Dual-crosslinked starch−poly(ester urethane)−oligochitosan films with high starch content: Application as biodegradable food packaging

This work offered a dual-crosslinking strategy to prepare starch−poly(ester urethane)−oligochitosan (SPEO) composites with high starch content (> 46 wt%). The crosslinking agent polycaprolactone-based polyurethane prepolymer (PPUP) with terminal −NCO groups and pendant −CHO groups was first synthesized. Then, the starch was modified by PPUP to form a crosslinked structure linked by urethane bonds. Oligochitosan (OCS) was subsequently introduced to produce another crosslinked structure linked by imine bonds, and the films were fabricated by compression molding. The chemical structures and physical performance of SPEO were characterized. Crystallinity, micromorphology and thermal analysis demonstrated that the dual-crosslinking structures enhanced the composite compatibility, which improved their mechanical properties. The surface hydrophilicity, water absorption, and in vitro degradation rate increased slowly with increasing OCS content, while a high crosslinking density provide a good barrier to water vapor permeation and therefore decreased the water vapor permeability (< 128 g/(m2∙24 h)). Due to the residual −NH2 groups of OCS, the composite film of SPEO-6 exhibited certain bacteriostatic activity against E. coli and S. aureus. Furthermore, the cytotoxicity of the composite films was assayed by the MTT method, and relative growth rates were much higher than 75% after incubation for 72 h, indicating high cytocompatibility and non-toxicity to food. The dual-crosslinked high-starch-content composites possessing high security, superior mechanical properties, good biodegradability, low water vapor permeability, and certain bacteriostatic capacity hold great potential to substitute petroleum-derived plastics for fresh-keeping food packaging applications.