Molecular segregation in dilute polycaprolactone with reduced entanglements

In polymer processing, low-molecular-weight chemical-identical oligomers are usually added to a polymer to improve its processability and mechanical properties by decreasing the concentration of entanglements. However, the potential crystallization-induced phase separation (i.e., molecular segregation effect) cannot be overlooked. In this work, we systematically examined the complex crystallization and melting behavior of a Poly(e-caprolactone), PCL, mixture composed of a relatively high molecular weight (83 kg/mol) fraction and a PCL oligomer with a mid-chain defect (2 kg/mol). Through Small Amplitude Oscillatory Shear (SAOS) rheological measurements, the entanglement concentration was quantitatively determined. Two kinds of molecular segregation effects were observed at different crystallization temperature ( T _c ) regimes. At relatively high (above 25 ^o C), a high molecular weight fraction with a small portion of oligomer tends to form mixed-chains crystalline lamellae. At the same time, the remaining oligomeric chains formed neat oligomeric lamellar crystals between the adjacent mixed-chain lamellae. At relatively low (below 30 ^o C), an unexpected additional melting peak at higher temperatures, related to the melting of thicker lamellae composed only of the long-chain PCL fraction, was observed. Because of the lack of entanglement constraints, it is speculated that long PCL chains tend to shrink locally, forming superfold lamellar crystals. Due to chain connectivity, two kinds of crystals (i.e., mixed chain and neat long-chain crystals) containing long chains are hypothesized to form within the same lamellae. Self-nucleation experiments demonstrate that the presence of even tiny fraction of long chain PCL80k could lead to the formation of stable self-nuclei after melting, leading to strong melt memory effect. Our results clearly elucidate that the melt memory effect in polar semi-crystalline polymers originate from the intramolecular interactions of adjacent chain folding, rather than entanglements or chain overlap which have been debated for long time.