Monomer particle

In 1944 Smith and Ewart published a paper analyzing the Harkins model quantitatively (3). They considered three cases in terms of average number of radical (/ ) in each polymer-monomer particle. 

Case 1, in which n is less than I, is realized when termination in aqueous phase takes place appreciably so that the radical concentration in water is kept low, forcing the radical in particles to diffuse back into the aqueous phase (Fig. 11.1.3A) (4). Case 2, in which n is 1, is found under the following condition (Fig. 11.1.3B) When one radical is polymerizing in a polymer-monomer particle and another radical enters the veiy polymer-monomer particle, the two radicals encounter in it and lead to termination reaction. That is, in each polymer-monomer particle, radicals entering... 

Smith and Ewart calculated the number of particles having been formed at the end of the first stage of polymerization. The number of particles is affected by the initiator decomposition rate (or radical formation rate) and total surface area of emulsifier to stabilize polymer-monomer particles. Smith and Ewart concluded that the number of particles is proportional to the 0.4 power of the initiator concentration and the 0.6 power of the emulsifier concentration, assuming that the surface area of total polymer-monomer particles is equal to the total surface area of emulsifier molecules when the last micelle disappears. 

Fate of Radicals Escaping from Polymer-Monomer Particles. The fate of a radical escaping from a polymer-monomer particle has been a hot topics and discussed quantitatively in the latest decade. An oligomer radical once entering a monomer-polymer particle seems to find it hard to escape from the particle because... 

In homogeneous copolymerization, the instantaneous composition of copolymer is decided only by monomer reactivity ratio. On the contrary, in emulsion copolymerization, the copolymer composition depends not only on the monomer reactivity ratio but also on the distribution of monomers between oil (polymer-monomer particles) and aqueous phases (18). 

In suspension polymerization water is used as a diluent and as a heat transfer aid. Suspending agents such as starch and melhylcellulose are used to keep the styrene monomer particles in suspension. The more efficient heat transfer of this process also allows for a narrower molecular weight distribution. 

By increasing the initial amounts of monomer, particle size decreases and particle number, as well as the formation rate, increases drastically (Table lii). ... 

For a successful incorporation of a pigment into the latex particles, both type and amount of surfactant systems have to be adjusted to yield monomer particles, which have the appropriate size and chemistry to incorporate the pigment by its lateral dimension and surface chemistry. For the preparation of the miniemulsions, two steps have to be controlled (see Fig. 14). First, the already hydrophobic or hydrophobized particulate pigment with a size up to 100 nm has to be dispersed in the monomer phase. Hydrophilic pigments require a hydro-phobic surface to be dispersed into the hydrophobic monomer phase, which is usually promoted by a surfactant system 1 with low HLB value. Then, this common mixture is miniemulsified in the water phase employing a surfactant system 2 with high HLB, which has a higher tendency to stabilize the monomer (polymer)/water interface. 

Fig. 14. Monomer placement in particles during emulsion polymerization, a, b, c, Various stages of polymerization (see text) d monomer particle stabilized by emulsifier diam. = 104 nm e monomer-polymer particle f, monomer solubilized in a micelle q, emulsifier. See p. 180 of ref. 126.

When all generated radicals enter the micelles forming monomer— polymer particles, then k = 0.53. When the radicals simultaneously diffuse into micelles and into monomer particles at rates proportional to the surface of each of these formations, then k = 0.37 (the experimentally found value of k in styrene polymerization is 0.40). 

The basic prerequisities of the Smith-Ewart theory are equal dimensions of all polymer-monomer particles, and their constant growth rate... 

There is a way to suspend even smaller monomer particles in water such that the monomer droplets are stable and do not aggregrate to form a separate layer. Essentially, a surfactant (soap) is used to form an emulsion. Surfactant molecules consist of a polar head (hydrophilic) group attached to a non-polar (hydrophobic) tail, such that it looks something like a tadpole, as depicted in Figure 3-46. 

SDS concentration (% compared to monomer) Particle diameter d (nm) Polydispersity index... 

The model proposed by Harkins and Yurzhenko doesn t take into accotmt, the intermolecular interactions on the interface, the most important factor determining behaviour of the colloidal system. Thus, this model assiunes that the molecule area of an emulsifier in a micelle and in an adsorption layer of a polymer-monomer particle have identical values, and the newly formed surface is stabilized immediately after its formation. As a result, the surface of particles per unit volume is defined as the surface oc-... 

Another reaction path involves the micellization of in situ surfactant. Both the micelles and primary particles can absorb monomer. Particle growth occurs by polymerization fed by entry into these loci of oligomers generated in the water. Figure 8-.3 is a schematic representation of these mechanisms, the details of which are still debatable [9-13]. 

An important further contribution to the analysis of steady-state reacticm systems has been made by Ugclstad et d. (1967), They have shown how account can be taken of the likely possihillty that radicals that exit from the reaction loci contribute to the stationary concentration of free radicals in the external phase which is available for entry into a reaction loci. For this purpose, it is necessary to distinguish blmolecular mutual termination between radicals that occurs in the reaction loci (i.e., within polymer/ monomer particles) from that which occurs in the external phase. The rate at which the former reaction occurs is characterized by the rate coefficient the rate of the latter reaction by. The total rate of entry of radicals into all loci within unit volume of reaction system is then expressed as the sum of three contributions. The first derives from the rate of formation of new "acquirable radicals within the external phase the second derives from the rate at which acquirable radicals become present in the external phase by the process of exit from the loci the third (which is negative) allows for the fact that radicals can be lost from the external phase by bimolecular mutual termination within the external phase. The resultant equation is... 

Monomer Initial SDDS cone in aqueous phase (moVdm x 10 ) Lates cone Cfl Coagulutn monomer (%> Particle diameter (um) 7 (mJ/m ) Additional adsorbed amount of SDDS (moVgof polymer x 10 ) Sliro Degree of adsorption saturation fe)... 

There are two main approaches used to simulate polymer materials molecular dynamics and Monte Carlo methods. The molecular dynamics approach is based on numerical integration of Newton s equations of motion for a system of particles (or monomers). Particles follow dctcr-ministic trajectories in space for a well-defined set of interaction potentials between them. In a qualitatively different simulation technique, called Monte Carlo, phase space is sampled randomly. Molecular dynamics and Monte Carlo simulation approaches are analogous to time and ensemble methods of averaging in statistical mechanics. Some modern computer simulation methods use a combination of the two approaches. 

For example, in a typical gas- or liquid-phase particle-formation process all the moments are identically null aX t = 0. In these cases, in fact, generally no particles are present at the beginning of the simulation, since they are produced through nucleation or inception mechanisms. In other cases (for example in seeded crystallization processes or in industrial coagulators) the initial moments correspond to the NDF of the seeds/monomers/particles initially present. 

In Fig. 13, a kinetic curve conversion degree — reaction duration Q, is shown schematically, on which the technique of estimation of the characteristic times t, and tj, corresponding to termination of initial section and autoacceleration section of curve Q, are indicated. The calculated according to the Eq. (4) of Chapter 1 D value, which is equal to 1.65, defines unequivocally polymer formation mechanism as diffusion-limited aggregation cluster-cluster. However, as it was indicated above, in polymerization process beginning at t = 0 monomers (particles) solution was the initial reactive mixture, where macromolecular coils were absent. Let us remind that for such coil formation a macromolecule should consist of, as a minimum, 20 monomer links [35], Therefore, it is obvious, that on the first stage of polymerization mechanism particle-clrrster should be realized and this mechanism will act until in solution macromolecirlar coils (clusters) sufficient number is not formed for mechanism cluster-clrrster realization [36], Hence it follows that... 

In such treatment the value is proportional to conversion degree Q and initial monomer (particles) concentration c [63] ... 

The presence of several growing radicals in polymer-monomer particles, which causes the gel effect appearance and an increase in the polymerization rate at high conversion degrees. 

Solids, % Viscosity mPa-s pH Mech stability min Free monom., % Particle size distr nm Average part size nm Zeta potential mV MFFT °C T g °C... 

Free ions are usually not included explicitly in the simulations, but their overall effects on monomer-monomer and monomer-particle interactions are described via the dependence of the inverse Debye screening length (m" ) on the electrolyte concentration according to... 

As very small particles (even monomer particles, which of themselves would form a liquid) are nearly always present, having the same chemical constitution as the polymer molecules, the ideal conditions are present for pure isogel-formation This differs therefore from the gel formation found with condensation polymers in which a mixture of isogels (the various particles of the polymer themselves) and of heterogels (water and the polymer) are present. 

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