Chain folded platelets

FIGURE 2.16 Spherulite structure showing the molecular-level lamellar chain-folded platelets and tie and frayed chain arrangements (a), and a more complete model of two sets of three lamellar chain-folded platelets formed from polyethylene (PE) (b). Each platelet contains about 850 ethylene units as shown here. 

Thermoreversible gels of ciystallizable polymers have been known for some time and are the topic of a recent review by Keller. An important contribution of Keller and co-workers was in identifying the physical associations, which are the network junctions, as being crystals. In particular, two kinds of crystallization were identified, the usual chain folded platelet crystals which are paniculate and form a turbid solution, and a second kind of crystallization which produces a... 

As a model of the nucleus in polymer crystallisation one often takes a rectangular prism. A breakthrough in this respect was the discovery and exploration of polymer single crystals (Schlesinger (1953) and Keller (1957)) which are indeed small prisms, platelets of polymeric chains, folded back and forth in a direction perpendicular to the basal plane (see Fig. 19.1)... 

In man-made fibres, any stretching will irreversibly alter the crystallinity and there is no control of the lateral size of polymer crystals. Semicrystalline polymer networks typically consist of platelet type crystals whose width exceeds their thickness by several order of magnitudes because only the thickness is controlled by the chain folding [61]. In contrast to synthetic fibres, spider silk does not need any mechanical treatment by external forces the constituents self-assemble directly during the spinning-process. These examples clearly demonstrate the need for more detailed control of the mesoscopic structures for further development of man-made materials. 

Polymer Crystals Crystalline regions (or crystallites) are plate-shape and have a chain-folded structure (Figure 14.12)—chains within the platelet are aligned and fold back and forth on themselves, with folds occurring at the faces. 

The chain direction within the crystal is along the short dimension of the crystal, indicating that the molecule folds back and forth, fire hose fashion, with successive layers of folded molecules accounting for the lateral growth of the platelets. 

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 crystal structures of fragment D from several recombinant fibrinogens has demonstrated that these mutant molecules are excellent models for the study of structure-function relationships, since the polypeptide chains are folded properly with only local changes (Kostelansky et al., 2002). Moreover, these studies allow testing of hypotheses about the effects of a particular mutation on structure and internal interactions within the molecule, which then can be correlated with the effects on fibrin polymerization or platelet aggregation (Kostelansky et al., 2004a,b). 

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