Inverse Emulsion Polymerization of Acrylamide Initiated by Oil- and Water-soluble Initiators: Effect of Emulsifier Concentration

Abstract

ABSTRACT:The nature of the reaction loci in inverse emulsion polymerization of acrylamide (AAm) initiated by oil (2,2 0 -azoisobutyronitrile, AIBN)-and water-soluble (ammonium peroxodisulfate, APS) initiators was investigated.The rate of polymerization (R p,max , Interval 2) increases with increasing the emulsifier concentration up to a certain critical concentration and then decreases. The rate of polymerization and the polymerization rate per particle are somewhat larger in the APS-initiated polymerization than in the AIBN-initiated one. For both initiators the polymerization rate per particle is proportional to the particle size and decreases with increasing the emulsifier concentration and the decrease is much more pronounced at the high emulsifier concentrations. The similar kinetic dependences for both initiators were discussed in terms of the formation of monomeric or oligomeric radicals with the similar nature, mobility via the reaction system and partitioning between the oil and water phases. The similar radicals carrying the initiator fragment were formed by the addition of one or two (or several) units to the primary radicals (SO 4 À or 2-cyanoisopropyl). The estimated average number of radicals per particle ( n n) is much below 0.5. The low radical concentration in the polymer particles was discussed in terms of the desorption of monomeric and emulsifier radicals. The desorption of radicals was confirmed by both the Nomura model (N model , k With the increased interest in the technological exploitation of inverse emulsion polymerization as a method for the preparation of water soluble polymers has come the need to understand microscopic kinetic events that govern both the particle nucleation and the polymer growth in the polymer particles. The conventional processes such the initiation, propagation, termination and chain transfer are all operative in the inverse emulsion or microemulsion polymerizations. The inverse microemulsion polymerization processes (water-in-oil systems, w/o) are well known.1-4 The polymerization is very fast and the process is described by two non-stationary rate intervals. This is ascribed to two opposing effects; the continual increase of particle number and continual decrease of monomer at the reaction loci. The microdroplets saturated with the aqueous solution of monomer act as reaction loci and the reservoir of monomer. The rate of inverse microemulsion polymerization increases with increasing the initiator concentration but strongly decreases with increasing the emulsifier concentration. The study devoted to the inverse miniemulsion (homogenized emulsion) polymerization led to the conclusion that the addition of lipophobe to the inverse miniemulsion generated the long term stability against Ostwald ripening.5 The inverse emulsion polymerization of acrylamide produced turbid, viscous and unstable dispersions at the outset and during the intermediate stages of the polymerization. [6][7][8] It was observed that the initial rate of inverse emulsion polymerization is first or...

On the Kinetics of Inverse Emulsion Polymerization of Acrylamide

“…The average number of radicals in a particle (n) was estimated from the polymerization rates divided by the number of particles and the appropriate constants (Table 2).n lies well below 0.5, which indicates strong desorption of monomeric radicals from particles [5]. The values ofn depend on the mechanistic events such as the decomposition rate of initiator, the entry of oligomeric radicals into the particles, the nature of the reaction loci, the chain transfer to monomer and the desorption of radicals from particles.…”

“…The details of these calculations are given in our previous papers [5,31]. The specific desorption rate constant k des (s −1 ) is related to k des (cm 2 /s) by the following equations [29]:…”

“…The separation of two SO 4 −• radicals from one another can be made by the diffusion of monomer of emulsifier segments into the cage. The diffusion of initiator radicals to the particle surface can start the polymerization due to the presence of AAm at the micelle shell (AAm is reported to be surface active [5]). The chain transfer to emulsifier can also suppress the cage effect and initiate the formation of reaction loci.…”

Rheological Properties in Aqueous Solution for Hydrophobically Modified Polyacrylamides Prepared in Inverse Emulsion Polymerization

“…Also, molecular weight is higher (Table 1), even though a smaller percentage of monomer is used in comparison with solution (1.5 versus 3 wt.%). This behavior is expected, because of compartmentalization of the reaction loci [32]. Results show that the conditions employed provide an emulsion system.…”

“…In this sense, the effect of initial surfactant concentration was investigated. Capek [32] found that the AIBN-initiated polymerization and molecular weight decrease at higher emulsifier concentrations. He argued that the decrease in the polymerization rate with increasing the emulsifier concentration is expected, because monomer concentration at the particle loci (diluted by the emulsifier molecules) diminishes.…”