Spherulites, form

Figure 4 Optical micrograph of spherulites formed by slow coagulation of an isotropic solution of PBT in MSA, viewed between crossed polarizers.

Fanta, G. F., Felker, F. C., Shogren, R. L., Saleh, J. H. (2006). Effect of fatty acid structure on the morphology of spherulites formed from jet cooked mixtures of fatty acids and defatted cornstarch. Carbohydrate Polymers, 66, 60-70. 

Figure 8.5. Modes of formation of spheruUtes [2], [3]. Spherulites formed (a) on a spherical particle of foreign material, (b) on a polycrystalline aggregate of the same species, and (c) by split growth.

Fig. 9. Drawing of a 2D projection of a spherulite formed from radially grown lamellae with a small degree of branching...

Spherulites. As a polymer melt solidifies, several folded chain lamellae spherulites form which are up to 0.1 mm in diameter. A typical example of a spherulitic structure is shown in Fig. 1.15. The spherulitic growth in a polypropylene melt is shown in Fig. 1.16. 

The spherulite formed from the sample with the longer stereosequence has a denser structure with a greater frequency of fibrillar branching than does the other spherulite from the sample with shorter stereosequence. This is in accord with the mechanism of spherulitic crystallization recently proposed by Keith and Padden (15). 

A preferentially and a sheaf-like aggregation with random in-plane orientation are observed for the thinner films (thicknesses of 0.1, 0.2 and 0.4 pm in panels a-c). By contrast thick films (0.6 pm and thicker, panel d) show a morphology that resembles the well known (bulk) spherulitic form with a banded structure, characteristic of linear polyethylene crystallized from the melt at moderately high undercooling. 

Fig. la-c. Schematic representation of typical morphology in partially crystalline polymers (a) stack of crystalline lamellae of thickness lc and amorphous regions of thickness la (L = long period) (b) spherulites formed by a radial symmetric arrangement of stacked lamellae, during main crystallization (c) spherulites after the end of main crystallization... 

The Avrami exponent, n, has a theoretical value of 3 when crystallization takes the form of spherulitic growth of nuclei that came into being at the samei instant in time. Integral values of n ranging from 1 to 4 can be attributed to other forms of nucleation and growth (Hay, 1971). The spherulites formed from a melt have different sizes and degrees of perfection, and they completely fill the volume of a well-crystallized material. 

Kyu et al. (2) and Ree et al. (3, 5) studied the crystallization behavior of blends of LLDPE-B (114,000 M , 4.50 PDl, and 18 ethyl branches per 1000 backbone carbons) and LDPE (286,000 M, 15.98 PDl, and 26 short and 1.6 long branches per 1000 backbone carbons) using DSC, WAXD, SAXS, SALS, and LAM. They found that the LLDPE-B and LDPE components crystallize separately. The SAXS and SALS results indicate that when the LLDPE/LDPE blends are cooled from the molten state, the LLDPE-B crystallizes first and forms volume-filling spherulites, which is closely followed by crystallization of the LDPE within the spherulites formed by LLDPE-B. Similar separate crystallization behavior was reported by Lee et al. (43) for blends of LLDPE-B (89,300 Mw, 3.8 PDl, and 15-16 branches per 1000 backbone carbons) with LDPEs (73,000-98,000 and 8.7-9.2 PDl 32-34 branches per 1000 backbone carbons). 

Figure 10-8. Time dependence of the number A sph of spherulites formed in melts of poly-(decamethylene terephthalate) at different temperatures. A sph is in arbitrary units.

Heterogeneous nucleation occurs when the crystallizing molecules or segments can wet the nucleus surface or deposit in cracks or holes on the nucleus. If the interaction between the nuclei and the crystallizing material is strong, then the number of nuclei or the spherulites formed from them is constant from the start of the crystallization and all spherulites are of the same size. 

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