Relaxation Dynamics and Strain Persistency of Azobenzene‐Functionalized Polymers and Actuators

Abstract

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/mame.201700256. Azo PolymerThe accumulation of photoinduced deformation in azobenzene functionalized polymers has received a significant amount of attention in recent years. Critically, the induced photomechanical deformation in these systems experiences varying degrees of relaxation. Control over the persistence of photomechanical strains is vital to the broader utility of these materials in shape programmable systems including soft robotics and engineered origami. Furthermore, investigations of relaxation in light responsive polymer systems triggered by UV light are more prominent than those triggered by blue-green light. In this study, the impact of chain mobility and initially induced photostrain on the relaxation dynamics of azobenzene-functionalized polyimides after irradiation with blue light is examined. A modeling effort coupling chromophore population dynamics to material strain is carried out to further explore the relationship between material structure, relaxation dynamics, and macroscopic deformation. The implications for controlling strain persistence are highlighted by simulating one example of a photoprimed bistable actuator. 3 of 11) www.advancedsciencenews.com www.mme-journal.de (

Reprocessable Photodeformable Azobenzene Polymers

“…In addition to the above-described reprocessable physically and DCB-crosslinked photodeformable azo polymers, some special reprocessable photodeformable azo polymers without the above dynamic crosslinking networks (briefly uncrosslinked photodeformable azo polymers) have also been reported, which mainly include linear azo polyimides developed by White and coworkers [ 72 , 73 , 74 , 75 , 76 , 77 ] and amphiphilic epoxy-based azo random copolymers developed by Wang and coworkers [ 78 , 79 , 80 , 81 , 82 , 83 , 84 , 85 , 86 ].…”

“…Later on, the same group further prepared a series of main-chain azo polyimides with different chemical structures and studied the effects of the backbone rigidity [ 74 ], segmental mobility [ 75 ], free volumes [ 76 ], and crosslinking [ 75 , 77 ] on the photomechanical bending and relaxation of these samples [ 77 ]. The following rules were obtained: (1) increasing the rigidity of the polymer backbone can lead to the increased magnitude of the generated stress but decreased bending angles of the cantilevers [ 74 ]; (2) the inclusion of a bulky cardo substituent into the azo polyimide can strongly increase its fractional free volume, which is conducive to the more efficient photoisomerization or reorientation of azo units and thus leads to comparatively larger photoinduced deformation and force generation [ 76 ]; (3) azo polyimide materials with larger segmental mobility can assimilate larger force generation and displacement; (4) crosslinking the rigid backbone polymer provides a network environment containing additional free volume, which is coupled with network connectivity of the crosslinked chains, leads to enhanced light-induced deformation [ 75 , 77 ].…”

“…Although they could exhibit good photomechanical and mechanical properties, they have some obvious drawbacks such as the typical complicated crosslinking processes, low flexibility in fabrication, and less freedom for further processing and recycling. To address these issues, a number of reprocessable photodeformable azo polymers have been developed on the basis of different design strategies [ 29 , 31 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 , 61 , 62 , 63 , 64 , 65 , 66 , 67 , 68 , 69 , 70 , 71 , 72 , 73 , 74 , 75 , 76 , 77 , 78 , 79 , 80 , 81 ,…”

“…They can be mainly classified into two types, i.e., the physically crosslinked ones with dynamic non-covalent bond-involved networks [ 29 , 31 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 ] and chemically crosslinked ones with dynamic covalent bond (DCB)-involved networks [ 61 , 62 , 63 , 64 , 65 , 66 , 67 , 68 , 69 , 70 , 71 ]. In addition, some uncrosslinked photodeformable azo polymers have also been reported and constitute one special type of reprocessable photodeformable azo polymers, whose photodeformation behaviors are mainly induced by the selective reorientation of the azo moieties (become perpendicular to the polarization direction of the polarized light) under the irradiation of either polarized blue light [ 72 , 73 , 74 , 75 , 76 , 77 ] or interfering polarized light [ ...…”

Photodeformable Azobenzene-Containing Polyimide with Flexible Linkers and Molecular Alignment

“…As the rising temperature accelerates both trans−cis photoisomerization and cis−trans thermal isomerization, the deformation at high temperature is a dynamic equilibrium of both isomerizations. 41 The effects of temperature on the PSS and the cis−trans thermal relaxation were evaluated by UV−vis spectroscopy (Figure S14). When the temperature increases, there is a higher content of trans isomers at the photostationary state; thus, a smaller contraction is generated and a slight macroscopic deformation is induced.…”