Photoplastic Self-Healing Polyurethane Springs and Actuators

Abstract

Typical cross-linked polymer networks have rubbery elasticity and do not exhibit thermoplasticity such as linear polymers. However, thermodynamically cross-linked polymer networks exhibit certain thermoplasticity that allows for thermal processing. Here, we introduce a new concept of photoplasticity, that is, under light illumination, the covalently cross-linked polymer elastomers become flexible and plastic-like linear polymers. A novel class of photoplastic polyurethane elastomers based on dynamically covalent crosslinker hexaarylbiimidazole (HABI) and permanent cross-linker glycerol is designed and synthesized. Under the dual actions of light and stretching, the photoplastic elastomer exhibits reversible elongation and contraction-like springs. Because of the photoinduced reversible dissociation/recombination of HABI, two cut samples of solvent-free elastomers can be healed under irradiation at room temperature. Under asymmetric illumination, the photoplastic elastomer exhibits the appreciative phototropism, which demonstrates that artificial crucifer actuators are selectively driven by light. Photoplastic elastomers integrate photodriven healing, reversible stretching, and bending deformation, which offers the potential for new applications as optical actuators through optimized design of molecular structure, composition, and geometry.

Challenges and Opportunities of Self‐Healing Polymers and Devices for Extreme and Hostile Environments

“…A novel class of photo‐plastic polyurethane elastomers based on dynamically covalent cross‐linker hexaarylbiimidazole (HABI) and permanent cross‐linker glycerol was designed and synthesized by Xiang et al. [ 105 ] Under the dual action of light and stretching, the photo‐plastic elastomer exhibited reversible elongation and contraction‐like springs. Two damaged samples were healed understand irradiation (405 nm irradiation) at room temperature, providing a solution to self‐healing soft actuators for severe irradiation environment.…”

Photomechanical polymer hydrogels based on molecular photoswitches

“…Furthermore, Zhu et al 106 modified HABI with four OH groups to form a 2‐Cl‐4‐diol‐HABI molecule. Photoplastic polyurethane elastomers (PPU) with 2‐Cl‐4‐diol‐HABI as dynamic crosslinking unit and glycerol as fixed crosslinking unit were prepared by one‐pot copolymerization of 2‐Cl‐4‐diol‐HABI, PTMG 1000 , HDI and glycerol (Figure 10A).…”

“…Photoplastic polyurethane elastomers (PPU) with 2‐Cl‐4‐diol‐HABI as dynamic crosslinking unit and glycerol as fixed crosslinking unit were prepared by one‐pot copolymerization of 2‐Cl‐4‐diol‐HABI, PTMG 1000 , HDI and glycerol (Figure 10A). 106 PPU can undergo photoplastic deformation under load, while a secondary light irradiation later can restore it to its initial shape, performing as a light‐controlled spring (Figure 10B). The photo‐driven deformation performance was explored.…”

Recent Advances in Stimuli‐Responsive Commodity Polymers

“…In segmented PURs photo‐responsive moieties are typically incorporated into main and side chains of hard and soft segments. Photo‐responsive groups can be also embedded into crosslinkers [ 104 ] as well as PUR blocks in block copolymers, [ 105 ] but mechanophores are typically incorporated into a backbone because mechanical forces can be efficiently transferred during mechano‐activation. Mechanophores can also be incorporated into crosslinking chains [ 106 ] and/or end groups of telechelic PURs.…”

“…These reversible transitions facilitated the opportunity for the development of photoplastics and photoactuators, such as dual‐crosslinking PURs containing glycerols as permanent crosslinker and four‐armed HABI as dynamic crosslinker. [ 104 ] Upon UV radiation and mechanical loading these elastomers elongate, indicating photoinduced plastic deformation. Although removal of the light sources causes incomplete retraction due to limited chain mobility, the process is reversible due to decrosslinking and stored entropy.…”