Molecular Segregation and Melt Memory in Dilute Polycaprolactone with Reduced Entanglements

Abstract

In polymer processing, low-molecular-weight chemically identical oligomers are typically added to a polymer to improve its processability and mechanical properties by manipulating its entanglement concentration. However, the complicated semi-crystalline morphologies and melt memory effect of less entangled bimodal dispersed mixtures cannot be overlooked. In this study, we systematically examined the crystallization and self-nucleation behavior of poly(ε-caprolactone) (PCL) mixtures composed of a relatively high-molecular-weight (83 kg/mol) fraction and a low-molecular-weight PCL with a mid-chain defect (2 kg/mol). The entanglement concentrations were quantified by linear rheological measurements. Both thermal analysis and small-angle X-ray scattering (SAXS) investigations revealed that pronounced crystallization-induced phase separation (i.e., molecular segregation) occurs because of the different nucleation energy barriers. In contrast to our previous investigation using a long chain fraction with a higher molecular weight (200 kg/mol) (Macromolecules 2024, 57, 1632–1641), the low-molecular-weight oligomer tends to crystallize between adjacent crystalline lamellae composed of long PCL chains due to the lack of intra-crystalline links, for example, entanglements and tie-molecules. The correlation between the melt memory effect and the entanglement concentration was evaluated using self-nucleation experiments. The 2 kg/mol PCL oligomer (with a central defect) did not exhibit melt memory. However, the introduction of high-molecular-weight components, even below the critical entanglement concentration, leads to a wider temperature range, preserving the ordered structure. Our results provide solid evidence that the melt memory effect in polar semi-crystalline polymers originates from the intramolecular interactions of adjacent chain folding rather than entanglements or chain overlaps.