Towards Polymers with Molecular Auxeticity

For many decades, it has been challenging to synthesize auxetic materials at the molecular level. Auxetic materials exhibit counterintuitive behavior; they expand perpendicularly to the direction in which they are stretched. An aromatic macrocycle containing a sequence of N-substituted and N-unsubstituted amides was designed to resemble the re-entrant structure found in macromolecular auxetic materials. Upon application of tensile force, bent cis amides change to linear trans amides. This was anticipated to trigger the expansion of the macrocycle perpendicular to the direction of the applied force. To investigate the proposed configurational change by atom force microscopy (AFM), we designed and prepared a cis-trans aramide motif incorporated into an end-functionalized polymer which ensured covalent attachment to the AFM tip. At large extensions, polymer chains were envisioned to unfold and induce cis-trans isomerization.