Nowadays, the food safety problems caused by food contamination have become one of the most prominent public health problems around the world with the accelerated global food circulation. Approximately 30% of the global population experience serious health problems through consumption of food containing pathogens 1. Furthermore, foods, especially fruits and vegetables, bring more security problems because they are perishable in the process of storage, transportation, and sale. Appropriate packaging material can maintain food quality and avoid possible contamination along the entire food supply chain 2. Various petroleum-based packaging systems such as PP and PE have been widely used in our daily lives3, 4, while these materials face a serious problem of environmental pollution because of their non-biodegradable nature5. Therefore, natural polymer materials have been considered as promising candidates to develop food packaging materials because of their non-toxicity, renewability, low cost, good biocompatibility, biodegradability, and high consumer acceptance 6–9.
As a natural biopolymer material, chitosan (CS), the most abundant alkaline polysaccharide in nature 10, has been extensively used in food, biomedicine, and other aspects due to its diverse bioactivities, such as biocompatibility, biodegradability, non-toxicity, and antibacterial activity11–14. As expected, CS has also drawn attention as an ideal environmental material regarding the replacement of petroleum-based materials15 and is becoming one of the key raw materials for preparing antibacterial packaging materials16,17. Despite the promise, strong intermolecular H − bonds existing in the crystalline structure of CS produced poor tensile properties18. Additionally, CS has limited solubility in water and most organic solvents, further restricting its application in packaging19–21. O − carboxymethyl chitosan (O − CMC), a derivative of CS, possesses similar chemical properties but offers improved water solubility, superior antibacterial activity, and enhanced chemical activity due to the introduction of the carboxymethyl group 22–25. However, O − CMC has also drawbacks such as brittleness and weak gas permeability, which hinder its application in the field of food packaging26,27. Modification of O − CMC can improve the tensile properties, gas barrier properties, and so on, so considerable work has been carried out to promote its properties of O − CMC28,29.
Polyethylene glycol (PEG) is a biocompatible and non-toxic polyether that can dissolve in water and various organic solvents without undergoing hydrolysis 30–33. The double-ended hydroxyl (− OH) structure of PEG allows for modifications with functional groups like carboxyl, hydroxyl, and amides34. As a result, PEG has been widely used as a modifier to enhance the properties of materials, including plasticity, hydrophilicity, and flexibility 35. In a recent work36, we reported a class of monomethoxyl PEG grafted O − CMC films, which simultaneously exhibited excellent tensile properties, satisfactory water vapor barrier properties, and good broad-spectrum antibacterial activities. The nontoxic materials hold significant potential for edible food packaging. However, the application range of the composites was greatly limited by their high water solubility.
To solve the limitations of the above CS-based film materials, a novel food packaging material was designed with a simple strategy in this study. The condensation reaction between PEG and excess diisocyanate readily accomplished to produce NCO − terminated PEG (OCN − PEG − NCO), which was used as a crosslinking agent to react with O − CMC in water under mild conditions to produce PEG − modified O − CMC (O − CMC − PEG) with crosslinked structure linked by urea bonds. The influences of the PEG content (or crosslinking degree) on the physicochemical features (thermal properties, tensile properties, water sensitivity, water vapor barrier properties, degradability) of the casted O − CMC − PEG films were extensively investigated. In addition, the antibacterial performance of the composite film was evaluated.