Phase behavior of partially miscible blends of linear and branched polyethylenes

Miscibility of Ziegler-Natta catalyzed high density polyethylene (HDPE) of broad molecular weight distribution with a homogeneous ethylene-octene copolymer (EO) of relatively narrow molecular weight distribution was studied as a function of constituent molecular weight and comonomer content. Blends were rapidly quenched from the melt to retain the phase morphology, and the phase volume fractions were obtained from AFM images. A homogeneous ethylene-octene copolymer with 5.3 mol % comonomer was completely miscible with high-density polyethylene, whereas copolymers with 8.5 and 12.3 mol % comonomer demonstrated a blend composition window for partial miscibility as indicated by phase volume fractions different from blend volume fractions. The temperature dependence of blend morphology confirmed the UCST behavior of EO/HDPE blends. The phase composition and the X interaction parameter were extracted by using an approach that considered the molecular weight distribution. The solubility parameters obtained from the analysis agreed with literature values. The study demonstrated the general consequences for phase behavior of the low and high molecular weight tails on the broad distribution of HDPE. Even in EO copolymers with relatively narrow molecular weight distribution, preferential solubility of low molecular weight fractions in the HDPE-rich phase was clearly evident.