Tailoring the PhotomechanicalResponse of Glassy,Azobenzene-Functionalized Polyimides by Physical Aging.

Photoresponsive polymers convert a light stimulus inputinto amechanical output (work). Photoinduced conformational changes, suchas within azobenzene, dictate molecular-level distortions that summateinto a macroscopic strain, which often manifests as a shape changeor motion. The transduction of the molecular-level response to a macroscaleeffect is regulated by mesoscopic features, such as chain packing,free volume, and local molecular orderfactors which dependon chemical composition as well as the process history ofthe material. Herein, we demonstrate the ability to widelytailor the photomechanical response of a photoresponsive polymer bymanipulating the energy state of the glass, rather than formulatingnew chemical compositions. Physical aging increases the density ofthe glass, reduces local free volume, and thus reduces the minimain local conformation space, thereby strongly influencing the azobenzenephotochemistry (trans–cis–trans isomerization). Thesubsequent change in the energy landscape of the system reduces thefraction of azobenzene able to undergo reconfiguration as well asincreases the probability that those photoinduced conformations willrelax back to the initial local environment. The result is a tuningof the magnitude of macroscopic strain and the ability to shift fromshape fixing to shape recovery, respectively. [ABSTRACT FROM AUTHOR]