Unit Cell

Polyethylene

• Only a small part (a C₂H₄ unit) of each chain lies in a unit cell, PC Fig. 7.22.
• The chains are in the conformation that is preferred on the basis of the short-range intramolecular interactions, which is the fully extended, planar all-trans zig-zag for polyethylene.
• The crystal is closely packed. Structural defects, such as short branches, are not easily accomodated in the crystal. A small amount of branching will greatly reduce the crystallinity of polyethylene.

Isotactic Polypropylene

• As with polyethylene, the chains of isotactic polypropylene crystallize in the conformation that is preferred on the basis of the short-range interactions. However, that conformation is different from the one in polyethylene. It has an alternation of trans and gauche states along the chain, with the gauche states all of the same sign. The conformation is a helix with three propylene units per turn, PC Fig. 7.23. This helix is therefore called a 3₁ helix.
• There is a regular close packed arrangement in the crystal.
• Atactic polypropylene cannot form this regular arrangement, and therefore atactic polypropylene remains amorphous at temperatures where the isotactic polymer crystallizes. Syndiotactic polypropylene also cannot form this structure. It crystallizes, but with a structure different from that of the isotactic polymer.

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