Adjacent reentry

The mesomorphous phase, also called an intermediate phase or a mesophase, is formed by molecules occurring in surface layers of the crystallites. It can be assumed that the mesophase is made up largely by regularly adjacent reentry folds. However, it cannot be excluded that the mesophase is also composed of some irregular chain folds, which are characterized by a long length and run near the crystal face in the direction perpendicular to the microfibril axis. 

There has been a sharp debate for many years on the best description of the real macroconformation. Much experimental research has been carried out on pure polymers using different techniques (225) [small angle and intermediate angle neutron scattering (226), electron microscopy, IR, etc.]. Yoon and Flory (40, 228-231) and Gawrisch et al. (232) held the view that the probability of adjacent reentry in polymeric lamella is rather low (<50%) and does not justify the validity of such a model. The trajectory of the chain extends across numerous lamellae and its macroconformation is not far from that of the random coil. In the view of Keller and co-workers (224, 233-236) the adjacent reentry, although not complete (3 1 with respect to other possibilities) largely prevails. 

A well-defined amount of co-crystallization is possible across the interface of two adjacent crystals by annealing two stacked, completely wetted, solution-cast films of UHMW-PE [32]. It was found that doubling of the lamellae across the interface enhances the peel energy to such a level that the films could not be separated anymore. By contrast, pre-annealing one side of the film prohibited co-crystallization across the interface and these films could still be separated easily. It was therefore concluded that a limited amount of chain diffusion across the interface occurs during doubling of the lamellae, as facilitated by the well-defined structure of the interphase due to the adjacent reentry that occurs upon crystallization from solution. 

FIGURE 8-56 Chain folding models (top) adjacent reentry and (bottom) switchboard. 

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 2.7 Folded-chain models for single crystals, (a) Regular adjacent reentry (b) nonregular random reentry.

From all these data analyses, we can definitely say that the D and H chain stems are distributed statistically randomly in the crystalline lamellae of the D/H cocrystallized blend. This conclusion is quite important in relation with the chain-folding problem, a controversial research theme that had been discussed for a long time (30). The random distribution of the D and H chain stems naturally supports the idea that the D and H chains reenter randomly into and out of the crystalline lamellae as shown in Fig. 5.7. The regular adjacent reentry model is impossible to apply at all as for as the melt-crystallized sample is concerned. 

Figure 5.7 Illustration of the chain-folding models, (a) A regular adjacent reentry model, (b) a random reentry model, and (c) a cluster model consisting of the mixed structures of (a) and (b). (From Reference 46 with permission from the Society of Polymer Science, Japan).

In Sect. 3 a set of experimental data on cyclic molecules is described supporting the basic discussions of Sects. I and 2. Central are the cycloalkanes that ultimately serve as a model for adjacent reentry, sharply folded polyethylene crystals. Chain-folded polyethylene was shown in the early 1960 s to thicken in the crystalline state by straightening as many as 100 to 1000 folds when brought to elevated pressure and temperature. This surprising observation found its explanation in the fast reptation possible in the condis-crystal state. 

Figure 5. Schematic of chain organization in lamellar single crystals. The chain folds are shown as regular with adjacent reentry although larger loops at the folds are also a possibility. Based on the known degree of polymerization of the nigeran (ca. 1000—2000 anhydroglu-cose units), the chain orientation in the crystals and crystals dimensions, chain folding must occur (14).

The regular folded array with adjacent reentry of the ehains, but with some loose folding and emergent chain ends or cilia that contribute to the disordered surface... 

FIGURE 11.5 Schematics of possible chain morphology in a single polymer crystal (a) regular folding with adjacent reentry of chains, (b) switchboard model with random reentry of chains. 

For very low degrees of super cooling, model A is postulated to predominate. The evidence for regular folding with adjacent reentry is ... 

The evidence cited in support of the model B structure also supports the model C fold surface structure. In addition, Flory and Yoon [19] have argued that the steric hindrance to forming a sharp bend over a few chain atoms precludes the prevalence of adjacent reentry. 

The relative amount of random reentry to adjacent reentry will vary with polymer chain structure. The energy required for a chain to form a bend determines the extent to which adjacent reentry is favored. The lower the energy expended to form a tight bend, the more adjacent reentry loops will occupy a fold surface [20]. 

A final difficulty with the adjacent-reentry model was thought to be of a kinetic nature in that the reeling-in of long-chain molecules from the melt onto a growth face of a lamella was considered to be far too slow to account for the observed growth rates (Flory and Yoon 1978). 

Finally, to more specifically explore the nature of the amorphous component of spherulitic polyethylene and the nature of the fold surfaces of the lamellae, Hoffman and collaborators (Guttman et al. 1981 Guttman and di Marzio 1982) performed a probabilistic choice computation on whether molecules in the amorphous component undergo adjacent reentry, undergo non-adjacent reentry... 

As discussed in Section 2.7.2 above, in chain-folded linear-chain polymers with spherulitic morphology, where lamellae could have a high concentration of chain folds with adjacent reentry, annealing of the polymer close to the melting... 

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