Physically Cross-Linked Main-Chain Ortho-Fluorinated Azobenzene Semicrystalline Polymer Photoactuators with Ultralarge Contraction and Multiple Reprogrammable Properties via Simple Thermal Annealing

Azobenzene (azo) polymer photoactuators with multiple reprogrammable properties are highly appealing for many photodriven applications, but their development remains challenging. Herein, we report a facile yet versatile strategy for fabricating such advanced azo polymer photoactuators with multiple reprogrammable properties (including photodeformation modes, photoinduced stress, and mechanical strength) from a physically cross-linked main-chain ortho-fluorinated azo semicrystalline polymer via simple thermal annealing. The main-chain ortho-fluorinated azo polymer capable of showing visible-light-induced trans–cis isomerization was efficiently prepared via Michael addition polymerization of a diacrylate-type ortho-fluorinated azo monomer and 1,4-dithiothreitol under mild condition. It can be readily processed into free-standing uniaxially oriented films with dual physical cross-linking networks (formed by both the vicinal diol unit-induced hydrogen bonding and crystalline domains) and strains up to 2483% via solution casting/mechanical stretching method. The stretched films could exhibit up to the unprecedented 87% shrinkage due to the photoinduced destruction of the crystalline domains in the stretched films under 470 nm blue light irradiation. Importantly, both the photomechanical and mechanical properties of the stretched films could be easily reprogrammed by their simple thermal annealing for different times, leading to visible-light-induced multiple photodeformation modes (from one-dimensional shrinkage to shrinkage plus bending and further to reversible bending/unbending at ambient temperature) and largely enhanced photoinduced stress and mechanical strength for a single sample, mainly owing to their thermal annealing-induced enhancement in crystallinity and interchain hydrogen bonding interactions. The strategy presented here offers new possibility for efficiently fabricating remoldable photoactuators with easily tunable photomechanical and mechanical properties from physically cross-linked azo semicrystalline polymers.