Development of Spherulites

What defines a spherulite is its shape and structural symmetry. All radii of an ideal spherulite are equivalent, at least at lengths greater than the 0.1-pm resolution of optical microscopy. The spherical (or circular) envelope of growth fronts is established by advance at rate G of separate fibrillar crystals into the melt from a common nucleus. This ideal shape is altered by growth restrictions presented by other spherulites (impingement/ truncation) or by melt interfaces (two-dimensional spherulites). 

Polymer spherulites have two additional features. The radially growing crystals are not fibrillar, but ribbon-Uke, with chain folding establishing the smallest dimension /c, which is itself an inverse function of undercooling. While difficult to establish with certainty, the internal 

This section discusses the primary nucleation and subsequent radial growth of spherulites. The latter concentrates on mechanisms by which an increasing number of ribbon-like crystals appear at larger distances from the spherulite center. The intriguing topic of lamellar twist is deferred to Section 3.4.4. 

As described briefly in the introduction to this section, spherulite growth requires approximately radially directed fibrillar or ribbon-like crystals and the creation of more crystals at larger distances from the center. Any new crystals spawned from existing ones must splay from the parent lamella or from one another in order to fill the gaps between diverging dominant lamellae. Keith and Padden in 1963 attributed what they 

Neighboring fingers with differently oriented defects were posited to grow in different directions, giving rise to noncrystallographic low-angle branching. 

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Carr, S. H., Geil, P. H., Baer, E. The development of spherulites from structural units in glassy poly(bisphenol-A-carbonate). J. Macromol. Sci.-Phys. B2,13-28 (1968). 

FIGURE 8-59 The initial development of spherulites (redrawn from D. C. Bassett, Principles of Polymer Morphology, Cambridge University Press, 1981). 

Many polymers crystallize in the form of spherulites. Hedrites, as early stages of development of spherulites, can reveal information about the growth and development of spherulites in time. As shown below, the morphological analysis is often... 

Figure 1. Cross polarized photomicrographs of films of EO-Is-EO block copolymers with relatively large fractions of EO segments revealing less perfect development of spherulitic crystalline texture as the Is content increases...

Figure 16.8 shows the development of spherulitic morphology in the 50/50 iPP/ EPDM blend (6) upon cooling from 230°C to ambient at a slow cooling rate of 0.5°C min As depicted in Fig. 16.8a, the polarized optical micrograph under the cross... 

A mathematical phase-field model for the kinetics of isothermal polymorphic crystallization has recently been proposed [47], according to which crystallization involves rapid relaxation of the metastable state followed by nucleation and growth of the polycrystalline phase. Computer simulations were used to obtain results which could be tested experimentally using X-ray scattering experiments. Growth rates of different polymorphic polymers have also been investigated [48]. Simultaneous development of spherulites of different polymorphs occurs at different rates under isothermal conditions. From observation of interspherulitic boundaries between the a- and y-forms of polypivalolactone. 

Another cause for tennis racket type patterns is the incompleteness of development of spherulites. Spherulites nucleate from bundle-like crystals which evolve into sheaves and eventually into complete spheres. Such evolution has been idealised by a model of sectors of spherulites" " ... 

The bulk haze contribution was measured by applying a very thin layer of silicone oil to the film surface prior to measurement. The predominant contribution to the haze was from the surface, and was the result of surface roughness. The surface haze of metallocene-catalysed PE films was increased by the development of spherulitic-like snrface features, which increased the surface roughness. However, Ziegler-Natta catalysed PE, exhibited relatively low total and surface haze values, which were attributed to the relatively smooth surface texture. 

Fig.65. Development of spherulite size ( ) with time for PCL crystallisation in blends with SMA with 90 wt % PCL, (A) SMA-14 and (O) SMA-25 taken from [148]...

In the following sections the development of spherulitic morphologies will be discussed both from a historical point of view and in the light of recent investigations. 

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