## 3 Results and Discussion  
The cryofractured surfaces of 3D-printed samples of neat TPU and the relative composites were investigated using FESEM, and the most representative micrographs are provided in Figure 2a-d. Moreover, in Figure 3a-d, FESEM images at higher magnification are shown in order to highlight the microstructural details of the interphase between microcapsules and the TPU matrix. At low magnification, the 3D-printed TPU (Figure 2a) and TPU_1 MWCNT (Figure 2b) specimens show a uniform and consistent arrangement of material layers deposited during the FFF process, with good adhesion between subsequent layers. Even in instances where the specimens were broken in liquid nitrogen, it is still possible to observe ductile fracture behavior due to the low glass transition temperature of TPU (about \(-\)50 \({}^{\circ}\)C). In the samples filled with microcapsules, a reduction in ductility is noticeable, with the appearance of a flat fracture profile, which can be attributed to the presence of M6D capsules acting as defects within the polymer matrix. This decreased ductility suggests compromised mechanical integrity and underscores the negative influence of the microcapsules on the failure properties of the resulting materials (Figure 2c,d). Nevertheless, it is possible to observe a triangularly shaped porosity corresponding to the edges of the filaments and good adhesion between the layers. No microcapsule agglomerates were detected, and the uniform M6D distribution allowed the morphological continuity of the matrix to be retained even at elevated M6D concentrations. This feature could be important both for mechanical behavior and the thermal diffusivity of these samples. At higher magnification levels (see Figure 3a-d), some peculiar microstructural features can be detected. Even in the material containing microcapsules, the presence of a ductile fracture surface originating from the plastic deformation of the TPU matrix can be noticed. Upon a closer examination, an intriguing interplay becomes apparent: while the M6D shell demonstrates strong adherence to the TPU phase, it also exhibits a brittle behavior, as evidenced by the presence of sharp cracks traversing the surface of the capsule shell (see Figure 3c,d). Moreover, it is important to note that throughout the entire study, no agglomeration of MWCNTs was found within the TPU matrix. This observation emphasizes the successful integration of the MWCNTs into the composite material, contributing positively to the overall mechanical properties without compromising structural integrity. The reason for this brittleness can be traced back to the intrinsic brittle nature of the melamine shell the microcapsules are composed of. This observation underscores the complex interplay between the brittle behavior of the microcapsules' shells and their good interfacial interaction with the TPU matrix. These contrasting effects can strongly influence the failure behavior of the resulting materials. Figure 2: Low-magnification FESEM micrographs of 3D-printed (**a**) TPU, (**b**) TPU_1 MWCNT, (**c**) TPU_30M6D_1 MWCNT, and (**d**) TPU_40M6D_1 MWCNT samples. In order to evaluate the thermal degradation resistance of the prepared nanocomposites, TGA was performed. In Figure 4a, thermogravimetric curves of neat TPU and the relative composites are shown, while in Figure 4b, the corresponding derivative thermogravimetric (DTG) curves are reported. At around 125 \({}^{\circ}\)C, there is an initial degradation step for the materials filled with microcapsules, and this mass loss (ranging from 2 wt% to 2.5 wt%) is proportional to the M6D content. This degradation step is related to the presence of residual paraffin leaking out from some microcapsules that broke during the manufacturing stage; this paraffin can easily evaporate at relatively low temperatures [34]. For all the samples, the major weight loss took place in the 280-485 \({}^{\circ}\)C range.