Polymer chains lamellae folding

The solution-grown crystal (SGC) from dilute solution forms diamond-like crystals of lateral dimension up to several micrometres, but their thickness is about 10 nm in which polymer chains are folded. The chain axes are parallel to the thickness direction of these lamellar crystals. In SGC materials precipitated from dilute solutions (Figure 3A), the lamellae are stacked and oriented parallel to the material surface. 

Design of Folded-Chain Lamellae, Folded-chain lamellar crystals are a dominant morphological feature of natural and synthetic polymers of repetitive chemical structure. In synthetic polymers, the folded-chain structure is dictated by the kinetics, rather than the thermodynamics, of the crystallization process, and is metastable. In our early experiments... 

Polymer crystals most commonly take the form of folded-chain lamellae. Figure 3 sketches single polymer crystals grown from dilute solution and illustrates two possible modes of chain re-entry. Similar stmctures exist in bulk-crystallized polymers, although the lamellae are usually thicker. Individual lamellae are held together by tie molecules that pass irregularly between lamellae. This explains why it is difficult to obtain a completely crystalline polymer. Tie molecules and material in the folds at the lamellae surfaces cannot readily fit into a lattice. 

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. 

The formation of the microstructure involves the folding of linear segments of polymer chains in an orderly manner to form a crystalline lamellae, which tends to organize into a spherulite structure. The SCB hinder the formation of spherulite. However, the volume of spherulite/axialites increases if the branched segments participate in their formation [59]. Heterogeneity due to MW and SCB leads to segregation of PE molecules on solidification [59-65], The low MW species are accumulated in the peripheral parts of the spherulite/axialites [63]. The low-MW segregated material is brittle due to a low concentration of interlamellar tie chains [65] and... 

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. 

The. folded-chain lamella theory arose in the last 1950s when polymer single crystals in the form of thin platelets termed lamella, measuring about 10,000 A x 100 A, were grown from polymer solutions. Contrary to previous expectations, X-ray diffraction patterns showed the polymer chain axes to be parallel to the smaller dimension of the platelet. Since polymer molecules are much longer than 100 A, the polymer molecules are presumed to fold back and forth on themselves in an accordionlike manner in the process of crystallization. Chain... 

The small ratio of lamellar thickness to the contour length of a polymer molecule clearly implies that chains must fold back and forth into stems with chain direction essentially perpendicular to the lamellar surface, as originally declared by Storks. The large surfaces of the lamellae containing the chain folds are called fold surfaces, and the thin surfaces are called lateral surfaces. 

Electron diffraction measurements indicate that polymer chains are generally oriented normal or very nearly normal to the plane of the lamellae [13,14]. As the molecules in the polymer are at least 1000 A long and the lamellae are only about 100 A thick, the most plausible explanation is that the chains are folded [13,15], Figure 22.3 illustrates the proposed models of the fold surface in polymer lamellae [13,16],... 

LLDPE rapidly crystallizes from the melt with the formation of sphemlites, small spherical objects 1-5 im in diameter visible only in a microscope. The elementary structural blocks in spheruliles are lamellae, small flat crystallites formed by folded linear segments in LLDPE chains, which are interconnected by polymer chains that pass from one lamella to another (lie molecules). Crystalline lamellae within sphemlites give LLDPE articles necessary rigidity, whereas the large amorphous regions between lamellae, constituting over 60% of tile splierulile volume, provide flexibility. 

Spherulites. As a polymer melt solidifies, several folded chain lamellae spherulites form which are up to 0.1 mm in diameter. A typical example of a spherulitic structure is shown in Fig. 1.15. The spherulitic growth in a polypropylene melt is shown in Fig. 1.16. 

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