Photo-induced Melting of Semi-Crystalline Polymers via Azobenzene Isomerization

Incorporation of photo-isomerizable molecules into macromolecular systems is a powerful method to harness Angstrom-level geometry changes induced by light to drive macroscopic shape changes and force generation. The most commonly explored material platforms exploit azobenzene isomerization-driven disordering of nematic polymer networks. While these materials generally perform better than isotropic networks due to cooperative nanoscale organization, higher ordering of azo-molecules into crystalline lattices presents a potential route to further enhance photogenerated work output. To this end, we have explored semi-crystalline azo polymers as a means to enhance ordering while preserving the processability inherent to polymer systems. We show that upon UV light exposure these materials undergo an isothermal melting of crystalline lamellae due to trans-cis isomerization, and that crystalline order can be restored upon re-isomerization back to the trans state. Interestingly, this process is found to be strongly suppressed at temperatures sufficiently far below the melting point. Finally, using shear alignment, we demonstrate the fabrication of well-aligned crosslinked fibers wherein reversible and directional crystallization can be harnessed for photoactuation.