Intermolecular Interactions and Intramolecular Motions in Photomechanical Effect: Nonlinear Thermo- and Photomechanical Behaviors of Azobenzene-Functionalized Amide–Imide Block Copolymers

To discern multiple intertwined effects, a set of azobenzene-functionalized amide–imide block copolymers, azo­(PA-co-PI)-x, where x is amide-block content, viz., [azoPA] = 25, 50, 75 mol %, was synthesized from 2,2-bis­{4-[4-(4-aminophenyldiazenyl)­phenoxy]­phenyl}­propane­(azoBPA), 4,4′-oxydibenzoyl chloride (ODBC), and 4,4′-oxydiphthalic anhydride (OPDA). Including homopolymers (azoPA and azoPI), this series of amorphous azopolymers possesses a high glass-transition temperature (T g > 210 °C) and a modulus (E′ ∼ 1.23–2.50 GPa). Their photobending (ca. 23–90°) and photostress (ca. 250–380 kPa) were assessed in the form of cantilevers with a linearly polarized 445 nm light. Nonlinear composition/[azoPA] dependencies of the thermo- and photomechanical properties are correlated. As [azoPA] increases from 0 mol %; T g, E′, photostress, and photobending angle initially decrease to reach four separate minima for azo­(PA-co-PI)-50; and then all increase with a higher [azoPA]. The trend considerations of film density, dynamic thermomechanical, Fourier transform infrared (FT-IR), and ultraviolet–visible (UV–vis) measurements implicate that (i) intermolecular association and intramolecular segmental mobility collectively influence the photomechanical outcomes and (ii) two types of hydrogen bonding (HB), namely, amide–amide [HB-AA] and amide–imide [HB-AI] coexist in azo­(PA-co-PI)-x copolymers, with [HB-AI] being largely responsible for photomechanical outcomes of azo­(PA-co-PI)-x with [azoPA] 40–50 mol %. We hypothesize that the “U-shaped” photomechanical effect apparently stems from the cooperative “unzipping” of H bonds in the [HB-AA]* excited state with H bonds in [HB-AI]* being stabilized by electrostatic interactions inherent in an excited intermolecular complex.