Adjacent re-entry

Figure 7.9 Mechanisms of laying down chain segments on the growing face of a crystallite a) tight folds with adjacent re-entry, b) independent deposition of chain segments... 

Fig. 8. Schematic representation of the folded chain model of crystalline domains in thermoplastics. a Lamella crystallized from dilute solution (multiple/adjacent re-entry) and b Lamella crystallized from melt (single entry/far distance re-entry)...

The implications of adjacent re-entry is realised in the drawability of the solution crystallized film of UHMWPE and the strain hardening behaviour of branched ethylene-octene copolymers, for example. 

The melt-crystallised sample skipped Process 1 due to the limited relaxation of its entangled molecules on the lamellar surfaces, rather than the adjacent re-entry folding for the solution-crystallised sample. 

Using atomic force microscopy (AFM), lamellar surface of normal and cyclic alkanes C33H68, C36H74, (CH2)48, and (CH2)72 was examined to atomic scale.114 In the cycloalkanes, the observed images are consistent with adjacent re-entry 100 folds in a monoclinic crystal structure. 

Figure 2.20 (a) Model of a lamellar crystal showing regular, adjacent re-entry folds, (b) Model of fold plane illustrating chain folding with imperfections which may occur in the structure. (From Ref. 21.)... 

Fig. 5.4 Schematic diagrams showing (a) reguiar chain foiding with adjacent re-entry, as envisaged for a perfect chain-foided crystaiiite and (b) the switchboard modei.

At higher degrees of super cooling, the fold surface, proposed in model B (Figure 8.14), is more likely. The evidence for irregular folding with adjacent re-entry is based on ... 

Moreover, the crystals are very thin, between 4 and 6 nm depending on crystallization temperature. This represents only six to ten crystallographic repeat units of two monomers each and, since the sample is known to be 80% crystalline, leaves just three or four monomer units in the amorphous phase to perform a complete 180° turn and allow the chain to re-enter the crystal. Clearly there is little room in the amorphous fold to allow anything but adjacent re-entry and this leaves the question of why the chains apparently zig-zag along the a-axis instead of continuing to fold linearly at 45° to it. 

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