In With the New!
In the late 1950s, researchers inferred that linear polyethylene, PE, crystallized with its chain contour ostensibly normal to the crystal surface. This picture was refined using diffraction and electron microscopy of collapsed, sectored, tent-like crystals obtained from solution which indicated that the all-trans polymer “stems” within crystalline lamellae are tilted relative to the lamella normal. These seminal inferences, in turn, implied that the chain transverses the crystal multiple times in a "folded chain crystal habit."
LEFT: Chain topologies in a typical cartoon of the amorphous phase of the lamellar morphology of semicrystalline polymers: (1) loop or loose fold; (2) entanglement; (3) tie molecule; (4) chain end sections in the amorphous interphase.
RIGHT: Chain topologies in a corrected cartoon of the amorphous phase of the lamellar morphology of semicrystalline polymers; (1) loop or loose fold; (2) entanglement; (3) tie molecule; (4) chain end sections terminate at the crystal surface.
From the outset, especially for melt-crystallized PE, the mechanism of crystallization and the detailed nature of the fold surface was a contentious subject; one that continues to challenge simulators. In semicrystalline PE the distinctions between the fold surface and the crystal’s amorphous surroundings are ambiguous in the two-phase "lamellar morphology" composed of crystalline lamellae embedded in a disordered amorphous phase.
Adjacent and randomly re-entered chain-folded polymer "stems" make up the lamellae and the amorphous phase is widely believed to be composed of topological defects, loops and "tie chains," and chain ends occluded from the crystalline lamellae.
In work published in Macromolecules, recently retired professors Maurice Brookhart, and Ed Samulski, show with solid-state deuterium nuclear magnetic resonance, 2H NMR, studies of specifically labeled PE that a large fraction of chain end sections, >0.8, are immobilized and therefore must reside in the crystalline lamellae.