Crystallinity folded-chain crystallites

Numerous studies of the structure and properties of drawn crystalline polymers have led to the microfibrillar model of fibrous morphology177 179 180. According to Peterlin 179) and Prevorsek et al. 180), the long and thin microfibrils are the basic elements of the fibrous structure. The microfibrils consist of alternating folded chain crystallites and amorphous regions. The axial connection between the crystallites is accomplished by intrafibrillar tie-molecules inside each microfibril and by inter-fibrillar tie-molecules between adjacent microfibrils. 

Semicrystalline polymers contain liquid-like amorphous and ordered crystalline phases. When solidified from the pure melt, these polymers show a spherulitic structure in which crystalline lamellae composed of folded chain crystallites radiate from the center of the spherulite in such a way that a constant long period or crystallinity is apvproximately maintained. The amorphous regions reside in the interlamellar regions in the form of tie chains, whose ends are attached to adjacent lamellae loop chains, whose ends are attached to the same lamella cilia chains with only one end attached to a lamella (or dangling chain ends), and floating chains which are not attached to any lamellae. This hierarchical structure is illustrated in Figure 1. 

The value of angles used in SAXS is from 1° to 5°. SAXS provides information on greater interatomic distances from 50 to 700 A. Consequently, SAXS is useful in detecting larger periodicities in a structure. For example, many polymeric materials crystallize with individual chains folding back and forth within a given crystalline region or crystallite [3]. Other examples of the utility of SAXS are in the study of lamellae crystallites or in the distribution of particles or voids in the material. 

It is now generally accepted that folding is universal for spontaneous, free crystallisation of flexible polymer chains. It was first of all found in crystallisation from very dilute solutions, but it is beyond doubt now, that also spherulites, the normal mode of crystallisation from the melt, are aggregates of platelike crystallites with folded chains, pervaded with amorphous material. "Extended chain crystallisation" only occurs under very special conditions in the case of flexible chains for rigid polymer chains it is the natural mode ("rigid rod-crystallisation" from the melt in case of thermotropic polymers, and from solution in case of the lyotropic liquid-crystalline polymers both of them show nematic ordering in the liquid state). 

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. 

The fundamental concept of molecular chains of cellulose extending along the direction of microfibrils, to form crystalline and amorphous or paracrystalline regions, was well accepted and set the trend for interpretation of all relevant data. However, folded-chain conformations have since been advocated as a strong possibility. Also, several other models based on the extended-chain concept have been proposed in order to define better the distribution of crystallites and the disposition of the molecular chain within the microfibrils. 

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 crystalline structure of PVA has been discussed in detail by Bunn [14]. On a molecular level, the crystallites of PVA can be described as a layered structure [15, 16]. A double layer of molecules is held together by hydroxyl bonds while weaker van der Waals forces operate between the double layers. A folded chain structure of PVA chains leads to small ordered regions (crystallites) scattered in an unordered, amorphous polymer matrix. Values representative of the crystallinity and thermal properties of PVA have been reported [4]. The crystalline melting range of PVA is between 220 and 240 °C. The glass transition temperature of dry PVA films has been reported at 85 °C. In the presence of water (and other solvents), the glass transition temperature decreases significantly [2]. 

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