Spherulites origins

In quiescent homogeneous melts, crystallization proceeds by the formation of isotropic quasi-spherical clusters known as spherulite. Figure 1 illustrates the final structure of a crystallized polymer (PHB), showing clearly the different spherulites originating from nucleation, whose growth competing to fill the space was stopped by impingement. 

Spherulites have been observed in organic and inorganic systems of synthetic, biological, and geological origin, including moon rocks, and are therefore not unique to polymers. 

The molecular chain folding is the origin of the Maltese cross which identifies the spherulite under crossed Polaroids. The Maltese cross is known to arise from a spherical array of birefringent particles through the following considerations ... 

In another study [31] it has been reported that haze of LLDPE is mainly a consequence of light scattered by the spherulites. Thus, origin of poor clarity is not the same in LLDPE as in HP LDPE. In HP LDPE the haze is produced by surface defects coming from processing rheology [48,54,55]. Processing variables, therefore, do not have the same effect on LLDPE as they do on HP LDPE. 

Figure 15 Morphological map of linear polyethylene fractions. Plot of molecular weight against crystallization temperature. The types of supermolecular structures are represented by symbols. Patterns a, b and c represent spherulitic structures with deteriorating order from a to c. Patterns g and d represent rods or sheet-like structures whose breadth is comparable to their length g or display a different aspect ratio d. Pattern h represents randomly oriented lamellae. Neither h nor g patterns have azimuthal dependence of the scattering. Reproduced with permission from Ref. [223]. Copyright 1981 American Chemical Society. (See Ref. [223] for full details.) Note the pattern a is actually located as o in the figure this was an error on the original.

Another technique used for obtaining macroscopically polar films involves mechanical extension of the material. Uniaxial plastic deformation induces a destruction of the original spherulitic structure into an array of crystallites in which the molecules are oriented in the deformation direction. In case of PVF2 when such deformation takes place below 90 °C the original tg+ tg chains are forced into their most extended possible conformation which is all-trans [32]. 

Pecora, W.T., Hearn, Jr., B.C. and Milton, C., 1962. Origin of spherulitic phosphatic nodules in basal Colorado Shale, Bearpaw Mountains, Montana. U.S. Geol. Surv., Prof. Pap., 450 B30—B35. 

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