## 4 Conclusions  
Encapsulated paraffin and MWCNTs were successfully melt-compounded with TPU and processed via FFF to prepare novel multifunctional 3D-printed materials for thermal energy storage applications. FESEM analysis revealed a uniform dispersion of the microcapsules throughout the polymer matrix and strong adhesion between consecutive 3D-printed layers. However, as the concentrations of M6D and MWCNTs increased, the printing quality worsened, with the appearance of some layer misalignment and surface irregularities. TGA tests highlighted an initial degradation at 125 \({}^{\circ}\)C in the microcapsule-filled materials due to residual paraffin leaking out from damaged microcapsules, while the introduction of MWCNTs resulted in a slight positive shift in the degradation temperature. DSC tests revealed that the thermal energy storage efficiency of the 3D-printed materials was slightly lowered (about 20%) by the paraffin leakage caused by the compounding and FFF processes but remained at an interesting level (i.e., a melting enthalpy up to 48 J/g with an M6D content of 40 wt%). The stiffening effect provided by the MWCNTs and the microcapsules seen in the DMA was confirmed through quasi-static tensile tests. The limited TPU/M6D interfacial adhesion, coupled with the degradation of the printing quality observed in the MWCNT- and microcapsule-filled materials, led to an evident embrittlement and a consequent decrease in the failure properties. The addition of MWCNTs and PCM exerted a notable influence on creep behavior, with an evident reduction in both the elastic and the viscoelastic components of the creep compliance. Therefore, this study demonstrates the feasibility of utilizing additive manufacturing technology to fabricate 3D-printed materials based on TPU composites with thermal management capabilities. These multifunctional materials could find wide applications in innovative sectors such as sensors or in sport applications.  
Conceptualization, A.D. and A.P.; methodology, D.R.; software, D.R.; validation, D.R., A.D. and A.P.; formal analysis, D.R.; investigation, D.R.; resources, D.R.; data curation, D.R.; writing--original draft preparation, D.R.; writing--review and editing, A.D. and A.P.; visualization, D.R.; supervision, A.D. and A.P.; project administration, A.D. and A.P. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
Dataset available on request from the authors.  
The authors declare no conflicts of interest.  
\begin{table}
\begin{tabular}{l c c c c} \hline  & \(\mathbf{D_{e}}\) & \(\mathbf{k}\) & \(\mathbf{n}\) & \(\mathbf{Adj\ R^{2}}\) \\  & (\(\mathbf{MPa^{-1}}\)) & (\(\mathbf{MPa^{-1}\ s^{-n}}\)) & & \\ \hline TPU & 262,091 & 275,612 \(\pm\) 1 & 0.1002 \(\pm\) 0.0007 & 0.9980 \\ TPU\_0.5 MWCNT & 175,707 & 194,208 \(\pm\) 1 & 0.0747 \(\pm\) 0.0008 & 0.9960 \\ TPU\_1 MWCNT & 158,298 & 172,607 \(\pm\) 1 & 0.0705 \(\pm\) 0.0001 & 0.9998 \\ TPU\_30 MG0\_0.5 MWCNT & 96,351 & 110,359 \(\pm\) 1 & 0.0709 \(\pm\) 0.0003 & 0.9992 \\ TPU\_30 MG0\_1 MWCNT & 74,770 & 83,555 \(\pm\) 1 & 0.0802 \(\pm\) 0.0004 & 0.9988 \\ TPU\_40 MG0\_0.5 MWCNT & 83,523 & 93,255 \(\pm\) 1 & 0.0790 \(\pm\) 0.0005 & 0.9984 \\ TPU\_40 MG0\_1 MWCNT & 65,904 & 74,451 \(\pm\) 1 & 0.0786 \(\pm\) 0.0005 & 0.9985 \\ \hline \end{tabular}
\end{table}
Table 4: Results of creep tests on the 3D-printed samples with fitting parameters based on [PERSON]’s model (see Equation (2)).