Graphene-Reinforced PE-RT Nanocomposites for Flexible Pipeline Barriers: Structure-Property-Performance and Aging

Polyethylene of raised temperature resistance (PE-RT Type II) is the preferred material for barrier layers in offshore flexible risers, yet its CO₂ permeability and long-term stability under hydrocarbon exposure remain critical performance gaps. Here, PE-RT nanocomposites reinforced with four graphene-based fillersgraphene oxide (GO), reduced graphene oxide (rGO), vinyltrimethoxysilane-functionalized GO (GOS), and graphene nanoplatelets (GNP)at 0.5-2.0 wt % were prepared by melt compounding and systematically characterized. Filler surface chemistry and structure were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Raman spectroscopy, thermogravimetric analysis (TGA), and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM/EDS), revealing distinct structural and interfacial features among the graphene-based fillers. SEM provided qualitative information on filler morphology and microscale distribution, while Raman spectroscopy and XRD were used to characterize the structural features of the graphene-based fillers. Because nanoscale dispersion within the PE-RT matrix was not directly quantified by TEM, AFM, or other image-based methods, relationships between filler dispersion and functional performance are discussed as structure-property interpretations rather than direct measurements of nanoscale dispersion. GOS delivered the greatest CO₂ barrier improvement, reducing the mean permeability by 56.5% at 1 wt % relative to neat PE-RT (2.13 ± 0.30 vs 4.90 ± 0.10 cm² s^-1 psi^-1; Tukey HSD, p < 0.001). The Nielsen tortuosity model yielded an effective platelet aspect ratio of ≈570, consistent with efficient platelet-induced tortuosity. GNP exhibited the highest mean stiffness at 1 wt %, with an elastic modulus increase of approximately 50.8% relative to neat PE-RT. Six months of immersion aging in synthetic hydrocarbon oil (ISO 23936-1, 80 °C) produced formulation-dependent changes in thermal, surface, dimensional, and molar-mass-related properties. The graphene-containing nanocomposites showed lower swelling than neat PE-RT, while GOS exhibited comparatively stable GPC parameters over the aging period. These results demonstrate enhanced aging resistance, although the specific mechanisms responsible for this stabilization were not directly determined. GNP showed comparatively poorer CO₂ barrier performance despite exhibiting the highest mean mechanical properties among the 1 wt % nanocomposites. These results establish quantitative structure-property-performance relationships and provide design guidelines for graphene-functionalized polyolefin barrier materials in demanding service environments.