## 4 Conclusions  
The effect of grafting polyethylene with acrylic acid on the interfacial interactions in zirconia nanocomposites for FFF and PIM has been investigated by MD simulations and experimental methods. The MD simulations show the increase in the adhesion to pure zirconia when polyethylene is grafted with acrylic acid, and the orientation of the acrylic acid side groups towards the oxygen in the zirconia. These results have been compared to the experimental characterization of a commercial acrylic acid grafted high density polyethylene and a comparable ungrafted high-density polyethylene and the nanocomposites of both materials with zirconia. The higher polarity of the acrylic acid-grafted high-density polyethylene compared to a similar ungrafted polymer has been observed in their interface with different zirconia surfaces. Moreover, the shift of the carboxyl group peak in the composite with acrylic acid grafted high density polyethylene demonstrates the formation of hydrogen bonds of the acrylic acid with the hydroxyl groups and the oxide in the powder surface. The increase in adhesion to the powder in the acrylic acid-grafted polyethylene produced by the increase in polarity, the improved interaction with the oxygen in the zirconia surface and the hydrogen bonding to the hydroxyl groups in the powder strongly affect the morphology and properties of the nanocomposites. All these effects result in an improvement in the dispersion of the powder in the polymer matrix in the nanocomposite with the acrylic acid grafted polyethylene. The combination of a higher adhesion between the matrix and filler and better powder dispersion for the acrylic acid grafted polyethylene than for the ungrafted one greatly affects the viscoelastic and mechanical properties of the nanocomposite. The restriction of the polymer chains movement and the formation of a polymer-filler network in the nanocomposite with grafted acrylic acid results in higher viscosity and higher storage modulus than for the ungrafted system, especially at lower shear rates. Furthermore, the reduction of the powder agglomerates that act as defect points and the improvement of the adhesion to the powder provide a higher strength and flexibility for the nanocomposite with grafted acrylic acid than for the one not containing it. These findings offer insight into the interfacial interactions and the improvement of the adhesion between polymer and filler in ceramic nanocomposites for FFF and PIM by the grafting of the backbone polymer with polar groups. It is shown that these modifications result in an improvement of the quality of the nanocomposite by increasing the filler dispersion and the improving the properties required for its processing. In future studies, the effect of the backbone grafting on the properties and processability of feedstocks for FFF and PIM with multicomponent binder systems will be evaluated. Conceptualization, S.C., A.G., C.K. and J.G.-G.; Investigation, S.C. and A.G.; Formal Analysis, S.C., A.G. and J.G.-G.; Writing-Original Draft, S.C.; Writing-Review & Editing, S.C., A.G., C.K., G.R., C.H. and J.G.-G.; Visualization, S.C.; Supervision, C.K. and C.H. ; Project Administration, C.K. and J.G.-G.; Funding Acquisition, C.K. and J.G.-G. All authors have read and agreed to the published version of the manuscript. This research was performed under the European project CerAMfacturing and the Austria-China bilateral cooperation projects FlexiFactory3 DP and 3 DMultiMat. The CerAMfacturing project has received the financial support of the European Commission in the frame of the FoF Horizon 2020 with financial agreement 678503. FlexiFactory3 DP and 3 DMultiMat has received funding from the Austrian Research Promotion Agency under the program Production of the Future, Grant Agreements No. 860385 and No. 875650. [PERSON] was supported by the Empa Internal Research Call 2018 in the framework of the \"ALLFIN\" project.