Semicrystalline polymers crystallization under quiescent

The major feature of polymers that have been bulk crystallized under quiescent conditions are polycrystalline structures called sphemlites. These are roughly spherical supercrystalline structures which exhibit Maltese cross-extinction patterns when examined under polarized light in an optical microscope. Spheruliies are characteristic of semicrystalline polymers and are also observed in low-molecular-weight materials that have been crystallized from viscous media. Sphemlites are aggregates of lamellar crystallites. They are not single crystals and include some... 

Supaphol et al. [216-218] reviewed a number of hypotheses that have been proposed to explain the occurrence of multiple melting endotherm phenomena. In the studies on isothermal crystallization under quiescent conditions (i.e., crystallization is only a function of temperature), the multiple melting behavior of these semicrystalline polymers have been attributed to the following four phenomena (1) the presence of two (or more) crystal modifications in the sample (2) the presence of two (or more) crystalline morphologies (3) the presence of two populations of crystal lamellae of different thicknesses and (4) the simultaneous melting, recrystallization, and re-melting of the lamellae initially formed at the crystallization conditions. 

Figure 4. Crystalline morphologies of semicrystalline polymers for different processing conditions (a) crystallized from a quiescent melt (spherulitic structures) (b) crystallized under moderate stress...

In the formation of crystals, polymer chains fold back and forth to form the crystalline lamellae. The crystalline lamellae and the amorphous phase are arranged in semicrystalline morphological entities, ranging from a micron to several millimeters in size. The most common morphologies that can be found in injection-molded polymers are spherulites, which usually form under quiescent conditions, and shish-kebab structures, which may appear under shear flow [see, for example, Eder and Janeschitz-Kriegl (1997), Zuidema et al. (2001) and Janeschitz-Kriegl (2009)]. 

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