Determination of the degree of substitution of cellulose esters via ATR‐FTIR spectroscopy

Cellulose derivatives are promising bio-based polymeric materials with possible applications ranging from thickening agents and stabilizers to optical films. The precise determination of their degree of substitution (DS) is crucial, as the DS substantially influences the material properties of such cellulose derivatives. Herein, the applicability of attenuated total reflection-Fourier transform infrared for DS determination of cellulose esters was investigated. Sixteen cellulose acetates with DS ranging from 0.41 to 2.99 were synthesized and three acetyl group-specific vibrational signals were evaluated quantitatively in reference to the cellulose anhydroglucose unit vibration ν(C-O)$_{AGU}$. A non-linear correlation for the absorbance intensity with the DS was observed and the data was fitted based on a newly developed model with correlation coefficients R$^2$ between 0.958 and 0.998, depending on the evaluated signal. This DS determination method is simple and efficient, since it does not need any prederivatization or extensive sample preparation and can furthermore be applied to non-soluble samples. In order to widen the scope, eight cellulose butyrates, six cellulose laurates and six cellulose benzoates with different DS were synthesized and the DS determination method could be analogously applied to these cellulose esters, but a separate calibration for the different esters was found to be necessary.