Low nucleation density

We suggest that both the lamellar and the microfibrillar morphologies are formed by a nucleation and growth process. A low nucleation density of crystal solvates results in a lamellar morphology, whereas a high nucleation density of the crystalline PBT results in formation of microfibrils. 

For the case of the PtChfa), fluorine in the ligand reacts with the underlying SiO Which results in an etching reaction of the Sio. This unwanted reaction was another reason for the low nucleation density. The surface morphology should be improved by some kind of surfade modification technique which increases the nucleation density. However, the Pt films deposited on BST films show a much better surface morphology due perhaps to the larger nucleation density on the BST film. 

Zhou and Hay [1993] investigated the crystallization in LLDPE/PP blends. They reported that the extent of crystallization in PP droplets is seriously hindered by the low nucleation density of PP, resulting in a serious drop of the degree of crystallinity during the isothermal measurements. From these experiments it could be predicted that cooling from the melt would result in a fractionated crystallization (30 wt% PP) or even homogeneous crystallization (10 wt% PP). Similar results had already been reported by Long et al. 

FIGURE 4-6 a-Al203 fiber with large grain size resulting from low nucleation density. Source 3M Company. 

When PHB is crystallized from the melt it forms large banded spherulites. Because of its biological origin and the extensive purification process used to separate it from cell debris, the polymer does not contain inorganic catalyst residues or other impurities which could act as heterogeneous nucleation centres. As a result it is very easy to obtain samples of PHB with low nucleation density that form massive spherulites on cooling from the melt. With a modicum of care, spherulites of several millimetres diameter that are visible to the naked... 

Early work on single particle nucleation and growth involved the use of either Pt or carbon microelectrodes on which was deposited a material which had a suitably low nucleation density, for instance, Ag, Hg, or Cu (11-13). Later work expanded this to look at other systems such as Pb02 on carbon (37), or PbS04 on Pb (34). Working with single particles allows analysis of the electrokinetics of deposition without the complication due to overlap of growing particles or their diffusion spheres. 

Because of the high surface energy of diamond, its nucleation on virgin substrates, in particular if the substance of the substrate does not have chemical affinity with carbon, is characterized by low nucleation densities (ca. 10 cm ) and long incubation times. ... 

As a result of the low nucleation density of the virgin polymers, PHB and its HV copolymers are ideal systems for the study of spherulite morphology. With care it is possible to grow spherulites several millimetres in diameter. PHB and the lower HV copolymers all form banded spherulites (see Fig. 5.7) the regularity and spacing of the bands depends on the exact crystallization conditions. 

Low nucleation densities also facilitate measurements of spherulitic growth rates (SGR). Typical plots of SGR as a function of crystallization temperature for several copolymers is shown in Fig 5.8. HV content has the effect of diminishing the overall rate profile at low HV contents but this trend is reversed at higher HV content [33]. This transition reflects the different crystal structures formed by the different copolymers. As already discussed, below 40% HV the crystal grows with a PHB lattice whilst above 40% HV a PHV lattice is formed. The variation in SGR with copolymer composition is due to the influence of... 

In comparison to other conventionally produced semicrystalline thermoplastics, PHB and the HB-HV copolymers have remarkably low nucleation densities in the absence of self-seeding or deliberately introduced nucleants (Table 5.6). This low nucleation density, which in practice means that it is possible to grow spherulites several millimetres in diameter on crystallization from the melt, is attributed to the purity of the fermentation-produced polymer, and in particular to the absence of inorganic catalysts residues. This absence of heterogeneous nucleation has led to a great deal of academic interest in the PHBV range of polymers as systems for the study of homogeneous nucleation kinetics, but the poor nucleation of the pure polymer is also of commercial... 

The BP a A" B compound shows a (111) polar axis just as cBN does. One would expect epitaxy to appear on the B-terminated surface and low nucleation density to be observed on the P-terminated (111) surface. Heteroepitaxy on cBN had a limited impact because substrate crystals are not easily available. The cBN and BP single crystals were synthesized by HP/HT processes. The largest eBN erystal has a linear dimension of 3 mm. The BP crystals grown from B solution in liquid P at 1.5 GPa have a dimension of about 1 mm. 

The nucleation density of the deposited atoms is an early indication of good or poor contact. A high nucleation density indicates strong chemical interaction of the deposited adatoms with the substrate surface and is desirable for good adhesion. A low nucleation density indicates poor interaction, the development of poor interfacial contact, and the formation of interfacial flaws, which leads to poor adhesion. 

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