The crystallization of polymers from entangled melts generally leads to the formation of semicrystalline materials with a nanoscopic non-equilibrium morphology consisting of stacks of alternating crystalline and amorphous layers. This morphology also determines the favorable mechanical properties of thermoplastic semicrystalline polymers. We have used a combination of advanced characterization techniques, in particular small-angle X-ray scattering and solid-state NMR, and a set of model polymers to elucidate the role of relevant but hitherto underestimated effects in the formation of the morphology, specifically those of intracrystalline chain diffusion (ICD) and of entanglements. We show that the interplay of crystal growth and crystal thickening enabled by ICD determines largely the crystal thickness, while the entanglements accumulated in the amorphous phase determine the thickness of the amorphous layers. The results also suggest an explanation for the large supercooling that is typically required for crystallization of polymers if entanglements cannot be resolved during crystallization.

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