## 4 Conclusions  
The filler content and the ball milling effect on the tensile and shear properties of short-chain branched-polyethylene/graphene nanoplatelets nanocomposites were examined in this study. Two series of SCB-PE/GNPs nanocomposites with various filler loadings (0.5-5 wt.%) were synthesized; the one was solely melt-mixed, and the other was ball-milled prior to the melt-mixing process. X-ray diffraction analysis revealed that the ball-milling step induced a lower crystallinity degree on the ball-milled samples than the SCB-PE/GNPs no BM counterparts. The nanocomposites presented improved tensile properties in any case. The SCB-PE/GNPs w BM nanocomposites presented a more significant enhancement of the elastic modulus and tensile strength compared to the solely melt-mixed counterparts despite having a lower crystallinity degree, meaning that the controlling factors of the improved tensile properties are the GNPs' dispersion and the interfacial adherence with the polymer matrix. The Ji micromechanical model successfully predicted the experimental elastic modulus of the SCB-PE/GNPs no BM nanocomposites, and the modified Halpin-Tsai model was the one that best described the elastic modulus behavior of the SCB-PE/GNPs w BM samples. Fractography studies suggest that, in the case of the ball-milled samples, the stress transfer between the filler and the matrix is more effective for lower filler content due to the better dispersion and interfacial adhesion of the GNPs and the polymer. Shear tests of the prepared materials showed that the nanocomposites' shear strength increased with filler content while the ball-milled nanocomposites presented more significant augmentation compared to the SCB-PE/GNPs no BM counterparts in any case. On the other hand, the nanocomposite samples were characterized by lower ductility with increasing filler content than the neat SCB-PE, while the SCB-PE/GNPs no BM nanocomposites were the least ductile among all the prepared materials in general. To sum up, the use of the ball-milling as a premixing step induced better mechanical properties even at low GNPs content, reducing the cost of the overall fabrication process of the nanocomposites.  
The following are available online at [[https://www.mdpi.com/article/10.3390/app11209420/s1](https://www.mdpi.com/article/10.3390/app11209420/s1)]([https://www.mdpi.com/article/10.3390/app11209420/s1](https://www.mdpi.com/article/10.3390/app11209420/s1)), Figure S1: Representative tensile stress vs strain curves of neat SCB-PE, (a) SCB-PE/GNPs no BM and (b) SCB-PE/GNPs w BM nanocomposites, Figure S2: Theoretical modeling of elastic modulus for the SCB-PE/GNPs no BM and SCB-PE/GNPs w BM nanocomposites' experimental data as a function of filler volume fraction, using various micromechanical models, Figure S3: SEM images of the fracture surfaces of (a) SCB-PE/1 GNPs no BM and (b) SCB-PE/1 GNPs w BM, Figure S4: SEM images of the fracture surfaces of (a) SCB-PE/5 GNPs no BM and (b) SCB-PE/5 GNPs w BM.  
Conceptualization, D.T., D.N.B. and K.C.; methodology, D.K., K.T. and M.-E.G.; formal analysis, D.K., K.T. and M.-E.G.; investigation D.K., K.T. and M.-E.G. and D.T.; writing--original draft preparation, D.K., K.T. and M.-E.G.; writing--review and editing, D.K., K.T. and M.-E.G., D.T., D.N.B. and K.C.; supervision, D.T., D.N.B. and K.C. All authors have read and agreed to the published version of the manuscript.  
The research work was supported by the Hellenic Foundation for Research and Innovation (HFRI) under the HFRI PhD Fellowship grant (Fellowship Number: 1123).  
Not applicable.  
The data presented in this study are available on request from the corresponding author.  
The authors declare no conflict of interest.