**4. Conclusions**

A high-performance novel graphene-based multimodal polyethylene nanocomposite was produced directly without any particular treatment to the composite constituents. Different characterization techniques such as TEM, SEM, optical microscopes, Raman spectroscopy, Rheology, density measurements, TGA, and CTE were employed in order to demonstrate the novelty of the fabrication method. Microscopic analysis showed that graphene platelets were homogenously dispersed and distributed within the polymer matrix. The electronic and optical microscopies showed that the graphene was dispersed with an average size of less than 450 nm, and distributed with a %area fraction of only 0.0063. The adhesion strength between the graphene sheets and polymer matrix examined by microscopic examination of the cryofractured surfaces of the nanocomposite, mechanical testing and stress-induced Raman band shifts, revealed a strong interfacial bonding attained through thermo-mechanical and thermo-oxidative degradation to a controlled level during the melt extrusion process. The cryofractured surface of the nanocomposite became rough with fibrils almost entirely absent. Deflection of the nanocomposite led the characteristic 2D and G Raman peaks of graphene to shift significantly towards high wavenumbers. This was confirmed further by the mechanical testing where the storage modulus of the polyethylene reinforced with 1 wt.% of graphene increased up to 118% at room temperature.

The thermal performance of the nanocomposite was investigated via rheology testing, TGA and thermal expansion analysis. Loading a polymer with 1 wt.% graphene has resulted in a significant shift of the crossover frequency point and crossover modulus point to the lower regions. The homogenous dispersion and distribution of graphene platelets within the polymer matrix, as well as the strong adhesion bonding, led to the formation of an interconnected 3D network in the polymer which accordingly restricted the movement and expansion of the polymer chains movements. This has resulted in a significant increase in the onset degradation temperature by more 31 ◦C as a consequence of a thermal barrier or synergetic effects induced by graphene. The nanocomposites exhibited almost zero thermal expansion below 100 ◦C. The results of this study change the idea of there being difficulty in using melt extrusion for producing well dispersed and disturbed graphene-based hydrophobic polymer nanocomposites. Furthermore, considering there is no need to chemically treat graphene or polymer, or in fact change anything in the existing plants, such a result can significantly attract the industry.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2079-4991/8/11/947/s1, Figure S1: Type and use of each element used in the present study, Figure S2: The cross-sectional area of a two-flighted screw element, Figure S3: XPS wide range spectrum of graphene powder (CAE=50eV). A C1s (CAE=50eV) narrow scan peak fit is also presented in the inset, showing the components and the degree of oxygen species. They were fitted by the Gaussian-Lorentzian (GL60) functions. Figure S4: TEM images of the as-received graphene powders at different magnifications.

**Author Contributions:** Conceptualization, I.A.A.; methodology, I.A.A.; validation, I.A.A.; formal analysis, I.A.A.; investigation, I.A.A.; resources, I.A.A.; writing—original draft preparation, I.A.A.; writing—review and editing, I.A.A., S.D., and R.V.K.; visualization, I.A.A., and S.D., and H-K.K.; supervision, R.V.K., K.K.K.K, and H-K.K.; project administration, I.A.A.; funding acquisition, I.A.A.", please turn to the CRediT taxonomy for the term explanation.

**Funding:** This research was funded by Abu Dhabi National Oil Company (ADNOC) and Borouge Pte. Ltd., grant code LJGK GAAA, and the APC was funded by ADNOC.

**Acknowledgments:** This work was conducted under the framework of the Research and Development Program of the Korea Institute of Energy Research (KIER) (B8-2432-04) and ADNOC.

**Conflicts of Interest:** The authors declare no conflict of interest.
