Crystal lamella chain folding

The earliest concepts of crystallization of semicrystalline polymers were obtained by studies of polymers crystallized from very dilute solutions and at atmospheric pressures. Electron microscopy studies revealed that the large macromolecules formed lamellar-shaped single crystals, which had a thickness of 5-50 nm, much less than the length of the extended chain. In 1957, Keller [18] and Fischer [19] proposed that the chains fold back and forth, forming folded chain lamellar crystals, as shown in Figure 19.1. In melt-crystallized lamellae, chain folding is also observed, except there are other conformations, such as loose chain folds and tie molecules, that exit one lamella and enter an adjacent... 

Moreover, the amorphous and crystalline components are not independent of each other. The amorphous surface layers between adjacent lamellae are formed by free chain ends, chain folds, and tie molecules. The free chain ends are fixed at the other end in the crystal lattice. Chain folds and tie molecules are fixed at both ends, the former on the same lamellae, the latter on different ones. These constraints modify deeply the properties of the amorphous layer. Even above Tgi the mobility of the amorphous chains is substantially less than in a rubber with the same average number of mers in the chain segments between subsequent crosslinks. 

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)]. 

The single crystal of a polymer is a lamellar structure with a thin plateletlike form, and the chain runs perpendicular to the lamella. The crystal is thinner than the polymer chain length. The chain folds back and forth on the top and bottom surfaces. Since the fold costs extra energy, this folded chain crystal (FCC) should be metastable with respect to the thermodynamically more stable extended chain crystal (ECC) without folds. 

For density values g > 0.92 g/cm3 the deformation modes of the crystals predominate. The hard elements are the lamellae. The mechanical properties are primarily determined by the large anisotropy of molecular forces. The mosaic structure of blocks introduces a specific weakness element which permits chain slip to proceed faster at the block boundaries than inside the blocks. The weakest element of the solid is the surface layer between adjacent lamellae, containing chain folds, free chain ends, tie molecules, etc. 

Then we extended the 2D-model to a 3D one [21]. We considered crystallization of a single polymer chain C500 from a vapor phase onto a solid substrate, taking into account detailed interactions between the chain and the substrate. Though the polymer molecule in a vacuum was collapsed, like in a very poor solvent, under the influence of bare van der Waals interactions between atoms, the molecule was found to show quick adsorption and crystallization into a rather neat chain folded lamella. 

Finally, we were led to the last stage of research where we treated the crystallization from the melt in multiple chain systems [22-24]. In most cases, we considered relatively short chains made of 100 beads they were designed to be mobile and slightly stiff to accelerate crystallization. We could then observe the steady-state growth of chain-folded lamellae, and we discussed the growth rate vs. crystallization temperature. We also examined the molecular trajectories at the growth front. In addition, we also studied the spontaneous formation of fiber structures from an oriented amorphous state [25]. In this chapter of the book, we review our researches, which have been performed over the last seven years. We want to emphasize the potential power of the molecular simulation in the studies of polymer crystallization. 

Molecular processes at the growth surface of the crystal are one of our greatest concerns. By melting a chain-folded lamella at 600 K for 200 ps, we prepared a 2D random-coil of the molecule. The random-coil was then instanta-... 

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