Folded chain lamellae

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. 

It is known that elevating the crystallization temperature [42,49] or annealing above the crystallization temperature [50] of PE results in a thicker folded-chain lamella of up to —200 nm. In addition to the higher temperature, if high pressure is applied, crystals can grow as thick as several micrometers in the chain axis direction [2-4,51]. 

Supramolecular structures formed during the crystallization of the melt under a tensile stress have already been described by Keller and Machin25. These authors have proposed a model for the formation of structures of the shish-kebab type according to which crystallization occurs in two stages in the first stage, the application of tensile stress leads to the extension of the molecules and the formation of a nucleus from ECC and the second stage involves epitaxial growth of folded-chain lamellae. 

Wunderlich30 and Zubov33 suppose that ECC under high pressures occur as a result of an isothermal thickening of folded-chain lamellae. However, this contradicts the later data of Wunderlich and of Japanese authors31 who have shown that folded-chain crystals (FCC) are formed after ECC, when the melt is cooled. According to Kawai22, crystallization under hydrostatic compression can he considered as a variant of the bicomponent crystallization. 

For linear PE, the initial structure formed is a single crystal with folded chain lamellae. These quickly lead to the formation of sheaflike structures called axialites or hedrites. As growth proceeds, the lamellae develop on either side of a central reference point. They continue to fan out, occupying increasing volume sections through the formation of additional lamellae at appropriate branch points. The result is the formation of spherulites as pictured in Figures 2.15 and 2.16. 

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... 

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. 

Further evidence for development of the folded-chain lamellae in the melt rather than by crystallization during cooling was obtained by reheating a previously well developed lamellar structure to 350 °C for an additional 30 min and then water-quenching it. As shown in Fig. 20, the lamellae remain well developed, with no sign of the fingered edges seen if quenched when first melted,... 

Many polymeric solids consist largely of folded chain lamellae and that the breadth of X-ray diffraction lines is caused by the crystallite size distribution and by the disorder within the lamella. 

The experiments of Kovacs et al. also demonstrate beautifully that although the extended chain crystal is the thermodynamically most stable form, when the undercooling is sufficient, kinetics favors folded chain lamellae. As we have seen, long chain polymers only crystallize at finite rates at high undercoolings, so only form folded chain structures. 

The morphology of crystalline isotactic polystyrene, i-PS, has been investigated by others, and they have concluded that i-PS normally crystallizes as stacks of folded chain lamellae which are arranged in volume filling spherulites. The melting point of lamellar polymer crystals depends on the lamella thickness, L, as follows (28 )... 

For polymers manifesting the most common type of crystalline morphology (folded chain lamellae), the "equilibrium" values (asymptotic limits at infinite lamellar thickness) of Tm, of the heat of fusion per unit volume, and of the surface free energy of the lamellar folds, are all lowered relative to the homopolymer with increasing defect incorporation in the crystallites. By contrast, if chain defects are excluded completely from the lamellae, the equilibrium limits remain unchanged since the lamellae remain those of the homopolymer, but the values of these properties still decrease for actual specimens since the average lamella becomes thinner because of the interruption of crystallization by non-crystallizable defects along the chains. 

We will now discuss the crystallization of polyethylene in more detail to illustrate some of the details previously mentioned. A linear polyethylene molecule, HDPE, will crystallize in an extended chain conformation in the crystallite if its molecular weight is below about 10,000 daltons. Above this molecular weight, the polymer forms folded chain lamellae, as shown in Fig. 3.40. The crystalline regions are closely packed chains that loop back on themselves. The region above and below the crystalline region is composed of two portions. That closest to the crystallite is... 

The crystallisation from strained melt as for instance in a blown film or in the jet during fibre spinning produces a row nucleated structure. " Linear nuclei are formed parallel to the strain direction. They contain more or less extended polymer chains. Secondary epitaxial nucleation on the surface of such linear row nuclei produces folded chain lamellae which are oriented perpendicular to the strain (Fig. 6). In such a case the sample exhibits a high uniaxial orientation of chain axes in the strain direction with random orientation of the a- and b-axes perpendicular to it. If the growing lamellae exhibit a helical twist the chain orientation in the strain direction is very soon replaced by the orientation of the axis of maximum growth rate (b-axis in the case of polyethylene) perpendicular to the strain direction and a more random orientation of the remaining two axes (a- and c-axes in the case of polyethylene) with a maximum in the strain direction. Such a row nucleated structure has parallel cylindrical spherulites (cylindrites) as its basic supercrystalline element. 

The row nucleated structure contains two types of crystals a small fraction of fibrillar crystals (row nuclei) with partially or even fully extended chains and the normal type folded chain lamellae. The existence of two types of crystals is detectable by calorimetry and the resistance to filmic nitric acid attack, high in the row nuclei and low in the surface layers of lamellae. The number of tie molecules between consecutive... 

What is the folded chain lamella An adjacent-reentry model A switchboard or a nonad-jacent-reentiy model ... 

In each case, the basic crystalline subunit is the folded-chain lamella, with a thickness of about 180 A. However, as the content of PS increases. 

Figure 6.10. Models of (a) single and (b) double folded chain lamellae in PEO/PS block copolymers. (Crystal et ai, 1970.)...

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