Conformational Freedom-Enhanced Optomechanical Energy Conversion Efficiency in Bulk Azo-Polyimides

Azobenzene-doped glassy polyimides (azo-polyimides) offer some of the most efficient optomechanical power densities to date rivaling electrostrictive polymers. Despite such potential attributes, the optomechanical efficiency remains low in comparison to other smart materials. Using high-fidelity coarse-grained molecular dynamics simulations, the authors reconcile both experimental and theoretical challenges to understand the limiting factors for the optomechanical conversion in photostrictive polymers. Interestingly, the ideal optomechanical efficiency of 10-24% for a single-chain azo-imide monomer predicted here is equal to or a little higher than experimental reports, suggesting experimental design space. The time-dependent optomechanical efficiency of bulk azo-polyimide is quantified, for the first time, to be strongly correlated with the initial free volumes, a measure of polymer conformational freedom. This trend is elaborated by conformational order parameters and viscoelastic relaxation moduli. Resembling the role of porosity in azobenzene-contained metal/covalent organic frameworks to enhance the photo-switching efficiency, a larger conformational freedom enables >10 times increase in optomechanical efficiency comparing to existing experiments. This is primarily due to facilitated viscoelastic relaxation after photo-switching which alleviates residual stresses quickly and reduces heat dissipation. These findings suggest opportunities to improve the optomechanical performance through targeted strategies, such as porosity control and thermal annealing.

Automated summary

The study utilized molecular dynamics simulations to understand the limiting factors for optomechanical conversion in photostrictive polymers. It found that conformational freedom has a significant impact on optomechanical efficiency, with materials possessing larger conformational freedom exhibiting over 10 times higher efficiency than existing experiments.