Enhancement of crystallization

On the other hand, the incorporation of a small content of ethylene units in the polypropylene chains has an influence on the regularity of the molecular structure. In fact, a change in tacticity induced by the shortening of isotactic sequences was observed (Zimmermann, 1993). Although this has a negative influence on crystallisation kinetics, an opposite effect should come from the enhanced mobility due to the presence of the ethylene sequences. As a result of these counteracting effects, a relatively narrow window of cooling rates exists in which an enhancement of crystallization kinetics sets in (Foresta et al., 2001). 

To understand the mechanism leading to enhancement of crystallization under shear, the structure of the so-call shish or row nuclei, the primary nucleus induced by flow, is essential. The common textbook picture is that the shish consist of extended-chain crystals that maybe defective [184-189]. It is generally accepted that structures with some degree of order are induced by the flow before the onset of crystallization, which can be long living at shallow supercooling [127-150]. These shear-induced precursors have been described... 

Isothermal and non-isothermal crystallization studies showed that the presence of nanofiller (multiwall carbon nanotubes, silver nanoparticles, nanodiamonds and copper-nanofibers) caused enhancement of crystallization rates and that the most effective nucleation was achieved using MWCNT. The crystals of a-type dominated, except in case of the sPS/MWCNT nanocomposite in which mairdy P-type crystals appeared. ... 

Anderson s theory [87] of local vacancy density enhancement of crystal imperfections and the Bose-Einstein condensation theory for the superelasticity and superfluidity of the individual segment of the He crystal supersolidity. 

In References [6,114,115] shear-induced crystallization of iPP-based nanocomposites with o-MMT was examined. In Reference [114] early stages of flow-induced isothermal spherulitic crystallization of intercalated iPP/PP-g-MA/o-MMT nanocomposites with 2 wt% of the clay were studied after application of shear flow for a short time of up to 30 seconds. The flow induced enhancement of crystallization was stronger in the nanocomposite than in neat iPP. However, acceleration of crystallization in iPP/PP-g-MA blend was also observed. 

An interesting observation associated with crystallization under uniaxial deformation is the marked enhancement of the crystallization rate at constant tempera-tures.(43,44,55,56) For example, the crystallization rate constant of natural rubber increases by six to nine orders of magnitude with extension ratio at constant temper-atures.(43) This enhancement is also reflected in the crystallization half-time, which also increases by several orders of magnitude.(44,46,47,55,56) This enhancement of crystallization rate is not limited to rubber-like polymers. It is also observed in poly(pentenamer) (50) and in poly(ethylene terephthalate) (47 9,57,57a) when crystallized under uniaxial deformation. The change in rate can be attributed in part... 

Small concentrations of vinylcarboxyhc acids, eg, acryhc acid, methacrylic acid, or itaconic acid, are sometimes included to enhance adhesion of the polymer to the substrate. The abihty to crystalline and the extent of crystallization are reduced with increa sing concentration of the comonomers some commercial polymers do not crystalline. The most common lacquer resins are terpolymers of VDC—methyl methacrylate—acrylonitrile (162,163). The VDC level and the methyl methacrylate—acrylonitrile ratio are adjusted for the best balance of solubihty and permeabihty. These polymers exhibit a unique combination of high solubihty, low permeabihty, and rapid crystallization (164). 

Purification. The objective of crystallization also can be purification of a chemical species. For example, L-isoleucine (an essential amino acid) is separated by crystallization from a fermentation broth that has been filtered and subjected to ion exchange. The recovered crystals contain impurities deleterious to use of the product, and these crystals are, therefore, redissolved and recrystalHzed to enhance purity. 

The data plotted in the figure clearly support the predicted positive dependence of crystal size on agitation rate. Precipitation in the crystal film both enhances mass transfer and depletes bulk solute concentration. Thus, in the clear film model plotted by broken lines, bulk crystal sizes are initially slightly smaller than those predicted by the crystal film model but quickly become much larger due to increased yield. Taken together, these data imply that while the initial mean crystal growth rate and mixing rate dependence of size are... 

Cleavage of the acetals 9 provides crystalline amino diol hydrochlorides. Remarkably, the (S.S.S)-diastereomer crystallized preferentially leading to a considerable enhancement of d.r. values. 

Moreover, the absorption-desorption hydrogen by a metal, able to form the hydride phase, leads to a cracking of metal crystallites (9, 4 )i the disclosure of new crystal planes, and increasing disintegration—the whole set of phenomena resulting in an enhancement of catalytic activity. 

To enhance the discussion even further, we showed them pictures of cleaving a crystal. Such experiments played an important part in the history of theory development in the area of crystal chemistry (see above). The students were able to draw the conclusion that splitting must be correlated to a separation of particles, but they did not draw any conclusions about the shape and order of the particles before and after the splitting. 

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