Low-Concentration Graphene Nanoplatelet/HDPE Nanocomposites with Enhanced Dispersion and Interfacial Bonding for Improved CO₂ Barrier and Mechanical Performance at Elevated Temperatures

High-density polyethylene (HDPE) is well known for its excellent moisture and chemical resistance, combined with a balance of ductility and strength, making it ideal for high-temperature, high-pressure applications. To enhance HDPE-based products under these conditions, improving barrier properties against greenhouse gases and increasing creep strength are essential. This study developed a graphene nanoplatelet (GNP)/HDPE nanocomposite with only 0.5 wt % GNP using a scalable melt-compounding method. By optimizing processing conditions and grafting GNP onto HDPE chains via maleic anhydride functional groups, significant improvements were achieved at 60 °C, including a 44.6% reduction in supercritical CO2 permeability, a 40.7% reduction in creep strain, and a 68.1% decrease in creep strain rate. Additionally, the onset degradation temperature increased by 25 °C, and the maximum weight loss rate improved by 54% compared to neat HDPE. Notably, these enhancements were achieved while maintaining elongation at break, with the GNP/HDPE nanocomposite exhibiting nearly 20% higher elongation at break than neat HDPE. Fourier transform infrared spectroscopy (FTIR) confirmed strong ester linkages between GNP and the polymer chain, resulting from a nucleophilic addition reaction that improved interfacial bonding and overall properties. This study offers a significant step toward more resilient and environmentally friendly engineering materials.