High-Temperature Size Exclusion Chromatography for Advanced Recycling: Systematic Errors in Analysis of Polyethylene─Alkane Mixtures

Polyolefins, which dominate the plastics marketplace, require high-temperature size exclusion chromatography (HT-SEC) to characterize their molar masses. Chemical recycling methods designed to deconstruct plastic waste into smaller molecules (i.e., depolymerization methods) rely on HT-SEC to characterize their products, but depolymerization methods often yield a complex mixture, including components below the typical separation range of commercial HT-SEC columns. Herein, we report the accuracy and limitations of triple detection HT-SEC to analyze model chemical recycling products. We examined the chromatographic separation and quantification of individual components and mixtures of short polyethylenes (apparent M_n= 465 and 2722 g/mol), hexatriacontane (C₃₆H₇₄, M = 507 g/mol), dotriacontane (C₃₂H₆₆, M = 451 g/mol), and octadecane (C₁₈H₃₈, M = 254 g/mol). Despite short alkanes exhibiting molar masses below the lower molar mass limit of the columns (500 g/mol), the separation of their mixtures was resolved via refractive index (RI) detection. However, the determined molar masses of these alkane mixtures did not agree with the known molar masses of the short alkanes or their prepared mixtures. Similarly, the SEC analysis of mixtures containing discrete alkanes with low molar mass PE revealed that their individual components are easily resolved in the HT-SEC chromatogram. No obvious shift in the elution volume of the blend components versus the elution volume of the pure components was observed. However, inaccurate molar masses and molar mass distributions for these alkane-PE mixtures are calculated from the HT-SEC data. These inaccuracies are attributable to two factors: (1) molar mass-dependent ∂n/∂c for low molar mass components and (2) the small size of the molecules in solution that limits light scattering. These results highlight the importance of considering measurement limitations for quantitative interpretation of triple detection HT-SEC data obtained from chemical recycling and depolymerization experiments that commonly contain complex mixtures of polymeric, oligomeric, and small molecule alkanes.