Entry in Emulsion Polymerization:  Effects of Initiator and Particle Surface Charge

Abstract

The rate coefficient for radical entry into latex particles in emulsion polymerizations is measured for styrene systems in which the entering species are anionic (from persulfate) and cationic (from 2,2′-azobis(2-methylpropionamidine) dihydrochloride, or V-50). These entry rate coefficients F are obtained by measuring rates in seeded emulsion polymerizations where the seeds have either cationic or anionic groups on the surface; "zero-one" conditions are employed, because these offer the advantage that particle size is sufficiently small (≈70 nm diameter) that intraparticle termination is not rate-determining. Data comprise steady-state rates with chemical initiator, combined with loss rates obtained using γ-radiolysis initiation and following the relaxation behavior after removal from the radiation source. Values for F as a function of initiator concentration can be meaningfully compared for different initiators through the dependence of initiator efficiency f entry on primary radical generation rate (radical flux). For the anionic latex, this dependence is seen to differ depending on the nature of the initiator used. This may be explained by the entry model [

Radical entry mechanisms in redox-initiated emulsion polymerizations

“…the particle phase. There is extensive evidence [1][2][3][4][5] that radical entry in conventional, thermallyinitiated, electrostatically stabilized emulsion polymerizations is well described by the so-called 'MaxwellMorrison' model [6]. This describes entry as arising from hydrophilic radicals I % formed in the water phase from initiator decomposition (e.g.…”

“…If indeed the rate coefficient for radical production between the two moieties of a redox couple is extremely large, then there is the possibility of carrying out studies of radical loss rates by switching off the initiator feed and observing the decrease in rate, analogous to the 'gradiolysis relaxation' method which has led to considerable insight into radical loss processes in a range of emulsion polymerization systems [5,17,18]. If the reaction rate between the components of the redox couple is sufficiently high, then radical flux would cease virtually immediately after feed has been switched off in a redox system.…”

Interaction of Surfactant and Initiator Types in Emulsion Polymerisations: A Comparison of Ammonium Persulfate and Hydrogen Peroxide

“…For example van Berkel et al [6] investigated the kinetics of seeded emulsion polymerisation using positively and negatively charged initiators, however, did not consider the role of the charges in particle initiation. Tauer and Müller [7] went further and compared the influence of anionic, cationic, and nonionic initiators on the emulsion polymerisation of styrene and methyl methacrylate (MMA).…”

A critical evaluation of reaction calorimetry for the study of emulsion polymerization systems: thermodynamic and kinetic aspects

“…One possible origin of this residual polymerization is 'spontaneous' polymerization, which is a well-known effect in emulsion polymerization whose origin is uncertain [20,[23][24][25][26][27][28][29][30]. A further possible source of some 'spontaneous' polymerization that could be considered is the impulse of energy from the calibration heater.…”

“…Extensive mechanistic knowledge has been obtained for emulsion polymerizations in systems involving thermal initiators, from dilatometric measurements of both the steady state and the time dependence of the polymerization rate, including use of gradiolysis initiation following removal from the radiation source ('relaxation' data) [19][20][21]. Equivalent knowledge cannot be obtained using dilatometry for systems with redox initiation, because this is perforce a semi-batch process; however, such experiments are possible using accurate rate data from reaction calorimetry.…”