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 R2
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.