Semiaromatic Polyamides with Re-Entrant Chain Folding Templated by “U-Turn” Repeat Units

Biopolymers
often show deterministic chain folding templated by
repeat unit sequences whose conformations are locked in by intramolecular
interactions. In this work, we have used repeat units based on rigid
anthracene or acridine scaffolds, which we refer to as “U-turn”
repeat units, to template chain folding reminiscent of β-serpentine
folds in bulk synthetic polyamides. Unlike the rigid kinked repeat
units present in certain commercial semiaromatic polyamides, the acridine-based
U-turn repeat units did not impede crystallization in the polyamides,
whose crystalline phase was shown to be stabilized by infinite arrays
of intermolecular hydrogen bonds and π–π interactions
between the acridine units. This led to a unique layered crystalline
structure characterized by a well-defined lamellar thickness that
depended on the length of the aliphatic spacers between the U-turn
repeat units and regular adjacent re-entrant chain folding, which
is not usually expected for semiaromatic polyamides crystallized at
high supercooling. Our work hence provides the first example of the
introduction of deterministic chain folding into a nonpeptidic polyamide
in order to tailor its crystalline morphology and may consequently
open up new perspectives for the molecular design of structural materials.