction order (the dependence of the rate of polymerization or the number of particles on the incident light intensity) was taken as a measure of particle nucleation efficiency. Particle size distribution was observed to be very narrow at low conversion and increased with increasing conversion.KEY WORDS Microemulsion j Polymerization I Alkyl (Meth)acrylates I Kinetics I Particle I Nucleation/ Microemulsion and miniemulsion polymerizations offer convenient access to well-defined microlatex particles being typically one order of magnitude smaller than polymer particles obtained by conventional emulsion polymerization. The fine monomer emulsion exhibits a large interface between monomer droplets and the aqueous phase. Under such conditions, continuous nucleation up to (very) high conversion is operative. There is no apparent constant rate period in the o/w microemulsion or miniemulsion polymerization. 1 -3 Ugelstadt et a/. 4 clearly demonstrated that monomer droplets could effectively compete with monomer-swollen micelles for capture of free radicals generated in the aqueous phase when the total surface area of the droplets became large enough. In conventional emulsion polymerization, the principle locus of particle nucleation is the aqueous phase or the monomer swollen micelles depending on the degree of water solubility of the monomers, an initiator and amount of emulsifier. Monomer droplets are considered only as monomer reservoir supplying monomer to the growing polymer particles. However, the small size of the monomer droplets (micro-or minidroplets) enables them to become the principle locus of particle nucleation ..The principle behind the formation ofmicroemulsions (droplet size I 0-50 nm) is penetration of coemulsifier into the water/oil interface, thereby decreasing surface tension and increasing interface area. Coemulsifier also decreases the rigidity of interface film due to increasing the molecular disorder. Coemulsifier promotes the formatio...

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“…When using SDS as the surfactant, a larger particle size was obtained for EA and BA and macroparticles were formed when polymerizing the more water soluble monomer MA. 41 Table 1 also shows that a larger particle size was obtained by increasing monomer concentration, an increase in particle size with increasing monomer content was also reported by Capek et al 39 in the microemulsion polymerization of methyl and nonyl methacrylate initiated by UV light and in the microemulsion polymerization of alkyl methacrylates and acrylates. 41, 42…”

“…Table 1 shows that by increasing the chain length (less water soluble monomer) more particles were obtained, but this data alone cannot be used to explain the increase in the reaction rate, because it is also needed to know which fraction of the particles are active. When the monomer water solubility increases, there is a grater possibility for monomeric radicals to exit polymer particles after chain transfer reactions, resulting in a lower fraction of active particles; 39, 41 therefore, for the above reasons, when the monomer water solubility decreases, more active particles are present. Furthermore, because of thermodynamic equilibrium and the small proportion of monomers used, the monomer concentration in the particles will be lower as the monomer water solubility increases resulting in lower reaction rates.…”

“…In the microemulsion polymerization of alkyl methacrylates and acrylates using the anionic surfactant sodium dodecyl sulfate (SDS), Capek et al 39, 40 found that there is an increase in polymerization rate with increasing chain length (lower water solubility). However, in the microemulsion polymerization at a lower temperature (25 °C) using the anionic surfactant SDS, and initiated by UV light, Capek 41 later reported that the reaction rate followed the order: ethyl acrylate (EA) > methyl methacrylate > nonyl methacrylate (lower reaction rate as water solubility decreased). Using a mathematical model, Mendizábal et al 42 compared their predictions with experimental data for the microemulsion polymerization of three monomers with different chemical structures and water solubilities and found that homogeneous nucleation becomes more important as the solubility of the monomer in the aqueous phase increases.…”