Decoupling the Roles of Chain Length, Entanglements, and Intermolecular Interactions on the Melt Memory of...

In polymer crystals, chains are closely packed within unit cells. If they are heated above their melting point, they require a specific temperature and time to revert their ordered conformations to isotropic random coils in the melt. When the temperature is slightly above the melting point and all crystals have melted, the chains may retain a memory of the conformations they had in the crystals, i.e., they remember some of the extended or oriented conformations that they had in crystallographic registry. This causes enhanced recrystallization, a property denoted melt memory. Its exact nature remains a central question in polymer crystallization. Here, we combine small-angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC) self-nucleation experiments to systematically investigate the molecular origin of melt memory in poly-(ε-caprolactone) (PCL) and poly-(ethylene oxide) (PEO) model samples, spanning a range of molecular weights from oligomers to highly entangled polymers. The entanglement molecular weights (M _e) were experimentally determined with rheological techniques using a large number of samples. To quantify intermolecular interactions and rheological constraints, we introduce a dimensionless interaction index that accounts for crystallinity-weighted intermolecular interactions and chain packing in the melt. This index rises sharply in oligomeric samples and attains a maximum near M _e. Without strong enough intermolecular interactions, melt memory cannot develop; for example, linear polyethylene does not exhibit melt memory. Conversely, in polar homopolymers, there is a critical chain length below which the intermolecular interaction density is not enough for memory to develop. Beyond this minimum chain length, melt memory is observed in polar homopolymers even in the absence of entanglements, in which case it is exclusively due to intermolecular interactions. Beyond M _e, the melt memory increases as entanglements preserve the melt's complexity, characterized by intermolecular interactions. These results establish a unified structure-property framework that links molecular weight, morphology, and intermolecular interactions to the melt memory of semicrystalline polar homopolymers.