Nucleating efficiency

While this is an effective nucleation mechanism for PET, the efficiency of this system is not stable and decreases significantly with melt mixing (compounding) time. This instability is due to a disproportion reaction in which the sodium chain ends react with each other to give disodium terephthalate. The subsequent decrease in ionic chain end concentration is directly linked to the loss in nucleation efficiency. 

An approach similar to that taken by Nomura and Harada was used by Samer to quantify the effects of droplet nucleation on emulsion polymerization kinetics in a CSTR. In their simplified analysis, it was assumed that radical capture by particles and droplets is proportional to the ratio of particle and droplet diameters. This assumption is reliable at low to moderate residence times, when polymer particles still closely resemble monomer droplets with respect to composition and surface characteristics. For predominant droplet nucleation, the maximum particle generation is limited by the concentration of monomer droplets in the feed. In Fig. 11 the steady state particle generation is given as a function of the residence time and temperature. Nucleation efficiency is defined as the number of particles divided by the number of droplets in the... 

A comparative study on the influence of chemical structure and solubility of a series of low-molecular-weight 1,4-phenylene-bisamides in i-PP was conducted to explore their performance as electret additives [30]. Bisamides are also capable of forming well-defined nano-aggregates via self-assembly from the i-PP melt upon cooling. Here, a comparison of a dicyclohexyl-substituted 1,4-phenylene-bisamide 5 and the asymmetrically substituted cyclohexyl/ -alkyl-l,4-phenylenebisamide 6 will be presented the chemical structures are included in Fig. 24. The symmetry of the molecule, the type of substitution, and the length of the alkyl chain have an influence on the solubility in the i-PP melt while the nucleation efficiency and the ratio of the a- to [3-crystal modification will have an effect on the charge storage properties. 

Understanding the effect of ions on nucleation would be very difficult if the ions specific chemical characteristic had a significant effect on their nucleating efficiency. Since frequently the number of molecules of the nucleating species in the critical-sized cluster is on the order of tens to hundreds of molecules, it is reasonable to assume that an ion buried near its center has little effect on the cluster s surface properties other than those due to its charge. Thus the ion creates a central force field that makes it more difficult for a molecule to evaporate than it would be from an uncharged but otherwise identical cluster. Ion self-repulsion and the typical low density of ions ensure that almost all such clusters contain only one ion. Molecules arrive at and evaporate from the ion-vapor molecule clusters. The critical cluster size for nucleation is that for which the evaporation rate of... 

Finally, the degree of nucleation at the amorphous/ semicrystalline interfaces was found to be temperature dependent. When the crystallization temperature was raised, the nucleating efficiency of the interface was found to decrease [Bartezak et al., 1987]. 

Fig. 3.3 a, b. Typical curves for the number of precipitated particles produced as a function of the initial precipitant concentration a, in the presence of No impurity particles of equal nucleation efficiency b, in the presence of No impurities of varying nucleation efficiency. Homogeneous nucleation occurs at a much higher level of supersaturation, S, than for heterogeneous nucleation, S So is the solubility of the precipitant ... 

S/PS SLS/CA Micro- fluidizer K2S2O8 70 20 Dissolved polymer enhances nucleation efficiency, approaching one (fc, t) [IS] Miller et al. 1994... 

Anderson, K.S. and HiUmyer, M.A. (2006) Melt preparation and nucleation efficiency of polylactide stereocomplex crystallites. Polymer, 47,2030-2035. 

As can be seen from the video test results, there seems to be a slight improvement in the nucleation efficiency with increasing P-value. However the drawback is that the higher P-values result in surfactants which will lead to a considerably negative effect on the burn performance of the resulting foam [43]. 

The summary of the data at this point seemed to indicate that the combination of good nucleation efficiency with the BT-structures and the advantage of high P-values seem to... 

Figure 2.15 Video imaging test nucleation efficiency with increasing P-value...

Comparison of structural siliconepolyether analogues with and without styrene oxide incorporation in surfactant test molecules showed an indication of a positive trend in the video test regarding the nucleation efficiency of those aromatic group substituted silicone polyether copolymers. 

That observation was rationalised with the following assumption. If indeed aromatic rings wonld increase the solubility of carbon dioxide then these types of structnres wonld be more solnble in a liquid carbon dioxide containing environment. This wonld resnlt in a decreased tendency to move to the surface of a such a liquid phase. The obvious result would be a decrease of surface activity, so the net result could very well be a higher nucleation efficiency combined with a lower activity during the rise time of the foam, leading to gas bubble coalescence. 

To some extent the presence of compatibilizers has also decreased the crystallization temperature [15]. It indicates the inhibition of the crystallization by the addition of compatibilized clay platelets. The presence of compatibilizers in the layer spacing during intercalation may reduce the nucleation efficiency. The shielding effect of compatibilizers over the surface of the clay will avoid the direct contact. 

Frensch and Jungnickel (1989) and Frensch et al. (1989) tried to elucidate the crystallization behavior of the minor phase in the binary PVDF/PA-6 blends, in relation to the final blend morphology. They reported that the crystallization of the PA-6 droplets was fractionated and/or retarded, depending on the number of mixing cycles and dispersion size. The smaller the PA-6 droplets, the more pronounced the retardation of the crystallization peak (AT 40 °C). Nevertheless, the melting endotherm remained unaffected. They concluded that part or all of the PA-6 phase finally coincidentally crystallized with the PVDF matrix due to the specific mutual nucleating efficiency of both components. 

Nucleating efficiency is normally determined isothermaUy from crystallization half-time, by use of peak crystallization temperature of the nucleated system measured during cooling and compared with that of the neat polymer. The peak crystallization temperature technique is based on a single-point observation. FDlon et al. proposed alternate approach for the evaluation of nucleating efficiency using two dynamic reference points to understand the crystallization behavior of a polymer (FOlon et al. 1993). These two reference points include (i) polymer s crystallization temperature when crystallized normally and (ii) the same when polymer nucleated ideally. 

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