Random re-entry

Flory based his ideas of re-entry on the tried and tested assumption that the coil follows random coil statistics in the melt. It is clear from the SANS data that the chains fold much more tightly than would be predicted by a random re-entry model.Infrared spectroscopy showed that 75% of the folds... 

However, Guttman et a/. claimed that the model used by Flory and Yoon to simulate the experimental data had a density anomaly in the amorphous phase and was therefore physically untenable. They showed calculations for two models that give results close to the experimental data one had a central core of adjacent stems with random re-entry wings, and the other involved variable sized clusters of adjacent stems. Both models suggested that 60—80% of the stems were adjacent. 

Peterlin " stated that half of the stems in a polyethylene crystal must be arranged with adjacent re-entry and one third with random re-entry to give both the required crystalline and amorphous densities and reproduce the neutron scattering results. For further discussions readers are referred to ref. 11. 

There has been considerable argument over the years concerning the way in which folding occurs and the nature of the fold plane. The different models that have been suggested to account for folding in polymer crystals are illustrated schematically in Fig. 4.11. The models range from random re-entry ones, where a molecule leaves and re-enters a crystal randomly, to adjacent re-entry models, whereby molecules leave and re-enter the crystals in adjacent positions. Two particular adjacent re-entry models... 

Fig. 4.11 Schematic illustrations of the different types of folding suggested for polymer single crystals, (a) Adjacent re-entry with sharp folds, (b) Adjacent re-entry with loose folds, (c) Random re-entry or switch-board model.

Fleming MF, Barry KL, Manwell LB, et al Brief physician advice for problem alcohol drinkers a randomized controlled trial in community-based primary care practices. J Am Med Assoc 277 1029-1043, 1997 Goldstein MF, Deren S, Sung-Yeon K, et al Evaluation of an alternative program for MMTP drop-outs impact on treatment re-entry. Drug Alcohol Depend 66 181-187, 2002... 

Figure 3.12 compares the chain re-enhy in folded-chain and switchboard models. In the folded-chain model, the polymer chains reenter the crystalline phase right next to where they left. On the other hand, in the switchboard model, the polymer chains do not have to follow adjacent re-entry. Instead, they can re-enter the crystalline phase randomly. The switchboard model requires minimum movement and re-organization of polymer chains. 

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