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The shape-shifting behavior of thin film under FPP. (a) Schematic of the DLP-based FPP experiment setup. (b) The front of the solidification interface propagates from the illuminated surface (bottom) into the resin during the process of FPP. The sequential volume shrinkage of the cured layer is due to the formation of the covalent bonds. (c) The cured layer deforms from a 2D to a 3D structure due to internal stress after removed from the substrate. (d) The conversion ratio varies with the distance to the illuminating surface under different exposure doses. The samples were cured under an incident light intensity of 4 mW/cm 2 for 10 s, 15 s, 20 s, and 25 s, respectively. The dashed line represents the critical conversion ratio. (e) The solidified thickness varies as a function of irradiation time. The light intensities used are 3.5, 4, and 5 mW/cm 2 , respectively. (f) Young's modulus varies as a function of the conversion ratio. (g) Shrinkage strain changes as a function of the conversion ratio.
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... of the DLP-based FPP process, where the light is generated by a projector and reflected by a reflector to cure the liquid resin upward from the bottom through a transparent glass slide. Upon illumination, the photoinitiator decomposes into free radicals, and these free radicals react with the polymer monomers to form a larger polymer network (Fig. 1b). The volume decreases during polymerization, resulting from the formation of the covalent bonds. Therefore, the volume shrinkage during FPP is a function of the polymerization ...
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... during polymerization, newly formed layers will contract under the constraint of the previously cured layers. Thus, the cured polymer film tends to bend toward the newly formed layers. But it remains a flat shape limited by the substrate. Once it is removed from the substrate, the residual stress inside the polymer film drives it to bend (Fig. 1c and Video S1). To better explain the internal stress formation and the shape transformation mechanism, we use the layer concept to distinguish the early cured part and the newly cured part. However, it should be noted that the film is cured under a single layer printing. To accurately control the internal stress and the resulting ...
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... (0 ≤ φ ≤ 1) along the thickness direction z of the cured film is measured. The origin of the z coordinate is set on the illumination surface of the cured film. The conversion ratios are measured using the FTIR spectra method. The experimental conversion ratios at z = 0.1 to 0.4 mm under different illumination times are presented by the markers in Fig. 1d. Noted that the monomer-to-polymer conversion only exits upon a critical threshold conversion φ c . This conversion ratio φ c is measured and set as ...
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... t is the irradiation time, K a material-related reaction constant, I 0 the incident light intensity and μ 0 the attenuation coefficient. By comparing the theoretical and experimental φ in Fig. 1d, K and μ 0 are fitted as K=0.025 cm 2 mW −1 s −1 and μ 0 =5.1 mm −1 . At the interface between solid and liquid, the conversion ratio is φ c . Substituting φ c into Eq. (1), the corresponding solidified thickness z f can be written ...
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... Fig. 1e, the theoretical predicted and experimental measured z f are compared for samples cured with different times under various light intensities 3.5, 4, and 5 mW/cm 2 ...
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... to the illumination gradient, the Young's modulus E varies along the thickness direction. We measured E of the cured sample at various φ as shown in the markers in Fig. 1f. An apparent gradient exists within E. The cured layer near the substrate has the largest modulus, while that at the solid-liquid interface shows a vanishing modulus. An exponential function is then used to fit the continuous change of E along z directions [3,20]. ...
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... obtain the distribution of the shrinkage ratio on the z direction, the shrinkage strain ε s of the cured film under different exposure doses D (the product of incident light intensity I 0 and irradiation time t, D = I 0 × t) are measured. The dependence of the measured ε s on φ is plotted in Fig. 1g and fitted by a linear function [3]: 4), the dependence of ε s on z can be obtained and is plotted in Fig. 2b. Similar to E, ε s varies significantly along z and can be denoted by E(z) and ...
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... Due to the limitation of scope, this review could not include all achievements related to this research area, and only a selection of representative examples will be introduced. Readers are encouraged to refer to other excellent review papers in the literature for additional examples [28][29][30], like photoresponsive smart materials [31], nonlinear photochromic materials [32], 3-dimensional (3D) printing of photochromic liquid crystal elastomers [33], photochromic photoinitiators [34], 3D printing of photochromic hydrogels [35] and photochromic optical control devices [36]. ...
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