Effects of synthesis temperature on structures and properties of epoxidized soybean oil oligomers and starch-based bioplastics

The work investigated the synthesis of the oligomers with citric acid (CA) and epoxidized soybean oils (ESO) at various temperatures and the effects of the oligomers on the structures and properties of starch-based bioplastics. CA was bonded effectively onto ESO via ring-opening polymerization at 90 degrees C as confirmed by the results of Fourier-transform infrared spectroscopy, thermogravimetric analysis, and carboxylic group contents of CA-ESO oligomers (CESO). The oligomers exhibited higher thermal stability than ESO. Regarding starch-based bioplastics, CESO disrupted the hydrogen bonding interaction within starch molecules and formed the esterification reaction with starch. The bioplastics containing CESO exhibited remarkably higher structural homogeneity and opacity as the synthesis temperatures of CESO increased. However, the thermal properties of the bioplastics with various CESO reduced, which might be due to the decomposition of starch by CESO. The films containing CESO also exhibited lower tensile strength than the film with ESO, which might be related to the decomposition, crosslinking, and plasticization effects of CESO on starch. The bioplastics with CESO exhibited lower degradation due to a more intense interaction of CESO and starch. The study demonstrated the potential of CA as an interfacial linker of starch/ESO-based bioplastics by adjusting the synthetic temperatures of CESO.