Diffusion of monomers

However, in the case of mini- and microemulsions, processing methods reduce the size of the monomer droplets close to the size of the micelle, leading to significant particle nucleation in the monomer droplets (17). Intense agitation, cosurfactant, and dilution are used to reduce monomer droplet size. Additives like cetyl alcohol are used to retard the diffusion of monomer from the droplets to the micelles, in order to further promote monomer droplet nucleation (18). The benefits of miniemulsions include faster reaction rates (19), improved shear stabiHty, and the control of particle size distributions to produce high soHds latices (20). 

The monomer concentration within the forming latex particles does not change for a long period due to the diffusion of monomer from the droplets to the polymerization loci. Therefore, the rate of the propagation reaction does not change and a constant polymerization rate period is observed in a typical emulsion polymerization system. 

There are some reports that values of kp are conversion dependent and that the value decreases at high conversion due to kp becoming limited by the rate of diffusion of monomer. While conversion dependence of kp at extremely high conversions is known, some data that indicate this may need to be reinterpreted, as the conversion dependence of the initiator efficiency was not recognized (Sections 3.3.1.1.3,3.3.2.1.3 and 5.2.1.4). 

So the handling of hydrocarbons presents serious fire hazards. There are many accidents linked to this in the industrial sector. For instance, a serious accident happened when polyethylene was stored. It appeared to be caused by the diffusion of monomer through the mass of polymer, which created an inflammable atmosphere in the storage container. Incorporating a mixture of oxygen and styrene in a reactor cause spontaneous ignition. 

Due to the reduced absorption of monomers and the low rate of polymerization in the micelles, the diffusion of monomer molecules from droplets to the growing particles is limited. Correspondingly, the probability of polymerization in the droplets increases. 

By using lipophilic initiators, such as 2,2 -azobis(isobutyronitrile) (AIBN), in the micro-ECP, diffusion of monomers is too slow compared with the reaction rate. Therefore, copolymerization is confined to the incoherent, lipophilic phase [112,113] and very small microgel particles with a rather uniform size result. 

In normal emulsion polymerization the diffusion of monomers from droplets allows particles to grow. The polymerization is usually initiated in the aqueous phase and the oligomeric radicals either enter micelles or merge with other growing species. In the crosslinking ECP of EUP the ratio EUP/comonomer and the solubility or insolubility of both components and the initiator in the aqueous and non-aqueous phases respectively are parameters which decide whether diffusion of the reactants in the aqueous phase plays a role and where the initiation takes place. 

In non-crosslinking ECP, monomers are supplied to the growing polymer species by diffusion of monomer from droplets. In crosslinking ECP, however, the gel effect increases the copolymerization rate in the droplets as well as in the growing microgel particles. As the diffusion rate of lipophilic monomers in the aqueous phase is lower than the copolymerization rate, monomer droplets may... 

The fact that the DP becomes independent of [PJ when this is great, and the temperature low (-30°), is interpreted by the authors - quite plausibly - in terms of polymer gel inhibiting diffusion of monomer to the reaction sites, and a concurrent build-up of temperature. 

Polymerization proceeds in the polymer particles as the monomer concentration in the particles is maintained at the equilibrium (saturation) level by diffusion of monomer from solution, which in turn is maintained at the saturation level by dissolution of monomer... 

A characteristic of aldehyde polymerization is the precipitation, often with crystallization, of the polymer during polymerization. Depending on the solvent used, polymerization rate, state of agitation, and other reaction conditions, the polymerization can slow down or even stop because of occlusion of the propagating centers in the precipitated polymer. The physical state and surface area of the precipitated polymer influence polymerization by their effect on the availability of propagating centers and the diffusion of monomer to those centers. 

The propagation kinetic constant remains relatively constant at low to moderate conversions where diffusion of the smaller monomer molecules is unhindered. As the critical free volume for the diffusion control of propagation is reached, kp begins to decrease. Diffusion of monomer molecules is now the rate controlling step for propagation. As polymerization continues and conversion increases (free volume decreases), the diffusion of the monomer and kp drastically decrease. 

A thermodynamic approach was put forward by one of us (10), based on the Flory-Huggins lattice theory of a polymer solution the chemical potentials of each monomer must be equal in each phase copolymerization increment causes a little change in the chemical potential in the particles diffusion of monomers from the water phase will reequilibrate the system and in turn diffusion from droplets to water phase takes place. For instance, expression from monomer 1 in the particles is ... 

Slow growth rates of graft radicals indicate that effective concentrations of monomer around active ends of graft radicals are quite low. The rate-determining step would be the diffusion of monomer molecules within and around the surface of microcrystals. Hence, it would be reasonable to assume that growth rate of graft radicals within crystalline parts is much slower than those located outside throughout reactions. 

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