Crystallization of polymers from the entangled melt always leads to the formation of semicrystalline materials with a typical nanoscopic morphology consisting of thin lamellar crystals separated by disordered amorphous layers. The crystallinity as a key parameter depends on the thickness of the crystalline and of the amorphous regions. While continued efforts have been made to investigate the mechanisms selecting a certain crystal thickness during crystallization, it is largely unknown, which factors determine the thickness of the amorphous layers. We present a study of the crystallization of a series of blends of high molecular weight polycaprolactone with its own unentangled oligomer. Rheological analysis confirms that the samples form homogeneous polymer melts with a reduced density of entanglement strands. At low crystallization temperature, both components crystallize without separation. Small angle X-ray scattering shows that the resulting semicrystalline morphology is characterized by a reduced thickness of the amorphous regions, while the crystal thickness remains largely unaffected. The thickness of the amorphous layers corresponds to a condition of constant density of entanglement strands in the central part of the amorphous layers. Corresponding to the higher crystallinity, the mechanical modulus of the resulting materials increases with decreasing polymer content.

The role of entanglements for the morphology of semicrystalline polymers

ACS Spring Meeting 2021

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