Growth of Spherulites

At sufficient supercooling, the sphemhtic superstmcture may be described with only one growth rate, v, as shown in Fig. 3.83. The increase in crystal volume, V, is given by the second equation and allows the computation of the enthalpy evolved by multiplication with the product of density and specific heat of fusion (Ahf in 1 g ). With nucleation data, the crystallization rate of the whole sample can be computed and linked to growth rates measured by dilatometry or calorimetry. Results for the LiPOj crystallization, which is discussed in Sect. 3.1.6, are also listed in Fig. 5.83 [15]. The changes of the growth rale with temperature are given in the table. Note that the crystallization of the polymer is in this case coupled with the polymerization reaction [1], and increases with temperature. 

The supercooling is sufficiently large to grow dendritic-type crystals 

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Mandelkern et al. (1968) have proved that the WLF formulation, which has had an outstanding success in explaining the segmental mobility and flow properties of completely amorphous polymers, is not applicable to the transport process involved in the growth of spherulites in melts of semi-crystalline polymers. Rather, a temperature-independent energy of activation, specific to a given polymer and dependent on its glass temperature, suffices to explain the experimental data now available. Mandelkern s equation reads ... 

FIGURE 8-61 Schematic diagram of the final stage of the growth of spherulites. 

The cooling of polymer melt in the presence of a foreign surface which can nucleate crystalline growth inhibits the lateral growth of spherulites. Crystallization occurs This is called transcrystallinity. It can im-... 

Analysis of DTA curves has shown that the main process of crystallization in the presence of stabilizer proceeds at much lower temperature. This is explained by nucleus activity of stabilizer and filling interspherulite space by it, preventing the growth of spherulites. It has been also obserbed on DTA curves that melting of stabilized PETP is observed at much lower temperature compared with unstabilized one, which is the result of less perfection of fine spherulite structure of stabilized PETP [270] and plasticizing action of the additive too. 

The usual and therefore most important situation where polymers crystallize is in melts eooled below the point of the fusion of a crystallite of infinite dimensions. Then, crystallization occurs by the nucleation and growth of spherulites. Another crystallization process is sometimes encountered in oriented melts and glasses. In such systems, the crystallization seems to occur at once in the whole sample and not at the interface between the growing crystallites and the amorphous matrix. Despite munerous studies, the crystallization process is not fully understood. Scattering measurements suggest a preliminary spinodal decomposition of the undercooled isotropic melt in phases with and without chain ends and chain defects before the formation of the crystallites [32]. 

Figure 16.7 Temporal evolution of the crystalline microstructure in the 50/50 iPP/EPDM blend, following a T-quench from the isotropic melt to a supercooled temperature below both the UCST spinodal gap, showing the growth of spherulitic front in the concentration field, but the overgrowth of this spherulitic boundary on the bicontinuous SD domain structures can be seen clearly only in the enlarged version.

Hobbs, J., McMaster, J., MUes, M. and Barham, R, Direct observations of the growth of spherulites of p>ol34hydrox34)utyrate-co-valerate) using atomic force microscopy. Polymer, 39, 2437,1998. 

The value of R found in this way is heavily weighted towards larger spherulites, because the intensity scattered by a spherulite is proportional to its volume. The method is useful for comparing samples or following the growth of spherulites, since the rate of increase of radius is usually independent of the size of spherulite. The theory has also been worked out for more complicated forms of spherulites, for truncated spherulites and for other non-spherulitic types of crystalline aggregates. 

Fig. 5.17 Early stages in the growth of spherulites (a) the sheaf-life stage in the growth of a polyethylene spherulite and (b) the beginnings of radial growth in a spherulite of poly(4-methylpentane). ((a) Reprinted by permission of Kluwer Academic Publishers (b) Cambridge University Press 1981.)...

According to reference the primary and secondary crystallization may be defined through the experimental techniques used, and consequently the kinetic parameters and reaction order for each process may be determined. Primary crystallization is associated with the growth of spherulites, while secondary crystallization is associated with additional crystallization within the boundaries of a spherulite as well as the crystallization of material trapped between spherulites. The re-... 

Illustration of five stages In crystallization showing spherulite growth. At time fo—the supercooled melt. At later times t, to fg—the growth of spherulites. Finally, at time f4— the specimen Is composed completely of spherulites. 

Nylon, nucleated n. A nylon polymerized in the presence of a nucleating agent, e.g., about 0.1% of finely dispersed silica, which promotes the growth of spherulites and controls their number, type, and size. Nucleated nylons have higher tensile strength, flexural modulus, abrasion, resistance, and hardness, but lower impact strength and elongation than their unnucleated counterparts. 

A clay-induced crystal transformation from a-phase to p-phase in PVFD was confirmed by x-ray diffraction and FTIR. Clay layers restricted the growth of spherulites. Montmorillonite nucleates the y-phase of PVDF. Montmorillonite increases nucleation efficiency. Montmorillonite has been used as a nucleating agent in invented PDVF with increased recrystallization temperature by 11°C. ... 

Fig. 3.5 Growth of spherulitic crystal pattern in SA-AA-Ce system, a After 13 h 15 min and b after 13 h 45 min [8]...

Yadav, N., Srivastava, P.K. Growth of spherulitic crystal pattern in Belousov-Zhabotinski type reaction system. New. J. Chem. 35, 1080 (2011)... 

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