Morphology‐Assisted Strengthening in NR / HDPE Composites: Nano‐ vs. Microfiller Effects

ABSTRACT The tensile strength of semicrystalline polymer composites is commonly correlated with crystallinity; however, this approach does not capture the size‐dependent strengthening behavior observed in nanocomposites. In this work, natural rubber/high‐density polyethylene (NR/HDPE) composites filled with micro‐ and nanoscale boron carbide were investigated to establish the relationship between filler size, crystalline morphology, and tensile properties. SAXS showed no evidence of significant agglomeration, while at 2 wt% filler loading, FESEM revealed pronounced spherulitic refinement in nanocomposites (61% reduction in diameter), compared to a more modest refinement in microcomposites (25% reduction). XRD analysis showed that nanocomposites preserved lamellar thickness, whereas microcomposites exhibited progressive thinning with increasing filler loading. These structural differences produced distinct mechanical responses, with non‐monotonic tensile behavior in nanocomposites and near‐monotonic strength reduction in microcomposites. A unified model is proposed in which tensile strength scales with the product of crystallinity and a morphology factor derived from spherulite size and lamellar thickness. The model captures the observed size‐dependent strengthening behavior, demonstrating the critical role of morphology in nanocomposite strengthening.