How to evaluate the pore size of microporous filtration membrane and what is its theoretical basis

The pore size of a microporous filtration membrane is a key parameter that determines its separation performance. Most methods for evaluating pore size use a variety of measurement methods. The data obtained through these relatively scientific methods reflect parameters such as pore size specifications and performance, but each Each measurement method has its limitations, here is a brief analysis for reference.

1. Measurement of filtration membranes using the bubble point method, a widely used method based on the principle of capillary flow in porous materials The desired pressure is then calculated using the Young-Laplace equation, which relates the pressure difference across the liquid interface in a capillary (or pore) to the surface tension and contact angle of the liquid and the radius of the tube (or pore), usually Used to repel water and allow air or gas to pass through, while preventing liquid from passing through hydrophobic filter membranes, usually the membrane materials involved include polytetrafluoroethylene (PTFE), polypropylene (PP) and polyethylene (PE) etc., membranes evaluated using bubble point measurement methods depend on factors such as the desired pore size range, compatibility with the liquid used, and specific testing requirements.

2. Use porosimetry (mercury intrusion porosimetry or gas adsorption), commonly used techniques mainly used to measure the pore size distribution and porosity of materials, such as mercury intrusion porosimetry (MIP) and gas adsorption (such as nitrogen or helium) gas), suitable for membrane materials that can withstand high pressure and allow detection fluids or gases to penetrate into the pores, such as porous ceramic membranes with an interconnected uniform pore network, such membranes have a well-defined and controllable pore structure, high chemical stability Porosimetry is a more suitable evaluation method for the characteristics of thermal stability and compatibility with various solvents and gases.

3. Scanning electron microscopy (SEM) measurement method. SEM can directly visually evaluate the membrane surface and pore size. It mainly relies on the interaction of electron beams. Various interactions will occur when the electron beam hits the surface of the membrane sample, such as membrane Some electrons from the outermost shell of atoms in the sample are re-emitted back, carrying information about the surface morphology and composition of the film. Similarly, backscattered electrons hitting the surface of the film sample also provide information about the atomic composition and density. . This type of evaluation method is usually suitable for membrane materials with larger pores and has limitations in the complete image of pore size distribution. The more suitable types of microporous filtration membranes such as polycarbonate (PC) membranes.

4. Atomic Force Microscopy (AFM) measurement method, which is a technique used to determine the characteristics of microporous filtration membranes at the atomic level. AFM can provide nanoscale 3D topographic images of membrane surfaces, which can be used to estimate pore size, pore depth, and pore size. Density, surface roughness, and the distribution of these properties can be evaluated, and it can also be used to measure mechanical properties, such as membrane stiffness or elasticity, etc., suitable for evaluation methods of microporous membranes such as silicon nitride (Si3N4) membranes.

5. Permeability measurement method, the flow rate of gas or liquid through the membrane can indirectly measure the pore size, pressure filtration method, constant volume filtration method, etc.

The evaluation methods for the pore size of microporous filtration membranes introduced above are all based on the principles of fluid dynamics, capillary flow, adsorption, microscopy, etc. Simply discussing a certain evaluation method has its advantages and limitations, and even It can be said that even the best measurement methods cannot fully represent the actual membrane pore size distribution, especially for membranes with polydisperse and irregularly shaped pores. For example, the bubble point method only provides information about the largest pores, while mercury porosimetry. Smaller pores may not be accurately represented because of the high pressure required for mercury to enter these pores. It is not difficult to see that the combination of multiple evaluation methods can provide a more comprehensive understanding of the pore size distribution of microporous filtration membranes. In practical applications, Due to limitations of various conditions, costs and other reasons, it is very difficult to comprehensively combine all evaluation methods. We can only make a more objective and closer to actual value based on membrane selection and application, specific membrane material characteristics, available evaluation resources, etc. Pore size evaluation method.