Retardation of nucleation

For liquid products (solvents), only polar molecules selectively absorb microwaves, because nonpolar molecules are inert to microwave dielectric loss. In this context of efficient microwave absorption it has also been shown that boiling points can be higher when solvents are subjected to microwave irradiation rather than conventional heating. This effect, called the superheating effect [13, 14] has been attributed to retardation of nucleation during microwave heating (Tab. 3.1). 

C. Ultrasound (200 kHz, 300 W) was stimulated to a 250-mL sample of CB at 32.3°C during cooling before crystallization. Form II occurred without ultrasonication, whereas Form V was observed when ultrasonication was done for 3 seconds. Further ultrasonication for 9 seconds formed a mixture of Form II and Form V, and only Form II was observed by the ultrasonication for 15 seconds. It is assumed that there are conflicting effects by ultrasonication promotion of nucleation by pressure effect and retardation of nucleation by thermal energy caused by absorption of attenuated ultrasound wave. The former effect may prevail at the ultrasonication for 3 seconds. The temperature rise, however, of the sample caused by absorption of attenuated ultrasound wave was 2°C for 9 seconds and 3.9°C for 15 seconds, and the latter effect may result in the case of cooling from above the melting point of CB. 

The rate of nucleation, I, has been estimated by Turnbull and Fisher [25] from the shape of AG and the influence of local viscosity, governed by a free enthalpy AG,. In Fig. 3.61 this equation is listed. The rate I applies to the case that nucleation is unhindered. The first exponent of the equation expresses retardation of nucleation due to viscosity effects with the given parameters (see Sect. 5.6). It stops nucleation as the glass transition temperature is approached. The nucleation described is homogeneous nucleation and creates a continuous stream of new crystals in the remaining melt or solution. For polymers it takes a supercooling of about 50 K to overcome the... 

Silicone foam thus formed has an open ceU stmcture and is a relatively poor insulating material. Cell size can be controlled by the selection of fillers, which serve as bubble nucleating sites. The addition of quartz as a filler gready improves the flame retardancy of the foam char yields of >65% can be achieved. Because of its excellent dammabiUty characteristics, siUcone foam is used in building and constmction fire-stop systems and as pipe insulation in power plants. Typical physical properties of siUcone foam are Hsted in Table 10. 

The crystalliza tion resistance of vulcaniza tes can be measured by following hardness or compression set at low temperature over a period of time. The stress in a compression set test accelerates crystallization. Often the curve of compression set with time has an S shape, exhibiting a period of nucleation followed by rapid crystallization (Fig. 3). The mercaptan modified homopolymer, Du Pont Type W, is the fastest crystallizing, a sulfur modified homopolymer, GN, somewhat slower, and a sulfur modified low 2,3-dichlorobutadiene copolymer, GRT, and a mercaptan modified high dichlorobutadiene copolymer, WRT, are the slowest. The test is often mn near the temperature of maximum crystallization rate of —12° C (99). Crystallization is accelerated by polyester plasticizers and delayed with hydrocarbon oil plasticizers. Blending with hydrocarbon diene mbbers may retard crystallization and improve low temperature britdeness (100). 

Marosi, G., Anna, P., Balogh, I., Bertalan, G., Tohl, A., and Maatoug, M. A. 1997. Thermoanalytical study of nucleating effects in polypropylene composites. 3. Intumescent flame retardant containing polypropylene. Journal of Thermal Analysis 48 717-26. 

The hydration of hemihydrate or anhydrite shows interesting similarities to and differences from that of C3S (G49). Many foreign ions accelerate or retard hydration, but the sequences of effectiveness differ markedly from those found for C3S. Excess SO4, but not excess of Ca, is a particularly strong accelerator. Murat el al. (MI07) found that, for anhydrite, the effectiveness of an added cation decreased with its ionic potential, z/r. and considered that the cations mainly affected the rate of nucleation. The relation to z/r is almost the reverse of what is found with C3S hydration. 

While observations such as these contribute to the necessary accumulation of knowledge in this area, their empirical nature is evidence that a comprehensive quantitative understanding of nucleation catalysis is still in the process of development. Many seemingly isolated phenomena are still treated separately, and rather formidable experimental difficulties retard their rationalization and correlation. This situation parallels that in the area of chemical catalysis, where theory seems to be somewhat more advanced, but practice is still often empirical. 

MC) and HPMC could significantly inhibit the crystallization of supersaturated hydrocortisone acetate (HA). The mechanism of nucleation retardation was believed to be due to the hydrogen bonding interactions between HA and the polymers. As to the... 

MAH-g-PP +FA-g-EBA +GMA-g-EEA 80/20 60/40 of binary blend and MAH-g-PP PA-6 dispersed coincident crystallization with PP SEES, EBA and E EA form immiscible interlayer between PA-6 and PP => no nucleation =>T =T c,PP c,bulk Compatibilization => serious reduction of PA-6 droplet size => lack of nucleating species => retarded cryst. [Ikkala et al, 1993]... 

Under adequate conditions determined for the preparation of the seeds, recrystallization was repeated and the obtained crystals were used as seeds. As the numter of recrystallization (N) increases the time of the start of purity drop was prolonged, implying that rec stallization is effective in retarding the nucleation of L-Thr. The effect, however, seems to have limits since there is httle improvement in the starting time of purity drop above, say N=5, which is not shown in Figure 11 to avoid confusion. 

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. 

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