Enabling phase transition in pectin through green heterogeneous synthesis: Structural and thermal insights

The flexible plastics market requires polymers capable of viscous flow processing. To replace synthetic plastics with natural polysaccharides like pectin, similar industrial processability is essential. In this work, a phase transition was induced in pectin by chemical modification via green heterogeneous synthesis using phosphoric acid as a modifier. Different amounts of phosphoric acid were mixed with pectin powder in ethanol as the reaction medium. Chemical and structural characterizations were performed through Fourier Transform Infrared (FTIR) spectroscopy, solubility test and X-ray diffraction (XRD). The results showed the formation of new diester-phosphate crosslinks, responsible for the increase in the degree of acyl esterification above 80%, a reduction in solubility (from 99.86 ± 0.07% to 55.66 ± 0.46%) and crystallinity (from 10 to 7%) as the amount of phosphoric acid increased. Furthermore, thermal analysis was performed to confirm the occurrence of any phase transition as a result of the chemical modification. Modified pectin powders showed an endothermic peak in the range from 216 to 202 °C as the amount of phosphoric acid increased, while the thermal degradation shifted to lower temperatures in the modified samples, from 238 °C to 216 °C, compared to pure pectin (239 °C). A trade-off between the formation of new diester-phosphate crosslinks, hydrolysis and depolymerization induced by phosphoric acid treatment was dependent on the phosphoric acid concentration. These findings established a processing window (T_ep-T_d) for modified pectins which would represent a significant advancement in their processing for viscous flow.