Aqueous phase reactions and models of Pickering emulsion polymerization are the same as those of conventional emulsion polymerization. Different reaction schemes can be considered, thus leading to different mass balances. The main hypothesis considered in this work are given in detail in a previous paper, dedicated to the modeling of emulsion polymerization of styrene using the same persulfate initiator.56 The main assumptions to be outlined here are the following: i) oligomeric species may attain a maximal length of monomer units in the aqueous phase, beyond which either radical capture or
low low solubility of styrene in water; iii) radical capture into polymer particles concerns only aqueous phase radicals of sizes , with and for styrene;49 and iv) no differentiation is done between the radical groups originating from initiator decomposition or those produced by transfer to monomer inside the polymer particles, i.e. desorbed monomeric radicals are considered as . According to these assumptions, the following material balances are obtained for aqueous phase radicals after applying the quasi-steady state assumption for the total concentration of radical species:
(13)
(14)
(15)
(16)
The total concentration of radicals in the aqueous phase is given by:
(17)
Monomer balance
The monomer balance is given by:
(18)
The reaction in the aqueous phase is considered to be negligible.
In the considered reaction period for modeling, polymer particles were assumed saturated with monomer all the time. Equilibrium is assumed fast enough to ensure reacted monomer within the polymer particles to be instantaneously replaced by monomer from droplets, and the saturation value is considered size independent. This assumption was experimentally validated for the present system based on a preliminary set of experiments where the stirring rate was varied between 350 and 600 rpm. It was observed that the stirring speed of 400 rpm (with the considered 3-blade impeller) does not induce any mass transfer limitation (as similar reaction rates and properties were obtained as with 500 rpm). Some shear-induced coagulation was observed at 600 rpm (increase in the particle size compared to the other stirring rates). and are thus maintained at their saturation values which are known. This allows all the coefficients to be considered constant with time, such as the propagation, termination and radical diffusion coefficients.
Radical desorption models
For many important emulsion polymerization systems, desorption represents the major cause of the loss of free-radical activity inside a particle as it decreases the concentration of radicals in the particles. As mentioned in the introduction, desorption mainly concerns monomeric radicals, derived from the transfer reactions to the monomer. Therefore, it may safely be assumed that desorption of monomer radicals would not be affected by the surfactant layer due to their small size (few monomer units) and to the fact that they are not charged (as the monomer is nonionic and monomeric radicals do not contain an
The desorption rate is given by the following equation:
(19)
where the desorption rate coefficient, , is supposed to reflect all necessary conditions for desorption.
Different precision levels were considered in the desorption models which gained in complexity with time, going from simple diffusion (Smith and Ewart (1948)31, Ugelstad et al. (1969)39, Chang et al.
(1981) 57, Morrison et al. (1994)58), to models taking into account competitive reactions inside the particles (e.g. Harada et al. (1971)52), and finally accounting for competitive radicals in the aqueous phase or fate of exited radicals (Asua et al. 198959, Hernandez and Tauer (2008)51, Ghielmi et al.
(2014)60). This last mechanism is the one admitted nowadays. In this mechanism, a radical is considered to be effectively desorbed from the particle only after it reacts in the aqueous phase. Desorbed radicals that are reabsorbed by a polymer particle before undergoing any reactions in the aqueous phase are not included in the desorption term. In this mechanism, the desorption rate coefficient takes the following form:
(20)
where is the frequency at which monomeric radicals (that may exit) are being formed, which mainly concerns transfer to monomer reactions (as well as to chain transfer agents, if present):
(21)
is the probability of the radical to escape the particle before undergoing other reactions (propagation and termination) inside the particle and to undergo reactions in the aqueous phase (Hernandez and Tauer 200851):
(22)
where the probability of radical reaction inside the particles is given by:
(23)
and the probability of radical reaction in the aqueous phase is:
(24)
where and are rate coefficients for the ith competitive reaction in the polymer and aqueous phase, respectively, e.g. . is the radical capture rate coefficient and is the equilibrium radical desorption rate coefficient given for instance by (Harada et al. 1971):52
(25)
where is the partition coefficient of the radicals between the polymer particles and the aqueous phase, usualy approximated by (or assumed as a tuning parameter). Another way of calculating was proposed by Brooks and Makanjuola (1981)61 as well as Hernandez and Tauer (2008)51 based on the energy barrier for desorption due to the difference in chemical potential of the radicals between the phases as well as the presence of surfactant layers around the particles.
Acknowledgement
The support of the Agence Nationale pour la Recherche is gratefully acknowledged (PickEP project, grant n° ANR-12-JS09-0007-01).
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