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

David H Wang; Kyung Min Lee; Deborah H Lee; Matthew Baczkowski; Jae Gyeong Lee; Jeong Jae Wie; Loon-Seng Tan

doi:10.1021/acsami.1c14511

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

ACS Appl Mater Interfaces. 2021 Oct 13;13(40):48127-48140. doi: 10.1021/acsami.1c14511. Epub 2021 Oct 3.

Authors

David H Wang¹, Kyung Min Lee¹, Deborah H Lee¹, Matthew Baczkowski¹, Jae Gyeong Lee², Jeong Jae Wie², Loon-Seng Tan¹

Affiliations

¹ Functional Materials Division, Materials & Manufacturing Directorate, Air Force Research Laboratory, AFRL/RXA, Wright-Patterson Air Force Base, Ohio 45433-7750, United States.
² Department of Polymer Science and Engineering, Inha University, Incheon 22212, South Korea.

PMID: 34601861
DOI: 10.1021/acsami.1c14511

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.

Keywords: amide−imidol tautomerization; charge-transfer complexation; excited-state structure; hydrogen bonding; multiblock copolymers; stimuli-responsive; sub-Tg motions.