Syntheses and properties of glucomannan acetate butyrate mixed esters

Glucomannan acetate butyrate mixed esters (GMAcBus) were prepared from konjac glucomannan (GM) with acetic acid, butyric acid, and trifluoroacetic anhydride. Eight GMAcBus were prepared with different degrees of substitution (DS) of acetyl group and butyryl group. The DS ratio of the two substituents could be controlled by the in-feed molar ratio of two carboxylic acids. Structural characterization of GMAcBus were carried out by 1H NMR analysis. Thermal properties were investigated by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA). The decomposition temperature of GMAcBus (ca. 360 °C) was improved from that of GM (274 °C), and the temperature was independent of the DS ratio. DSC thermograms showed no melting point for all samples, indicating GMAcBus were all amorphous. The glass transition temperature (Tg) of GMAcBus increased with increasing DS of acetyl group in a range between the Tgs of GMBu (107 °C) and GMAc (178 °C). All GMAcBus formed transparent films upon solvent-casting. Tensile tests of the films revealed that tensile strength increased with increasing DS of acetyl group, and film elongation tended to increase with increasing DS of butyryl group. Thermal and tensile properties of GMAcBus could be controlled by the DS ratio of acetyl and butyryl groups.