Development and assessment of an improved SEC-FTIR detector and flow cell for measuring short chain branching in polyethylene copolymers

Abstract

Determination of short chain branching distribution (SCBD) is important in modifying the mechanical properties of the polyethylene (PE) copolymers in order to meet the end-user requirements especially for high density polyethylene (HDPE). In the current PE business landscape, determining SCBD in a fast and efficient manner is critical to maintain a competitive advantage in product development. Coupling size-exclusion chromatography (SEC) to Fourier transformed infrared (FTIR) or optical filter-based infrared (IR) spectrometers, has brought significant improvements in this area in comparison to the offline hyphenated method of preparative fractionation and subsequent nuclear magnetic resonance (NMR) analysis of the fractions.

This research describes the design and implementation of a new FTIR detector which measures the carbon-hydrogen stretching IR absorbance (3000-2700 cm⁻¹) of PE samples at each MW slice. The detector was innovatively designed so that the flow cell is placed inside the column oven to ensure uniform heating and employs a thermoelectrically cooled detector. Furthermore, machine learning techniques such as Hierarchical Cluster Analysis (HCA) and Partial Least Squares models (PLS-1) were applied to the spectral data to establish methyl group qualification limits (due to sample concentration) in terms of Me/1000 Total Carbon (TC) and accuracy of the measured SCB/1000 TC values, respectively. Results show that initial partial least squares models (PLS-1) can accurately estimate SCB content to ± 1 SCB/1000 TC down to a sample concentration of 0.035 mg/mL at the cell for various different types of HDPE samples.