Nucleating systems

Additional functions of the nucleating system include the following ... 

We reported in the previous papers [8, 9] that the effect of the operational factors such as temperature and solvents on the polymorphic crystallization of a thiazole derivative - 2-(3 -Cyano-4-(2-methylpropoxy)-phenyl)-4-methyl-thiazole-5-car-boxylic acid (BPT) - which is an enzyme inhibitor. In this paper, we synthesized the esters of BPT and studied the effect of the molecular structure on polymorphic nucleation systemically, and at the same time we also examined the solvent effect on the polymorphic nucleation of the ester. 

If a miniemulsion could be run at 100% droplet nucleation (or near to this), then a very robust nucleation system would result. The number of particles could be determined by the number of initial monomer droplets, and this can be controlled by adjusting surfactant, costabilizer and shear levels. In this case, the number of particles would be independent of radical flux. In fact, the most compelling evidence for droplet nucleation is experimental evidence that the number of polymer particles is independent of the initiator level. (Once the radical flux is high enough to nucleate all, or nearly all of the droplets, then changes in radical flux caused by inconsistent initiator or unknown inhibitors will not affect the final particle number.) We will discuss the results of such robust nucleation later. 

A typical nucleating system consists of a homogeneous vapor at a defined pressure p and temjjerature T. If p exceeds the vapor pressure p of some condensed phase of the substance at the working temjjerature, then the vapor is unstable to the formation of the condensate. Although unstable, the vapor may be metastable, often with an exceedingly long lifetime. For metastable vapors it is common to define the sujjersaturation S of the vapor by... 

Because of the slow rate of condensation the composition of a typical nucleating system is dominated by monomers. Consequently the growth of clusters is governed by monomer capture,... 

Continuous processes involving tubular reactors have been reported in the literature (General References 2, 7, and ). Continuous tubular reactors have been used in three ways. Gonzalez (9) used a tubular prereactor to feed a CSTR system. The tubular prereactor served as a particle nucleation system and thus solved the problem of conversion oscillations often observed in CSTR systems. A tubular prereactor also can be used to generate a higher particle concentration than would be produced with the same recipe in a CSTR system. 

With regard to PET fraction, the potential applications are strongly dependent on its purity (as discussed earlier). Applications like films, fibers, or straps are not recommended when a high concentration of impurities are present In this case, the PET fraction can be employed for structural applications as an engineering polymer with the addition of other components like glass fibers, impact modifiers, and/or nucleating systems. However, reuse of the PET fraction implies that the amount of residual PVC must be kept below 50 ppm to avoid undesirable polymer degradation that results in poor surface appearance and loss of mechanical properties of the manufactured products. 

PLCs have been shown to nucleate systems which are hard to crystallize by any other means. Polyethersulfone (PES) is a high temperature thermoplastic which, even when cast from solution, is unable to crystallize [84], as opposed to, for example, PC. However, when solution blended with 30 wt% wholly aromatic longitudinal PLC (structure not reported), spherulites of the PES can be formed and it is suggested that the combined effect of solvent molecules to lubricate the motion of the crystallizing PES molecules and a nucleant (PLC) is required, either condition on its own not being sufficient. Similar attempts to crystallize the blend from the melt failed. 

Mesoscopic approaches based on master equations have been applied to other problems in aerosol physics including coagulating systems [2.18] and nucleating systems [2.19]. These areas will be discussed below. 

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. 

Fig. 19.34. Common logarithm of the overall crystallization rate constant for sporadic nucleation system as a function of common logarithm of molecular weight and temperature. Curves are calculated on the bases of PESU data [76]...

Modified AZ. Modified AZ systems have been developed which offer improved performance and increase versatility in a wide variety of applications. Each system has a formulated cell nucleation system (usually silica) and gas yield is approximately the same as unmodified AZ. Modified types are also available in several particle size grades. 

The nucleating system for poly(p-phenylene sulfide) includes a combination of an inorganic crystalline compound and an aromatic amide oligomer. Because of the improved crystallization rate, the thermoplastic composition can be molded at lower temperatures to still achieve the same degree of crystallization. The inorgartic compound is boron nitride. Boron nitride used alone has also been proposed for PPS. ... 

J is proportional to n times the solubility expressed in number of molecules per unit of volume, Nq. v is the frequency with which nuclei of critical size r become supercritical by addition of a molecule and develop into crystals. The term nNav can be simply described as a pre-exponential factor K -Equation (9.3) shows that the frequency of nucleation depends not only on the supersaturation (3 but also on the concentration of molecules nNa. All things being equal, supersaturation included, the higher the probability of inter-molecular contact, the easier is nucleation. Systems with high solubility, and thus low y, meet this condition. For systems with low solubility, the solute molecules are separated by larger distances and by a greater number of solvent molecules. The probability that the molecules will come into contact and form a nucleus is thus lower. 

Here we summarize the conclusions of our work, detailed in Ref. [14], on the nucleation system ... 

The nucleation efficiency can be calculated from dynamic DSC measurements at a constant cooling rate by comparing the crystallization temperatures of the nucleated system with that of the self-nucleated matrix. 

Figure 18.4 SAXS profiles obtained for HISPS/nucleator systems in MD and TD.The mold temperatures are (a) 80°C and (b) 200°C.

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