KR20000007593A - Method of making membrane having microscopic holes differs in size to direction of thickness - Google Patents

Abstract

PURPOSE: A method of making membrane is provided to increase the stability of membrane as an isolation membrane of battery cell and improve the shut down function. CONSTITUTION: The method of membrane making comprises the step of projecting the high energy ions on the surface of a high polymer membrane in a vacuum circumstance to form microscopic hole in the membrane. In the membrane, the size of microscopic hole is decreasing with the advance of distance from the surface of membrane. The method improves the shut down function of membrane caused by the dispersion and size of the microscopic hole and make a membrane of better mechanical property with less physical damage without contamination to circumstance.

Description

Preparation of Microporous Membrane with Pore Size Difference in Thickness Direction

The present invention relates to the production of microporous membranes having a pore size difference in the thickness direction.

The battery separator basically separates the positive electrode from the negative electrode and prevents a short circuit due to fusion bonding between the two electrodes, and serves to pass electrolyte or ions. Although the material itself is inert which does not contribute to electrical energy, its physical properties greatly affect battery performance and safety. Various kinds of separators are used depending on the type of battery chemistry, but various studies have recently been conducted in lithium batteries because they require different characteristics from those used in other batteries.

The basic properties required as separators are to isolate the positive and negative electrodes, to facilitate the passage of electrolytes or ions to lower the electrical resistance, the wettability of the electrolyte, the mechanical strength required for battery assembly and use, and the film thickness for high density charging. May be reduced. In the case of highly reactive lithium ion batteries, the safety of the separator is particularly demanded. In addition to the basic function of the aforementioned separator, a function of closing the battery circuit by closing micro pores when a large current is suddenly introduced for reasons such as an external short circuit is caused. Say The cutting function of the battery circuit due to such fine pore closure is referred to as shut-down characteristic of the separator. In terms of safety against external short-circuits, the membrane's shut-down characteristics, as well as the melt-integrity of the membrane when temperature rises after shutdown is a very important factor. Will act as. If the shut-down occurs completely, then the residual current becomes zero, but it is very difficult and the temperature generally increases rapidly, making it difficult to control. Therefore, maintaining the film shape above the melting temperature is of great importance, and losing the shape too early is a dangerous condition because it causes direct welding of the electrode.

As a factor influencing the safety of the separator such as shut-down characteristics and melt-integrity, the material of the separator may be mentioned first. The faster the shutdown, the easier it is to suppress the temperature rise due to the closing of micropores. Currently, lithium ion batteries often use polyethylene with a low melting point, and sometimes the shutdown characteristics of the separator. In addition, polyethylene and polypropylene may be used together in consideration of melt-integrity and mechanical properties.

As a method of manufacturing a lithium ion battery separator by laminating polyethylene and polypropylene, European Patent Nos. 715,364, 718,901 and U.S. Patents 5,240,655, 5,342,695, 5,472,792, and Japanese Unexamined Patent Publication No. 4-181651 etc. are mentioned. However, there is a difficulty in thinning the film thickness, processing technology is also difficult, and the adhesive strength between the polyethylene layer and the polypropylene layer is weak, so that it is easily separated between layers.

Also described in US Pat. Nos. 5,385,777 and 5,480,745 are methods for making microporous membranes using polyethylene / polypropylene blend systems. However, this method is also not commercialized and widely used.

In addition, methods for dispersing and presenting a low melting point thermal fuse material on a general porous membrane matrix are disclosed in US Pat. Nos. 4,650,730, 4,731,304, 4,973,532, and 5,2440,655. And 5,453,333. Here, the thermal melting material is a material having a significantly lower melting temperature than the porous membrane base material, and examples thereof include low density polyethylene, ethylene-vinylacetate copolymer, waxes, and the like. However, there are various problems such as difficulty in the manufacturing process, weak shutdown characteristics, reduction of mechanical properties, and the like, and thus have difficulties in actual use.

In the present invention, unlike the conventional method using the characteristics of such a material, by irradiating the surface of the polymer with ionic particles with energy under a vacuum state to produce a membrane having a different pore size in the thickness direction of the material, safety as a battery separator, in particular shutdown We want to improve the (shut-down) function. When the separator is manufactured using the present invention, the shut-down characteristic becomes wider in addition to the material itself. Therefore, the mechanical properties and melt-integrity properties are excellent, but materials such as polypropylene, which have been limited in use due to shut-down properties, can be used as separators.

1 is an electron micrograph of the ion beam irradiation surface of the microporous membrane.

FIG. 2 is an electron micrograph of an opposite side not irradiated with an ion beam in the same microporous membrane as in FIG. 1.

FIG. 3 is a graph showing pore size distribution in the film thickness direction of a microporous membrane prepared by irradiating an argon ion beam on one side (FIG. 3A) or both sides (FIG. 3B) of a polypropylene disc film.

Figure 4 is a graph comparing the shutdown (shut-down) characteristics of the battery separator prepared by the present invention and the conventional separator made of conventional polyethylene and polypropylene.

When the microporous membrane for battery separator is manufactured by irradiating high energy ion particles, the number and degree of microcracks may be controlled by the number and energy of ion particles irradiated on the surface of the polymer, and the pores may be formed using an stretching process. When used in conjunction with the dry method for manufacturing, it is possible to control a variety of pore size and shape, thereby obtaining a membrane having a high pore density and porosity. In addition, there is little physical damage to the membrane has excellent mechanical properties, and has the advantage of being an environmentally friendly process compared to other methods using a solvent.

Method of modifying polymer surface by ion beam assist reaction consisting of reducing the contact angle to water on the surface of polymer or increasing adhesion by irradiating ionic particles to polymer surface while blowing reactive gas under vacuum The modified polymer is disclosed in Korean Patent Application No. 96-2356. However, the present invention uses an ion beam together with a reactive gas to impart hydrophilicity or hydrophobicity of the surface of the polymer, and is not related to the formation of microporous membranes. In the present invention, by using the ion beam used to modify the surface of the polymer for the production of microporous membranes to improve the safety function of the battery separator by controlling the pore size distribution in the membrane thickness direction.

According to the method of the present invention, when the microporous membrane is manufactured by using an ion beam irradiation and a dry method process, the number of microcracks and the degree of cracking can be controlled by the number and energy of ion particles irradiated on the surface of the polymer, and thus the pores. It is possible to produce a membrane having a very small size, a large number thereof, and a high porosity. In particular, the pore size tends to decrease in the thickness direction according to the degree of ion beam energy reaching from the surface to which the ion beam is irradiated, and such a small pore enables pore clogging even at a temperature lower than the melting temperature of the membrane material. In this case, only the shutdown temperature is lowered while maintaining the melt-integrity characteristic of the membrane, which greatly contributes to battery safety.

Most studies to date have focused on improving shut-down characteristics through material changes, but in the present invention, the morphological factors of the membrane, that is, the decrease in pore size, may cause pores to be blocked at lower temperatures. With this in mind, we tried to improve the shut-down characteristics. When the pore size is reduced, the mechanical properties are also improved, but it can be expected that the electrolyte and ions can not pass easily. In the present invention, this problem can be solved by increasing the number of pores, improving porosity, and adjusting pore size in the film thickness direction. Membrane prepared for the present invention, there are a lot of pores of a relatively large size on the surface of the membrane, the more pores of a smaller size as the inside enters, the shape characteristics as if laminated with a film having a different pore size ( morphology). Therefore, the pores with a large size of the membrane surface improve the permeability of the electrolyte solution and ions, and the shut-down characteristics and the mechanical properties can be improved by the small pores inside the membrane.

The method used in the present invention specifically consists of the following method.

1. Preparation of disc film: A disc film is prepared by using an extruder having a T-die attached to a synthetic resin.

2. Annealing: In order to increase the crystallinity and elastic recovery rate of the prepared original film, it is annealed at a temperature below the melting point of the polyolefin in a drying oven.

3. Surface irradiation: After the original film is put into a vacuum chamber maintained at high vacuum, ion generating gas is injected into the ion gun to produce particles with energy, and then energy is changed while changing the ion beam current. Particles with are irradiated on one or both surfaces of the polymer surface. At this time, the power supply connected to the ion gun is adjusted so that the energy of the energetic particles is 0.01-10 keV. Fine cracks are formed in the polymer irradiated with the ion beam. On the other hand, in order to modify the surface of the polymer simultaneously with the formation of microcracks, hydrophilic or hydrophobic treatment is performed by injecting a reactive gas with a reactive gas injector while changing the amount of reactive gas injected into the film around 0.5-20 ml / min with ion beam irradiation. . This surface modification step may be carried out separately after the preparation of the microporous membrane.

4. Low temperature stretching: The film irradiated with an ion beam is stretched uniaxially at a temperature below room temperature using a roll or a biaxial stretching machine to grow the micro cracks prepared above.

5. High temperature stretching: By using a biaxial stretching machine, the microcracks prepared by ion beam irradiation and low temperature stretching are uniaxially or biaxially stretched below the melting point of the polymer to form micropores having a desired size. Imparts mechanical properties to the membrane.

6. Heat fixation: After high temperature stretching, heat fix for a certain time under tension under the melting point of polymer.

The above steps describe the overall process for the preparation of the membrane with optimal properties, and some steps may be omitted or additional steps may be added depending on the final properties desired.

The microporous membranes prepared using this method were analyzed based on the following items.

1) thickness

2) air permeability: JIS P8117

3) Porosity: ASTM D2873

4) Pore size: SEM, TEM

5) Pore density: SEM, TEM

6) Tensile strength and tensile modulus: ASTM D882

7) puncture resistance

8) shut-down (SD) temperature

9) melt-integrity temperature

10) Water absorption speed: JIS L-1096

Example 1

Formation of Polyethylene Microporous Membranes Using Ion Beams

High density polypropylene used isotactic homopolymer, melt index was 2.0 g / 10 min, density was 0.90 g / cc. Polypropylene precursor films were prepared using a single screw extruder and a take-up device with a T-die attached. The extrusion temperature was 230 ° C., the cold roll temperature of the winding device was 70 ° C., and the winding speed was 60 m / min, at which time the winding ratio was 120. The prepared original film was annealed at 140 ° C. for 1 hour in a drying oven. After the annealing, the original film was placed in a vacuum chamber maintained at 10 ⁻⁵ −10 ⁻⁶ torr, and then argon particles (Ar ⁺ ) were irradiated on both sides of the original film using an ion gun to form pores. At this time, the energy of the ion beam was 2.5 keV, and the ion irradiation amount was 5 × 10 ¹⁸ ions / cm ² . The physical properties of the obtained microporous membrane are shown in Table 1.

Example 2

Formation of Polypropylene Microporous Membrane Using Ion Beam Irradiation and Low Temperature Stretching

The same high density polypropylene as in Example 1 was used. The high density polypropylene original film was manufactured using the same extruder and winding apparatus as Example 1, the cooling roll temperature of the winding apparatus was 60 degreeC, the winding speed was 30 m / min, and the winding ratio was 70. The obtained original film was annealed at 140 ° C. for 1 hour in the same drying oven as in Example 1. After annealing, fine cracks were made using the same vacuum and ions as in Example 1. At this time, the energy of the ion beam was 1 keV, and the ion irradiation amount was 5 × 10 ¹⁷ ions / cm ² . After the ion beam irradiation was completed, uniaxial stretching was performed at a draw ratio of 100% with respect to the initial length at room temperature using a roll stretching method. After the stretching was completed, by using an annealing roll fixed at 140 ℃ heat fixed for 2 minutes in the state under tension to cool to prepare a fine pore membrane. Various physical properties of the obtained microporous membrane are shown in Table 1.

Example 3

Formation of Polypropylene Microporous Membrane Using Ion Beam Irradiation and High Temperature Stretching

A high density polypropylene disc film was prepared in the same manner as in Example 2. The obtained original film was annealed at 140 ° C. for 1 hour in the same manner as in Example 2. After annealing, fine cracks were made using the same vacuum and ions as in Example 2. At this time, the energy of the ion beam was 1.5 keV and the ion irradiation amount was 5 × 10 ¹⁷ ions / cm ² . After the ion beam irradiation was completed, high temperature uniaxial stretching was carried out at 140 ° C. at a stretching ratio of 100% using a biaxial stretching machine manufactured by Toyoseiki Co., Ltd. After stretching, the microporous membrane was prepared by heat-fixing for 2 minutes while being tensioned at 140 ° C. Various physical properties of the obtained microporous membrane are shown in Table 1.

Example 4

Formation of Polypropylene Microporous Membrane Using Ion Beam Irradiation and Low Temperature and High Temperature Stretching

An original film of high density polypropylene was prepared in the same manner as in Example 2 and then annealed. After annealing, fine cracks were made using the same vacuum and ions as in Example 2. After the ion beam irradiation was completed, uniaxial stretching was performed at a draw ratio of 30% of the initial length at room temperature using a roll stretching method. After the stretching at room temperature, a high temperature uniaxial stretching was carried out at 140 ° C. at a stretching ratio of 70% using a biaxial stretching machine manufactured by Toyoseiki Co., Ltd. Subsequently, heat fixation was performed for 2 minutes while the tension was applied at 140 ° C., followed by cooling to prepare a microporous membrane. Various physical properties of the obtained microporous membrane are shown in Table 1.

Example 5

Formation of Polyethylene Microporous Membrane Using Ion Beam Irradiation and Low Temperature and High Temperature Stretching

High density polyethylene was used, with a melt index of 0.05 g / 10 min and a density of 0.951 g / cc. The high density polyethylene disc film was produced using the same extruder and winding device as in Example 1. The extrusion temperature was 180 ° C., the cold roll temperature of the winding device was 50 ° C., and the winding speed was 35 m / min, at which time the winding ratio was 70. The prepared original film was annealed at 110 ° C. for 1 hour in a drying oven. After annealing, the original film was placed in a vacuum chamber maintained at 10 ⁻⁵ −10 ⁻⁶ torr, and then argon particles (Ar ⁺ ) were irradiated on both sides of the original film using an ion gun to form fine cracks. At this time, the energy of the ion beam was 1 keV, and the ion irradiation amount was 10 ¹⁷ ions / cm ² . After the ion beam irradiation, it was uniaxially stretched at a draw ratio of 30% at room temperature and 70% at 115 ° C in the same manner as in Example 4. Subsequently, in the same manner as in Example 4, the microporous membrane was prepared by heat-fixing for 2 minutes under tension at 115 ° C. and cooling. Various physical properties of the obtained microporous membrane are shown in Table 1.

Example 6

Formation of Microporous Membranes Composed of Polypropylene / Polyethylene Blends

The original film was manufactured using the same extruder, T-die, and winding apparatus as Example 4 and 5 using the polypropylene and high density polyethylene used in Examples 4 and 5. At this time, the composition ratio was polypropylene / high density polyethylene = 80/20% by weight. On the other hand, the extrusion temperature was 230 ℃, the cold roll temperature of the winding device was 55 ℃, and the winding speed was 50 m / min, the winding ratio of the original film produced at this time was 100. The obtained original film was annealed at 120 ° C. for 1 hour in the same drying oven as in Example 1. Fine annealing was made using the same vacuum and ions as in Examples 1 and 2 after annealing. At this time, the energy of the ion beam was 1.25 keV, and the ion irradiation amount was 2.5 × 10 ¹⁷ ions / cm ² . After the ion beam irradiation was finished, uniaxial stretching was performed in the same manner as in Example 4 at a draw ratio of 30% at room temperature and 70% at 125 ° C. Subsequently, the film was heat-fixed for 2 minutes while being tensioned at the same temperature and then cooled to prepare a microporous membrane. Various physical properties of the obtained microporous membrane are shown in Table 1.

Example 7

Formation of microporous membrane consisting of polypropylene / polyethylene lamination

The polypropylene disc film was prepared in the same manner as in Example 4, and the high density polyethylene disc film was made in the same manner as in Example 5 so that the thickness of each was 12 μm. The two original films thus prepared were laminated in the order of polypropylene / high density polyethylene / polypropylene at high temperature using a press. The temperature of the press was 130 degreeC, and the pressure was 50 kg / cm <^2> . The laminated disc film was irradiated with an ion beam in a vacuum in the same manner as in Example 6 to form fine cracks on the surface. Various properties of the obtained microporous membrane are shown in Table 1.

Example 8

Formation of Polyethylene Microporous Membrane Using Ion Beam Irradiation and Stretching with Hydrophilic Reactive Gas

An original film was prepared in the same polypropylene and production method as in Example 4, followed by annealing. After annealing, the same vacuum state and ions as in Example 4 were used to generate fine cracks on the surface, and at the same time, the amount of reactive gas (oxygen) injected around the film was injected by a reactive gas injector to perform surface treatment. At this time, the energy of the ion beam was 1.2 keV, and the ion irradiation amount was 2 × 10 ¹⁶ ions / cm ² . After the ion beam irradiation was finished, uniaxial stretching was performed at room temperature and high temperature in the same stretching manner as in Example 4. Subsequently, heat fixation was performed for 2 minutes while the tension was applied at 140 ° C., followed by cooling to prepare a microporous membrane. Various physical properties of the obtained microporous membrane are shown in Table 1.

Comparative Example 1

Preparation of Polypropylene Microporous Membrane Using General Dry Method

In the same manner as in Example 8, the microporous membrane was prepared by the general dry process, and unlike the previous example, the ion particles and the reactive gas treatment were not performed. Various physical properties of the obtained microporous membrane are shown in Table 1.

Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Thickness (μm) 20 24 24 25 25 25 Pore size (μm) 0.01 × 0.01 0.1 × 0.04 0.05 × 0.02 0.08 × 0.03 0.12 × 0.06 1.08 × 0.04 Porosity (%) 27 39 36 45 40 43 Pore density (poredensity, pores / cm ² ) 18 × 10 ⁹ 15 × 10 ⁹ 15 × 10 ⁹ 15 × 10 ⁹ 17 × 10 ⁹ 16 × 10 ⁹ Breathability (sec / 100 cc) 960 670 720 650 620 640 Puncture resistance (g) 580 515 540 535 510 520 Tensile strength (kg / cm ² ) (MD / TD) 1850/195 1530/165 1600/180 1720/185 1580/190 142/175 Tensile modulus (kg / cm ² ) 8100 7800 7500 7700 6650 7200 Non-Cooling Temperature (SD temp, ℃) 132 135 136 136 110 121 Membrane breaking temperature (℃) 180 179 179 179 150 174

Example 7 Example 8 Comparative Example 1 Thickness (μm) 26 24 25 Pore size (μm) 0.08 × 0.03 0.09 × 0.04 0.13 × 0.06 Porosity (%) 46 47 37 Pore density (pores / cm ² ) 17 × 10 ⁹ 15 × 10 ⁹ 5 × 10 ⁹ Breathability (sec / 100 cc) 635 620 660 Puncture resistance (g) 530 525 460 Tensile strength (kg / cm ² ) (MD / TD) 1620/205 1600/190 1500/160 Tensile modulus (kg / cm ² ) 7400 7600 6800 Non-Cooling Temperature (SD temp, ℃) 117 137 159 Membrane breaking temperature (℃) 180 178 181

By using the method for producing a microporous membrane using the ion beam of the present invention, the pore size can be adjusted in the film thickness direction, thereby improving safety when used as a battery separator. While the melt-integrity property of the membrane is maintained, the pores are blocked at a lower temperature by the smaller pores inside the membrane, thereby improving the shut-down characteristics. In addition, having a pore size distribution in the film thickness direction, it is possible to simultaneously realize excellent physical properties that can be expected at each pore size, that is, high electrolyte and ion permeability, improvement of mechanical properties and the like.

Claims (31)

In a polymer membrane having fine pores formed by irradiating a surface with ion particles having high energy under a vacuum state, A polymer microporous membrane, characterized in that the size of the pores decreases away from the surface of the polymer membrane. The method of claim 1, Microporous membranes with spherical or oval pores. The method of claim 1, A microporous membrane having a pore size of 0.01 to 20 μm and a pore size of 0.001 to 5 μm inside the membrane. The method of claim 1, A microporous membrane in which fine pores are formed on one or both surfaces of the polymer surface. The method of claim 1, The microporous membrane of the polymer membrane comprises a material selected from nylon (nylon), polyester (polyester), polysulfone (polysul fone), polyimide (polyimide). The method of claim 1, The microporous membrane, wherein the polymer membrane comprises a material selected from polypropylene, high density polyethylene, low density polyethylene, linear low density polyethylene. The method of claim 1, The microporous membrane wherein the polymer membrane comprises a blend of polyolefin resin. The method of claim 1, The microporous membrane wherein the polymer membrane comprises a laminate of polyolefin resin. In the method of producing a polymer membrane having fine pores by irradiating ion particles on the surface of the polymer under a vacuum state, A method for producing a polymer microporous membrane, characterized in that the size of the micropores is reduced by moving the energy of the ion particles from 0.001 to 10 keV so as to move away from the surface of the polymer membrane. The method of claim 9, Method for forming a microporous membrane wherein the ion particles are selected from argon, oxygen, air, krypton, N ₂ O and mixtures thereof The method of claim 9, A method of forming a microporous membrane having a vacuum degree of 10 ⁻³ to 10 ⁻⁶ torr and an irradiation distance of 25 to 55 cm when the ion particles are irradiated onto a polymer surface. The method of claim 9, The method of forming a microporous membrane having a vacuum degree of 10 ⁻⁶ torr or more and an irradiation distance of 55 cm or more when the ion particles are irradiated onto a polymer surface. The method of claim 9, The method of forming a microporous membrane having a vacuum degree of 10 ⁻³ torr or less and an irradiation distance of 25 cm or less when the ion particles are irradiated onto a polymer surface. The method of claim 9, A method of forming a microporous membrane, wherein the energy of the ion particles is 0.01 to 10 keV. The method of claim 9, Wherein the ion dose of the particles 10 ⁵ ~10 ²⁰ ions / cm ² of the microporous film forming method. The method of claim 9, Blowing a reactive gas to the polymer surface to increase the hydrophilicity or hydrophobicity of the membrane. The method of claim 16, A method of blowing fine gas into the surface of a polymer while forming fine pores. The method of claim 16, A method of blowing a reactive gas onto the surface of a polymer after forming micropores. The method of claim 16, The reactive gas is selected from oxygen, nitrogen, hydrogen, ammonia, carbon monoxide, carbon tetrafluoride, N ₂ O and mixtures thereof. The method of claim 16, The injection amount of the reactive gas is 0.5 to 20 ml. a) extruding the synthetic resin; b) winding and cooling the extruded resin to prepare a disc film; c) annealing the original film; d) irradiating ionic particles on the surface of the annealed film; e) cold drawing the original film; f) hot stretching the original film; And g) heat fixing the disc film Method for producing a polymer microporous membrane comprising a. The method of claim 21, Extrusion method using a single or twin screw extruder with a T-Die or tubular die at a temperature of 10 to 100 ℃ higher than the melting point of the polymer. The method of claim 21, A method of winding using a cast roll having a linear velocity of 5 to 120 m / min at a temperature of 10 to 150 ° C. The method of claim 21, Annealing at a temperature of 5 to 100 ℃ lower than the melting point of the polymer for 10 seconds or more so that the elastic recovery rate at 25 ℃ to 40% or more. The method of claim 21, Cold drawing at a temperature below minus 20 ℃ to 60 ℃ lower than the melting point of the polymer. The method of claim 21, High temperature stretching at a temperature of 5 to 40 ℃ lower than the melting point of the polymer. The method of claim 21, Melting point of the polymer Let's heat fix for at least 30 seconds under tension at a low temperature of 5 to 80 ℃. The method of claim 21, Performing step f) before step c). a) extruding using a single or twin screw extruder with a T-Die or tubular die attached at a temperature of 10 to 100 ° C. above the melting point of the polymer; b) winding using a cast roll having a linear velocity of 5 to 120 m / min at a temperature of 10 to 150 ° C; c) annealing at least 5 seconds at a temperature lower than the polymer melting point for 10 seconds to obtain an elastic recovery rate of at least 35% at 25 ° C; d) The energy of the ion particles in the range of 10 ^-2 to 10 ^-7 torr and the ion beam irradiation distance of 15 to 60 cm is 0.001 to 10 keV, and the ion particle irradiation is in the range of 10 ⁵ to 10 ²⁰ ions / cm ² . Irradiating the ion beam with the annealed disc film; e) uniaxial cold stretching of at least 30% of the initial length at temperatures below 20 ° C. to 40 ° C. below the polymer melting point; f) uniaxially hot stretching at least 50% of the initial length at a temperature 5-40 ° C. below the melting point of the polymer; And g) heat fixing at a temperature of 5 to 80 ° C. below the melting point of the polymer under tension for at least 30 seconds; Method for producing a polymer microporous membrane for a battery separator comprising a. In a microporous membrane for secondary battery separators in which the pore size decreases as the distance from the surface is irradiated with ion particles having an energy of 0.001 to 10 keV on the surface of the polymer in a vacuum state, the distance from the surface of the membrane Pore membranes characterized in that the size of: y = ax + b Where y is the pore size, x is the distance from the membrane surface, and a and b are any constants. The method of claim 30, A pore membrane with a low porosity of 5 to 70 ° C. having a shut-down temperature of the polymer melting point.