Adjacent re-entry folds

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. 

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.)... 

Small-angle neutron scattering (SANS) studies on melt crystallized linear polyethylene, isotactic polypropylene, and poly(ethylene oxide) have shown that deuteriated molecules in a hydro-chain matrix have the same radius of gyration, and so molecular conformation, as in melt. This is evidently inconsistent with a regular adjacent re-entry folded chain lamellae. Recently Stamm et in comparing the scattering of neutrons at intermediate angles, of... 

Napolitano et al. [65] investigated the adjacent re-entry folds of chains of SPS crystallized in the P form by molecular mechanics. Various models of chain fold along bilayers have been found. The results are in agreement with the literature experimental data indicating that the fold surface is... 

Hamada, N., Tosaka, M., Tsnji, M., Kohjiya, S., Katayama, K. Crystallization of syndiotactic polystyrene in P-form. 2. Qnantification of stacking faults in the solution-grown single crystals. Macromolecules, 30(22), 6888-6892 (1997). Napolitano, R., Pirozzi, B. Models of adjacent re-entry folds in the beta form of syndiotactic polystyrene. Conformational and packing analysis by molecular mechanics. Macromol. Theor Simul., 11(5), 472-480 (2002). 

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)...

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. 

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. 

There is still considerable discussion about the existence of chain folding in semi-crystalline polymers." Using a Monte Carlo method, Flory and Yoon have shown " that adjacent re-entry could not reproduce the experimental results from polyethylene" and polypropylene" in the intermediate q range. An analytical computational technique, involving a model having a Gaussian distribution of inter-stem distances in the lamella, has been shown to fit these data" and can replicate the earlier Monte Carlo calculations. The root mean square, inter-stem distances derived from the fits are 22 A for polyethylene and 28 A for polypropylene. 

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