Adjacent chain-folding model

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

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).

Fig. 10.11 Illustration of the metastable polymer conformation in the crystalline regions. From left to right are the fringed-micelle model, the lamellar crystal with adjacent chain folding, the switchboard model and the variable-cluster model...

The result was a type of block copolymer with alternating epoxy and double-bonded segments. NMR analyses showed that for the two samples studied the chain-folded portion was about 2.4 and 3 mers thick, whereas the stems were 15.2 and 40.8 mers thick, respectively. Since the number of mer units to complete the tightest fold in this polymer has been calculated to be about three (51), the NMR study strongly favors a tight adjacent reentry fold model for single crystals. 

Fig. 4.2 Illustration pictures on the models of (a) fringed-micelle (Hermann et al. 1930) and (b) adjacent chain-folding (Keller 1957)...

Model of crystallinity in which chain folds regularly connect model adjacent stems. 

The structure of the H8 helix and the adjacent sequence in 306-311 is essentially identical in the A and B chains of the crystal lattice (Fig. 4B). The differences in the computed values of fsa between the A and B molecules along this sequence (Fig. 12B, upper panel) are simply due to the absence of the C-terminal domain in the B chain model, which in the A chain folds back along H8 and partially occludes side chains from about N311 to T320. 

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.

By following the discussed experiments, a model for the Ir-Cu(lOO) surface has been suggested and is presented in Fig. 14. As a matter of fact, a two dimensional epitaxial sub-surface alloy has developed and consists of adjacent chains of Ir and Cu atoms along the [Oil] directions to form an ordered (2x1) periodicity. The Ir-Cu sub-surface layer happens to be buried under a monolayer of copper. Remarkably enough, although the surface crystallography of Cu(lOO) expresses four-fold symmetry, a two fold symmetric pattern is showing up for the chains of subsurface Ir to resemble the (2x1) superstructure. 

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.)... 

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. 

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. 

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