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. 2023 May 18;28(10):4174.
doi: 10.3390/molecules28104174.

Fully Room Temperature Reprogrammable, Recyclable, and Photomobile Soft Actuators from Physically Cross-Linked Main-Chain Azobenzene Liquid Crystalline Polymers

Shengkui Ma  1 , Lei Wang  1 , Yan Zhou  1 , Huiqi Zhang  1
Affiliations

Fully Room Temperature Reprogrammable, Recyclable, and Photomobile Soft Actuators from Physically Cross-Linked Main-Chain Azobenzene Liquid Crystalline Polymers

Shengkui Ma et al. Molecules. .

Abstract

Fully room temperature three-dimensional (3D) shape-reprogrammable, recyclable, and photomobile azobenzene (azo) polymer actuators hold much promise in many photoactuating applications, but their development is challenging. Herein, we report on the efficient synthesis of a series of main-chain azo liquid crystalline polymers (LCPs) with such performances via Michael addition polymerization. They have both ester groups and two kinds of hydrogen bond-forming groups (i.e., amide and secondary amino groups) and different flexible spacer length in the backbones. Such poly(ester-amide-secondary amine)s (PEAsAs) show low glass transition temperatures (Tg ≤ 18.4 °C), highly ordered smectic liquid crystalline phases, and reversible photoresponsivity. Their uniaxially oriented fibers fabricated via the melt spinning method exhibit good mechanical strength and photoinduced reversible bending/unbending and large stress at room temperature, which are largely influenced by the flexible spacer length of the polymers. Importantly, all these fibers can be easily reprogrammed under strain at 25 °C into stable fiber springs capable of showing a totally different photomobile mode (i.e., unwinding/winding), mainly owing to the presence of low Tg and both dynamic hydrogen bonding and stable crystalline domains (induced by the uniaxial drawing during the fiber formation). They can also be recycled from a solution at 25 °C. This work not only presents the first azo LCPs with 3D shape reprogrammability, recyclability, and photomobility at room temperature, but also provides some important knowledge of their structure-property relationship, which is useful for designing more advanced photodeformable azo polymers.

Keywords: azobenzene; main-chain liquid crystalline polymers; photomobile; physically cross-linked; recyclable; reprogrammable; room temperature.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Scheme 1
Scheme 1
Synthetic routes and chemical structures of the diacrylate-type azo monomer bearing an amide group (M-Azo) and the main-chain azo liquid crystalline PEAsAs (prepared via the Michael addition polymerization of M-Azo and α,ω-alkanediamines (i.e., NH2(CH2)nNH2 (n = 2, 6, 12)).
Figure 1
Figure 1
1H NMR spectra of M-Azo (A), PEAsA-6 (B), and A-PEAsA-6 (C) in CDCl3.
Figure 2
Figure 2
(a) DSC thermograms of PEAsA-n (n = 2 (a1,a4), 6 (a2,a5), 12 (a3,a6)) from the second heating scan (a1–a3) and first cooling scan (a4–a6) (±10 °C min−1). (b) XRD spectra of PEAsA-n (n = 2, 6, 12) (first cooling/second heating rate: ±10 °C min−1): (b1,b2) XRD spectra of PEAsA-2 obtained after being annealed at 84 °C (b1) and 68 °C (b2) for 30 min (during the first cooling process); (b3–b5) XRD spectrum of PEAsA-6 obtained after being annealed at 59 °C (b3) (during the first cooling process), as well as those obtained after being annealed at 80 °C (b4) and 95 °C (b5) for 30 min (during the second heating process); (b6) XRD spectrum of PEAsA-12 obtained after being annealed at 77 °C for 30 min (during the first cooling process).
Figure 3
Figure 3
UV-vis spectral changes in dependence of time for the thin PEAsA-6 film (cast on the quartz glass plate, film thickness: 740 nm) upon its exposure to 365 nm of UV light (90 mW cm−2) (a) and upon irradiating the polymer film at the photostationary state with visible light (λ > 510 nm, 35 mW cm−2) (b) at 25 °C.
Figure 4
Figure 4
(a) POM images of the textures of a PEAsA-6 fiber taken at room temperature. Sample angle to the analyzer: θ = 0° (left) and 45° (right). (b) Polarized UV-vis absorption spectra of the uniaxially oriented PEAsA-6 fiber. (c) XRD spectrum of the uniaxially oriented PEAsA-6 fiber.
Figure 5
Figure 5
(a) Stress–strain curves of the uniaxially oriented PEAsA-n fibers (length: 10 mm, diameter: 20 μm) measured at 25 °C at a stretching rate of 10 mm min−1. (b) Dependence of the photoinduced stress of the uniaxially oriented PEAsA-n fibers (length: 10 mm, diameter: 25–30 μm) on the irradiation time of 365 nm of UV light (90 mW cm−2) at 25 °C. An external stress of 200 kPa was initially loaded on the fibers to keep their length constant.
Figure 6
Figure 6
(a) Photographs of a representative PEAsA-6 fiber that exhibits photoinduced bending and unbending upon irradiation with 365 nm of UV light (90 mW cm−2) and visible light (λ > 510 nm, 35 mW cm−2) at 25 °C (fiber size: 10 mm × 20 μm). (b) The reversible deformation of the PEAsA-6 fiber characterized by tracing the bent distance from its straight state at 25 °C.
Figure 7
Figure 7
Dependence of the UV light-induced bending times and visible light-induced unbending times (for reaching the maximum bending and restoring the initial straight state, respectively) and bending amplitudes (L) of PEAsA-n fibers on the flexible spacer length in the polymer backbones.
Figure 8
Figure 8
Three-dimensional shape reprogramming of a uniaxially oriented PEAsA-6 fiber and photomobile behaviors of the resulting reshaped photoactuator. (a) Reshaping a uniaxially oriented PEAsA-6 fiber (diameter: 24 μm) into a fiber spring (the wire diameter: 5 mm) at 25 °C and its shape (top) and homogeneous azo mesogen alignment (below, the reshaped fiber was untwisted for POM observation) remain unchanged after being kept at 25 or 60 °C for 10 days. (b) Photomobile behaviors of the reshaped fiber spring at room temperature under the irradiation of UV light (365 nm, 40 mW cm−2) and visible light (λ > 510 nm, 30 mW cm−2).
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