Growth bimodal

On this basis, one question is unanswered. How much can the lifetime be modified by the presence, number and efficiency of these impurities, when compared to the mean field prediction Answering this question will allow us to model heterogeneous growths (bimodal or demixtion) reliably and very much extend the knowledge of emulsion stability since these types of destruction are the most frequently encountered. However, we believe the mean field description which is considered here, and its ability to descibe homogeneous growths, gives a firm basis to the theory of emulsion stability. 

Fig. 31 Structural formation model for the initial stage of polymer crystallization [19], N G nucleation and growth of oriented domains, SD spinodal decomposition into oriented and unoriented domains, Tb, Ts, and Tx bimodal, spinodal, and crystallization temperatures, respectively I isotropic, N smectic, and C crystalline...

As indicated in Figure 10.2, there is a distinct change in the slope of the line at carbon numbers 8 to 12, and this has also been observed by other researchers.2-3 This change in the slope cannot be explained by the ASF model, which is based on the premise that the chain growth probability factor (a) is independent from the carbon number. Some further developments of the ASF model by Wojciechowski et al.3 made use of a number of abstract kinetic parameters for the calculation of a product spectrum. Although it still predicts a straight line for the a plot, they suggested that the break in the line is due to different mechanisms of chain termination and could be explained by the superposition of two ideal distributions. This bimodal distribution explained by two different mechanisms... 

The carbon number distribution of Fischer-Tropsch products on both cobalt and iron catalysts can be clearly represented by superposition of two Anderson-Schulz-Flory (ASF) distributions characterized by two chain growth probabilities and the mass or molar fraction of products assigned to one of these distributions.7 10 In particular, this bimodal-type distribution is pronounced for iron catalysts promoted with alkali (e.g., K2C03). Comparing product distributions obtained on alkali-promoted and -unpromoted iron catalysts has shown that the distribution characterized by the lower growth probability a, is not affected by the promoter, while the growth probability a2 and the mass fraction f2 are considerably increased by addition of alkali.9 This is... 

Under these conditions incorporation of 1-alkenes is negligible. As discussed in Section 11.2, for a high degree of conversion, a deviation from this strict bimodal ASF distribution is observed due to incorporation of these compounds and their subsequent chain growth. 

In Fischer-Tropsch synthesis the readsorption and incorporation of 1-alkenes, alcohols, and aldehydes and their subsequent chain growth play an important role on product distribution. Therefore, it is very useful to study these reactions in the presence of co-fed 13C- or 14 C-labeled compounds in an effort to obtain data helpful to elucidate the reaction mechanism. It has been shown that co-feeding of CF12N2, which dissociates toward CF12 and N2 on the catalyst surface, has led to the sound interpretation that the bimodal carbon number distribution is caused by superposition of two incompatible mechanisms. The distribution characterized by the lower growth probability is assigned to the CH2 insertion mechanism. 

The fracture mode of both forgings was by the microvoid nucleation and growth process (Figure 4). Microvoids nucleate at nonmetallic inclusions in the steel (sulfides, oxides, etc) and grow under strain until they coalesce at fracture (10). While the CF heats had a fairly uniform microvoid size distribution on the fracture surface, the HERF steel fracture surface had a bimodal distribution of microvoids with large microvoids surrounded by clusters of fine microvoids. 

The previous discussion has shown that the CIPS technique allows one to produce macroporous epoxy networks with either a narrow or bimodal size distribution. However, no indication has been given on the type of phase separation mechanism to yield these morphologies. As discussed earlier, the formation of a closed cell morphology can result either from a nucleation and growth mechanism or from spinodal decomposition. 

Kerminen, V.-M., and A. S. Wexler, Growth Laws for Atmospheric Aerosol Particles An Examination of the Bimodality of the Accumulation Mode, Atmos. Environ., 29, 3263-3275 (1995). 

Bimodal pore size distribution in MCM-4I has been observed by several groups in the last few years [22-24], However, the relation between two types of mesopores were never fully understood. In a recent TEM study of an MCM-41-type silicate with a bimodal mesopore system, a paint-brush like morphology of the particles was observed (Figure 7) [25], It was then proposed that the two types of pores with the pore diameters of 2.5 nm and 3.5 nm respectively coexist and are parallel to each other in the particles. Due to different rates of crystal growth, the lengths of these two groups of mesopores are different, resulting in such a novel structure only on the (001) surface. 

A strong similarity is found for the present blends with a PPE/PS ratio of 50/50, as reflected by a similar bimodal cell size distribution for all SAN contents. Small differences can be related to the distinct foaming kinetics of the PPE/PS blend phase. Compared to the PPE/PS 75/25 blend phase, the higher content of PS in the PPE/PS 50/50 phase leads to a cell nucleation and growth kinetics close to the SAN phase. Nevertheless, the PPE/PS phase still appears to restrict the cell growth and expansion in the SAN phase to some extent, and smaller cells are found within the cell walls. Independent of the SAN content, cell growth within the dispersed SAN phase proceeds under the constraints of the continuous, higher Tg PPE/PS phase. 

In order to overcome this drawback, the concept of selective blending was exploited. Selective blending of PPE with low-viscous PS allowed one to control the microstructure, to refine the phase size, and to adjust the foaming characteristics of the individual phases of PPE/SAN blends. Appropriate blend compositions allowed simultaneous nucleation and cooperative expansion of both phases, generally leading to bimodal cell size distributions in the micron range. Due to cell nucleation and growth in both blend phases, the density could be further reduced when compared to PPE/SAN blends. Moreover, the presence of coalesced foam structure and particularly macroscopic defects could be avoided, and the matrix of the foamed structure was formed by the heat resistant PPE/PS phase. 

Thorpe, J.E., Talbot, C. and Villarual, C. (1982). Bimodality of growth and smolting in Atlantic salmon. Aquaculture 28,123-132. 

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