Polyethylene regular chain folding

The discussion of the influence of the interphase need not be limited to just linear polyethylenes. Interphases of several nm have been reported in polyesters and poly-hydroxy alkanoates. One major difference between the interphase of a flexible polymer like polyethylene and semi-flexible polymers like PET, PEN and PBT is the absence of regular chain folding in the latter materials. The interphase in these semi-flexible polymers is often defined as the rigid amorphous phase (or rigid amorphous fraction, RAF) existing between the crystalline and amorphous phases. The presence of the interphase is more easily discerned in these semi-flexible polymers containing phenylene groups, such as polyesters. 

Most polymer crystals will exhibit facets and some like polyoxymethylene form hollow pyramids. The occurrence of the hollow pyramid is for similar reasons to that proposed for polyethylene and is a direct consequence of the constraints on the chain folding. The smooth surfaces observed for many crystal systems are evidence of regular chain folding, but are not proof that this occurs. 

The hollow pyramid shape typical of a single crystal of polyethylene indicates that the chain axis is not parallel with the normal of the lamella. The chain axis is generally at an angle, about 30°, with respect to the lamella normal. The reason for the chain tilt is essentially that a certain type of regular chain fold requires a small vertical displacement of the linear chain in the adjacent position (Fig. 7.14). 

FIGURE 19.2 Polyethylene lamella crystallized from the melt state showing some regular chain folds, loose loops, and tie molecules (switchboard model). Image courtesy of Eric Wysocki, Exponent, Inc. 

A detailed model for the single crystal was introduced by Keller in 1957 for polyethylene. The chains (Figure 4.16) are more or less regularly folded in lamellae. 

It was initially thought that chain folding in polymers like polyethylene would take place in some sort of regular, ordered fashion, as observed in certain cyclic paraffins (Figure 8-55), with some specific sequence... 

The crystallization of isolated polyethylene chains was first reported by Kavassalis and Sundararajan (204) using MD with the Nos Hoover thermostat. More recently, single polyethylene chains driven by Langevin dynamics have been used by Muthukumar and co-workers (205,206) to model the early stages of nu-cleation and growth of crystals in solution. But even simpler models of chains will form ordered domains, and hence they have been explored as a means to understand some of the aspects of crystallization (122,138). A lattice model has been used in conjunction with an MC technique to model crystallization in short polyethylene-like chains (207). The crystallite-amorphous interface is foimd to be rough, and to have a relatively low concentration of regular folds for even this lattice model. 

Figure 17. Schematic lepiesentation of a lamellar polyethylene single ciystal. (a) and (b) show enlaiged features of its stnicture in (o) the zigzag conformation of the macromolecular chain inside the crystal in (b) a hypothetical model for the regular folding, drawn according to the calculation reported in ref. 224a.

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