## 3 Results and Discussion
### Chemical-Physical and Morphological Characterization of the SLS Manufactured Foams  
SLS processing involves the selective melting of the particle surface by using a laser beam. In this process, the TPU particles coalesce with each other, building up the desired 3D structure [1]. Since the TPU melt is highly viscous and no stress is generated during the process, the particle morphology is not significantly changed. Therefore, similar to the CNTs [1], the GE sheets remain entrapped in between the particle boundaries, thereby forming a percolated conductive network, as sketched in Figure 3a. Low magnification SEM images (Figure 3b) show that the wall structure of the holes in the porous specimens consist of sintered TPU particles. High magnification images (Figure 3c) clearly demonstrate that the surface of the TPU particles is covered by GE platelets. TEM observations provide additional information on the morphology and microstructure of the samples. Figure 3d and 3e show the GE percolated network due to the filler segregation between the sintered TPU particles, with a thickness ranging from 200 to 500 nm. Raman spectroscopy was used to get insight on the effect of SLS processing on the structure of the GE platelets within the sintered porous structures. The analysis has been conducted by using a  
Figure 3: **(a)** Schematics of the microstructure of the fabricated porous TPU/GE composites (GE content \(1.0\) wt%), highlighting the percolated GE network at the interparticle boundary (red dashed line). SEM images of the **(b)** wall structure of the composite, and the **(c)** GE-wrapped TPU particle surface. **(d,e)** TEM images of the percolated GE network in the SLS-processed composites. Figure 2: Visualization of the three designed porous structures. CAD-designs of the unit cells (left column); CAD-designs of \(3\times 3\times 3\) assembled structures (center column); photos of the TPU/GE SLS fabricated structures (right column). Raman microscopy, in order to selectively focus on the TPU particles or onto the segregated graphene between particles. Raman spectra of TPU, GE powder before processing, and GE in the fabricated porous structures are reported in Figure 4. The spectrum of pristine TPU shows the typical peaks of polyurethane, including the absorption peak of aromatic rings (1470-1440 cm\({}^{-1}\)), the absorption peak at 1665 cm\({}^{-1}\) corresponding to the C=C stretching, the C=O bending peak of the ester group at 1740 cm\({}^{-1}\), and the peak at about 3000 cm\({}^{-1}\) due to C-H bonds [36]. In the Raman spectrum of graphene, it is possible to observe the G (1580 cm\({}^{-1}\)) band, which is a primary in-plane vibrational mode of carbon-carbon bonds in graphene sheets, the D (1350 cm\({}^{-1}\)) band ascribed to disordered carbon in graphene and 2D (2690 cm\({}^{-1}\)) band, which is a second-order overtone of D band ascribed to AB-stacked graphene (where AB-Stacked refers to misorientation of graphene nanoplatelets) [37]. Figure 4 demonstrates that no dramatic change occurs in the GE spectrum when graphene nanoplatelets are assembled onto the TPU particle surface and then sintered during SLS processing. In any case, a slight decrease of \(\mathrm{I_{D}}/\mathrm{I_{G}}\) ratio is observed, which results in 0.064 for GE powder and 0.035 for the GE in the composite realized by SLS. This variation may be tentatively ascribed to an effect of the sintering process, which likely reduces the extent of defects of GE platelets assembled onto TPU powder particles (the effect of TPU in the measurements of the \(\mathrm{I_{D}}/\mathrm{I_{G}}\) ratio is negligible) [38; 39]. Thermal properties of SLS-fabricated TPU-based and TPU/GE porous samples were investigated by DSC and TGA. Figure 5a shows the cooling and heating DSC curves of the reference TPU porous structure.