Lamellae and their organization in melt-crystallized polymers

Significant advances in knowledge of lamellae and their organization in meltcrystallized polymers have stemmed from the ability to examine internal morphologies systematically with the transmission electron microscope. Spherulites form because the first-forming (dominant) lamellae branch repetitively, often at giant screw dislocations, then diverge substantially creating a skeleton to which later-forming lamellae must accommodate. This sequence promotes chain-folding, invites fractional crystallization and modulates chemical, mechanical and thermal properties of spherulites at the inter-dominant spacing. The key feature of lamellar divergence at screw dislocations is present in individual crystals, probably deriving from pressure of uncrystallized molecular cilia; growing lamellae will also distort very substantially to gain material. If necessary, spacefilling is achieved without lamellar and crystallographic continuity by nucleating new growth at large misorientations. Individual melt-grown crystals have been studied both after extraction from a quenched matrix and in situ in thinned specimens. For polyethylene different lamellar profiles have been placed in context while their fine structure provides insights into molecular mechanisms of growth.