| The barrier property of food packaging materials received significant attention as the guarantee of food quality and safety.Therefore,the design of food packaging with high barrier property has become one of the key scientific problems which were urgently needed to be solved in nowadays food science area.The barrier property of food packaging was determined by the mass transfer of small molecule penetrants.The barrier property against water vapor and oxygen was very crucial to food packaging.Based on the development of biodegradable packaging materials,this study put forward the using of nanoparticles chemical composition,size and aspect ratio to regulate the multi-scale structures of the nanocomposite films in order to control their barrier property.From the view of film multi-scale structures,the mechanism about the barrier property against water vapor and oxygen was clarified,which had important academic value and guiding significance for the rational design of starch-based nanocomposite films with various barrier property..According to the mass transfer process and mechanism of water vapor and oxygen,the cellulose nanocrystals and carnauba wax nanoparticles were selected.The various starch-based nanocomposite films were prepared by the change of nanoparticle chemical composition,size and aspect ratio.In the simulation of real food storage conditions,the relative permeability against water vapor and oxygen of starch-based nanocomposite films were tested based on the national standards.The results showed the water vapor and oxygen relative permeability decreased after the addition of nanoparticles,but they were all obviously deviated from the tortuous model approximation.The relative permeability of these two small molecules showed obvious difference for the same nanocomposite film.It indicated that the improvement of barrier property cannot be accurately explained only by the impermeability of nanoparticles.As the tortuous path theory neglected the influence of film multi-scale structures change after nanoparticles addition,the influence of chemical composition,size and aspect ratio of nanoparticles on starch molecular interaction,short-range molecular conformation,crystalline structure and micro-ordered region structure in the nanocomposite films were systematically investigated with modern analytical technologies.The physical model between multi-scale structure change and barrier property of starch-based nanocomposite films was established.The structural functional relationship was found from molecular level thus molecular mechanism of film barrier property was revealed.The results suggested that,for the molecular interaction,the hydrogen bonding was the most crucial secondary interaction for starch-based nanocomposite films.The chemical composition of the nanoparticles was the key factor for the hydrogen bonding formation with starch molecules.The nanoparticles could occupy the hydrogen bonding forming position in the starch molecules thus the inter-and intra-starch molecule hydrogen bonding was changed.For the short-range molecular conformation,the hydrogen bonding stabilized the starch helix and restricted the mobility of random coil starch chains.The change of hydrogen bonding would result in the portion change of double helices,single helix and random coil in the films.Compared with chemical composition and size,the aspect ratio of nanoparticles achieved the subtle regulation of short-range molecular conformation.For the crystalline structure,the double helix was the major component of the starch crystalline domain.The increase of double helix portion favored the rise of starch relative crystallinity.But there was not a simple linear relationship between double helix portion and starch relative crystallinity.The nucleating effect and steric hindrance effect of the nanoparticles together determined the starch relative crystallinity.Compared with chemical composition,the aspect ratio and size of the nanoparticles played more notable role in regulating starch relative crystallinity.For the microordered region,it was the characterization of film aggregation structure in a larger scale than crystalline structure.The size of micro-ordered region was affected by starch relative crystallinity and inter-and intra-starch molecule hydrogen bonding.Compared with nanoparticle size,the effect of chemical composition and aspect ratio of nanoparticles on micro-ordered region size was more obvious.Owing to the variation of migration pattern of water vapor and oxygen,the key structure factors from the view of film multi-scale structure that affected the barrier property against these two molecules were not the same.Water vapor was a typical polar molecule that could form hydrogen bonding with non-crystalline starch chains in the micro-ordered region.The starch molecules was further plasticized which was in favor of water vapor permeation,thus the barrier effect of micro-ordered region against water vapor was relative low.Starch crystalline structure was so dense packed that the water vapor could not further plasticize the starch chains in crystalline domain for water vapor permeation.The increase of starch relative crystallinity favored the water vapor barrier property.The arrangement of starch chains in amorphous region was relative loose.The further amorphization in this region after the addition of nanoparticles offered more absorption sites for water vapor which favored its permeation.But oxygen was a typical non-polar molecule that could not further plasticize the starch chains by forming hydrogen bonding.The permeation of oxygen needed the wide open of starch chains thus it was more closely correlated with the cohesive energy density(CED)in the films.The oxygen could not permeate the micro-ordered region with relative high CED.Thus the increase of micro-ordered region size contributed to oxygen barrier property and it was more obvious than the improvement of water vapor barrier property.The amorphous region with relative low CED was the main permeation path for oxygen.The further amorphization in this region was in favor of oxygen permeation but the rise was not as much as water vapor.This study scientifically connected the multi-scale structure of starch-based materials with mass transfer process of typical small molecule penetrants.The method and mechanism of barrier property regulation against water vapor and oxygen were obtained,which provided the technical support and theoretical foundation for the development of new biodegradable food packaging with permselective barrier property. |
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