Folded lamellar crystal

Semicrystalline polymers may crystallize from solution, as well as from the melt, in the form of chain folded lamellar crystals. The high spatial resolution of ATM enables one to assess lamellar thicknesses from images of these lamellar crystals in edge-on and flat-on orientation. As discussed in this section, images of flat-on oriented lamellae are particularly suitable for a quantitative determination of lamellar thicknesses. 

Fig. 10.27 Illustration of the lateral growth profile of chain-folding lamellar crystals, with the secondary nucleation barrier at the top and the excess lamellar thickness harvested instantly at the root...

Chain folded lamellar crystals were described early 1950 by Prof. Andrew Keller who recognised that in flexible macromolecules the thermodynamically most favourable conformation is a chain folded lamellar crystal. The figure gives the SEM photo and a drawing of his model. 

Lamellar Crystals. Chain-folded lamellar crystals are regarded as the intermediate hierarchical level in polymer morphology between macromolecule and higher order multicrystal aggregates, such as spherulites. Because of the central importance of lamellae in the field of polymer crystallization and the relation of polymer crystallization to, eg, mechanical properties, lamellae have been widely studied by SFM in all important imaging modes. 

Crystallization from the melt also results in chain-folded lamellar crystals and for numerous polymers single lamellae have been studied by SFM approaches. Since typical lamellar thicknesses, depending on the polymer and the crystallization temperature, are in a range of 10 nm to several tens of nanometers, the finite size of the SFM probe tip leads to convoluted data, if lamellae are viewed edge-on (33). Among the coimtless examples are isotactic PS (157,158), PEO (159,160), PP (161-163), PE (164), polycarbonate (PC) (165), etc. 

Chain-Folded Lamellar Crystals from Periodic Polypeptides. 201... 

Figure 1.12 Crystallites with folded lamellar crystals of thickness f in the direction of the c axis for (a) regular folding and (b) irregular folding of the chain molecules.

Fig. 8.10. Blundell s schematic diagram of the morphologies above and below the crystal melting point for (a) rigid-chain nematic polymer, and (b) conventional polymer with chain-folded lamellar crystals. The thicker parts of the lines represent regions where the chains form 3D crystal lattices. ...

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. 

One of the most remarkable features of polymer crystallization is that such chain molecules can form lamellar crystals that contain heavily folded polymer chains. In experiments, the structural analysis of these lamellar crystals became possible when polyethylene single crystals were first prepared from a solution [100-102]. It was found that the orientation of the polymer chains... 

It is now possible to obtain the average lamellar thickness L. Assuming that the folds are incorporated into the lamellar crystal after a proper conformational straightening, and attributing a 0.127 nm axial advancement to each... 

Such a model for lamellar crystallization is now investigated. Initially the calculations are restricted to the simple lamellar model shown in Figure 1, but multiple fold lamellae are briefly considered even though they present a serious problem stemming from insufficient information about the number of folds. 

Electron diffraction by lamellar, single crystals leads to a two-dimensional, tetragonal unit-cell with a = b = 22.9 A (2.29 nm). From X-ray diffraction data obtained from a film of sedimented, lamellar crystals, it was found that the c axis spacing (7.8 A 780 pm) is equivalent to that in 6-fold and 7-fold amylose helices. The true helical diameters of the 1-butanol, isopropyl alcohol, and 1-naphthol complexes were calculated from experimental data. The ratios of 6 7 8 indicated that the 1-naphthol complex has eight D-glucose residues per turn. The diversity of helical orientations in V-amylose crystals was discussed. 

Note A lamellar crystal is usually of a thickness in the 5-50 nm range, and it may be found individually or in aggregates. The parallel-chain stems intersect the lamellar plane at an angle between 45° and 90°. The lamellae often have pyramidal shape owing to differences in the fold domains, as a result, one can deduce different fold planes and fold surfaces from the lamellar morphology. 

Note The sectors of lamellar crystals frequently represent fold domains. 

Crystallization from the melt often leads to a distinct (usually lamellar) structure, with a different periodicity from the melt. Crystallization from solution can lead to non-lamellar crystalline structures, although these may often be trapped non-equilibrium morphologies. In addition to the formation of extended or folded chains, crystallization may also lead to gross orientational changes of chains. For example, chain folding with stems parallel to the lamellar interface has been observed for block copolymers containing poly(ethylene), whilst tilted structures may be formed by other crystalline block copolymers. The kinetics of crystallization have been studied in some detail, and appear to be largely similar to the crystallization dynamics of homopolymers. 

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