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).

The most obvious question that needs to be answered about polymer crystallites is question (i) of section 5.1, How can long molecules give rise to small crystallites . Two principal types of answer have been given they lead to the fringed-micelle model and the chain-folded model for polymer crystallites. A further type of crystallite, the chain-extended crystal, can also occur when samples are prepared in special ways. These three types of crystallite are considered in the following sections. 

The third term is the crystallization energy of the B block. In this chain folding model, it can be expressed as... 

Arzhakov et al. proposed chain folding as an alternative to Kargin s bundle model to provide for eiongaticm of pcrfymeis in the glassy state. The bundle and simple chain folded models are wn in Figure 13. This latter model leads to polymer dimensions proporticmal to N and hence must be discarded in die fcmn depicted. 

Lindenmeyer does not illustrate his chain folded model but we assume it to be similar to that of Privalko and Lipatov, Fig. 14, and of Yehl°Ll02 shown in Figure 15. Lindenmeyer did quantify his model by estimating that one folded macromolecule would interact with at most 5-10 other macromolecules instead of 50-100 other macromolecules as in the random coil situation. 

The irregularly chain folded model appears consistent with a number of known facts about atactic polymers, as summarized in Table IV. This model does not appear inconsistent with Tjj phenomena but more consideration of this tentative conclusion is needed. 

As noted in Section V, Privalko and Lipatov questioned the conventional view of chain entanglement via random coils and proposed, without elaboration, a chain folded model as being more consistent with entanglement facts. We had also questioned (page 323 of ref. 1) without citing ref. 44, the seeming incompatibility of emanglement distances and a random coil modd. 

The several facts just presented do not offer proof that the chain fdding model of Privalko and Lipatov provides an explanation of N. phenomena. As we show in Table IV of Section VI, N. is one part of the cumulative evidence suggestive of a chain folded model for the amorphous state. It would still be necessary to examine if a folded model satisfies the known facts about rubber elasticity, including the energy term. (See latter part of Section VI.)... 

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