Preparation and properties of starch oxalate half-ester with different degrees of substitution

Oxalic acid, a strong dicarbonic acid mainly come from starch, was used to esterify gelatinized corn starch under nonaqueous conditions to give a material with degrees of substitution (DS) ranging from 0.08 to 0.87 depending on the oxalic acid/starch molar ratios used. The reaction product was washed by extraction with 95% ethanol solution and then characterized by using spectroscopic techniques (FT-IR, H-1-NMR and C-13-NMR). The results from these analyses revealed the presence of carbonylic groups, indicating that the esterification reaction was successful. SEM showed that granules of corn starch swelled on gelatinization and the particles had a porous structure favorable to esterification. The effect of different degree of substitution on properties of oxalate starch half-ester was studied by intrinsic viscosity measurement, wide angle X-ray scattering (WAXS), thermogravimetry (TG) and humidity absorption. X-ray diffraction studies revealed the loss of the ordered A-type crystalline structure, characteristic of corn starch. With the increase in DS, Mw (estimated by intrinsic viscosity), the thermal stability of starch oxalate half-ester decreased markedly while its ability to take up water increased due to the introduction of the carboxylic group.