Folded lamella

According to Hosemann-Bonart s model8), an oriented polymeric material consists of plate-like more or less curved folded lamellae extended mostly in the direction normal to that of the sample orientation so that the chain orientation in these crystalline formations coincides with the stretching direction. These lamellae are connected with each other by some amount of tie chains, but most chains emerge from the crystal bend and return to the same crystal-forming folds. If this model adequately describes the structure of oriented systems, the mechanical properties in the longitudinal direction are expected to be mainly determined by the number and properties of tie chains in the amorphous regions that are the weak spots of the oriented system (as compared to the crystallite)9). 

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-... 

The structure of the fold surface has long been a most controversial topic (ever since the finding of the chain folded lamellae). What kind of fold structure will the direct MD simulation predict Figure 29 shows (a) the crystalline domains and (b) the fold loops at 330 K after a sufficiently long time period of 38.4 ns the crystallinity reaches about 52%. We noticed that most of the fold loops near the substrate are rather short. The presence of looser and longer loops and abundant cilium in the middle of the MD cell is obviously... 

Such a model for lamellar crystallization is now investigated. Initially the calculations are restricted to the simple lamellar model shown in Figure 1, but multiple fold lamellae are briefly considered even though they present a serious problem stemming from insufficient information about the number of folds. 

Yamamoto, T. Molecular Dynamics Modeling of the Crystal-Melt Interfaces and the Growth of Chain Folded Lamellae. Vol. 191, pp. 37-85. 

Usually, synthetic polymers crystallize11 j15 from a melt or a solution in form of folded lamellae. Under specific circumstances it is sometimes also possible to obtain extended chain crystals which is the preferred arrangement in the crystallites of many natural polymers (cellulose, silk). Recently it has been found33 31 that in some cases another crystalline modification can be obtained, the so-called shish-kebabs, which are a sort of hybrid between folded lamellae and extended chain crystals. These shish-kebabs are obtained by shear-induced crystallization, a process in which the polymer crystallizes from solution under the influence of an elongated flow. 

Among the numerous challenges faced in understanding the formation and evolution of hierarchical structures in polymer crystallization, we restrict ourselves to explain the essential basic features of folded lamellae. Specihcally, we consider (1) molecular origin of enhanced scattered intensity before any crystallographic features are apparent, (2) spontaneous selection of small lamellar thickness, (3) molecular details of growth front, and (4) formation of shish-kebab structures in the presence of a flow. 

The shape of macromolecules within a folded lamella is not the same for all polymers. In crystalline polyethylene, for example, the chains assume a planar zigzag conformation, but in some other polymers like polypropylene and polyoxymethylene the chains prefer a helical shape, as in proteins. The helix might have three, four, or five monomer units per turn, i.e., the helices are three-, four-, or five-fold (Fig. 1.12)... 

By variation of the conditions of crystallization (see Sect. 1.3.3.3) polyethylene can be obtained either as folded lamellae, as extended chain crystals (high strength fiber), or as so-called shish kebabs (fibrils with a morphology similar to cellulose). All these variants differ in properties. 

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