Processing–Nanostructure–Property Relationships of All‐Polyethylene Composites Reinforced by Flow‐Induced Oriented Crystallization of UHMWPE

Abstract All‐polyethylene composites exhibiting substantially improved toughness/stiffness balance are readily produced during conventional injection molding of high density polyethylene (HDPE) in the presence of bimodal polyethylene reactor blends (RB40) containing 40 wt% ultrahigh molar mass polyethylene (UHMWPE) dispersed in HDPE wax. Scanning electron microscopy (SEM) and differential scanning calorimetry (DSC) analyses shows that flow‐induced crystallization affords extended‐chain UHMWPE nanofibers forming shish which nucleates HDPE crystallization producing shish‐kebab structures as reinforcing phases. This is unparalleled by melt compounding micron‐sized UHMWPE. Injection molding of HDPE with 30 wt% RB40 at 165 °C affords thermoplastic all‐PE composites (12 wt% UHMWPE), improved Young's modulus of 3400 MPa, tensile strength of 140 MPa, and impact resistance of 22.0 kJ/m2. According to fracture surface analysis, the formation of skin‐intermediate‐core structures accounts for significantly improved impact resistance. At constant RB40 content both morphology and mechanical properties strongly depend upon processing temperature. Upon increasing processing temperature from 165 °C to 250 °C the average shish‐kebab diameter increases from the nanometer to micron range, paralleled by massive loss of self‐reinforcement above 200 °C. The absence of shish‐kebab structure at 250 °C is attributed to relaxation of polymer chains and stretch‐coil transition impairing shish formation.