Unveiling the Significance of Graphene Nanoplatelet (GNP) Localization in Tuning the Performance of PP/HDPE Blends

High-density polyethylene (HDPE) and polypropylene (PP) blends are widely used in industries requiring mechanically durable materials, yet the impact of processing parameters on blend performance remains underexplored. This study investigates the influence of blending sequence and screw speed on the properties of blends of HDPE and PP filled with 1.25 wt.% graphene nanoplatelets (GNPs). Changes in crystallization behaviour, tensile strength, and viscoelastic responses with blending sequence are studied. The addition of GNP increases the crystallization temperature (T_c) of PP in the PE/PP blend by 4 °C when GNP is pre-mixed with PE to form (PE+GNP)/PP blends. In contrast, when GNP is pre-mixed with PP to create (PP+GNP)/PE blends, the T_c of PP rises by approximately 11 °C, from 124 °C for the neat PE/PP blend to 135 °C. On the other hand, the T_c of PE remains unchanged regardless of the blending sequence. XRD patterns reveal the impact of blending regime on crystallinity, with GNP alignment affecting peak intensities confirming the more efficient interaction of GNPs with PP when premixed before blending with PE, (PP+GNP)/PE. Tensile moduli are less sensitive to the changes in processing, e.g., screw speed and blending sequence. In contrast, elongation at break and tensile toughness show distinct variations. The elongation at the break of the (PP+GNP)/PE blend decreases by 30% on increasing screw speed from 50 to 200 rpm. Moreover, the elongation at the break of (PE+GNP)/PP prepared at 100 rpm is ~40% higher than that of the (PP+GNP)/PE. (PE+GNP)/PP displays a 'quasi-co-continuous' morphology linked to its higher elastic modulus G' compared to that of the (PP+GNP)/PE blend. This study highlights the importance and correlation between processing and blend properties, offering insights into fine-tuning polymer composite formulation for optimal performance.