Nucleation catalysts

The influence of foreign bodies, surfaces, crevices and similar entities that accelerate the nucleation process has been discussed earlier in this chapter. These heterogeneities are fortuitous since they have not been deliberately introduced into the system. In many instances strong efforts are made to remove them. However, there are also cases where specific species are deliberately introduced in order to accelerate the crystallization and alter properties. Such substances have been termed nucleation catalysts. They are usually low molecular weight organic and inorganic 

Cooling rate studies establish that a significant increase in crystallization rates can be achieved by the addition of appropriate compounds. A more quantitative analysis 

Nucleation catalysts also have a strong influence on the overall crystallization kinetics, as is demonstrated in the experimental isotherms illustrated in Figs. 9.69 and 9.70 for poly(caprolactam) (238) and poly(ethylene terephthalate) (253) respectively. The isotherms for poly(ethylene terephthalate) are for a fixed crystallization temperature with different types of nucleation catalyst at the indicated weight percent. In all cases the isotherm shapes are similar to one another. There is an enhancement of the crystallization rate that is also found with other catalysts for 

In most studies the derived Avrami equation (Eq. 9.31 a) has been used to analyze the isotherms. According to Eq. (9.31) a double logarithmic plot is usually made with the data. Most commonly, a nonintegral value of the Avrami exponent was obtained. Despite the inherent shortcoming in analyzing the data by this method, the conclusion can be made that for a given polymer the Avrami n value is independent of the nature and concentration of the catalyst. Thus, the isotherms are superposable with one another and that of the pure polymer. 

Concentration of solid in sample, % CaO Ti02 MgO BaS04 Si02 AI2O3 

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There are two approaches to nucleation kinetics that have been utilized to explain intraglobular fat crystallization—homogeneous and heterogeneous nucleation. During homogeneous nucleation, there are no impurities present to act as nucleation catalysts (6). Heterogeneous nucleation involves the presence of impu-... 

It is only in recent years that suitable techniques have been devised for studying the kinetics of homogeneous nucleation. The main difficulties have been the preparation of systems free from impurities, which might act as nucleation catalysts, and the elimination of the effects of retaining vessel walls which frequently catalyse nucleation. 

An early attempt to study homogeneous nucleation was made by Vonnegut (1948) who dispersed a liquid system into a large number of discrete droplets, exceeding the number of heteronuclei present. A significant number of droplets were therefore entirely mote-free and could be used for the study of true homogeneous nucleation. The dispersed droplet method, however, has many attendant experimental difficulties concentrations and temperatures must be measured with some precision for critical supersaturations to be determined the tiny droplets (< 1 mm) must be dispersed into an inert medium, e.g. an oil, which will not act as a nucleation catalyst and any nuclei that form in the droplets have to be observed microscopically. 

A simple microfluidic reactor system He et al. [124] for the effective synthesis of enzyme-functionalized nanoparticles offers many advantages over batch reactirais, including excelloit enzyme efficiencies. Better control of the process parameters in the microfluidic reactor system ovct batch-based methodologies enables the production of silica nanoparticles with the optimum size for efficient enzyme immobilization with long-term stability. The synthetic approach used glucose oxidase and two different nucleation catalysts of similar molecular mass the natural R5 peptide, and PEI polymer... 

Crystal growth by sublimation is extensively used for obtaining high purity crystals. The nucleation and growth of crystals from the vapor invariably occurs by grafting to some solid support, usually in practice some cold spot on the walls of a container. A likely hypothesis is then that the solid surface somehow acts as a nucleation catalyst. In principle, nothing forbids the simulation of the vapor-solid equilibrium along the same lines as for vapor-liquid equilibria. 

Carefully conducted experiments have shown that it is possible for spherulites to develop sporadically in both time and space in thin polymer films.(22,23,101,102) Repetitive experiments have indicated that spheruhtes do not necessarily form in identical positions if complete melting of the sample is ensured. However, a study of poly(decamethylene terephthalate) has shown that although spherulites are formed sporadically in time, they appear in identical positions within the sample.(103) Experimental evidence showed that for this sample the spheruUtic centers are initiated from a fixed number of heterogeneities. There is a strong tendency for spherulites to appear in the same position in the field of view after successive crystallizations.(73,74,104-106) In some cases this observation is solely a result of incomplete melting.(33,36,102) In others, it can also be due to the presence of a finite number of nucleation catalysts in the polymer melt. 

Fig. 9.71 Spherulite growth rates of isotactic poly(propylene) with different sorbitol compounds as nucleation catalysts, o pure polymer dibenzylidene sorbitol o- (p-chloro, p-methyl) dibenzylidene sorbitol, -o bis (p-ethylbenzylidene sorbitol. (Data from (249))...

A scale has been proposed by which the effectiveness of a nucleation catalyst can be assessed. In addition to comparing the crystallization temperature on cooling the nucleated polymer with that of the pure polymer, the crystallization temperature of the self-seeded polymer is also taken into account.(268,269) The self-seeded polymer (270) is taken to represent the ideal nucleated system. The efficiency of a nucleating agent, E, expressed as a percentage, is given as (268,269)... 

The deviations that are observed from either the free-growth or Avrami relations can be attributed in part to the general problem encountered in homopolymer crystallization, i.e. the role of chain entanglements. In addition, there is a major contribution to the deviations due to the decreasing availability of eligible sequences as the transformation proceeds. This is due to the decrease in the undercooling at constant temperature. As a consequence, in contrast to homopolymer crystallization, copolymer isotherms are not superposable. Deviations from theory are observed at much lower levels of crystallinity, although the same basic type of nucleation is involved with both homopolymers and copolymers. Nucleation catalysts influence copolymer crystallization in a similar manner to that of homopolymers.(33b)... 

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