Chain folding regular

While, in recent years, many laboratories have demonstrated that nearly every synthetic polymer can crystallize in the form of single crystals (13) consisting of lamellae formed by regular chain folding (Figure 3), it is clear that extreme condi-tions-i.e., very slow crystallization or very dilute solutions, are required for these structures to form. Under normal conditions, such as those encountered in any industrial process, the polymer usually crystallizes in the form of less ordered, large structures, called spherulites. 

Fig. 5 Schematic drawing of single crystal with regular chain folding...

The discussion of the influence of the interphase need not be limited to just linear polyethylenes. Interphases of several nm have been reported in polyesters and poly-hydroxy alkanoates. One major difference between the interphase of a flexible polymer like polyethylene and semi-flexible polymers like PET, PEN and PBT is the absence of regular chain folding in the latter materials. The interphase in these semi-flexible polymers is often defined as the rigid amorphous phase (or rigid amorphous fraction, RAF) existing between the crystalline and amorphous phases. The presence of the interphase is more easily discerned in these semi-flexible polymers containing phenylene groups, such as polyesters. 

Fig. 11-4. Possible conformations of poly inerchains al the surfaces of chain-folded single crystals, (a) Adjacent reentry model with smooth, regular chain folds, (b) adjacent reentry model with rough fold surface, and (c) random reentry (switchboard) model.

Figure 2.17 Schematic representation of (a) fofd plane showing regular" chain folding, (b) ideal stacking of lamellar crystals, (c) interlamellar amorphous model, and (d) fringed micelle model of randomly distributed crystallites.

Most polymer crystals will exhibit facets and some like polyoxymethylene form hollow pyramids. The occurrence of the hollow pyramid is for similar reasons to that proposed for polyethylene and is a direct consequence of the constraints on the chain folding. The smooth surfaces observed for many crystal systems are evidence of regular chain folding, but are not proof that this occurs. 

Figure 2.17 Schematic representation of (a) fold plane showing regular chain folding, (b)...

The hollow pyramid shape typical of a single crystal of polyethylene indicates that the chain axis is not parallel with the normal of the lamella. The chain axis is generally at an angle, about 30°, with respect to the lamella normal. The reason for the chain tilt is essentially that a certain type of regular chain fold requires a small vertical displacement of the linear chain in the adjacent position (Fig. 7.14). 

The crystallographic (110) fold, which was first proposed by Bassett, Frank and Keller (1963), is essentially a path in the diamond lattice. Hence, the hollow pyramid itself is indicative of the dominance of regular chain folding in single crystals. Bassett showed that the fold surfaces in the no sectors were parallel to 312 planes and that the fold surfaces in the lOO sectors were parallel to 20l planes. A match between 312 and 20l is obtained only for a certain fixed ratio of H0 and lOO growth. A preference for such a growth ratio was indicated... 

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