Chains crystallization from dilute solution

Fig. 8. Schematic representation of the folded chain model of crystalline domains in thermoplastics. a Lamella crystallized from dilute solution (multiple/adjacent re-entry) and b Lamella crystallized from melt (single entry/far distance re-entry)...

Whereas crystallization from dilute solutions may result in the formation of single polymer crystals, this perfection is not achieved when deahng with polymers cooled from the melt. The basic characteristic feature is still the lamellar-hke crystallite with amorphous surfaces or interfaces, but the way these are formed may be different, based on the careful investigation of melt-crystallized polymers using neutronscattering techniques. The two models that have been proposed to deseribe the fine structure of these lamellae and their surface characteristics in semicrystalline polymers differ mainly in the way the chains are thought to enter and leave the ordered lamellae regions. These are ... 

In the previous sections it was observed that when polymers are crystallized from dilute solutions, they form lamellar-shaped single crystals. These crystals exhibit a folded-chain habit and are of the order of 100 to 200 A thick. From somewhat more concentrated solutions, various multilayered dendritic structures are observed. 

Section 6.4.2 described Keller s early preparations of single crystals from dilute solutions. Since the crystals were only about 100 A thick and the chains were oriented perpendicular to the flat faces, Keller postulated that the chains had to be folded back and forth. 

Crystallization from dilute solution flexible chains... 

---