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Crystallites lamellae

Explain the hierarchy of crystalline regions using the words unit cell, crystallite, lamellae, cylindrites and spherulites. [Pg.153]

Polymer fiber technology is a field of research branching out into many diverse applications. Fashion, healthcare, smart materials, electronics and load bearing elements both on then-own and in composite materials are just a selection of some of the areas where they can be found. Regardless of their application, the smallest practical size of any pol5rmer fiber can be specified as approximately 50 nm. This can be inferred from the fact that the typical size of a polymer crystallite (lamella) has dimensions in the order of 5 to 50 nm [4]. Below this value (the case of a polymer crystallite) the structure has become so small that it takes on the form of an ordered array of atoms rather than that of a fiber. However, in most melt crystallized polymers it is these lamellae that aggregate and grow linearly to form fibrils [4]. [Pg.644]

Fig. 14 shows the MW changes of PCL with in vitro degradation. M w decreased steadily from initial 58,700 to 7,000 after 200 weeks. After 133 weeks, the MW distribution became trimodal due to the selective degradation of amorphous zones and of the crystalllite edges. The MW of the three peaks were 2,600, 5,200 and 8,800, respectively, corresponding to one, two and three times the thickness of the crystallite lamellae. [Pg.103]

Figure 2 The lamellar substructure of a fibril. (a) Reciprocal positions of crystalline lamellae as a result of fiber annealing. (b) The situation after relaxation of stress affecting TTM. ai.2 - average angle of orientation of TTM CL - crystalline lamellae CB - crystalline blocks (crystallites) mF -border of microfibrils and F - fibril. In order to simplify it was assumed that (1) there are the taut tie molecules (TTM) only in the separating layers, (2) the axis of the fibril is parallel to the fiber axis. Figure 2 The lamellar substructure of a fibril. (a) Reciprocal positions of crystalline lamellae as a result of fiber annealing. (b) The situation after relaxation of stress affecting TTM. ai.2 - average angle of orientation of TTM CL - crystalline lamellae CB - crystalline blocks (crystallites) mF -border of microfibrils and F - fibril. In order to simplify it was assumed that (1) there are the taut tie molecules (TTM) only in the separating layers, (2) the axis of the fibril is parallel to the fiber axis.
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). [Pg.212]

The order in PA can be studied by WAXS and SAXS. WAXS gives insights into the unit cell sttucture, the crystallinity, the crystallite size (if not too large), and the crystallite orientation. SAXS gives a more accurate measure of the lamella thickness. In a temperature sweep, the changes in structure with temperature can be followed. [Pg.163]

The dimensions of crystallites vary widely some measure only a few nanometers in any direction, while others, known as lamellae , are platelets with lateral dimensions of several tens of nanometers and thicknesses of a few nanometers. The chain axes in lamellae typically span the thickness of the crystallite. With reference to the unit cell illustrated in Fig. 7.2 a), the c direction corresponds to the thickness of the crystallite. [Pg.137]

Primary crystallization occurs when chain segments from a molten polymer that is below its equilibrium melting temperature deposit themselves on the growing face of a crystallite or a nucleus. Primary crystal growth takes place in the "a and b directions, relative to the unit cell, as shown schematically in Fig. 7.8. Inevitably, either the a or b direction of growth is thermodynamically favored and lamellae tend to grow faster in one direction than the other. The crystallite thickness, i.e., the c dimension of the crystallite, remains constant for a given crystallization temperature. Crystallite thickness is proportional to the crystallization temperature. [Pg.141]

Nylon crystallites consist of sheets of chains that are hydrogen-bonded to their neighbors. On a supermolecular scale, crystallites have a lamellar structure, that is they are many times longer and broader than they are thick. When nylon crystallizes from an isotropic molten state, it generally forms spherulites, which consist of ribbon-like lamellae radiating in all directions... [Pg.364]

The volume inside the semicrystalline polymers can be divided between the crystallized and amorphous parts of the polymer. The crystalline part usually forms a complicated network in the matrix of the amorphous polymer. A visualization of a single-polymer crystallite done [111] by the Atomic Force Microscopy (AFM) is shown in Fig. 9. The most common morphology observable in the semicrystalline polymer is that of a spherulitic microstructure [112], where the crystalline lamellae grows more or less radially from the central nucleus in all directions. The different crystal lamellae can nucleate separately... [Pg.159]

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. [Pg.302]

Maltese cross (Blanshard, 1979). The crystallinity of starch is caused essentially by amylopectin pol)Tner interactions (Banks and Greenwood, 1975 Biliaderis, 1998 Donald, 2004 Hizukuri, 1996). An illustration of currently accepted starch granule structure is given in Fig. 5.5. It is believed that the outer branches of amylopectin molecules interact to arrange themselves into "crystallites" forming crystalline lamellae within the granule (Fig. 5.5 Tester et al., 2004). A small number of amylose polymers may also interact with amylopectin crystallites. This hypothetical structure has been derived based on the cluster model of amylopectin (Hizukuri, 1986 Robin et ah, 1974 Fig. 5.1). [Pg.228]


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See also in sourсe #XX -- [ Pg.387 , Pg.389 ]

See also in sourсe #XX -- [ Pg.387 , Pg.389 ]




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