Decoupling the Roles of Chain Length, Entanglements, and Intermolecular Interactions on the Melt Memory of Semicrystalline Polar Homopolymers

TL;DR

This study explores how chain length, entanglements, and interactions affect the melt memory of polar polymers, revealing how these factors influence their recrystallization behavior.

Contribution

A new dimensionless interaction index is introduced to link molecular weight, entanglements, and intermolecular interactions to melt memory in polar homopolymers.

Findings

Melt memory in polar homopolymers is absent below a critical chain length due to insufficient intermolecular interactions.

Melt memory increases with chain length and entanglements, which preserve the melt's structural complexity.

Linear polyethylene lacks melt memory due to weak intermolecular interactions.

Abstract

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.

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