Miscibility in Polyethylene Blends Influenced by Weight Fractions and Short-Chain Branching: A Molecular Dynamics Study

Abstract

Polyethylene (PE), a versatile polymer, exhibits varying degrees of miscibility depending on molecular weight distribution, relative component concentrations (split ratio), and short-chain branching (SCB) content. This study investigates the thermodynamic factors governing miscibility in bimodal PE blends composed of low-molecular-weight polyethylene (LMwPE) and high-molecular-weight polyethylene (HMwPE), with emphasis on the effects of SCB and split ratio. Molecular dynamics (MD)-based solubility parameters, mixing energy ($\Delta E_{mix}$), and the Flory-Huggins interaction parameter ($\chi$)were used to determine miscibility thresholds in PE systems. Molecular weight influences chain interaction energies, free volume, and cohesive energy density, thereby affecting blend compatibility. In the absence of SCB, equal-weight blends exhibit higher miscibility than disproportionate blends, demonstrating the importance of the split ratio. SCB content further affects miscibility depending on its concentration in the blend components and the split ratio. In disproportionate blends where SCB-containing components are minor, increased SCB enhances miscibility, whereas in equal-weight blends, SCB reduces compatibility. SCB disrupts chain packing and promotes chain interpenetration by increasing free volume, improving compatibility at specific blend ratios. Overall, SCB influences packing efficiency, interaction energies, and phase-separation behavior, providing molecular-level insights for designing bimodal high-density polyethylene systems with controlled phase behavior.