UV-induced structural changes and mechanical reinforcement in benzoyl-modified chitosans: A comparative physicochemical study

Chitosan and functionalized chitosans with 3, 4-dihydroxybenzoyl groups (CS-DHBA) and 3,4, 5-trihydroxybenzoyl groups (CS-THBA) were exposed to ultraviolet (UV) irradiation. The physicochemical and mechanical properties of chitosan (CS), CS-DHBA, and CS-THBA, including chemical structures, molecular weights, polydispersity index, intrinsic viscosity, and mechanical properties before and after UV-irradiation, were investigated and compared. The results showed that the molecular weight of chitosan decreased from 782,401 to 522,012 g/mol with increasing UV irradiation time. In contrast to chitosan, the molecular weight of CS-DHBA and CS-THBA increased to 826,187 g/mol and 845,310 g/mol, respectively, with increasing UV irradiation time. In addition, the polydispersity index (Mw/Mn) of chitosan decreased from 7.41 to 5.62 after 12 h of UV irradiation, but the polydispersity index of CS-THBA increased from 9.84 to 9.94. The surface morphology and structural analysis of UV-irradiated chitosan, CS-DHBA, and CS-THBA by SEM, FT-IR, and XRD confirmed that the chemical structure of irradiated chitosan was not significantly changed. In contrast, the chemical structures of irradiated CS-DHBA and CS-THBA were altered. The tensile strength (TS) of chitosan films decreased after 12 h of radiation, but UV-irradiated CS-DHBA and CS-THBA showed a significant adhesive capacity and enhanced TS.