Chain-folded crystallization

Note Fibrous crystals may comprise essentially extended chains parallel to the fibre axis however, macroscopic polymer fibres containing chain-folded crystals are also known. 

The molecular processes that take place during polymer crystallization can be investigated from different viewpoints. A global approach determines the shape and conformation of chains in the polymer melt, and infers processes that lead to the lamellar chain folded crystal. Alternatively, analysis of the final crystal structure may help us to work backwards and gain insights into processes that take place in the last stages of crystal build-up. It is clear, however, that the two approaches should lead to the same conclusions, since the crystallization processes that occur should not, and certainly do not, depend on the viewpoint adopted for their analysis. 

There is experimental evidence in favour of two-dimensional secondary nucleation of chain-folded crystals. 

The crystallization of 3D-ordered crystalline phases from thermotropic mesophases, envisaged as stable pre-crystalline partially ordered intermediates, is an additional interesting issue which should be considered with care experimentally, theoretically, and with appropriate simulation approaches. Depending upon the nature of the mesophase it can be seen as a crystal-crystal transition or, for conformationally disordered, columnar mesophases, it approaches a true crystallization process. It is quite clear that the preexisting order will play a major role for example if the mesophase is chain-extended, bundle equilibria and chain-folding should not play any role. Indeed available experimental evidence supports this idea. Mechanistic and kinetic features should in general differ widely from the standard chain-folded crystallization processes yielding thin lamellar structures. In a number of cases (polyphosphazenes, polysiloxanes, see below) the crystalline polymorphs obtained from the chain-extended precursor differ from those obtained from solution. 

As mentioned earlier, in addition to /v-alkanes, pure monodisperse nylon 678 and nylon 6,679 oligomers have also been successfully synthesized with sufficient length to form chain-folded crystals. Once-folded chain crystals of nylon 6 nonamer and decanter,80-81 ancj twice-folded crystals of nylon 6 17-mer82 have been obtained from solution and studied by means of X-ray and electron microscopy and diffrac-... 

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