Polyethylene chain fold

Rg. 5.6 A model of a polyethylene chain folded using four gauche bonds, as suggested by Frank. Upper, view normal to the plane of folding lower, view along the plane of folding. (Reprinted by permission from John Wiley Sons Limited.)... 

An interesting question arises for those copolymers that crystallise with lamellar structures. Do the polyethylene chains fold so that the chain axes are perpendicular to the lamellar surface, as they do in the homopolymer, or parallel to it The answer need not be the same in all cases, even for a single type of copolymer, but there is certainly evidence that the chains fold so that their axes are perpendicular to the lamellar planes in some cases. Such evidence has, for instance, been obtained for PE-PEP-PE triblocks by studying the development of WAXS and SAXS patterns as samples were oriented by drawing. 

Fig. 22.6. A schematic drawing of a largely crystalline polymer like high-density polyethylene. At the top the polymer has melted and the chain-folded segments hove unwound.

Thin polymer films may also be investigated by TEM and high resolution images are obtained for e.g. thin films of liquid crystalline polymers [64]. Usually thin microtome cuts from bulk samples are investigated, but also epitaxial growth of polyoxymethylene on NaCl [152], chain folding of polyethylene crystals [153], epitaxial crystallization of polypropylene on polystyrene [154] or monomolecular polystyrene particles [155] are observed. The resolution is, however, in most cases not comparable to STM. 

FIGURE 2.16 Spherulite structure showing the molecular-level lamellar chain-folded platelets and tie and frayed chain arrangements (a), and a more complete model of two sets of three lamellar chain-folded platelets formed from polyethylene (PE) (b). Each platelet contains about 850 ethylene units as shown here. 

The block copolymers shown in both Table V and VI were hydrogenated. The B-lU block produced polyethylene and the polyisoprene block produced ethylene propylene alternating copolymer. The physical properties of this copolymer, composed of crystalline polyethylene block and a soft elastomeric segment made of an EPR block, is tabulated in Table VII. The data in this table illustrate the fact that a diblock of hydrogenated polybutadi ene-polyisoprene gave excellent physical properties. This is a further illustration of the new concept of soft chain interpenetrating the crystalli zable polyethylene chain via chain folding. 

First, it is necessary to define the structure. The structure of a planar zig-zag polyethylene chain is shown in Fig. 2, together with its symmetry elements. These are C2 — a two-fold rotation axis, C — a two-fold screw axis, i — a center of inversion, a — a mirror plane, and og — a glide plane. Not shown are the indentity operation, E, and the infinite number of translations by multiples of the repeat (or unit cell) distance along the chain axis. All of these symmetry operations, but no others, leave the configuration of the molecule unchanged. 

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