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Polyethylene adjacency fold-chain

Small-angle neutron scattering (SANS) studies on melt crystallized linear polyethylene, isotactic polypropylene, and poly(ethylene oxide) have shown that deuteriated molecules in a hydro-chain matrix have the same radius of gyration, and so molecular conformation, as in melt. This is evidently inconsistent with a regular adjacent re-entry folded chain lamellae. Recently Stamm et in comparing the scattering of neutrons at intermediate angles, of... [Pg.224]

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. [Pg.43]

Important experiments have been carried out with isolated lamellae, also called crystal cores, of which the chain ends could be removed chemically GPC analysis has shown that the cross-links mainly occur at the folds and that the G(cl) in the crystal cores is only ca 0.2 (lOOeV)" The fact that cross-links are formed with such a low yield in the crystal is attributed to the fact that the carbon atoms on the adjacent chains are too far apart (> 0.41 nm) for interchain C—C bonds (0.15 nm) to be formed. It is generally accepted now that the cross-linking in polyethylene mainly takes place at the fold surfaces and in the amorphous regions. [Pg.777]

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... [Pg.2907]

There is still considerable discussion about the existence of chain folding in semi-crystalline polymers." Using a Monte Carlo method, Flory and Yoon have shown " that adjacent re-entry could not reproduce the experimental results from polyethylene" and polypropylene" in the intermediate q range. An analytical computational technique, involving a model having a Gaussian distribution of inter-stem distances in the lamella, has been shown to fit these data" and can replicate the earlier Monte Carlo calculations. The root mean square, inter-stem distances derived from the fits are 22 A for polyethylene and 28 A for polypropylene. [Pg.205]


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Chain folding

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Polyethylene chain folding

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