In the application of film materials, the barrier properties of polymers against water and oxygen directly affect product shelf life and performance. However, a common phenomenon is that materials like PE and PP exhibit excellent water vapor barrier properties but relatively high oxygen transmission rates, while materials such as PA and EVOH have outstanding oxygen barrier performance but suffer significant performance degradation in high-humidity environments. The core of this difference lies in the distinct barrier mechanisms of polymers against water vapor and oxygen.
     The essence of barrier performance is determined by the permeability coefficient P, with the core relationship being P=S×D (where S is the solubility coefficient and D is the diffusion coefficient), meaning it is jointly determined by two processes: the dissolution and diffusion of medium molecules in the material. The key to water vapor barrier lies in molecular interactions and segment activation: water molecules are highly polar, can form hydrogen bonds, and easily interact with polymers containing polar groups such as -OH and -COOH. Additionally, as small-molecule plasticizers, water molecules weaken intermolecular forces and lower the glass transition temperature, leading to a significant increase in the diffusion coefficient. This explains why the water vapor barrier performance of some materials declines in high-humidity environments.
     In contrast, oxygen molecules are non-polar with low reactivity and lack strong interactions with most polymers. Their barrier performance is mainly dominated by the diffusion coefficient, relying on the material's structural density and free volume characteristics. High oxygen barrier materials typically feature high segment rigidity, regular structure, and a high crystalline region ratio. However, such structures are susceptible to the influence of water molecules, resulting in the deterioration of oxygen barrier performance in high-humidity environments.
     In engineering applications, improving water vapor barrier requires reducing material hydrophilicity, increasing the glass transition temperature, or constructing multi-layer composite structures. Enhancing oxygen barrier, on the other hand, involves optimizing segment packing efficiency and orientation. Due to thermodynamic contradictions in the structural design for the two types of barrier properties, a single polymer material can hardly achieve both low water vapor transmission rate and low oxygen transmission rate in high-humidity environments. Solutions such as multi-layer composites and barrier coatings have become the preferred choices for high-barrier applications in food, pharmaceuticals, and other fields.

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