Folded-Chain Lamellar Crystals

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... 

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. ...

Because of chain folding, melt-crystallized polymers are not as strong as they could be. One can envisage that under a load a sample will at some point yield, with chains in the amorphous domains becoming oriented in the draw direction while the lamellar arms of the spherulite undergo shear and whole sections are pulled ont. This process is illustrated in Figure 8-65. 

In Chapter 8, in the section on morphology, we have seen that polymers are only partially crystalline and when crystallized from solution or the melt they form chain-folded lamellar structures, as illustrated schematically in Figure 10-17 earlier. This is a consequence of the kinetics of crystallization, which we will explore here. 

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.

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