Crystallization and Molecular Folding: The properties of high polymers are very sensitive to the way molecules fold during crystallization.

The long-chain nature of the polymer molecule, together with the usual broad distribution of molecular lengths, makes it kinetically very unlikely that the molecule will achieve, on crystallization, its thermodynamically most stable form. Under most crystallization conditions a large portion of the polymer molecules crystallize into folded-chain crystals. Only a small fraction of the molecules of lower molecular weight, initially rejected from the growing folded-chain crystals, succeed in achieving the stable, fractionated, extended-chain crystalline form. The physical properties of the resulting polycrystalline structure are very strongly dependent upon the amount and kind of molecular folding which has occurred. In general, folded-chain crystals are tough and ductile and tend to produce low modulus and high elongation, whereas extended-chain crystals are brittle and tend to have higher modulus.