Radical Desorption Kinetics in Emulsion Polymerization. 1. Theory and Simulation

Abstract

Radical desorption is one of the most important physical processes influencing the kinetics of free-radical emulsion polymerization. Desorption of any molecule from a polymer particle can only take place if the molecule reaches the particle surface during its random diffusion through the polymer phase. For doing this the molecule must survive all possible competitive reactions taking place simultaneously inside the particle. Once at the surface it must also overcome a certain energy barrier in order to leave the particle. This energy barrier is determined mainly by the difference in the chemical potential of the molecule between the particle and the continuous phase (including interfacial tension effects) and by additional barriers such as the presence of a stabilizer layer. In this molecular picture of the desorption process all molecules inside the polymer particle can desorb from the particle at a rate determined by its energy barrier and diffusion coefficient. It is for this reason that desorption of the smaller and more hydrophilic molecules is predominant. In this paper we propose the use of Brownian dynamics (BD) algorithms for simulating the desorption of radicals from polymer particles and estimate the corresponding desorption rate coefficients. The results obtained in simple systems are found to be in very good agreement with the rate coefficients of desorption determined theoretically. For more complex systems, such as core-shell particles, nonspherical particles, hollow particles, or particles with a gradient in monomer concentration, BD simulations open the possibility to obtain easy, reliable estimations of the desorption rate coefficients, which are difficult to obtain using the experimental or theoretical methods currently available. This article is also intended to be a comprehensive and critical review of the different available theories of radical desorption in emulsion polymerization.

Radical Desorption Kinetics in Emulsion Polymerization, 2 – Brownian Dynamics Simulation of Radical Desorption in Non‐Homogeneous Particles

“…[18,19] This case was already discussed in the first part of this series. [3] It is important to remark that this example, as well as many other examples considered, are simplistic approximations to reality. A real surfactant layer around a particle may not be perfectly spherical, may not be uniform, may be anisotropic and also may be dynamic and its shape and concentration will vary with time.…”

“…[1,2] Higher rates of radical desorption lead to lower average number of radicals per particle and in consequence, to slower rates of polymerization. Previously, [3] the theory and simulation of the kinetics of radical desorption have been reviewed, and the use of Brownian dynamics (BD) simulation methods for the estimation of radical desorption kinetics has been proposed. In the present contribution, BD simulation is applied to specific non-homogeneous systems in order to demonstrate the suitability of this method for the determination of desorption rate coefficients in more complex, but also more realistic, emulsion polymerization systems.…”

“…Some important definitions of radical desorption were introduced in the first part of these series. [3] These definitions are also briefly presented here. cf.…”

“…Part 1 ref. [3] One of the most important events in free-radical emulsion polymerization is desorption of radicals from the polymer particles to the aqueous phase. Desorption takes place by diffusion of radicals inside the particle toward the surface and transfer to the aqueous phase.…”

“…As can be seen in the first part of the series, this is enough since the remaining rate coefficients can be calculated using the value of k 0 and the kinetic rate coefficients of the additional competitive reactions or processes considered. The rate coefficient of simple radical desorption for homogeneous particles can be calculated using the following theoretical expression: [3] …”