Humidity-responsive
polymeric actuators have gained considerable
interest due to their great potential in the fields including soft
robotics, artificial muscles, smart sensors, and actuators. However,
most of them can only exhibit invariable shape changes, which severely
restricts their further exploration and practical use. Herein, we
report that programmable humidity-responsive actuating behaviors can
be realized by introducing photoprogrammable hygroscopic patterns
into shape memory polymers. Poly(ethylene-co-acrylic
acid) is selected as a model polymer and the solvent-processed thin
films are soft and elastic, whose external shapes can be programmed
by a modified shape memory process. On another aspect, an Fe3+–carboxylate coordinating network formed by surface treatments
can be spatially dissociated under UV, resulting in transient hygroscopic
gradients as active joints for moisture-driven actuation. Moreover,
we show that the shape memory effect can be an effective means to
adjust the direction as well as the amplitude of the moisture-driven
actuating behavior. The proposed strategy is convenient and can be
generally extended to other shape memory polymers to realize programmable
moisture-responsive actuating behaviors.
Most
humidity-responsive polymeric actuators can only exhibit shape
transformations between a planar shape in the dry state and a bended
three-dimensional (3D) shape when exposed to moisture, and it is challenging
to design and prepare hygroscopic actuators with programmable actuating
behaviors displayed from sophisticated 3D structures. Herein, we demonstrate
that the integration of shape memory property and surface treatment
enabled hygromorphic responsivity endows a single-component polymer
film with programmable moisture-driven actuating behaviors. The solvent-processed
polyethylene-co-acrylic acid (EAA) copolymer film
is soft and stretchable at room temperature, and has a good thermal-responsive
shape memory property. By surface treatment using base/acid solutions,
the reversible gradient conversion between carboxyl groups and carboxylate
salts along the thickness direction enables the film to exhibit designed
hygroscopic actuations. The shape memory property and moisture-driven
actuating behaviors can be combined to realize 3D-3D morphing by first
programming the films into 3D shapes and then conducting the surface
treatments. Both shape programming and surface treatment processes
can be reprogrammed to make the actuation behavior readily tunable.
We also show that the created surface patterns can act as moisture-sensitive
conducting paths to detect human breathes, and the combination of
shape memory, moisture-responsive morphing and conductivity change
leads to some interesting applications such as smart switch in conducting
circuit. This work provides a new and general strategy for the design
of advanced humidity-responsive actuators.
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