# Composite Films of Polydimethylsiloxane and Micro-Graphite with Tunable Optical Transmittance
## 3 Results and Discussion
### Mechanical Stretching of Trapezoid-Shaped Composite Films with Uniform Thickness  
Mechanical loading can be used to control the transmittance of the sample by varying its thickness. Both uniaxial stretching and biaxial stretching can be used for this purpose. [PERSON] [29] verified the  
Figure 4: Cuneiform specimen (concentration \(c\) = 0.2 %; thickness, 0.3 mm \(<h<\) 2.0 mm), placed on top of paper with printed colored fonts. Figure 5: Characterization of transmittance versus sample thickness. feasibility of this method. However, because their sample had uniform thickness, it could only provide uniform transmittance on any given sample, even after being stretched. To obtain a wide transmittance range on a specific sample, we tentatively proposed a more flexible method: first, we cut the uniform-thickness composite film into different trapezoidal shapes in plane and then stretched the resulting samples in the direction perpendicular to the base line and the top line. We tailored the samples to produce three different trapezoidal shapes as shown in Figure 6a. All three trapezoidal samples had the same base line length of 30 mm and the same height of 60 mm, but the lengths of their top lines were 10 mm, 15 mm and 20 mm. Henceforth, we called the three trapezoidal samples listed above A, B and C, respectively. We then fixed sample A using its base line and added a mechanical stretching load on its top line. This trapezoidal specimen was stretched out to an elongation of 20%; in other words, the height of the stretched specimen increased to 72 mm. The measurements were repeated and taken for the other two samples, B and C, in turn. For simplicity, we considered the case in which stretching occurred in the plane perpendicular to the direction of propagation of the light when loading was applied. When we stretched the sample, because of its incompressibility, the concentration of the microcrystalline graphite powder remained unaffected; however, the sample's thickness gradually decreased, i.e., the optical path was reduced by degrees. To be precise, the sample's thickness changed gradually from top to bottom because the forces that acted at the different locations were different. In this way, we acquired a type of composite film with a special feature in that its transmittance varied gradually from one side of the sample to the other. After stretching, we took the base line of the sample to be the initial position, i.e., x = 0, and then measured the transmittance values of the three trapezoidal samples at their central line along the stretching direction X at 10 mm intervals. Figure 6b shows the results. If the stretched trapezoid-shaped samples maintain their normal trapezoidal shapes, then the thickness variation in the X direction will be linear. However, in the experiments, both the sides and the centers of the samples showed inhomogeneous shrinkage along the stretching direction. This led to inhomogeneous thickness gradients on all three stretched composite films, where the rate of variation in thickness increased as the measurement point became closer to the top side. The variation of transmittance shown in Figure 6b was consistent with the variation of thickness of the stretched composites. Furthermore, we found that samples with different trapezoidal shapes have different transmittance tuning capabilities. In addition, both the rate and the range of the variation in transmittance in sample A, which had the shortest top length, were maxima. In contrast, both the rate and the range of the variation in transmittance of sample C, which had the longest top length, were  
Figure 6: (**a**) Three different trapezoid-shaped samples with top line dimensions of (A) 10 mm, (B) 15 mm and (C) 20 mm; (**b**) transmittance at the central line for stretched composite films A, B and C along their stretching directions at intervals of 10 mm; the inset shows a diagram of the stretched sample. minimal.