Reprocessable Photodeformable Azobenzene Polymers

Multifunctional UV-NIR Dual Light-Responsive Soft Actuators from a Main-Chain Azobenzene Semi-Crystalline Poly(ester-amide) Doped with Polydopamine Nanoparticles

The development of soft photoactuators with multifunctionality and improved performance is highly important for their broad applications. Herein, we report on a facile and efficient strategy for fabricating such photoactuators with UV-NIR dual light-responsivity, room-temperature 3D shape reprogrammability and reprocessability, and photothermal healability by doping polydopamine (PDA) nanoparticles into a main-chain azobenzene semi-crystalline poly(ester-amide) (PEA). The PEA/PDA nanoparticle composite was readily processed into free-standing films with enhanced mechanical and photomechanical properties compared with the blank PEA films. Its physically crosslinked uniaxially oriented films showed rapid and highly reversible photochemically induced bending/unbending under the UV/visible light irradiation at room temperature in both the air atmosphere and water. When exposed to the NIR light, they (and their bilayer films formed with a polyimide film) exhibited photothermally induced bending even at a temperature much lower than their crystalline-to-isotropic phase transition temperature based on a unique mechanism (involving photothermally induced polymer chain relaxation due to the disruption of their hydrogen bonds). The room-temperature 3D shape reprogrammability and reprocessability and photothermal healability of the composite polymer films were also demonstrated. Such multifunctional dual light-responsive photoactuators with well-balanced mechanical robustness, actuation stability, 3D shape reprogrammability/reprocessability and photothermal healability hold much promise in various photoactuating applications.

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

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 (T_g ≤ 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 T_g 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.

Understanding the formation of surface relief gratings in azopolymers: A combined molecular dynamics and experimental study

The formation of surface relief gratings in thin azopolymeric films is investigated using atomistic molecular dynamics simulations and compared to experimental results for the specific case of poly-disperse-orange3-methyl-methacrylate. For this purpose, the film is illuminated with a light pattern of alternating bright and dark stripes in both cases. The simulations use a molecular mechanics switching potential to explicitly describe the photoisomerization dynamics between the E and Z isomers of the azo-units and take into account the orientation of the transition dipole moment with respect to the light polarization. Local heating and elevation of the illuminated regions with the subsequent movement of molecules into the neighboring dark regions are observed. This leads to the formation of valleys in the bright areas after re-cooling and is independent of the polarization direction. To verify these observations experimentally, the azopolymer film is illuminated with bright stripes of varying width using a spatial light modulator. Atomic force microscopy images confirm that the elevated areas correspond to the previously dark areas. In the experiment, the polarization of the incident light makes only a small difference since tiny grain-like structures form in the valleys only when the polarization is parallel to the stripes.

Reconfigurable, Reversible, and Redefinable Deformation of GO Based on Quantum-Confined-Superfluidics Effect

Graphene oxide (GO) films with natural "quantum-confined-superfluidics" (QSF) channels for moisture actuation have emerged as a smart material for actuators and soft robots. However, programming the deformation of GO by engineering QSF nanochannels around 1 nm is extremely challenging. Herein, we report the reconfigurable, reversible, and redefinable deformation of GO under moisture actuation by tailoring QSF channels via moisture-assisted strain-induced wrinkling (MSW). The shape fixity ratio of a general GO film can reach ∼84% after the MSW process, and the shape recovery ratio is ∼83% at room temperature under moisture actuation. The flexible shaping and deformation abilites, as well as the self-healing property of GO make it possible to fabricate soft robots using GO. Besides, as a proof-of-concept, passive electronics and soft robots capable of crawling, turning, switching circuit, and automatic somersault are demonstrated. With unique shaping and deformation abilities, GO may bring great implications for future soft robotics.

Smart Responsive Azo-Copolymer with Photoliquefaction for Switchable Adhesive Application

The development and utilization of switchable adhesives are considered to be an essential target to solve the problems of their separation and recycling in some specific service environments, such as the preparation or repair process of electronic devices. Intelligent materials with controllable phase transition are utilized to fabricate switchable adhesives because of the significantly diverse adhesion strengths in different phase states. Photoresponsive azobenzene and its derivatives usually possess different melting temperatures (T_m) or/and glass transition temperatures (T_g) of the cis-trans isomers, which are beneficial to making the photoinduced solid-liquid phase transition for switchable adhesive application possible. Here, a novel three-component azo-copolymer (PNIM-Azo) with fast and reversible photoinduced solid-liquid phase transition has been designed and synthesized. PNIM-Azo possesses reversible bonding/debonding processes, resulting from the different adhesion strengths between trans-configuration PNIM-Azo in the solid state and cis-configuration in the liquid state. Moreover, by incorporating commercialized 2-methoxyethyl acrylate and N-isopropylacrylamide with O and N heteroatoms into the copolymer, the trans-configuration PNIM-Azo possesses the highest adhesion strength (∼11 MPa between two glass substrates) among all of the reported azobenzene-based switchable adhesives, which could be attributed to the increase in the entanglement effect because of the H-bond in the polymer chains formed by introducing heteroatoms. Our synthesized PNIM-Azo copolymer provides an alternative for designing and developing switchable adhesives with high adhesion strength for some electronic production processes.

Fully Room-Temperature Reprogrammable, Reprocessable, and Photomobile Soft Actuators from a High-Molecular-Weight Main-Chain Azobenzene Crystalline Poly(ester-amide)

Azobenzene (azo) polymer photoactuators with full room-temperature reprogrammability, reprocessability, and photomobility are highly desirable for large-scale applications, but their development remains a daunting challenge. Herein, a strategy is first presented for fabricating such advanced photoactuators from a high-molecular-weight main-chain azo crystalline poly(ester-amide) (PEA) prepared via Michael addition polymerization. This azo PEA can be readily processed into both physically cross-linked, uniaxially oriented fibers and films with high mechanical robustness and reversible photoinduced bending/unbending at room temperature. Importantly, the presence of both amide unit-induced hydrogen bonding and crystalline domains in such films and fibers endows them with dynamic, yet stable cross-linking points, which enable their easy reprogrammability under strain at room temperature into various three-dimensional (3D) shapes (e.g., film helicoid and spiral ribbon, fiber spring) capable of showing completely different shape-dependent photomobile modes. In particular, these reshaped photoactuators can maintain their accurate 3D shapes and highly reversible photoinduced motions even after being kept at 80 °C for 20 days or at 100 °C for 2 days. They can also be reprocessed and recycled from solution at room temperature. Such a multifunctional main-chain azo crystalline PEA can serve as a versatile platform for fabricating various photoactuators with desired 3D shapes and motion modes under mild ambient conditions.

Enhancing the performances of physically cross-linked photodeformable main-chain azobenzene poly(ester-amide)s via chemical structure engineering

The development of physically cross-linked photodeformable main-chain azobenzene poly(ester-amide)s with enhanced performances via chemical structure engineering and obtention of their detailed structure–property relationship are first described.