Morphology of Polymer Crystals

When polymers crystallize at high temperatures, due to the difficulty of spontaneous crystal nucleation, the practical crystal nucleation often utilizes the foreign surfaces provided by the impurities. The sizes of the heterogeneous nuclei are normally large, so they can induce the growth of multi-layer lamellar crystals. One can see that the multi-layer lamellar crystals spread from the same center, just like an opened book. Such morphology of multiple stacking of lamellar crystals is called axialites, as illustrated in Fig. 10.17. 

If we take the radiation-growing axialites at high temperatures as the dominant lamellae, and the empty spacing is filled with the subsidiary lamellae grown at low temperatures, we can obtain the sphere-like crystals with dense filling, often called the type-I spherulites. Besides this kind of spherulites obtained by sequential formation of dominant and subsidiary lamellae during cooling, there exists another 

There are still hot ongoing debates about the formation mechanism of shish structures in the oriented flow field. Recently, Hashimoto and his coworkers proposed a new scenario for flow-induced phase transitions in polymer solutions to form the hierarchical structures of shish-kebabs (Hashimoto et al. 2010 Murase 

Sometimes, the shish fibers can be so thin that they are invisible under the present microscopes, leaving an array of parallel-oriented lamellar crystals, called the row-structure. The molecular simulations demonstrated that even a single prealigned polymer chain can play the role of shish in inducing the growth of kebab crystals, as demonstrated in Fig. 10.20. 

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The morphology of polymer crystals has been studied extensively by many experimentalists, and an enormous variety and richness has been unveiled (see e.g. Ref. [5]). It is beyond the scope of this review to lay out the evidence in detail. We can only concentrate on some selected points and refer the interested reader to the literature, in particular the review articles by Keller [6], Bassett [7], Dosiere [8] and most recently Phillips [9],... 

G. Reiter et al. Morphologies of Polymer Crystals in Thin FilmSy Lect. Notes Phys. 714, 179-200 (2007)... 

The here presented examples of morphologies of lamellar polymer crystals strongly suggest that the concept of DLA is the most appropriate way of describing how these crystals have been formed. As this concept can be ap-pUed to most crystalhsing materials, we conclude that polymer crystallisation can be treated in such a unified way. It is not necessary to invoke special approaches to explain the various morphologies of polymer crystals. 

Wunderlich (1973, 1976, 1980) has discussed the questions of the thermodynamics, kinetics, and morphology of polymer crystallization. 

Polymers and TAGs crystallize far more quickly when they are exposed to a shear. This phenomenon has been observed experimentally [26-30]. Crystallization under shear is still not fully understood on the theoretical side, yet this is a key parameter for the modeling of industrial processes. A first simple idea is that in the presence of shear the chains of the molecules rotate faster in the liquid and are therefore more easily incorporated into the intracrystaUine network. Morphologies of polymer crystals vary depending on the shear They crystallize preferentially in lamellas under shear whereas they are mostly crystallized as spherulites if they solidify statically, i.e., in quiescent conditions. 

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