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

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 degrees 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/m(2). 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 degrees C to 250 degrees C the average shish-kebab diameter increases from the nanometer to micron range, paralleled by massive loss of self-reinforcement above 200 degrees C. The absence of shish-kebab structure at 250 degrees C is attributed to relaxation of polymer chains and stretch-coil transition impairing shish formation.