Nucleation period

The rate of polymerization with styrene-type monomers is directly proportional to the number of particles formed. In batch reactors most of the particles are nucleated early in the reaction and the number formed depends on the emulsifier available to stabilize these small particles. In a CSTR operating at steady-state the rate of nucleation of new particles depends on the concentration of free emulsifier, i.e. the emulsifier not adsorbed on other surfaces. Since the average particle size in a CSTR is larger than the average size at the end of the batch nucleation period, fewer particles are formed in a CSTR than if the same recipe were used in a batch reactor. Since rate is proportional to the number of particles for styrene-type monomers, the rate per unit volume in a CSTR will be less than the interval-two rate in a batch reactor. In fact, the maximum CSTR rate will be about 60 to 70 percent the batch rate for such monomers. Monomers for which the rate is not as strongly dependent on the number of particles will display less of a difference between batch and continuous reactors. Also, continuous reactors with a particle seed in the feed may be capable of higher rates. 

The primary nucleation process is divided into two periods in CNT one is the so called induction period and the other is the steady (or stationary) nucleation period (Fig. 2) [16,17]. It has been proposed by CNT that small (nanometer scale) nuclei will be formed spontaneously by thermal fluctuation after quenching into the supercooled melt, some of the nuclei could grow into a critical nucleus , and some of the critical nuclei will finally survive into macroscopic crystals. The induction period is defined as the period where the nucleation rate (I) increases with time f, whereas the steady period is that where I nearly saturates to a constant rate (fst). It should be noted that I is a function of N and t,I = I(N, t). In Fig. 2, N and N mean the size of a nucleus and that of the critical nucleus, respectively. The size N is defined... 

Size control of the hematite particles must be performed during this nucleation period by controlling the temperature or pH, or by adding very fine seeds of ot-Fe203. 

The induction time is marked as 1 and includes the time taken for crystal nuclei to form which are not visible to macroscopic probes. The induction time is defined in practice as the time elapsed until the appearance of a detectable volume of hydrate phase or, equivalently, until the consumption of a detectable number of moles of hydrate former gas. The induction time is often also termed the hydrate nucleation or lag time (Section 3.1). (The induction or lag time is the time taken for hydrates to be detected macroscopically, after nucleation and onset of growth have occurred, whereas nucleation occurs on too small a size scale to be detected. Therefore, the term nucleation time will not be used in this context. Instead, the term induction time or induction period will be used. The induction time is most likely to be dominated by the nucleation period, but also includes growth up to the point at which hydrates are first detected.)... 

The rate of dispersion (co)polymerization of PEO macromonomers passes through a maximum at a certain conversion. No constant rate interval was observed and it was attributed to the decreasing monomer concentration. At the beginning of polymerization, the abrupt increase in the rate was attributed to a certain compartmentalization of reaction loci, the diffusion controlled termination, gel effect, and pseudo-bulk kinetics. A dispersion copolymerization of PEO macromonomers in polar media is used to prepare monodisperse polymer particles in micron and submicron range as a result of the very short nucleation period, the high nucleation activity of macromonomer or its graft copolymer formed, and the location of surface active group of stabilizer at the particle surface (chemically bound at the particle surface). Under such conditions a small amount of stabilizer promotes the formation of stable and monodisperse polymer particles. 

In agreement with the findings reported in another study (36). seeding proved efficient by considerably shortening the nucleation period of ZSM-20 "Figure 4". Moreover, the nucleation time is even more reduced when ZSM-20 seed crystallites (0.5 wt. % with respect to dry gel) were added after the complete evaporation of ethanol, thus confirming the inhibiting role of the latter in ZSM-20 crystallization. 

ZSM-20 is a highly metastable zeolite. Its preparation necessitates the use of very specific and drastic conditions low synthesis temperatures, adequate nucleation period and a careful selection of the ingredient nature and composition. A severe and simultaneous control of all the synthesis parameters is indispensible to obtain pure ZSM-20 in high yield and reproducible conditions. 

The various latexes were characterized with respect to particle size and size distribution, surface charge and functional group density, and electrophoretic mobility behavior. As observed by transmission electron microscopy all latexes were found highly monodisperse with a uniformity ratio between 1.001 and 1.010, a property due to the short duration of the nucleation period involved in the various radical-initiated heterogeneous polymerization processes. The surface charge density was determined by a colorimetric titration method reported elsewhere [13]. 

Ozdeger et al. studied the role of the nonionic emulsifier Triton X-405 (octyl-phenoxy polyethoxy ethanol) in the emulsion homopolymerization of St [99] and n-butyl acrylate (n-BA) [ 100], and in the emulsion copolymerization of St and n-BA [101]. In the emulsion homopolymerization of St, they noted two separate nucleation periods, resulting in bimodal PSDs. Although the total concentration of the emulsifier was maintained at a level above its CMC based on the water phase in the recipe, the portion of the emulsifier initially present in the aqueous phase was below the CMC due to partitioning between the oil and aqueous phases. Due to the nature of this emulsifier, the first of the two nucleation periods was attributed to homogeneous nucleation, while the second was... 

Polymerization rate represents the instantaneous status of reaction locus, but the whole history of polymerization is engraved within the molecular weight distribution (MWD). Recently, a new simulation tool that uses the Monte Carlo (MC) method to estimate the whole reaction history, for both hnear [263-265] and nonlinear polymerization [266-273], has been proposed. So far, this technique has been applied to investigate the kinetic behavior after the nucleation period, where the overall picture of the kinetics is well imderstood. However, the versatility of the MC method could be used to solve the complex problems of nucleation kinetics. 

After the nucleation period, three types of kinetic processes determine the kinetics of emulsion polymerization radical entry, radical desorption, and polymer chain formation in the polymer particles. The kinetics of emulsion polymerization are fully described by the following five dimensionless parameters ... 

Stabilize new particles, thereby increasing the total number of particles. Since the nucleation period is lengthened, the polydispersity increases. Figure 14 shows that the dependence of the inhibitor concentration on the number of particles is 0.176 0.010. Conversion time curves indicate that an induction period results from the presence of the inhibitor. Since polymer-stabilized miniemulsion polymerization occurs via droplet nucleation, it should be less sensitive to oil-phase inhibition. Initiator radicals will enter the droplet one after the other until all of the inhibitor is used up, and the monomer polymerizes. This does not affect the number of droplets or particles. As seen in Fig. 15, the number of particles is proportional to the DPPH concentration raised to the power of 0.0031 0.0001. Therefore, the number of particles is essentially independent of the presence of inhibitor. 

The effect of mass transfer of vinyl versatate on the mini/macroemulsion polymerization of VAc/VEOVA in batch and semibatch systems was explored. For the batch experiments, the addition of neat VEOVA formed poor dispersions of VEOVA, which resulted in smaller particles, lower polymerization rates and different polymer composition tracks compared to normal mini/macroemulsion polymerization of VAc/VEOVA. The well-dispersed VEOVA seemed to help the monomer-swollen particle to gain more radicals in the nucleation period. 

Typical particle sizes of the resulting lattices are between 50 nm and 500 pm. Generally, the size distribution of the latex particles is broad [252], Lattices with a very narrow size distribution can be achieved by a short nucleation period followed by a long growth period in the absence of coagulation [250]. Because the polymerization takes place within the outer periphery (shell) of the particle, latex polymers with a core-shell structure can be prepared, the core consisting of a cross-linked polymer, surrounded by a shell of tethered linear non-cross-linked polymer of different chemical composition. Recent reviews deal with the preparation and application of these core-shell polymers [212,253]. 

If K and R. are constant during the nucleation period, Eq. (81) may be integrated numerically. Assuming that as an approximation radical absorption may be neglected in the caladation of R we have... 

The probability P of nucleation, which can be identified with the reciprocal of the average nucleation period, in a sequence of nucleation events, is obviously... 

At low overvoltages, two factors play a dominant role in determining the growth mode (i) the average nucleation period nuc = CM), used in the following as... 

Nucleation and crystallisation kinetics generally follow S-shaped crystallisation curves as shown for zeolite A in Figs. 8.28 and 8.29. This means that a rather long incubation time or nucleation period precedes the crystallisation (compare Figs. 8.28b for zeolite A and 8.29b for ZSM5). The general trend in the kinetics of zeolite (ZSM5) synthesis can be summarised as follows ... 

I 0-10 Present Present Increases Increases Nucleation period... 

Growth of particles. After the rapid nucleation period, the polymerization of the residual monomer may take place at various points (i.e., water phase, on the surface of the formed particle, and in the swelled parts of the particles). In fact, the partition of the monomer between the water phase and polymer particles should be considered (1) polymerization in the water phase leads to the formation of small oligomers which can be cross-linked on the formed particles when there is... 

This is the nucleation period. Ideally, all M/P particles are generated in stage I. The particles grow in volume and adsorb surfactant molecules, resulting in diminution of micelles. If the area occupied by each surfactant molecule is the same on particle and micellar surfaces and the particles are completely covered by surfactant, the combined areas of micelles and particles in unit volume of the reaction medium will remain constant The particle surface area will grow at the expense of micellar surface area and when the total surface area of particles equals the total surface area occupied by the surfactant initially added, all micellar soap will have disappeared and further nucleation will cease. 

---