Inside PEF: Chain Conformation and Dynamics in Crystalline and Amorphous Domains

A thorough vibrational
spectroscopy and molecular modeling study
on poly­(ethylene 2,5-furandicarboxylate) (PEF) explores its conformational
preferences, in the amorphous and crystalline regions, while clarifying
structure–property correlations. Despite the increasing relevance
of PEF as a sustainable polymer, some of its unique characteristics
are not yet fully understood and benefit from a deeper comprehension
of its microstructure and intermolecular bonding. Results show that
in the amorphous domains, where intermolecular interactions are weak,
PEF chains favor a helical conformation. Prior to crystallization,
polymeric chains undergo internal rotations extending their shape
in a zigzag patternan energetically unfavorable geometry which
is stabilized by C–H···O bonds among adjacent
chain segments. The zigzag conformation is the crystalline motif present
in the α and β PEF polymorphs. The energy difference among
the amorphous and crystalline chains of PEF is higher than in PET
poly­(ethylene terephthalate) and contributes to PEF’s higher
crystallization temperature. The 3D arrangement of PEF chains was
probed using inelastic neutron scattering (INS) spectroscopy and periodic
DFT calculations. Comparing the INS spectra of PEF with that of poly­(ethylene
terephthalate) (PET) revealed structure–property correlations.
Several low-frequency vibrational modes support the current view that
PEF chains are less flexible than those of PET, posing greater resistance
to gas penetration and resulting in enhanced barrier properties. The
vibrational assignment of PEF’s INS spectrum is a useful guide
for future studies on advanced materials based on PEF.