Method for measuring the diameter of through-pores in porous membranes

Abstract

To evaluate the diameter of a through-hole of several nm to several-hundreds μm.SOLUTION: In a method for measuring the diameter of a through-hole of a porous film, a porous film is processed into a complex body by using a porous supporting body and a sealing material and the diameter of a through-hole of the porous film is measured by measuring the diameter of a thin hole of the complex body by using a mercury penetration technique.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for measuring the diameter of through-pores in a porous membrane using mercury intrusion porosimetry. [Background technology]

[0002] The pore structure of porous materials affects their fluid transport performance and can be characterized using techniques such as mercury intrusion porosimetry and bubble point porosimetry. Mercury intrusion porosimetry uses a pressurized chamber to inject mercury into pores connected to the surface of a porous material, measuring pore diameters (ranging from a few nanometers to a few hundred micrometers) and pore volume (Non-Patent Document 1). When injecting mercury into a given porous material surface, a method has been developed in which the rest of the material is coated with epoxy resin (Non-Patent Document 2). Conventional mercury intrusion porosimetry characterizes pores that are blocked within the material (hereinafter sometimes referred to as blocked pores) or pores that are interconnected from one surface to another on the material surface (hereinafter sometimes referred to as through pores).

[0003] The bubble point method is a method for selectively measuring the diameter of through-pores in a porous membrane (Non-Patent Documents 3 and 4). The diameter of through-pores, ranging from several tens of nanometers to several tens of micrometers, is measured from the relationship with the air flow rate when the air pressure applied to a porous membrane whose pores are filled with an organic solvent or the like is gradually increased.

[0004] In the development of membranes with fluid transport capabilities, membranes with the ability to separate and select nano-substances such as molecules, proteins, and viruses require a method for evaluating the nano-scale pore size, which affects performance. Furthermore, because defects such as pinholes in porous membranes also affect fluid transport performance, a method for evaluating the macro-pore size is also necessary for the development of superior membranes. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] ISO 15901-1:2016(en)Evaluation of pore size distribution and porosity of solid materials by mercury porosimetry and gas adsorption - Part 1: Mercury porosimetry [Non-patent document 2] Yuya Sakai, Choji Nakamura and Toshiharu Kishi: Evaluation of mass transfer resistance of concrete based on representative pore size of permeation resistance, Construction & Building Materials, Vol.51, 31, pp.40-46, 2013 [Non-patent document 3] ASTM F316-86:1986 Standard Test Methods for pore size characteristics of membrane filters by Bubble Point and Mean Flow Pore Test [Non-patent document 4] Jena, Akshaya and Gupta, Krishna: Characterization of Pore Structure of Filtration Media, Fluid Particle Separation Journal, Vol. 14, 3, pp.227-241, 2002 Summary of the Invention [Problem to be solved by the invention]

[0006] However, conventional mercury porosimetry cannot measure through-pore diameters selectively, and the bubble point method has the problem of being limited to measuring through-pore diameters of several tens of nanometers to several tens of micrometers. [Means for solving the problem]

[0007] To solve the above problems, we discovered a method for measuring the through-pore diameters of porous membranes, which are several nm to several hundred μm, by processing a porous membrane into a composite using a porous support and a sealant and applying mercury intrusion porosimetry. The composite is characterized by covering all surfaces except for the optional porous surfaces with a sealant, and mercury that penetrates from any porous membrane surface flows into the porous support through the through-pores of the porous membrane.

[0008] That is, the present invention is a method for measuring the through-pore diameter of a porous membrane, in which a porous membrane is processed into a composite using a porous support and a sealing material, and the pore diameter of the composite is measured using mercury intrusion porosimetry, thereby measuring the through-pore diameter of the porous membrane. [Effects of the Invention]

[0009] The present invention enables evaluation of the through-pore diameter of a porous membrane, which is several nanometers to several hundred micrometers, by applying mercury intrusion porosimetry to a composite containing the porous membrane. Furthermore, for membranes with asymmetric pore structures, the anisotropy of the through-pore diameters can be used to characterize the pores involved in functional expression. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is an example of a cross-sectional view of a composite in which a porous membrane is produced using a porous support and a sealing material. [Figure 2] 10 is another example of a cross-sectional view of a composite in which a porous membrane is produced using a porous support and a sealing material. [Figure 3] The pore permeability of each porous membrane is shown. [Figure 4] Mercury intrusion data for the composite is shown. [Figure 5] The bubble point pressure of the microporous membrane and the pressure threshold of the MCE composite are shown. [Figure 6] The through-pore diameters are estimated based on the Washburn equation from mercury intrusion data for two types of Al-PO composites. DETAILED DESCRIPTION OF THE INVENTION

[0011] The present invention will now be described in detail.

[0012] The porous membrane of the present invention is composed of at least one solid substance or a plurality of solid substances selected from the group consisting of inorganic, organic, and metal-organic frameworks, and is preferably a plate-like material with a pore radius in the range of about 1.5 nm to about 300 μm.

[0013] The porous support in the present invention is composed of at least one solid substance or a plurality of solid substances selected from the group consisting of inorganic, organic, and metal-organic frameworks, and preferably has a pore radius larger than that of the porous membrane, and is 1 mm 3 It is a material having a pore volume of at least 1000 sq.m.

[0014] The sealing material in the present invention is composed of at least one single or multiple solid or liquid substances selected from the group consisting of inorganic, organic, and metal-organic frameworks, and is preferably a material in which the intermolecular forces such as van der Waals force and Coulomb force acting on the contact surface with the porous membrane are greater than the gravity acting on the sealing material itself, and the radius of the pores connecting to the non-contact surfaces with the porous membrane and the porous support is smaller than that of the porous membrane.

[0015] Schematic cross-sectional views of the composite of the present invention are shown in Figures 1 and 2. The composite is composed of a porous membrane, a porous support, and a sealant, and is characterized by the pore size increasing in the order of the sealant, the porous membrane, and the porous support. Figure 1 is a schematic diagram of a composite in which mercury that has penetrated from the upper surface of the porous membrane can flow into the porous support through the lower surface of the porous membrane. Figure 2 is a schematic diagram of a composite in which mercury that has penetrated from the upper surface of the porous membrane can flow into the porous support through the lower surface, or the side, or both of the porous membrane.

[0016] Mercury passing through the composite's porous membrane exerts sufficient pressure to penetrate the porous support, and the mercury intrusion data for the composite indicates a pressure threshold at which the amount of mercury intrusion increases sharply. The pressure threshold can be used to estimate the through-pore size of the porous membrane based on the Washburn equation. Furthermore, in the composite shown in Figure 2, composites in which the top and bottom surfaces of the porous membrane are inverted may exhibit anisotropy in the through-pore size. This anisotropy is due to the asymmetric pore structure of the porous membrane. Therefore, in membranes with an asymmetric pore structure, the pores involved in functional expression can be characterized from the anisotropy in the through-pore size.

[0017] Mercury intrusion porosimetry is a method for measuring pore size and volume by forcibly infiltrating mercury, which is non-wettable for many materials, into pores on a solid surface using pressure. Specifically, a sample is placed in a glass cell consisting of a sample chamber and a capillary, which is attached to the pressure chamber of the instrument and evacuated. The glass cell is filled with mercury using the pressure difference between the mercury storage chamber inside the instrument and the glass cell as the driving force. As the pressure inside the glass cell increases, mercury infiltrates the pores, reducing the amount of mercury in the capillary. The capillary's outer surface is covered with metal, forming a capacitor with the mercury column inside. The capacitance of the glass cell is calibrated against a known amount of mercury, corresponding to the amount of mercury infiltrated into the pore. The pressure is controlled over a range from approximately 2 kPa to approximately 400 MPa, and the amount of mercury intrusion versus pressure (mercury intrusion data) is recorded. The dynamic range of pore radius that can be characterized by mercury intrusion porosimetry is approximately 1.5 nm to approximately 300 μm at a contact angle of 130°, based on the Washburn equation.

[0018] The pore diameter in the present invention is defined as the diameter or radius of the base of a through pore or a closed pore when the pore is assumed to be cylindrical.

[0019] A through hole in the present invention is defined as a pore that has both ends connected to two outer surfaces of a solid material.

[0020] A closed pore in the present invention is defined as a pore that is connected at one end to one outer surface of a solid material and is surrounded at one end by a solid.

[0021] From the above, by processing a porous membrane into a composite using a porous support and a sealant and applying mercury intrusion porosimetry, it is possible to measure the through-pore diameter of the porous membrane in the range of several nm to several hundred μm. Furthermore, for membranes with asymmetric pore structures, the anisotropy of the through-pore diameter can be used to characterize the pores involved in functional expression. [Example]

[0022] The present invention will be described below with reference to examples. Four types of cellulose mixed ester porous membranes (MCE#1 to MCE#4) and one type of polyolefin porous membrane (Al-PO) coated on one side with alumina were used as samples. The thickness of MCE#1 to MCE#4 was approximately 100 μm as measured by a digital micrometer, and the thickness of the Al-PO was 21.8 μm (alumina layer 3.6 μm, polyolefin layer 18 μm) as measured by cross-sectional SEM photographs.

[0023] The air permeability and pore size of each porous membrane were evaluated using a PMI Perm Porometer compatible with the bubble point method (JIS K3832). For air permeability measurements, a dry porous membrane with a diameter of 2.5 cm was placed in a sample holder, the pressure difference between the top and bottom sides of the membrane was increased, and air permeation data was recorded. For pore size measurements, prior to the measurement, a porous membrane with a diameter of 2.5 cm was immersed in Silwick (surface tension γ: 19.1 dyne / cm), and the pores were filled with Silwick by vacuum degassing. The membrane with pores filled with Silwick was placed in the sample holder, and the pressure difference between the top and bottom sides of the membrane was increased to measure the minimum pressure at which Silwick was expelled (bubble point pressure). Bubble point pressure P B Based on the following formula, the through hole radius r B was estimated. r B =1.43γ / P B (1).

[0024] Mercury intrusion data for each sample was measured using an Autopore IV (Micromeritics). The pressure P was increased from 4 kPa to 400 MPa, and the mercury intrusion volume V was recorded as a function of P. The pore radius r was estimated from P using the following Washburn equation:

[0025] r=-2γcosθ / P (2).

[0026] Here, the contact angle θ between the mercury and the sample was set to 130°, and the surface tension γ of mercury was set to 480 dyne / cm. The above pressure range corresponds to a pore radius of 1.5 nm (400 MPa) to 150 μm (4 kPa) according to equation (2).

[0027] Figure 3 shows the pore permeability of each porous membrane. MCE#1 to #4 have similar thicknesses, suggesting that the lower the permeability, the denser the MCE. Furthermore, Al-PO has the lowest air permeability, with a thickness roughly one-fifth that of MCE#4, suggesting that it has a denser structure than MCE#4. In fact, the pore diameters of MCE#1 to MCE#4 and Al-PO decreased with decreasing air permeability (Table 1). For Al-PO, which exhibited the lowest air permeability, the bubble point pressure was above the upper limit of measurement (1 MPa), and therefore the pore radius could be estimated to be less than 30 nm using equation (1).

[0028] [Table 1]

[0029] For MCE#1 to MCE#4, whose bubble point pressures were detected, each membrane was processed into a composite as shown in Figure 1, and the obtained mercury penetration data is shown in Figure 4. With increasing pressure, the amount of mercury penetration into the MCE composite exceeds a certain pressure threshold P t Considering the structure of the composite, the sudden increase in the amount of mercury intrusion is due to the mercury passing through the inside of the MCE and flowing into the porous support.

[0030] Figure 5 plots the bubble point pressure of the microporous membrane and the pressure threshold of the MCE composite. t and P B Since the two are proportional, they are affected by the same structural characteristics. B Since is a value that reflects the through-hole diameter (Non-Patent Document 4), P of the MCE composite t The pore diameter estimated from equation (2) corresponds to the through-pore diameter of the MCE.

[0031] Therefore, this technology, which applies mercury intrusion to composites obtained by processing microporous membranes, targets through-pore diameters ranging from several nanometers to several hundred micrometers.

[0032] Next, we will show an example of capturing the anisotropy of the nanopore diameter of a porous membrane by changing the structure of the composite. As shown in Figure 2, two types of Al-PO composites were created with different surfaces exposed from the sealing material. By inverting and arranging the Al-PO within the composite, we created membrane composites with i) the alumina surface exposed and ii) the polyolefin surface exposed. In both cases, mercury penetrates from the top surface (exposed surface) and can penetrate into the porous support through the side or bottom surface.

[0033] Figure 6 shows the through-pore radius estimated based on equation (2) from mercury intrusion data for Al-PO composites. Both types of Al-PO composites have nanoscale through-pore diameters, and the alumina-exposed composite exhibited a larger mercury intrusion radius than the polyolefin-exposed composite. The anisotropy of the through-pore diameter indicates differences in the mercury penetration pathways, suggesting that the alumina layer has a coarser pore structure than the polyolefin layer. In other words, because the mercury penetration pressure into the polyolefin layer is lower than that into the alumina layer, mercury that penetrates through the alumina surface escapes through the alumina side, while mercury that penetrates through the polyolefin surface can be reasonably predicted to pass through the side or the alumina layer.

Claims

1. The porous membrane is processed into a composite with a porous support and a sealant; the sealing material is composed of at least one solid or liquid substance selected from the group consisting of inorganic, organic, and metal-organic frameworks; A method for measuring the diameter of through-pores in a porous membrane, comprising applying mercury intrusion porosimetry to the composite to measure the diameter of the through-pores in the porous membrane.

2. A porous membrane is processed into a composite using a porous support and a sealing material, The anisotropy of the pore diameter is evaluated by changing the surface to be measured of the porous membrane exposed from the sealing material in the composite; A method for measuring the diameter of through-pores in a porous membrane, comprising applying mercury intrusion porosimetry to the composite to measure the diameter of the through-pores in the porous membrane.

3. 3. The method for measuring the through-pore diameter of a porous membrane according to claim 1 or 2, wherein the porous membrane and the porous support are both composed of at least one solid substance selected from the group consisting of inorganic, organic, and metal-organic frameworks, and the pore diameter of the porous support is larger than the pore diameter of the porous membrane.

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