What Is The Preferred Arrangement Of The Macromolecules For Crystallization?

The preferred arrangement of the macromolecules is, a lamellar arrangement in which the chains lie parallel at a constant distance from each other. The length of the lattice units in the direction of the chains is 0.25 nm for Polyethylene, 0.65 nm for Poly Propylene. 0.8 nm for Poly Amide 6, 1.73 nm for Poly Amide 66, and 1.95 nm for Poly tetra fluro ethylene. With measured lamella thickness in Polyethylene of 15 nm, the number of repeat units is about 60, but with Poly Amide 66 only 3.5 to 4.0. Since the macromolecules are much longer than the lamellae, they must either fold back into the lamellae or become part of adjoining lamellae.

For Polyethylene, the fold or loop contains at least six structural elements (monomer units). These loops and the chain segments that lie between lamellae are unable to form regularly aligned crystalline structures and, therefore, remain amorphous.

The height of the folds may vary considerably. Most frequently, the height is between 1 and 10 nm and depends particularly on temperature and pressure during the crystallization process. With pressures greater than 250 MPa, macromolecules may even crystallize in the form of elongated chains and have heights of several micro meters.

Polymers that contain both crystalline and amorphous regions are called semi-crystalline polymers. For PE and several other thermoplastics, the amorphous regions can be etched out because suitable etching reagents can only penetrate the amorphous, more open regions, especially when these are on or near the surface of a polymeric material. The remaining crystallites are brittle. Under normal conditions, 100% crystalline polymers do not exist.