Spherulites crystals

When more experiments were considered, a good correlation could be established between the aluminium concentration and the variation of the crystal habit. Figure 6 reports data collected from seven crystallizations. Spherulites exhibiting high-index faces existed for the higher supersaturation levels. When the aluminium content became lower than 4-5 mmol/1, (001) faces appeared, while (100) faces formed only below 2-2.5 mmol/1. 

Fig. 8. Schematic design of a spherulite which is formed in the course of polymer crystallization. Spherulites contain blocks of crystalline zones, embedded in an amorphous matrix. The size can be in the pm range and larger...

In crystallization from the melt, polycrystalline regions sometimes occur which are called spherulites because of their spherical form and optical properties. Microtome sections show that their internal structure is radially symmetric. Circular structures of similar internal construction occur in the crystallization of thin films (Figure 5-23). They are therefore likewise termed spherulites, since they can be considered as cross sections of bulk-crystallized spherulites. 

Spherulitic growth is a special case in crystallization. Spherulites form only within a specific temperature range for example, with it-poly(propylene) with a melting point of ITO C, they are first formed below IIS C. With spherulites, the rate of advance of the spherulite boundary is followed. This boundary encloses the crystalline portion of the spherulite. Since spherulites also contain noncrystalline material, however, the spherulite growth rate thus corresponds to the linear crystal growth rate. As the molecular weight increases, the rate of crystallization falls, since the rate of diffusion of segments and molecules decreases. 

Nucleating agents are used in polymer systems to increase the rate of crystallization. These agents are added to partly crystalline polymers and change the polymer s crystallization temperature, crystal spherulite size, density, clarity, impact, and tensile properties. These intentional contaminates achieve these functions by acting as sites for crystalline formation. 

Fig. 24. DTA melting peak temperature as a function dt heating rate for poly-ethylenes of different crystal morphologies. Curve 1, attended diain crystak curve 2, slowly cooled melt crystallized spherulites curve 3, fast cooled mdt crystallized spherulites curve 4, solution grown folded chain single crystals.

Melt Crystallized.—Spherulites are observed to consist of radiating fibrils or lamellae which in melt-crystallized polymers are 100—500 A thick, depending on crystallization temperature, and /um in breadth. The c axis, i.e, the chain direction, coincides with the thickness of the lamellae and, by analogy to single crystal morphology, the molecular chain folds backwards into the lamellae. Amorphous polymer fills the regions between lamellae and can account for a substantial amount of the content. Painter et al. have shown that these inter-lamellar regions are very similar to melt-crystallized polymer in their i.r. spectrum. 

Morphology It is the study of the physical form or structure of a material the physical microstructurc of a bulk polymer. Common units arc lamella, spherulitc, and domain. In turn there arc thermoplastic (TP) and thermoset (TS) plastics. Lamella is a thin, flat scale layer of polymers. Spherulitc is a rounded aggregate of radiadng lamellar crystals. Spherulites exist in most crystalline plastics and usually impinge on one another to form polyhedrons. They range in size from a few tenths of a micron in diameter to several millimeters. Domain is a microphase of one polymer in a multiphase system. 

Moreover, the increase of in composites with respect to the neat matrix can be attributed to the nucleating ability of the cellulosic fiber on the polymeric material as was observed by means of optical microscopy. For this purpose, in Fig. 59 an optical micrograph of iPPMA spherulites growing on a NaOH extracted fiber is reported. The figure shows a pronounced nucleating effect of the cellulosic fiber on the polypropylenic matrix. It is possible to note that the radius of the bulk crystallized spherulites and of the fiber nucleated spherulites seem to be comparable in their dimensions, so that only the nucleation and not the growth process is influenced by the presence of cellulosic fiber. 

Here, i/gph is the hardness value of the spherulites, is the hardness of the amorphous interspherulitic regions, and is the volume fraction of crystallized spherulites. During primary crystallization, Hgph remains constant and hardness is directly proportional to the volume occupied by the spherulites (12). The hardness variation in the course of isothermal crystallization of PET and poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) has been shown to follow Avrami law (13,14). 

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