Bulk polymer/solvent interactions for polyethylene and EVA copolymers, below their melting temperatures

Abstract

In this work, the Flory–Huggins parameters corresponding to the amorphous phase of a polyethylene (PE) and two ethylene–vinyl acetate (EVA) copolymers (with 18 and 33 % vinyl acetate content, respectively) samples, with different solvents have been determined below the melting temperature of the polymers, in order to quantify the bulk interactions of these polymer/solvent systems. The employed solvents were a dispersion solvent (cyclohexane), a polar solvent (vinyl acetate) and an association solvent (methanol). Initially, the inverse gas chromatography measurements allowed obtaining the retention volumes, activity coefficients and overall Flory–Huggins parameters of every polymer/solvent system. According to these parameters, in all cases, the more compatible solvent was cyclohexane, so it was selected as the probe to calculate the percentages of crystallinity at room temperature, whose results were in agreement the literature data (35 % for PE, 29 % for EVA18, and 12 % for EVA33). The percentage of crystallinity allowed determining the amorphous Flory–Huggins parameters which are the ones which take into account just the bulk interactions in a polymer/solvent mixture. The Flory–Huggins parameter results show that, to accurately study the vapor–liquid equilibrium between a polymer and a solvent (bulk interactions), when the range of studied temperatures is below the melting point of the polymer, it is crucial to calculate the amorphous contribution (χ amorphous) on the overall Flory–Huggins parameter. In the case of this study, the lower the vinyl acetate content (higher crystallinity), the higher the difference between the overall and amorphous Flory–Huggins parameters is. Analyzing the interactions between the three polymeric materials and the solvents it can be noticed that, for the most compatible solvent (cyclohexane), χ amorphous represents the less contribution, or the highest correction, to the overall Flory–Huggins parameter (around 50 % for PE and EVA18, and 79 % for EVA33, the less crystalline polymer).