Mechanically mutable polymer enabled by light

Unusual photo-tunable mechanical transformation of azobenzene terminated aliphatic polycarbonate

Human substance needsśś have been enriched by the development of smart-responsive materials possessing unique responsiveness and mechanical variability. However, acquiring these features in photoresponsive energy-driven elastomers is challengeable but highly desirable. Here, we report fabrication of physically-crosslinked elastomers based on an aliphatic polycarbonate terminated with one azobenzene derivative as the end group. Upon irradiation of UV light, the aliphatic polycarbonate shows unusual mechanical transformation from trans-azobenzene-rich elasticity to cis-azobenzene-rich plasticity, which is contrary to the photo-triggered mechanics of other azopolymers. This indicates that stronger interaction may be established between the terminated cis-azobenzenes and the benzene rings in the side chain of polymer, leading to a higher crosslinking density appeared in the cis-azobenzene-rich sample. This azobenzene-terminated polymer is an energy-driven elastomer, which has photo-switchable supramolecular interactions, showing photo-tunable mechanical properties (the half-life period of the cis-azobenzene is 16.9 h). More interestingly, the photoinduced mechanical change occurs at room temperature, enabling the aliphatic polycarbonate to behave as non-thermally switchable ultra-strong adhesive for different substrates, which is specifically suitable for smart dressings to promote wound healing. This switchable mechanical feature of elastomers may be a reference for smart elastomers towards advanced applications.

Liquid Crystal Promoted Self-Assembly of Statistical Copolymers into Diverse Nanostructures with Precise Dimensions

In both natural and synthetic systems, the segregation of multicomponent entities is vital for regulating functions and the ultimate usage of materials. To accomplish the desired properties via nanosegregation or microphase separation, great effort is usually demanded in the synthesis. For example, microphase-separated block copolymers rely on the delicate controlled/living polymerization of different monomers in sequence. Here, we demonstrate that a facile one-pot copolymerization can generate statistical side-chain copolymers exhibiting well-defined and diverse nanostructures. Two hemiphasmidic (or wedge-shaped) cyclooctene monomers were designed, differing in the peripheral tails of the wedges (dodecyl vs. tetraethylene glycol), with lengths of ca. 1 nm. When combining the two monomers together, the statistical copolymers can show columnar liquid crystal (LC) phase and microphase-separated structures of the two monomers, including sphere, cylinder, double gyroid, and lamella. To the best of our knowledge, this is the first time the gyroid phase has been achieved in statistical copolymers. We further demonstrate that changing the side chains to calamitic (or rod-like) mesogens or the backbone to less flexible polynorbornene, the statistical copolymers can also undergo microphase separation of the side chains. The intrinsic self-assembly scheme of statistical copolymers with mesogenic side chains, which are chemically accurate, affords the resultant nanostructures with precise periodicities at the 10- or sub-10-nm scale. Given the small chemical difference between the side-chain tails, microphase separation is promoted by the anisotropic packing of mesogens. It is validated that the statistical side-chain LC copolymers can be a versatile platform for creating nanostructured materials with tailored functionalities.

Two-way photoswitching norbornadiene derivatives for solar energy storage

Molecular photoswitches of norbornadiene (NBD) derivatives have been effectively applied in molecular solar-thermal energy storage (MOST) by photoisomerization of NBD to a quadricyclane (QC) state. However, a challenge of the NBD-based MOST system is the lack of a reversible two-way photoswitching process, limiting conversion from QC to thermal and catalytic methods. Here we design a series of NBD derivatives with a combination of acceptor and donor units to achieve two-way photoswitching, which can optically release energy by back-conversion from QC to NBD. Highly efficient photoconversion yields from NBD to QC and QC to NBD are up to 99% and 82%, respectively. The energy storage density of two-way photoswitching NBD is up to 312 J g-1 and optically controlled two-way photoswitching devices are demonstrated for the first time both in flow and in thin films, which illustrate a promising approach for fast and robust energy release in both solution and solid state.

Photocontrolled Reversible Solid-Fluid Transitions of Azopolymer Nanocomposites for Intelligent Nanomaterials

Intelligent polymer nanocomposites are multicomponent and multifunctional materials that show immense potential across diverse applications. However, to exhibit intelligent traits such as adaptability, reconfigurability and dynamic properties, these materials often require a solvent or heating environment to facilitate the mobility of polymer chains and nanoparticles, rendering their applications in everyday settings impractical. Here intelligent azopolymer nanocomposites that function effectively in a solvent-free, room-temperature environment based on photocontrolled reversible solid-fluid transitions via switching flow temperatures (Tfs) are shown. A range of nanocomposites is synthesized through the grafting of Au nanoparticles, Au nanorods, quantum dots, or superparamagnetic nanoparticles with photoresponsive azopolymers. Leveraging the reversible cis-trans photoisomerization of azo groups, the azopolymer nanocomposites transition between solid (Tf above room temperature) and fluid (Tf below room temperature) states. Such photocontrolled reversible solid-fluid transitions empower the rewriting of nanopatterns, correction of nanoscale defects, reconfiguration of complex multiscale structures, and design of intelligent optical devices. These findings highlight Tf-switchable polymer nanocomposites as promising candidates for the development of intelligent nanomaterials operative in solvent-free, room-temperature conditions.

Patterned Photonic Actuators with Dynamic Shape-Morphing and Color-Changing Capabilities Fabricated by Athermal Embossing Technology

Stimuli-responsive patterned photonic actuators, characterized by their patterned nano/microscale structures and capacity to demonstrate synergistic color changes and shape morphing in response to external stimuli, have attracted intense scientific attention. However, traditional patterned photonic actuator systems still face limitations such as cumbersome and time-consuming preparation processes and small-scale deformations. Herein, we introduce a facile approach involving an athermal embossing technique to rapidly fabricate patterned photonic actuators based on near-infrared (NIR) light-responsive liquid crystal elastomers. The resulting patterned photonic actuators demonstrate remarkable features, including brilliant angle-dependent structural color, complex three-dimensional actuation, and good color durability under NIR light stimulation. As illustrative demonstrations of the proof-of-concept, we fabricate two light-fuelled patterned photonic soft actuators: a butterfly-inspired actuator that can produce wing-flapping dynamic changes in structural color, and an origami crane-shaped actuator with shape memory, structural color information storage, and dynamic display properties. This strategy provides distinct insights into the design and fabrication of various patterned photonic soft robotic devices and intelligent actuators.

Responsive Liquid Crystal Network Microstructures with Customized Shapes and Predetermined Morphing for Adaptive Soft Micro-Optics

Stimuli-responsive materials have garnered substantial interest in recent years, particularly liquid crystal networks (LCNs) with sophisticatedly designed structures and morphing capabilities. Extensive efforts have been devoted to LCN structural designs spanning from two-dimensional (2D) to three-dimensional (3D) configurations and their intricate morphing behaviors through designed alignment. However, achieving microscale structures and large-area preparation necessitates the development of novel techniques capable of facilely fabricating LCN microstructures with precise control over both overall shape and alignment, enabling a 3D-to-3D shape change. Herein, a simple and cost-effective in-cell soft lithography (ICSL) technique is proposed to create LCN microstructures with customized shapes and predesigned morphing. The ICSL technique involves two sequential steps: fabricating the desired microstructure as the template by using the photopolymerization-induced phase separation (PIPS) method and reproducing the LCN microstructures through templating. Meanwhile, surface anchoring is employed to design and achieve molecular alignment, accommodating different deformation modes. With the proposed ICSL technique, cylindrical and spherical microlens arrays (CMLAs and SMLAs) have been successfully fabricated with stimulus-driven polarization-dependent focusing effects. This technique offers distinct advantages including high customizability, large-area production, and cost-effectiveness, which pave a new avenue for extensive applications in different fields, exemplified by adaptive soft micro-optics and photonics.

Photoresponsive Viscoelasticity of the Granular Materials of Azobenzene-Bearing Molecular Nanoparticles

The large sizes of granular particles lead to their slow diffusive dynamics and significant interparticle friction, bringing enormous difficulty to tune the mechanical properties and processability of the granular materials (GMs). Herein, 1 nm polyhedral oligomeric silsesquioxane (POSS) particles functionalized with azobenzene are designed as structural units, and the obtained GMs show unique photoswitchable viscoelasticity. The azobenzene group can undergo a reversible trans-cis conformation switch while the π-π stacking among the azobenzene fragments is only favored by the trans-conformation due to molecular geometrical requirements. The POSS units from neighboring assemblies close pack to form microdomains, and the POSS is under confinement by both the supramolecular bonding and the other POSS in the microdomains. The simultaneous breaking of the two types of confinement is difficult and, therefore, the free diffusion of POSS is hindered, leading to the elasticity of the GMs of trans-POSS. For cis-POSS, the interparticle supramolecular interaction is weak and the POSS unit can undergo free diffusion, contributing to their high flowability at room temperature. The photoswitching viscoelasticity of GMs is further used for self-healing and photoswitchable adhesion. This work paves new pathways for the regulation of material viscoelasticity and the design of GM-based smart materials.

Light-Responsive Supramolecular Liquid-Crystalline Metallacycle for Orthogonal Multimode Photopatterning

The development of multimode photopatterning systems based on supramolecular coordination complexes (SCCs) is considerably attractive in supramolecular chemistry and materials science, because SCCs can serve as promising platforms for the incorporation of multiple functional building blocks. Herein, we report a light-responsive liquid-crystalline metallacycle that is constructed by coordination-driven self-assembly. By exploiting its fascinating liquid crystal features, bright emission properties, and facile photocyclization capability, a unique system with spatially-controlled fluorescence-resonance energy transfer (FRET) is built through the introduction of a photochromic spiropyran derivative, which led to the realization of the first example of a liquid-crystalline metallacycle for orthogonal photopatterning in three-modes, namely holography, fluorescence, and photochromism.

Manipulation of photoresponsive liquid-crystalline polymers and their applications: from nanoscale to macroscale

We summarize the molecular design of photoresponsive liquid-crystalline polymers, manipulation at multiple scales and various applications based on their intrinsic properties, providing an opportunity for future development in this field.

Kinetics of UV Radiation-Induced Fast Collapse and Recovery in Thermally Cycled and Rehydrated Light- and Thermo- Double-Responsive Copolymer Films Probed by In Situ Neutron Reflectivity

The kinetics of UV radiation-induced fast collapse and recovery in thermally cycled and rehydrated light- and thermo- double-responsive copolymer films of poly(oligo(ethylene glycol) methyl ether methacrylate-co-6-(4-phenylazophenoxy)hexyl acrylate), abbreviated as P(OEGMA300-co-PAHA), are probed by in situ neutron reflectivity (NR). The copolymer film is exposed to a thermal treatment starting at a temperature of 60 °C, which is well above its transition temperature (TT = 53 °C) before the temperature is rapidly decreased from 60 to 23 °C. Based on the applied protocol, the initially collapsed P(OEGMA300-co-PAHA) film is rehydrated due to the switching of polymer chains from a more hydrophobic to a more hydrophilic state when the temperature falls below its TT. The whole rehydration process can be divided into 3 stages: D2O absorption, chain rearrangement, and film reswelling. After rehydration, the thermally cycled P(OEGMA300-co-PAHA) film is switched by UV irradiation via setting the UV radiation on and off. Considering the UV-induced collapse and recovery, both processes are slower than those observed in freshly hydrated films without any thermal stimulus history. Therefore, the experienced thermal history of the film should be considered in the design of sensors and detectors based on double-responsive copolymer films.

Molecular Photoswitching of Main-Chain α-Bisimines in Solid-State Polymers

Photoisomerization of chromophores usually shows significantly less efficiency in solid polymers than in solution as strong intermolecular interactions lock their conformation. Herein, we establish the impact of macromolecular architecture on the isomerization efficiency of main-chain-incorporated chromophores (i.e., α-bisimine) in both solution and the solid state. We demonstrate that branched architectures deliver the highest isomerization efficiency for the main-chain chromophore in the solid state─remarkably as high as 70% compared to solution. The macromolecular design principles established herein for efficient solid-state photoisomerization can serve as a blueprint for enhancing the solid-state isomerization efficiency for other polymer systems, such as those based on azobenzenes.

Repairable Macroscopic Monodomain Arrays from Block Copolymers Enabled by Photoplastic and Photodielectric Effects

Upon exposure to UV light (120 mW/cm², λ = 365 nm), a trans-cis isomerization occurs in a cylinder-forming, azobenzene-containing block copolymer of polydimethylsiloxane-b-poly((4(phenyldiazenyl)phenoxy)hexyl acrylate) (PDMS-b-PPHA) that enables the generation of monodomains of healable, long-range ordered arrays of nanoscopic domains over macroscopic distances. The trans-cis isomerization gives rise to a significant increase in the dielectric constant (from 6.52 to 19.8 at 100 Hz, photodielectric behavior) and a dramatic decrease in the T_g (from 54 to 1 °C, photoplastic behavior) of the PPHA block. By combining these characteristics with an in-plane electric field, macroscopic monodomains of near-perfectly aligned cylindrical microdomains are achieved at low temperatures, and a damage repair is clearly uncovered, where the 300 nm wide scratches can be completely healed at 40 °C, leaving a smooth, uniformly thick film where the continuity and orientation of the aligned microdomains are restored. Subsequent exposure to visible light causes a cis-trans isomerization, increasing the matrix T_g to 54 °C, producing highly oriented and aligned PDMS cylindrical microdomains in a PPHA matrix.