A series of chemically modified starches has been synthesized in the present study. The hydroxyl groups in starch molecules were substituted by ionic groups (acetate sodium or trimethylammonium chloride) and hydrophobic groups (hexanoyl group or benzoyl group). The FTIR spectra showed that the hydrogen bonds present in natural starch were weakened by the chemical modification. Through the calculation from the 1 H-NMR spectra, the degree of substitution of the modified starch prepared was determined.
The modified starches, including AS-B, AS-LF, CS-B, and CS-LF, were characterized by DSC. The DSC thermograms showed that the chemical modification has lowered the glass transition temperature of starch. Additionally, there was no melting peak in the DSC thermograms of modified starches, suggesting an amorphous morphology in the modified starches.
The thermal stability of the modified starches was characterized by TGA. When starch was modified only with ionic groups, the thermal degradation temperature of starch was largely decreased from over 300 o C to around 200 o C. On the other hand, the starches modified with both ionic groups and hydrophobic groups have a higher thermal degradation temperature than the starch modified only with ionic groups.
Natural starch is known to be very hydrophilic, which limits its use in conventional thermoplastic processing. The substitution of hydrophobic groups (hexanoyl groups or benzoyl groups) decreased the hydrophilicity of starch. We characterized the moisture absorbance of the modified starches in an environment having constant humidity over time. The results showed that the chemical modification has changed the starch from hydrophilic to hydrophobic.
We also investigated whether or not the biodegradability of a starch was influenced by the chemical modification. The weight loss of a modified starch during composting in a given period clearly has shown biodegradability of the modified starch. However, the weight loss of a modified starch is slower than natural starch, suggesting that the biodegradability of modified starches is lower than natural starch.