Conclusiones parciales del capítulo 3

Emulsion polymerization is a heterogeneous polymerization method where particles of 50-600 nm in diameter are formed. The term emulsion polymerization is in fact a misnomer; polymerization does not occur in monomer droplets as the name suggests, but in monomer swollen particles. Emulsion polymerization is split into three intervals describing the main characteristics (Figure 1.2).2-7 Interval I; the nucleation period describes the formation of all particles, assuming secondary nucleation (the formation of a secondary crop of particles) does not occur. In this interval particle number and particle size is increasing and monomer droplets are present. The nucleation stage is typically short and occurs in 0-10 % conversion, short nucleation periods yield a monomodal size distribution. Initiation occurs in the aqueous phase where the water soluble initiator species undergoes homolysis and reacts with the finitely water-soluble monomer; this oligomer propagates until it reaches a critical chain length (jcr), at this point particle nucleation proceeds in one of two ways, through micellar or homogeneous nucleation.

Figure 1.2 Scheme of the three intervals in emulsion polymerization.8

1.1.1.1.Homogeneousnucleation

Once the oligoradicals have reached jcr in the aqueous phase, the hydrophobic attributes outweigh the hydrophilic, and the polymer collapses to form a primary particle. Once a sufficient number of particles exist, capture of surface active oligoradicals (z-mers) occurs and overtakes new particle formation. Brownian motion, the random movement of particles, leads to particle collision; if the surface charge, obtained from ionic initiator species or surfactant, is not great enough these collisions result in the particles coagulating and fusing together. This coagulation occurs until enough particles have fused to result in a high enough surface charge density to warrant stable particles.9-11 This was first described in the Fitch-Tsai theory:

where N is the number of particles dm-3, b is the compensation of radical loss, Riw is the rate of appearance of primary radicals, Rc is the rate of radical capture and Rf is the rate of particle coagulation.12

1.1.1.2.Micellarnucleation13

Where there is surfactant present at a concentration greater than the critical micelle concentration (CMC), micellar nucleation occurs. Surface active oligoradicals, formed in the aqueous phase, enter monomer swollen micelles to produce primary particles. At a constant rate of initiation it was found that the concentration of surfactant affects the number of particles with a dependence on monomer hydrophobicity. This led to the theory of particle formation based on radical capture by surfactant micelles described by the Smith-Ewart-Roe equation:

(1.2)

where N is the number of particles, K is a constant with the value of 0.53 for purely micellar capture and 0.37 if capture by new particles is taken into account, µ

is the rate of particle volume growth which is assumed to be constant, αs is the area

covered by the surfactant molecule and S is the total number of surfactant molecules.14, 15 In this case coagulation was not considered due to the high concentration of surfactant. Roe showed that this also applied for systems without micelles. They suggested that particle nucleation stops once the polymer-water interfacial area stabilized by surfactant equals the area which can be covered by the amount of surfactant.

Figure 1.3 Scheme of kinetic processes taking place in a typical emulsion polymerization.16

Once particles have formed, reversible entry of z-mers occurs until they reach

jcr, at which point they are irreversibly trapped (Figure 1.3). Entry into monomer droplets, and thus monomer droplet nucleation, is unlikely due to their comparatively small surface area. Interval I ends and interval II begins when particle nucleation has stopped; the number of particles are now approximately constant. The particles swell with monomer, which has diffused from the still present reservoirs of monomer droplets. During this stage the rate of polymerization is considered constant, due to a constant concentration of monomer with respect to polymer in the particles. Particle size is increasing throughout this interval.

In Interval I and II the small latex particles cannot accommodate more than one radical, on entry of a second radical instantaneous termination occurs resulting in no radicals in the particle, hence zero-one kinetics is observed. In this case (Smith- Ewart case 2, Figure 1.4 a) the average number of radicals per particle (n) is considered to be ½. If there is significant chain transfer to small molecules the

resulting radical, typically smaller than jcr, can exit the particle further reducing the probability of the presence of a radical in the particle, in this case (Smith-Ewart case 1, Figure 1.4 b) n < ½.14 Interval III begins when there are no monomer droplets remaining, all monomer is now found in the polymer particles; monomer concentration in the particles is now decreasing and correspondingly the viscosity within the particles is increasing. This increased viscosity slows the rate of termination; more than one radical can be present in a particle at one time, n >> ½ (Smith-Ewart case 3, Figure 1.4 c), the kinetics at this stage resembles that of solution polymerization. Due to the reduced termination, an increased rate in reaction is observed, a phenomena known as the Trommsdorff-Norrish or gel effect.

Figure 1.4 Scheme illustrating (a) Smith-Ewart case 2, (b) Smith Ewart case 1 and (c) Smith-Ewart

Rate of polymerization is faster for emulsion polymerization than for the corresponding bulk polymerization due to the compartmentalization effect, as radicals are isolated in separate particles. Compartmentalization yields fast polymerization rates and high molecular weights.16