Sizing Spherulite size

The greater the percentage crystallinity the higher the yield point and tensile modulus. It has also been shown that by raising the quench temperature the spherulite size is increased and that this greatly decreases the impact toughness. 

The polymer is liable to depolymerisation at temperatures just above T. In the case of pure polymer there is a tendency for the few spherulites to grow to sizes up to 1mm diameter. Spherulite size may be reduced by the use of nucleating agents and by fast cooling. 

We can nucleate crystallization from the melt by incorporating finely ground inorganic crystalline compounds such as silica. Nucleation of injection molded nylons has three primary effects it raises the crystallization temperature, increases the crystallization rate, and reduces the average spherulite size. The net effect on morphology is increased crystallinity. This translates into improved abrasion resistance and hardness, at the expense of lower impact resistance and reduced elongation at break,... 

The self supporting wafers of zeolites (3 to 4 mg.cm ) presented an i.r. absorption continuum depending on the particle size. The most intense absorptions were obtained for the samples 5 and 6 with particle size of ca. 6 pm and a spherulitic shape. 

It is of course important to note that the overall rate of crystallization is not only determined by the growth rate of the spherulites, but also by the amount of nuclei being present in the system. This possibility is used as an effective method to influence the total crystallization rate of commercial polymeric materials in a controlled manner and to influence the size of spherulites and thus the physical properties of finished articles made from semicrystalline polymers. 

Both the rate of nuclei formation and the crystal growth rate can also be expected to influence the spherulite size. It has been reported (hat, in the temperature range 130-180 °C, the spherulite size increases with increasing temperature [74], This trend can be expected to extend to higher temperatures as the nucleation rate decreases. On the other hand, the presence of nucleating... 

Haze is generally caused by the scattering of light in crystalline polymers. Optical inhomogenities with dimensions in the wavelength range of visible light cause haze. The latter often corresponds to the spherulite volume fraction, spherulite size and crystallinity. An increased size of spherulites results in... 

The morphology of the spherulites was in the form of a Maltese Cross , which was confirmed by the Avrami exponent value in the DSC study. The spherulite size of the binary blends was smaller than that of pure PET and PEN. 

Crystalline polymers exhibit the following basic properties They are opaque as long as the size of the crystallites or spherulites, respectively, lies above the wavelength of light. Their solubility is restricted to few organic solvents at elevated temperature. The following crystalline polymers have attained technical importance as thermoplastic materials polyethylene, polypropylene, aliphatic polyamides, aliphatic/aromatic polyamides, aliphatic/aromatic polyesters, poly-oxymethylene, polytetrafluoroethylene, poly(phenylene sulfide), poly(arylene ether ketone)s. 

In the crystallization of isotactic polypropylene from the melt, the number and size of the spherulites (and hence the rate of crystallization) can be influenced by the addition of certain nucleating agents.The smaller the spherulites, the greater is the transparency of the polypropylene film.The mechanical properties can also be affected in some cases. 

The effect of heterogeneous nucleation on the crystallization of isotactic polypropylene from the melt can be easily established as follows. A small amount of powdered polypropylene is well mixed with about 0.1 wt% of sodium benzoate in a mortar or by means of an analytical mill. Some of the mixture is transferred with a spatula to a microscope slide and melted at about 250 °C on a hot block. A cover slip is pressed on to the melt with a cork to obtain as thin a film as possible.The sample is held at 200-250 °C for some minutes and then allowed to crystallize at about 130 °C on the hot stage of the microscope an unadulterated polypropylene sample is crystallized in the same way. Both samples are observed under a polarizing microscope during crystallization,the difference in spherulite size between nucleated and untreated polypropylene can be seen very clearly. An ordinary microscope can also be used by placing polarizers on the condenser and eyepiece, and adjusting these to give maximum darkness. 

In the following derivation we will assume an almost complete wetting of the substrate by the material, in such a way that a continuous amorphous condensed film is formed at a thickness h smaller than the critical size of nucleation. In order to evaluate dG/dN of the process of incorporation of molecules from the amorphous condensed film to the spherulite, that is the ordered phase, we will hypothesize that the thickness of the amorphous film increases linearly with time, h(t) = Uhf, where the velocity is a constant, and that the spherulite has a cylindrical shape of radius R and height h, as illustrated in Fig. 5.10. 

From vv = 15 to iv = 19. spherulites remain in equilibrium with isooctane. The spherulite size differs markedly (from 100 nm to 8000 nm). Syntheses in this phase region (15 < vv < 20) show formation of particles having a higher polydispersity in size and in shape than those observed at low water content. As matter of fact, triangles, squares, cylinders, and spheres are observed. 

Ultra-high-pressure metamorphic rocks. Diamonds occur sporadically in crystals of garnet or zircon in various ultra-high-pressure metamorphic rocks formed in deep subduction zones. Crystals are of micrometer size, and the morphology is mostly spherulitic or cuboid, but octahedral is also... 

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