Graphene‐Based Polymer Bilayers with Superior Light‐Driven Properties for Remote Construction of 3D Structures

3D structure assembly in advanced functional materials is important for many areas of technology. Here, a new strategy exploits IR light‐driven bilayer polymeric composites for autonomic origami assembly of 3D structures. The bilayer sheet comprises a passive layer of poly(dimethylsiloxane) (PDMS) and an active layer comprising reduced graphene oxides (RGOs), thermally expanding microspheres (TEMs), and PDMS. The corresponding fabrication method is versatile and simple. Owing to the large volume expansion of the TEMs, the two layers exhibit large differences in their coefficients of thermal expansion. The RGO‐TEM‐PDMS/PDMS bilayers can deflect toward the PDMS side upon IR irradiation via the cooperative effect of the photothermal effect of the RGOs and the expansion of the TEMs, and exhibit excellent light‐driven, a large bending deformation, and rapid responsive properties. The proposed RGO‐TEM‐PDMS/PDMS composites with excellent light‐driven bending properties are demonstrated as active hinges for building 3D geometries such as bidirectionally folded columns, boxes, pyramids, and cars. The folding angle (ranging from 0° to 180°) is well‐controlled by tuning the active hinge length. Furthermore, the folded 3D architectures can permanently preserve the deformed shape without energy supply. The presented approach has potential in biomedical devices, aerospace applications, microfluidic devices, and 4D printing.


Supplementary
. Young's modulus, tensile strength, and coefficient of thermal expansion (CTE) for polymer composites.

Materials
Young's modulus (MPa) Tensile strength (MPa) CTE (10 -6 K -1 ) PDMS 1.21 1.00 266-310 [S1] RGO-TEM-PDMS This table provides some important physical parameters of some polymer composites. These experimental data are an important part of the main text by demonstrating that the active layer has the highest CTE and a larger modulus. Figure S1. Optical photographs of the dynamic bending behaviors of the RGO-TEM-PDMS/PDMS bilayer sheets at different deformation process under IR irradiation. The bilayer polymeric composites always bend to PDMS side when the lamp irradiated either side. The three images correspond to (a) the initial state, (b) the state after 15 s IR irradiation, and (c) TEMs in active layer fully expanded (following 15 s IR irradiation), the specimen reached maximum deformation state. Scale bars are 10 mm.

Model for the Bilayer Sheet
Classical beam theory by Timoshenko was first used to model bilayer bending under thermal expansion. [S2] The bilayer sheet is depicted in Figure S7. The sheet of total thickness h has a passive layer of PDMS with thickness h p and an active layer of RGO-TEM-PDMS with thickness h a so that h p +h a =h. The width of sheet is denoted as w. When the bilayer film is uniformly heated from temperature T 0 to T, it bends to a configuration with radius of curvature ρ due to unequal thermal expansion of the constituent layers. The model is based on five assumptions: 1) the thickness of the beam is small compared to the minimum radius of curvature; 2) the strain throughout the bilayer is determined only geometrically by the curvature; 3) there is a linear relationship between stress and strain of the material; 4) Young's modulus and the actuation coefficient of expansion of the active layer are constant and do not depend on spatial location inside each layer; and 5) the curvature along the width of the hinge can be neglected, and deflection is only a function of length.
The relationship between the obtained radius of curvature ρ and the geometric parameters and material properties of the film was found by Timoshenko to be   where m = h p /h a and n = E p /E a . α and E denote the thermal expansion coefficient and elastic modulus of the sheets, respectively. Subscript p and a represent the passive layer (PDMS layer) and active layer (RGO-TEM-PDMS layer), respectively. Here, the passive layer provides negligible strain compared with active layer, that is, the expanding ability of the passive layer (PDMS) α p is assumed to be zero. [S3] Thus we use the following simple where ε is the actuation strain.
The folding angle θ (°) can be calculated with the simple rules and starting from the radius of curvature as: where l is the hinge lengths (arc length, θ). The folding angle is proportional to the hinge length l. That is, as hinge length increases, the folding angle of the hinge actuation bilayer sheet increases. The experimental data are well described by a linear relationship across the