Quantitative Analysis of the Etherification Degree of Phenolic Hydroxyl Groups in Oxyethylated Lignins: Correlation of Selective Aminolysis with FTIR Spectroscopy

PEGylated or oxyethylated lignins (OEL) have recently become a hot topic as precursors for novel lignin-based and sustainable materials or active substances such as hydrogels, aerogels, carbogels, dispersants, and surfactants. Since functional properties of OEL and the resulting materials are strongly affected by the degree of oxyethylation (DOE) of phenolic hydroxyl groups (OHphen), analytical techniques for its determination are crucial. OELs with different levels of modification were obtained by reacting lignins from different pulping procedures with varying amounts of poly­(ethylene) glycol-α,ω-diglycidyl ether (PEGDGE). Parent lignins and OELs were characterized by means of selective aminolysis that is subsequent preacetylation and selective deacetylation of aromatic acetates of preacetylated lignin/OEL with pyrrolidine. The reaction product 1-acetyl pyrrolidine was quantified using GC/FID. The DOE of OEL, obtained by subtraction of OHphen content before and after lignin oxyethylation, was found to be in the range between 53.4% and 70.0%. Selective aminolysis has been shown to be very accurate for OEL analysis but is very time-consuming. Thus, it was the aim to investigate the extent to which FTIR features of acetylated lignin and OEL relate to OHphen contents and DOE of OEL as obtained by aminolysis. Strong linear correlations (R 2 = 0.94–0.97) were found between OHphen contents of lignin/OEL and IR vibrations related to phenolic and aliphatic acetoxy groups. The results demonstrate that, with appropriate calibration, FTIR spectroscopy combined with sample preacetylation is a promising tool for a rapid and accurate determination of the DOE of OELs.