Radical Entry in Emulsion Polymerization: Estimation of the Critical Length of Entry Radicals via a Simple Lattice Model (original) (raw)
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Free radical exit in emulsion polymerization. I. Theoretical model
Journal of Polymer Science Part A: Polymer Chemistry, 1994
The exit or desorption of free radicals from latex particles is an important kinetic process in an emulsion polymerization. This article unites a successful theory of radical absorption (i.e., initiator efficiency), based on propagation in the aqueous phase being the rate determining step for entry of charged free radicals, with a detailed model of radical desorption. The result is a kinetic scheme applicable to true "zero-one" systems (i.e., where entry of a radical into a latex particle already containing a radical results in instantaneous termination), which is still, with a number of generally applicable assumptions, relatively simple. Indeed, in many physically reasonable limits, the kinetic representation reduces to a single rate equation. Specific experimental techniques of particular significance and methods of analysis of kinetic data are detailed and discussed. A methodology for both assessing the applicability of the model and its more probable limits, via use of known rate coefficients and theoretical predictions, is outlined and then applied to the representative monomers, styrene and methyl methacrylate. A detailed application of the theory and illustration of the methodology of model discrimination via experiment is contained in the second article of this series.
Model Discrimination of Radical Exit in Emulsion Polymerisation
2011
Analysis of published experimental data on monomeric radical diffusion in the emulsion polymerisation of styrene shows that it can be quantitatively described equally well by non-equilibrium diffusion from particles, where all parameters are derived from properties of the discrete phase, or by steady-state diffusion where all parameters are derived from properties of the continuous phase. The non-equilibrium model better describes an observed experimental trend to a reduced desorption rate coefficient at higher weight fraction of polymer in the particles. The theoretical upper bound of the non-equilibrium model is also higher than the theoretical upper bound of the steady-state model allowing fits to experimental data which must be discarded as anomalous in the continuous phase model.
Polymer, 1988
For emulsion polymerization of a 'zero-one' system, the method of moments is applied to the model proposed by Lichti et al. to describe the particle size distribution (PSD). Using the explicit expressions so obtained for the first four moments, the average number of free radicals par particle, ~, and kinetic parameters of the system involving the rate coefficients for adsorption and desorption of free radicals, p and k, and the propagation rate coefficient, kp, can be obtained easily by use of PSD data without involving a complicated curve fitting procedure as was required in their work. Detailed calculations on the styrene system data of Lichti et al. show that both p and k are proportional to particle surface area. The result for k is opposite to that proposed by those workers, in which k was considered to be inversely proportional to the particle surface area. After further manipulation and approximation of the expressions so obtained, explicit expressions for the number-average volume, 6n, the standard deviation, tr, and the skewness, u~, in terms of surfactant and initiator levels, temperature and reaction time are also obtained. The effects of the variations of these variables on the three characteristic parameters of the PSD can be determined.
Model Discrimination of Radical Desorption Kinetics in Emulsion Polymerisation
Macromolecular Theory and Simulations, 2011
Analysis of published experimental data on monomeric radical diffusion in the emulsion polymerisation of styrene shows that it can be quantitatively described equally well by non-equilibrium diffusion from particles, where all parameters are derived from properties of the discrete phase, or by steady-state diffusion where all parameters are derived from properties of the continuous phase. The non-equilibrium model better describes an observed experimental trend to a reduced desorption rate coefficient at higher weight fraction of polymer in the particles. The theoretical upper bound of the non-equilibrium model is also higher than the theoretical upper bound of the steady-state model allowing fits to experimental data which must be discarded as anomalous in the continuous phase model.
Radical capture efficiencies in emulsion polymerization
Journal of Polymer Science: Polymer Chemistry Edition, 1981
A theoretical procedure is developed that allows the importance of bimolecular termination in the aqueous phase of an emulsion polymerization to be determined. This shows that with sparinglysoluble and slowly propagating monomers like styrene, significant termination occurs in the aqueous phase at high initiator concentrations, as found experimentally. With more water soluble monomers, aqueous phase termination is likely to be small.
Effect of Pickering stabilization on radical entry in emulsion polymerization
Aiche Journal, 2018
The production of latexes stabilized by solid particles, so-called Pickering stabilizers, has attracted considerable attention due to its benefits, including the enhanced mechanical properties of the polymer films. Clays for instance were found to enhance particle stabilization in emulsion polymerization, in a comparable way to conventional surfactants. Their concentration thus determines the polymer particles size and number, and consequently the reaction rate. In this work, we investigate the impact of the presence of such rigid and big platelets at the polymer particle's surface on radical exchange between the aqueous phase and the polymer particles. It was found for the system underhand, that the average number of radicals per particle () was independent of the stabilizer layer. Therefore, a radical capture model independent of the clay concentration could be used to simulate reactions involving different clay concentrations and predict the evolution of the monomer conversion, particle size, and .
The role of aqueous-phase kinetics in emulsion polymerizations
Progress in Polymer Science, 1993
The kinetics of species in the aqueous phase control many events in emulsion polymerization: the rate of entry of free radicals into particles (equivalent to initiator efficiency), the rate of exit (desorption) of free radicals from particles, the fate of desorbed free radicals and of free-radical species derived directly from aqueous-phase initiator. Aqueous-phase kinetics also dominate particle nucleation and re-seeding (secondary nucleation), and the in situ formation of surfactant. The mechanisms of each of these events are discussed, and it is shown how general methods can be constructed to deduce the ratedetermining events for each of these. The methodology is then applied extensively to styrene, which leads to the following conclusions. (a) The aqueous-phase events which govern entry (initiator efficiency) are propagation and termination, with entry occurring irreversibly when a critical degree of propagation z is reached so that the resulting species (a di-or tri-styrenesulfonate species in the case of styrene with persulfate initiator) is sufficiently surface-active that, once adsorbed onto the particle it does not desorb before it propagates; the actual adsorption event is sufficiently rapid so as not to be rate-determining except during nucleation. (b) Exit of free radicals is governed by transfer inside the particle to form a monomeric radical which may desorb and diffuse irreversibly away from the parent particle before it propagates therein. (c) The fate of desorbed free radicals in the wide range of styrene systems examined is to re-enter another particle and remain therein, rather than the other possible fates (aqueous-phase termination or re-exit until intra-particle termination eventually occurs). (d) Below the cmc, nucleation is by the homogeneous-coagulative mechanism, while above the cmc, nucleation is through a process which combines the essential features of both homogeneous-coagulative and micellar-entry models. (e) Analysis of the aqueous-phase products produced in an emulsion polymerization shows that the species involved in termination, entry and exit also undergo subsequent reactions: hydrolysis and reaction with persulfate.
Accumulation and desorption of free radicals in emulsion copolymerization
Journal of Applied Polymer Science, 1989
The emulsion copolymerization of styrene with acrylonitrile, seeded on a polybutadiene support and initiated either with potassium persulfate or y-rays has been studied. The original technique of support activation consists of two steps: the start of the copolymerization in the presence of a small fraction of monomers, followed by the addition of the rest of monomers, which leads to a great increase in the reaction rate. The experimental data provide evidence for a new mechanism that takes into account the desorption-reentry of the free oligoradicals into another particle, their transfer to the polybutadiene support, which results in the accumulation o,f trapped free radicals. As a consequence of free radicals' accumulation, the overall copolymerization rate increases. The accumulation as well as the desorption processes place this type of copolymerization far from Smith-Ewart theory case 11.