Speaking of the waterproof and breathable principle of microporous membrane, we can look at it from a molecular perspective, by considering the structure of the membrane material and the interaction between the molecules on the surface of the membrane to analyze, in summary, from the following aspects:
I. Membrane material structure:
1. The structure with polymer chains, the characteristics of these polymer chains play an important role in determining the waterproof and breathable properties of the membrane, the polymer is composed of long chain repeating units, these units constitute the main chain of the membrane material, arranged to form a structure with interconnected pores, and these pores only allow gas or water vapor to pass through, while preventing liquid water from passing through, commonly used membrane materials such as polytetrafluoroethylene (PTFE), polypropylene (PP) or polyvinylidene fluoride (PVDF), etc. These polymers are composed of long chain repeating units, these units constitute the main chain of the membrane material
2. The structure of membrane pores. The microporous structure of the membrane consists of interconnected pores that allow certain molecules to pass through, such as gas or water vapor, while blocking other molecules, such as water molecules. The size and distribution of these pores play a vital role in determining the waterproof and breathable properties of the membrane.
2. The principle of membrane waterproofness:
1. Hydrophobicity. From the structural analysis above, it can be seen that the waterproof properties of microporous membranes are mainly attributed to the hydrophobicity of polymer chains. Because of the non-polarity of polymer chains and their interaction with water molecules, they repel water molecules. Therefore, there are many microporous filtration membranes that exhibit hydrophobic properties.
2. Intermolecular forces. The hydrophobic interaction between polymer chains and water molecules is weaker than the cohesive force between water molecules. Therefore, water molecules tend to form water beads on the hydrophobic surface instead of wetting it, minimizing the intrusion of water, thereby achieving the function of waterproofing.
3. Principles of membrane water permeation:
1. Selective permeation. Although microporous membranes are waterproof, they are designed to allow certain molecules or gases to pass through, such as air or water vapor. Gas molecules are smaller than the pore size of the membrane, which means they can pass through the membrane smoothly. Similarly, water molecules are larger than the pore size of the membrane and are retained by the membrane. Therefore, this selective permeation is facilitated by the size and structure of the pores in the membrane.
2. Diffusion characteristics. Water permeation is the diffusion of water vapor molecules through the micropores of the membrane. The movement of water vapor is driven by the concentration gradient, and molecules diffuse from areas with higher concentrations (e.g., inside the membrane) to areas with lower concentrations (e.g., outside the membrane).
4. Interaction of molecules on the membrane:
1. Surface energy. The surface energy of the membrane material affects its interaction with water molecules. The hydrophobic surface has a lower surface energy and tends to repel water, while the hydrophilic surface has a higher surface energy and may interact more favorably with water. This is also the microscopic difference between hydrophilic and hydrophobic.
2. Hydrogen bonds. The hydrophobic surface of the membrane will destroy the hydrogen bonds between water molecules, resulting in the formation of water droplets on the membrane surface instead of a continuous film. This destruction contributes to the waterproof effect.
In practical applications, we can design or customize membranes with waterproof and moisture permeability based on the molecular interactions and structures of microporous filtration membranes and combine the several aspects of the structures and principles introduced above to achieve specific applications in various industries such as water treatment, air filtration and medical equipment.
