Loop reentry

The specific nature of the reentry loops is not the point of this illustration. The sketch shows both hairpin turns and longer loops. Problem 7 at the end of the chapter examines the actual nature of the reentry loop. 

FIGURE 6-3. The process of initiation of reentry. (1 a) Two pathways for impulse conduction, with bidirectional block in one pathway (shaded area), resulting in a non-viable reentrant loop. 

It is generally accepted that the predominant mechanism of atrial fibrillation and atrial flutter is reentry. Atrial fibrillation appears to result from multiple atrial reentrant loops (or wavelets), and atrial flutter is due to a single, dominant reentrant substrate (counterclockwise circus movement around the tricuspid annulus). Atrial fibrillation or flutter usually occurs in association with forms of organic heart disease that causes atrial distension. Forms of heart disease that commonly lead to atrial stretch and precipitate atrial fibrillation or flutter include... 

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 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]. 

Fig. 14.4. Reentry distribution at the undeformed state of polyethylene as a function of reentry orientation at two temperatures light bar 350K, dark bar 450K. The last entry rest lumps together all the remaining loops not explicitly considered in the other sets. The shortest reentry vector length in a particular direction increases from left to right. See text for notation. Reproduced from [29] with written permission from ACS Publications...

Physical Properties. All polyethylene above 0.86 g/cm density is semicrystalline. The basic crystalline structure for most commercial LLDPE is chain-folded lamellae (Fig. 7). The body of the crystal consists of polymer backbone segments, and the surfaces are a collection of chain folds, loose cilia, and tie chains (chains incorporated into more than one crystal). When crystallized isothermally, it has been foimd that 95% of the lamellae in a given sample are within 5% of the same thickness (10). There is some debate over the mechanism of chain folding and of the subsequent fold loops. The most likely model includes adjacent reentry, loose adjacent reentry, and nonadjacent reentry. Short-chain branch length... 

The structure of the interphase has already been discussed in some detail. It is a diffuse region that extends some 10-15 A above the basal plane. Illustrated in the schematic representation are sequences in adjacent reentry, neighboring returns, long loops as well as sequences of units which escape the interfacial region. The chain dispositon within the interphase leaves some vestiges of order. " ... 

Figure 11 (a) Loose loops with adjacent reentry. (From Ref. 96.) (b) Composite model. 

Figure 19 (a) Tight folds with adjacent reentry. (From Ref. 33.) (b) Switchboard model. (From Ref 34.) (c) Loose loops with adjacent reentry. (From Ref 96.) (d) Departure of chains from immediate environs of crystallite. (From Ref 45.)... 

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