Radical capture efficiencies

It is accepted that the radical entry rate coefficient for miniemulsion droplets is substantially lower than for the monomer-swollen particles. This is attributed to a barrier to radical entry into monomer droplets which exists because of the formation of an interface complex of the emulsifier/coemulsifier at the surface of the monomer droplets [24]. The increased radical capture efficiency of particles over monomer droplets is attributed to weakening or elimination of the barrier to radical entry or to monomer diffusion by the presence of polymer. The polymer modifies the particle interface and influences the solubility of emulsifier and coemulsifier in the monomer/polymer phase and the close packing of emulsifier and co emulsifier at the particle surface. Under such conditions the residence time of entered radical increases as well as its propagation efficiency with monomer prior to exit. This increases the rate entry of radicals into particles. 

Longf ieict moaeiXwithout taking into account the radical capture efficiency of micelles(or the Nomura and Harada model with Case C for e) are given, for comparison, as follows ... 

The concept of radical capture efficiency was further elaborated on by Hansen et al. [15-17]. By applying the theory of mass transfer with simultaneous chemical reactions, they proposed the following expression to represent the net rate of radical absorption by a particle, introducing an ""absorption efficiency factor F into Eq. 8... 

On the other hand, Nomura et al. [14] proposed a different approach for predicting the number of polymer particles produced, where the new concept of radical capture efficiency of a micelle relative to a polymer particle was proposed. The assumptions employed were almost the same as those of Smith and Ewart, except that the volumetric growth rate p of a polymer particle was not considered to be constant. It was also assumed that all of the radicals formed in the aqueous phase enter either micelles or polymer particles with negligible termination in the aqueous phase. In this approach, the following elementary reactions and their respective rates were defined. 

As we discussed in Sect. 3.1.1, Hansen et al. [15] made significant improvements to the concept of the radical capture efficiency proposed by Nomura et al. [ 14]. Taking this concept into consideration, they examined the effect of radical desorption on micellar particle formation in emulsion polymerization [ 65 ]. Assuming that radical entry is proportional to the x power of the micelle radius and the polymer particle radius, they proposed the following general expression for the rate of particle formation ... 

On the other hand, in vinyl acetate emulsion polymerization the value of p. was 1.2 X 10 (Nomura et al., 1976), This value is also about 10 times greater than that predicted hy the diffusion theory. The reason for this may be that radicals have greater difficulty m entering micelles than polymer panicles, or it may be that radicals, having entered a micelle, may escape from the micelle too rapidly to cause initiation, because the micelle has too small a volume. Both factors will decrease the apparjsnt value of k, and hence increase the value of c. Therefore, e can be regarded as a factor that represents the radical capture efficiency of a micelle relative to a particle. ... 

Hansen and Ugelstad [27,41] proposed that the radical capture efficiencies may not only be different for micelles and particles, but also among particles because of different sizes, surface charges, and monomer and radical concentrations. The rate of absorption of radicals in micelles and small particles may be lower than expected from diffusion-controlled irreversible absorption. Radicals arriving at the surface of the micelles can adsorb, thai desorb back into the bulk or diffuse further into the micelle. Because of the small size of the micelles, desorptirai can take place at a relatively fast rate. For ionic surfactants, the radicals approach the micelles, adsorb and desorb at almost equal rates, and essentially pass through the micelles. For non-ionic micelles, the presence of a condensed layer at the water side of the micelle can cause a higher resistance to radical capture and thus result in an even lower radical capture efficiency than for ionic miceUes. 

The overall activation energy, Ea, of polymerization was determined for both monomers as a function of the nature of the initiator [66,79,81,90,92]. For styrene polymerization in cationic microemulsions, Ea was found to be much higher for KPS systems (E 95 kJ/mol) than for AIBN systems (48 kJ/mol) in spite of similar decomposition energies. This difference was attributed to different radical capture efficiencies between the anion radicals of KPS and the uncharged AIBN radicals and the positively charged... 

The initiation process, similar to other free-radical vinyl polymerizations, involves the chemical decomposition of unstable peroxides - azocompounds, or persulfates - into free radicals which can react rapidly with monomer to begin the propagation of polymer chains [4]. In the case of a water-soluble initiator, the radical concentration in polymer particles is related to the initiator concentration in water and the radical capture efficiency of latex particles. The radical capture efficiency of monomer droplets is very small and, therefore, their contribution to overall polymerization process is negligible. Thus, the small surface area of monomer droplets and/or high concentration of radicals in monomer droplets disfavor the growth events. Using an oil-soluble initiator, the radical concentration in particles and monomer droplets is related to the initiator concentrations in both phases. The initiator concentration between these phases is usually expressed in terms of an initiator partition coefficient. 

After depletion of monomer droplets and a decrease in the amount of monomer in the water, the rate of initiation decreases due to the increase of the water-phase termination and the decrease of radical capture efficiency of latex particles. 

It has been shown that the radical capture efficiency for a monomer with high water-solubility such as acrylonitrile and, therefore, also for vinyl chloride is very high (close to 100%) [40]. 

Taking into account the effect of desorption of free radicals and the concept of free radical capture efficiency on the formation of particle nuclei, the rate of particle nucleation can be expressed as follows [25] ... 

The concept of free radical capture efficiency was incorporated into the work of Hansen and Ugelstad [26,27], Based on the mechanism of mass transfer with simultaneous chemical reactions, the net rate of absorption of free radicals by a single latex particle (pJNp) can be written as... 

Another useful expression for the free radical capture efficiency factor F) is shown below ... 

Unzueta and Forcada [31] studied the emulsion copolymerization of methyl methacrylate and n-butyl acrylate. It was assumed that both micellar nucle-ation and homogeneous nucleation are operative in this emulsion polymerization system. Based on the experimental data and computer simulation results, the values of the free radical capture efficiency factors for monomer-swollen micelles (f ) and polymer particles (Fj) that serve as adjustable parameters in the kinetic modeling work are approximately 1(T and 10, respectively. The reason for such a difference in the free radical capture efficiency factors is not available yet. Table 4.2 summarizes some representative data regarding the absorption of free radicals by the monomer-swollen micelles and polymer particles obtained from the literature. 

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