Primary nuclei

But there are two important unresolved questions regarding the mechanism of crystallization the first question is are the primary nuclei actually formed from the earliest stage ( induction period ) of crystallization and the second is what is the role of the topological nature of polymers in the polymer crystallization mechanism ... 

The appearance and disappearance of embryos of the ether-soluble (ES) fraction of stereoblock PP were observed by Schonherr et al. [73]. Features 1, 2, and 3, which are clearly visible in Fig. 5a, disappear in Fig. 5b. A stable nucleus, for example, Feature B in Fig. 5a, develops into a lamella. These studies have clearly demonstrated that the primary nuclei of polymers can be observed using AFM. 

The crystallization curves can be broken down into three parts. There is an initial induction period during which the primary nuclei are formed. These primary nuclei are the smallest crystalline entities that are stable enough to allow further growth at that temperature (i.e., do not re-melt). This induction period is followed by a period of fast spheru-lite growth called primary crystallization (Figure 10-23). (If you haven t observed... 

Once primary nuclei are formed the ensuing spherulites grow radially at a constant rate. Primary crystallization, which occurs initially on the surface of the primary nucleus and then on the surface of the growing lamellar, also involves a nucleation step, secondary nucleation. It is this step that largely governs the ultimate crystal thickness and which forms the focus of most kinetic theories of polymer crystallization. 

Here Ni E) is the spatial density of cosmic-ray nuclei of mass i, and rip is the number density of target nuclei (mostly hydrogen) in the interstellar medium, Qi(E) is the number of primary nuclei of type i accelerated per cm3 per second, and Oi and are respectively the total and partial cross sections for interactions of cosmic-ray nuclei with the gas in the interstellar medium. The second term on the r.h.s. of Eq. 1 represents losses due to interactions with cross section a, and decay for unstable nuclei with lifetime t,. The energy per nucleon, E, remains constant to a good approximation in the transition from parent nuclei to nuclear spallation products, which move with velocity (5c and Lorentz factor 7 = E/mp. 

It should be pointed out that there is no direct physical relation between the phenomenon of fractionated crystallization and the number and the size of spherulites in the pure polymer. Whereas the occurrence of fractionated crystallization is related to the ratio between the number densities of dispersed polymer particles and primary nuclei, the size and the number of spherulites are additionally influenced by the cooling rate and the crystallization temperature. There is, therefore, also no relation between the fractionated crystallization and the type of the arising crystalline entities (complete spherulites, stacks of lamellae,...) both in the pure and in the blended material. There is, finally, no relation between the scale of dispersion which is necessary for the occurrence of fractionated crystallization and the spherulite size in the unblended polymer. 

Figure 3.35. Influence of the amount of dispersed phase, mixing time and crystallization temperature, T, on the amount primary nuclei active for crystallization at T in a PP/PS blend. All samples have been molten up at 220°C a) 2x mixing, b) 3x mixing T was set to 119X (V), 123°C (A), 125X (o) and 130°C ( ) [Bartczak et al, 1987],...

Long et al. [1991] investigated the crystaUiza-tion behavior in blends of PP with LLDPE. They found the crystallization temperature of the PP matrix, T, to decrease slightly upon the addition of LLDPE. However, the degree of crystaUinity, X, and the spheruUte growth rate, G, were not affected. The authors concluded that the overall crystaUiza-tion rate of PP in the matrix decreased due to a decreasing primary nuclei density. The latter was confirmed in O. M. experiments by the increased size of the PP spherulites upon the addition of LLDPE. However, Zhou and Hay [1993] reported that with the addition of LLDPE to PP, the crystallization rate remained similar as for the PP homopolymer. 

TEM results give evidence that the intensive formation of primary nuclei of... 

The reaction of tin cementation on zinc is reversible. The codeposited zinc reduces again tin ions from the solution. Tin films are formed with thickness of about 2-4 pm and more. The films include zinc in the near substrate zone (up to 10-50 at.%). These repeated processes of dissolution-deposition provide the low temperature recrystallization of the primary nuclei into grains and the grains into secondary agglomerates or crystallites of about 2 pm. The latter are separated by narrow and deep channels with the width not more than 30-50 nm. 

The initial conditions have been shown to be very important as they determine how rapidly the calcium oxide dissolves [22.1]. When quicklime is mixed with water, the most highly reactive particles dissolve producing a very high level of super-saturation with respect to calcium hydroxide. This results in heavy primary nucleation (i.e., the formation of a very large number of calcium hydroxide nuclei). Less reactive particles dissolve more slowly and produce a lower degree of supersaturation, which largely results in crystal growth on the primary nuclei. 

As described in section 15.4, commercial quicklimes consist of particles with a distribution of apparent densities. The rate of solution of a given particle depends on its apparent density (or reactivity) and its particle size. Thus finely divided par-ticles/those of high reactivity dissolve and hydrate first, producing primary nuclei. Coarser particles/those of lower reactivity dissolve and hydrate more slowly and contribute to crystal growth. 

The above practices relate to batch slaking. In continuous slakers, primary nuclei are always being recycled and reduce the levels of super-saturation. As a result, for the same conditions, continuous slakers tend to produce coarser particle size distributions than batch slakers. If required, this effect can generally be offset by adjusting the slaking conditions. 

Such a porosity formation via nucleation and subsequent aggregation of primary nuclei was described for the first time by Kun and Kunin [311] at the end of 1960s. However, later it was found [312-314] that, being prepared under slightly different conditions, macroporous resins may acquire an entirely different texture. Fig. 3.9a shows a schematic diagram initially suggested by Haupke and Pientka [312], in which, as a function of the DVB and diluent concentrations, two borderlines confine domain II where aU the above three levels of porosity may be observed. Below the lower boundary (domain I), where the concentrations of DVB and the diluent are smaller... 

The influence of temperature on the porosity of a molded monolith prepared with a precipitating porogen is described by a simple rule the higher the temperature of polymerization, the smaller are the pores (Fig. 4.6) [383]. This tendency may be understood in terms of the classical mechanism of porosity formation via nucleation and subsequent aggregation of the primary nuclei. Phase separation occurs when the growing polymer becomes insoluble in the monomer—diluent mixture. 

Primary nuclei that precipitate at early polymerization stages are formed mostly by a divinyl monomer the reactivity of which is known... 

In addition purity of fine particles was found always lower in comparison with those of larger or middle sized particles as shown in Figure 12. This suggests that the particles generated in the later stages are mixtures of primary and secondary nuclei, where the primary nuclei would be of the unseeded enantiomer while the secondary nuclei the mixtures of seeded and unseeded enantiomers. 

Consequently, the selective production of crystals of a metastable type is desirable. Accordingly, it is necessary to study the crystallization behavior of the polymorph appearing first (primary nuclei) in a supersaturated solution. 

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