Liquid Chromatography‐Infrared and Size Exclusion Chromatography‐infrared Analysis for Polymer Characterization

Abstract

Fourier transform infrared spectroscopy (FTIR) can be used as a detector in liquid chromatography, giving detailed information on the molecular structure of the effluents after separation. Achieving structural details of chemical compounds after their separation makes the coupling of interactive liquid chromatography (IC) and size exclusion chromatography (SEC) with IR spectroscopy a versatile analytical tool superior to refractive index (RI), ultraviolet (UV), and evaporative light scattering detectors (ELSDs). LC/IR offers an alternative to LC/MS and LC/NMR to study the chemical composition of the eluted compounds. The hyphenation can be realized in two ways, namely, the on flow with an IR flow cell and off‐line via solvent elimination. Both approaches are discussed with regard to their advantages and limitations and different solvent‐elimination interfaces are compared. The coupling of LC with FTIR using the IR flow cell and the solvent‐elimination interfaces is demonstrated on selected samples, including interactive and size exclusion models at ambient and high temperatures.

Characterization of Unsaturated Polymers Using High‐Temperature Size Exclusion Chromatography Coupled with Ultraviolet‐Evaporative Light Scattering Dual Detection

“…[15] Therefore, it is not surprising that IR detection has been coupled to SEC in various forms to quantify an average value of the chemical composition of the different molar mass fractions. [16][17][18] Frijns-Bruls et al determined the short chain branching as function of molar mass using filter-based IR detector coupled to SEC for several copolymers of polyethylene covering the entire range of chemical composition. [16] Several research groups have described coupling of FTIR detectors to SEC at high temperature by means of heated transfer lines and flow-through cells using the absorbance ratio of the bands due to methyl (CH 3 ) and methylene (CH 2 ) groups to determine the number of methyl groups per one thousand total carbons (CH 3 /1000TC), [19] or the ratio of the CH 3 band absorbance over the total CH absorption.…”

“…The off-line FTIR method (LC-Transform) is powerful for qualitative studies and may work for the determination of the amount of unsaturation, given that the full sample spectrum is available for analysis. [17,31] Plass et al [17] observed that the flow-cell FTIR interface may provide reliable chemical composition data across the MMD in a single analysis, due to the high accuracy and reasonable detection limits and provide structural information to some extent. However, they concluded that complete spectral information without solvent interference was still best obtained with solvent-elimination SEC-FTIR.…”

“…[ 15 ] Therefore, it is not surprising that IR detection has been coupled to SEC in various forms to quantify an average value of the chemical composition of the different molar mass fractions. [ 16–18 ] Frijns‐Bruls et al. determined the short chain branching as function of molar mass using filter‐based IR detector coupled to SEC for several copolymers of polyethylene covering the entire range of chemical composition.…”

“…[ 17,31 ] Plass et al. [ 17 ] observed that the flow‐cell FTIR interface may provide reliable chemical composition data across the MMD in a single analysis, due to the high accuracy and reasonable detection limits and provide structural information to some extent. However, they concluded that complete spectral information without solvent interference was still best obtained with solvent‐elimination SEC–FTIR.…”

“…However, they concluded that complete spectral information without solvent interference was still best obtained with solvent‐elimination SEC–FTIR. [ 17 ] While the LC‐Transform approach for SEC‐IR may work in the case of unsaturated polymers, it has limitations for quantitative application. This is mainly due to the difficulties with solvent evaporation and consistent deposition of the solute on the rotating disk.…”