Effects of Cooling Rates on Self‐Reinforced Polyethylene Composites via Multiscale Experimental Characterizations

ABSTRACT This study investigates cooling rate effects on self‐reinforced polyethylene composites (PE‐SRCs), focusing on macroscopic thermal and mechanical properties, and micro‐ to sub‐micron morphologies. Unlike conventional fiber‐reinforced composites (e.g., carbon and glass fiber based), cooling rate effects on SRCs remain less studied. PE‐SRCs were fabricated from ultrahigh molecular weight polyethylene (UHMWPE) fabric and high‐density polyethylene (HDPE) matrix with cooling rates from 0.76°C/min to 425°C/min. Tensile properties, including modulus (~9.5 GPa), strength (~360 MPa), and ultimate elongation (~6%), showed no significant cooling rate dependence. However, thermal analysis revealed melting temperature of HDPE matrix decreased from ~130°C to ~126.5°C with total crystallinity reducing from 84% to 75%. Wide‐angle X‐ray scattering confirmed decreasing crystallinity but at lower values (67%–59%), as thermal analysis might overestimate crystallinity. Small‐angle X‐ray scattering showed long period decreased from 24.3 to 17.4 nm, attributed to thinner lamellae and closer packing, suggesting limited crystal growth and faster nucleation. While cooling rates alter melting point and crystalline structures of HDPE matrix, these changes do not translate to substantial impact on macroscale mechanical properties of PE‐SRCs. This insensitivity might be related to low interfacial shear strength at PE/PE interfaces, enabling process optimization while maintaining mechanical performance.