Photoplastic Transformation Based on Dynamic Covalent Chemistry

Abstract

The magical fantasy of decades-old transformer characters is becoming closer to scientific reality, as transformable materials can change their shapes in response to thermal, mechanical, electrical, and chemical stimuli. However, precise and prompt control of plastic shaping remains to be wanted. Photoresponsive materials provide a promising alternative for rapid optomechanical shaping with limited success. Here, we report a new class of photoplastic transformation based on dynamic covalently crosslinked polytriazole (PTA) networks, in which crosslinking points are comprised of photocleaveable hexaarylbiimidazole (HABI). Upon sub-500 nm light irradiation, HABI is dissociated into two triphenylimidazole radicals (TPIRs) followed by spontaneous recombination back to the initial state. This photoswitching effect is demonstrated to generate nonthermal shape change in the PTA-HABI gel network at will upon light stimulus. A unique photoalignment phenomenon has also been discovered which can form oriented nanoscale patterning in the PTA-HABI gel network upon laser irradiation. The solvent-free PTA-HABI elastomer exhibits photoenhanced automatic self-healing properties at temperatures ranging from 25 °C to freezing points, which is attributed to the dynamic equilibrium between TPIRs and HABI. A photoplastic spring is fabricated and exhibits photoswitchable plastic behavior, i.e., a reversible transformation between plastic strain and elastic strain upon light irradiation. HABI-based polymer networks, including solvated gel and solvent-free elastomer, are promising as smart materials for nonthermal photoactivated shape changing, transformation, and self-healing applications.

Mechanochromophore‐Linked Polymeric Materials with Visible Color Changes

“…The conversion from HABI to TPI can also be triggered by heat, light, or force, and the response to light has been used for self-healing. [79,80] The generated TPIs upon C-N bond cleavage endow the polymeric materials with a blue color.…”

Photomechanical polymer hydrogels based on molecular photoswitches

“…Zhu et al 102 paid more interest to the photomechanical performance of HABI‐based photosensitive polymers. They demonstrated that HABI‐based polymers with photoplastic properties could be used in photo‐processing, self‐healing, photoplastic springs and photomechanical hydrogels.…”

“…They have designed and prepared 2‐Br‐tetrapropargyl‐HABI, which is copolymerized with dipropargyl polyethylene glycol ether and 1, 6‐diazide hexane by aid of click chemistry to obtain the cross‐linked polymer of PTA‐HABI gel (Figure 8). 102 A range of photo‐transformation and photo‐processing applications based on PTA‐HABI gels were conducted, including light‐induced swelling, light‐induced melting, light‐induced self‐healing, light cutting, light drilling, light reshaping, and micro‐etching 102 . To be important, light remodeling and light‐induced self‐healing are of great significance for the recycling of materials.…”

“…After removing the light source and standing in the dark for minutes, the hydrogel gradually became solidified. The solid–liquid conversion cycles with light participation can be repeated many times 102 . Such a smart material could achieve a large degree of light‐controlled shape deformation, repeatedly reshape and self‐heal after fracture, which inspired much interest in light‐controlled mechanical motions with self‐healing and recyclability at the same time.…”

“…Solid‐liquid conversion cycling process of PTA‐HABI gel. Adapted with permission 102 . Copyright 2019, American Chemical Society.…”