Intermolecular Interactions between Polyethylene, Water, and Potential Antistatic and Slip Additives: A Molecular Dynamics Study

Abstract

Additives are essential to enhance or modify the properties of plastics for target applications. However, finding appropriate additives may be challenging, since we lack knowledge on their interactions with plastics and moisture, and the interplay between them. In this work, we study stearoyl diethanolamine (commercial antistatic additive for polyolefins) as well as two amphiphilic molecules as potential new additives for their antistatic or slip properties in polyethylene by means of atomistic molecular dynamics simulations. We reveal that additive/water interactions and relative solubility are strongly determined by their relative ratios. The polyethylene model thin film adopts a crystalline core and an amorphous-like surface, with polymer chain terminations predominantly located at the surface of the slab. Water forms a layer on top of the polymer surface or droplets when its concentration is lowered, but it never enters the polymer matrix. All additives interact with water mainly by their polar heads, with water acting as a hydrogen bond acceptor or donor depending on the additive. The additives studied exhibit remarkably different structures when they are mixed with the polymer: two of them enter the polymer matrix to various degrees, either by intercalating their chains with the polyethylene ones or by forming micellar-like structures, while the third one stays at the surface. When water is incorporated into the system, the structure of some of the additive/polyethylene systems changes. The magnitude and nature of these changes depend on the relative concentrations of all species and on the nature of the additive. The additives penetrate and organize the polymer to some extent, depending on whether water is present. We find that the ethoxylated amine additive orients its polar heads to the surface and its non-polar tail to the center of the film, consistent with its antistatic properties and migratory behavior. We propose that one of our two modeled molecules could have promising properties as a slip agent, as its behavior in the PE matrix resembles that of the industrial slip agent erucamide. We hope that our predictions will spark interest in testing these molecules in the laboratory as polyethylene additives and in performing similar studies for other additive/polymer pairs.