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

Abstract

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 %, and [HB-AA] for [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.

Flexible Azo-Polyimide-Based Smart Surface with Photoregulatable Surface Micropatterns: Toward Rewritable Information Storage and Wrinkle-Free Device Fabrication

“…6,22,41,42 Recently, azobenzene main-chain polymers, such as azobenzene-containing polyimide (azo-PI), with excellent comprehensive performance have been used in the fabrication of smart soft actuators. 43−46 Although the light responsiveness of the azo-PI with different chain structures has been systematically researched, 43,45,46 the azobenzene main-chain polymers utilized for the fabrication of smart surfaces with photoregulatable surface morphology or photoswitchable optical properties have rarely been reported. Herein, we have synthesized a novel azobenzene main-chain polyimide (azo-PI) with a siloxane chain as a flexible segment and used it for the fabrication of photoresponsive surfaces.…”

“…Zhang et al synthesized a novel photoresponsive polymer with enhanced mechanical properties and photohealing ability by introducing a long hexamethylene spacer and a long n -hexadecyl tail into the azobenzene-containing side chain . In addition, different azobenzene side-chain polymers and copolymers were also used to fabricate smart surfaces for applications in information storage, sensors, and light-responsive soft photonics along with microfluidic devices. ,,, Recently, azobenzene main-chain polymers, such as azobenzene-containing polyimide (azo-PI), with excellent comprehensive performance have been used in the fabrication of smart soft actuators. − Although the light responsiveness of the azo-PI with different chain structures has been systematically researched, ,, the azobenzene main-chain polymers utilized for the fabrication of smart surfaces with photoregulatable surface morphology or photoswitchable optical properties have rarely been reported.…”

Shape-Dependent Photomechanical Motions of Cyanostilbene-Based Molecular Crystals

“…Innovative mechanical-responsive materials, exhibiting mechanical responses under the external stimulus of heat, light, solvent, or external force, are essential to mechanical devices in biomimetics, actuation, and soft microfluidics. − Up to now, light-driven mechanical behaviors have exhibited many conspicuous advantages because of their high spatiotemporal resolution and various manipulating methods that can be applied instantly, precisely, and invasively. Various photomechanical molecular crystals with highly ordered structures and fast responses − are developed from anthracenes, azobenzenes, and diarylethenes, and their diverse photomechanical motions such as bending, − twisting, − curling, − and jumping , are observed.…”

Photoinduced deformation of amorphous polyimide enabled by an improved azobenzene isomerization efficiency

“…The polarized-light-induced deformation of azo-PIs has been widely studied, in which the repeated trans-cis-trans isomerization causes the reorientation of azobenzene perpendicular to the polarization direction (the Weigert effect). [20][21][22][23][24][25][26][27][28] This reorientation process leads to the conformational changes of polymer networks in the polarization direction, thereby resulting in deformation. White et al first synthesized a deformable polyimide with azobenzene as the crosslinker, and it exhibited a bending angle of ∼20°after irradiation with polarized bluegreen light for 30 min.…”

“…20 Subsequently, some efforts have been made to investigate the influence of the molecular structure on the photomechanical response of azo-PIs. [21][22][23][24][25][26][27] For example, freely rotating structures 24 or bulky substituents 25 increase the segmental mobility as well as free volume, which promotes the ability of azobenzene to undergo efficient reorientation and induce conformational changes, endowing azo-PIs with large deformation. However, as they lack the cooperative effect of azo-LCPs, the deformation of azo-PIs through reorientation is relatively slow, which is unfavourable for light-driven actuators.…”