## 3 Results and Discussion
The main mechanical properties of the prepared materials are summarized in Table 3. According to the ANOVA results, the addition of M6D significantly increased the stiffness of the 3D-printed samples (_p_-value = 10\({}^{-9}\)), while the inclusion of MWCNTs had a positive but very limited effect that is statistically non-significant (_p_-value > 0.05), and this could be attributed to the low MWCNT content introduced (1 wt%). The enhancement in material stiffness at room temperature upon M6D introduction is attributable to the increased rigidity of the melamine shell containing the paraffin in the microcapsules [34]. On the other hand, the failure properties of the 3D-printed samples were negatively influenced by the presence of microcapsules, with an evident drop in both stress and strain at break upon the introduction of M6D. Consequently, the specific tensile energy at the breaking point is noticeably reduced. This reduction in ductility could be partly due to the non-optimal interfacial adhesion between the microcapsules and the TPU matrix. Also, the inclusion of MWCNTs in the TPU matrix engendered a reduction in the strain at break (_p_-value < 0.001), even if to a lower extent with respect to M6D addition. This embrittling effect could also be caused by a worsening of the printing quality and a consequent reduction in the effective cross-section, as observed in the SEM micrographs of the nanofilled samples (Figure 2). In Figure 7, creep compliance curves of the 3D-printed samples at 30 \({}^{\circ}\)C are reported. Accordingly, compared to the results of the DMA and tensile tests, the dimensional stability clearly improved with the introduction of the MWCNTs, as the creep compliance value measured at 3600 s moved from 0.62 MPa\({}^{-1}\) for the neat TPU up to 0.35 MPa\({}^{-1}\) and 0.31 MPa\({}^{-1}\) with a nanofiller content of 0.5 wt% and 1 wt%, respectively. Moreover, the simultaneous introduction of microcapsules and carbon nanotubes decreased the \begin{table}
\begin{tabular}{l c c c c}  & \(\mathbf{T_{8}}^{*}\) (\({}^{\circ}\)C) & \(\mathbf{E^{\prime}}\) at -60 \({}^{\circ}\)C (GPa) & \(\mathbf{E^{\prime}}\) at 25 \({}^{\circ}\)C (MPa) & **tanS Peak** \\ TPU & \(-\)39.3 & 2.856 & 6.1 & 0.80 \\ TPU\_0.5 MWCNT & \(-\)37.9 & 3.134 & 8.1 & 0.75 \\ TPU\_1 MWCNT & \(-\)36.5 & 3.422 & 9.2 & 0.75 \\ TPU\_30 MbC\_0.5 MWCNT & \(-\)36.9 & 2.672 & 17.1 & 0.57 \\ TPU\_30 MbC\_1 MWCNT & \(-\)36.4 & 2.647 & 18.3 & 0.56 \\ TPU\_40 MbC\_0.5 MWCNT & \(-\)36.5 & 2.319 & 20.7 & 0.50 \\ TPU\_40M6D\_1 MWCNT & \(-\)33.3 & 2.206 & 23.4 & 0.47 \\ \end{tabular} \({}^{\star}\)Evaluated as inflection point of E\({}^{\prime}\) curves.  
\end{table} 
Table 2: Results of DMA tests conducted on the prepared 3D-printed samples. Figure 6: DMA thermograms of 3D-printed samples. Trends of (**a**) storage modulus (E’) and (**b**) loss tangent (tan8) as a function of the temperature. creep compliance to 0.14 MPa-1 in the TPU_40M6D_1 MWCNT sample. It can also be interesting to fit creep data according to the existing models in the literature. In particular, the total creep compliance in terms of isothermal tensile creep in the linear viscoelastic region \(D(t)\) is generally considered to consist of two components, with \(D_{el}\) representing the reversible and instantaneous elastic response and \(D_{vc}(t)\) denoting the time-dependent viscoelastic response, if no plastic deformation is produced during the creep test, as reported in Equation (1) [42]:  
\[D(t)=D_{el}+D_{ve}(t) \tag{1}\]  
In order to characterize the viscoelastic creep behavior, we used [PERSON]'s model, derived by extending the Kohlrausch-Williams-Watts (KWW) model, which is typically represented by a Weibull-like function in series form, with a focus on the primary term and disregarding the subsequent terms, as shown in Equation (2) [43]:  
\[D(t)=D_{el}+kt^{n} \tag{2}\]  
where \(k\) is a coefficient related to the magnitude of the underlying retardation process and \(n\) is an exponent tuning the time dependency of the creep process.