Asymmetric hollow fiber membrane

Abstract

Asymmetric hollow fiber membranes are described that include a porous substrate layer and a skin layer. The skin layer is a copolymer of polymethylpentene and polypropylene. Gas separation components fabricated using such hollow fiber membranes, as well as methods for fabricating and using such hollow fiber membranes and gas separation components, are also described.

Description

[Technical Field]

[0001] The present disclosure relates to hollow fiber membranes having a porous substrate layer and a skin layer comprising a copolymer of polymethylpentene and polypropylene. Gas separation components manufactured using such hollow fiber membranes, and methods of making and using such hollow fiber membranes and gas separation components are also described. Summary of the Invention

[0002] Briefly, in one aspect, the present disclosure provides an asymmetric hollow fiber membrane comprising a porous substrate comprising a polyolefin polymer and having a plurality of pores surrounding an internal lumen, and a skin layer overlying the porous substrate, wherein the skin layer comprises a copolymer comprising at least 80 weight percent 4-methyl-1-pentene units and at least 5 weight percent propylene units, the total weight percent of 4-methyl-1-pentene units and propylene units being at least 98%, based on the total weight of monomer units in the copolymer. In some cases, an additional porous substrate layer may overlie the skin layer.

[0003] In another aspect, the present disclosure provides a separation component comprising a plurality of such asymmetric hollow fiber membranes arranged generally parallel in an array pattern and secured together.

[0004] In yet another aspect, the present disclosure provides a method for producing such an asymmetric hollow fiber membrane. [Brief explanation of the drawings]

[0005] [Figure 1] 1 shows a cross section of an exemplary asymmetric hollow fiber membrane. [Figure 2] 1 shows a cross section of another exemplary asymmetric hollow fiber membrane. [Figure 3] 1 shows a cross section of another exemplary asymmetric hollow fiber membrane. DETAILED DESCRIPTION OF THE INVENTION

[0006] Microporous hollow fibers can be used to separate components from a fluid stream based on, for example, size, phase, and charge. Microporous hollow fibers are often used in materials with controlled porosity and pore sizes in the range of a few micrometers, and have many uses, including separation, filtration, diffusion, and barrier applications. These wide-ranging applications have found practical application in medical devices, electrochemical devices, chemical processing equipment, pharmaceutical equipment, and water purification, to name a few.

[0007] The performance of microporous hollow fiber membranes is often a complex function of the specific end use and composition and structure of the hollow fibers (e.g., hollow fiber diameter, wall thickness, porosity, and pore size). These and other variables are often tailored to the specific end use. For example, membranes with gas-permeable separating layers can be used to selectively pass gas / gas and gas / liquid.

[0008] Membrane contactors useful for gas / liquid separation applications can be fabricated using asymmetric microporous hollow fiber membranes. Asymmetric microporous hollow fiber membranes, i.e., hollow fiber membranes comprising two or more layers, selectively allow specific dissolved gases to pass through while blocking liquid water or other aqueous liquids. Such hollow fiber membranes can be used in membrane contactors to achieve gas / liquid separation in applications such as degassing aqueous printing inks during printing, and separation of dissolved gases such as carbon dioxide or methane from brines used to enhance oil recovery. Other exemplary applications include electroplating or microelectronic components, control of dissolved gases in beer, wine, and spirits, extracorporeal membrane oxygenators, oxygen enrichment of air, and removal of dissolved gases from fluids used in carbon capture.

[0009] U.S. Patent No. 10,010,835 (B2) describes an asymmetric hollow fiber membrane including a thermoplastic polymer substrate defining a plurality of micropores and a polymethylpentene (PMP) polymer skin disposed on the thermoplastic polymer substrate. International Publication No. WO 2020 / 136568 (A1) describes an asymmetric hollow fiber membrane including a porous substrate having numerous pores and a skin layer overlying the porous substrate. The porous substrate includes a first semi-crystalline thermoplastic polyolefin (co)polymer and a nucleating agent effective to achieve nucleation. The skin layer includes a second semi-crystalline thermoplastic polyolefin copolymer derived from polymerizing up to 98 weight percent of 4-methyl-1-pentene monomer with at least 2 weight percent of linear or branched alpha-olefin monomer.

[0010] Despite the success of these and other asymmetric hollow fiber membranes, further improvements remain necessary. For example, it would be desirable to provide the necessary porosity levels, improve the adhesion of the interlayer between the substrate and the skin layer without the selective use of nucleating agents, and increase tensile strength (breaking strength). Surprisingly, the inventors have discovered that one or more of these advantages can be achieved when using a polyolefin substrate layer in combination with a skin layer comprising a polymethylpentene copolymer containing at least 5 wt.% polypropylene.

[0011] Generally, asymmetric hollow fiber membranes comprise at least two layers that form a boundary around the lumen of a hollow central section. A cross-section of an exemplary asymmetric hollow fiber membrane is shown in FIG. 1. A boundary 180, comprising a porous substrate 150 and a skin layer 160, surrounds the lumen 110 of the asymmetric hollow fiber membrane 100. In this example, the porous substrate 150 comprises the surface 120 of the lumen 110, i.e., the surface of the boundary 180 immediately adjacent to the lumen. The skin layer 160 comprises the outer surface 130 of the asymmetric hollow fiber membrane 100, i.e., the surface of the boundary 180 that is exposed to the ambient environment.

[0012] In some embodiments, the relative positions of these layers may be reversed. For example, as shown in Figure 2, boundary 280 comprises porous substrate 250 and skin layer 260 surrounding lumen 210 of asymmetric hollow fiber membrane 200. In this example, porous substrate 250 comprises outer surface 230 of asymmetric hollow fiber membrane 200, and skin layer 260 comprises surface 220 of lumen 210.

[0013] As used herein, the terms "overlapping" and "overlapping" refer to layers that surround a common lumen and require any particular spatial orientation of those layers. Thus, with reference to Figure 1, porous substrate 150 is overlaid with skin layer 160, and skin layer 160 is overlaid with porous substrate 150. Similarly, with reference to Figure 2, porous substrate 250 is overlaid with skin layer 260, and skin layer 260 is overlaid with porous substrate 250.

[0014] Typically, the porous substrate comprises a polyolefin polymer, such as a polyolefin homopolymer or copolymer. In some embodiments, the polymer comprises a polypropylene, such as a polypropylene homopolymer. Generally, any suitable polypropylene can be used. Suitable polypropylene homopolymers include, for example, those available from Total Petrochemicals (Houston, Texas) under the trade name FINA, those available from Lyondel-Basell Industries (Pasadena, California) under the trade name PRO-FAX, those available from INEOS Olefins & Polymers (Carson, California) under the trade name INEOS, and those available from Exxon-MoBil Chemical Company (Spring, Texas).

[0015] The thickness of the porous substrate layer can depend on the particular application for which the microporous asymmetric hollow fibers are used. In some embodiments, the porous substrate is at least 5 micrometers thick, e.g., at least 10 micrometers or at least 20 micrometers thick. In some embodiments, the porous substrate is 20 micrometers or less thick, e.g., 100 micrometers or less thick, or even 50 micrometers or less thick, e.g., 5 μm to 200 μm, 10 μm to 100 μm, 15 μm to 75 μm, 20 μm to 50 μm, or even 25 μm to 35 micrometers thick.

[0016] The skin layer comprises a copolymer of 4-methyl-1-pentene and propylene repeating units. Generally, the copolymer of the skin layer (also referred to as skin resin) comprises at least 80 weight percent of 4-methyl-1-pentene units and at least 5 weight percent of propylene units, and the total weight percent of 4-methyl-1-pentene units and propylene units is at least 98 weight percent based on the total number of monomer units in the skin resin. As used herein, the term "monomer unit" refers to the structure of a monomer in its polymeric form. For example, for propylene (CH2=CH-CH3), the corresponding propylene unit is -CH2-CH(CH3)-.

[0017] Generally, the skin resin comprises primarily 4-methyl-1-pentene units, i.e., at least 80% by weight. In some embodiments, the skin resin comprises at least 84% by weight, e.g., at least 90% by weight, of 4-methyl-1-pentene units, based on the total weight of monomer units in the skin resin. In some embodiments, the skin resin comprises 95% by weight or less, e.g., 94% by weight or less, of 4-methyl-1-pentene units (e.g., 84% to 94% by weight, or even 90% to 94% by weight of 4-methyl-1-pentene units).

[0018] Unlike prior art PMP-based skin resins, the skin resins of the present disclosure contain a relatively high amount of polypropylene, i.e., at least 5 weight percent, e.g., at least 6 weight percent, of polypropylene, based on the total weight of monomer units in the skin resin. In some embodiments, the skin resin contains 20 weight percent or less, e.g., 16 weight percent or less, 10 weight percent or less, or even 8 weight percent or less propylene units (e.g., 6 weight percent to 16 weight percent, e.g., 6 weight percent to 8 weight percent propylene units).

[0019] In some embodiments, the total weight percent of 4-methyl-1-pentene units and propylene repeat units is at least 99 weight percent, at least 99.5 weight percent, or even 100 weight percent, based on the total weight of monomer units in the skin resin. In some embodiments, the skin resin can contain additional comonomers, such as at least one additional α-olefin comonomer, such as ethylene, 1-hexene, and 1-octene. When present, the skin resin may include a combined amount of such additional comonomers in a combined amount of from 0.05 weight percent to 2 weight percent, e.g., from 0.1 weight percent to 1 weight percent, or even from 0.1 weight percent to 0.5 weight percent.

[0020] In some embodiments, the skin layer is impermeable to the liquid, e.g., a solid skin without pores, or a microporous skin that is impermeable to the liquid but permeable to gas. In some embodiments, the skin layer can efficiently facilitate gas absorption and / or degassing of the liquid even though the skin layer is impermeable to the liquid.

[0021] The thickness of the skin layer can depend on the particular application in which the microporous asymmetric hollow fiber is used. Generally, reducing the thickness of the skin layer increases gas flux and makes the asymmetric microporous asymmetric hollow fiber membrane more efficient. In some embodiments, the skin layer can be 20 micrometers or less in thickness, e.g., 5 micrometers or less, or even 3 micrometers or less in thickness. In some embodiments, the skin layer is at least 0.1 micrometers thick, e.g., at least 0.5 micrometers thick, e.g., 0.1 micrometers to 20 micrometers thick, e.g., 0.1 micrometers to 10 micrometers thick, 0.5 micrometers to 10 micrometers thick, 0.5 micrometers to 5 micrometers thick, or 1 micrometer to 3 micrometers thick.

[0022] In some cases, the boundary may include one or more additional layers, any one of which may include the exterior surface, the luminal surface, or an intermediate layer of an asymmetric hollow fiber membrane. Similarly, when one or more additional layers are present, one or both of the porous substrate and the skin layer may be intermediate layers. In either case, each layer may be described as overlying the other layer.

[0023] For example, an asymmetric hollow fiber membrane can include at least three layers that form a boundary around the lumen of the hollow central section. A cross-section of such an exemplary asymmetric hollow fiber membrane is shown in FIG. 3. A boundary 380 includes a first porous substrate 350, a skin layer 360, and a second porous substrate 370 that surround the lumen 310 of the asymmetric hollow fiber membrane 300. In this example, the first porous substrate 350 includes the surface 320 of the lumen 310, i.e., the surface of the boundary 380 that is directly adjacent to the lumen. The second porous substrate 370 includes the outer surface 330 of the asymmetric hollow fiber membrane 300, i.e., the surface of the boundary 380 that is exposed to the ambient environment. The skin layer 360 is sandwiched between the first porous substrate 350 and the second porous substrate 370.

[0024] In some cases, the first porous substrate and the second porous substrate comprise the same material. In some cases, different materials can be used to form the first porous substrate and the second porous substrate. For example, in some cases, the materials used to form the first porous substrate and the second porous substrate can be independently selected from the materials described above.

[0025] The second porous substrate can act as a protective layer to protect the thin skin layer from damage during subsequent processing, such as stretching. Generally, the thickness of the second porous substrate is thinner than the thickness of the first porous substrate. In some cases, the thickness of the second porous substrate is 10 micrometers or less, for example, 5 micrometers or less, or even 3 micrometers or less. In some cases, the thickness of the second porous substrate is at least 0.5 micrometers, for example, at least 1 micrometer, or even 2 micrometers or less. In some cases, the second porous substrate is 0.5 micrometers to 10 micrometers thick, for example, 0.5 micrometers to 5 micrometers thick, 0.5 micrometers to 3 micrometers thick, or even 1 micrometer to 3 micrometers thick.

[0026] The hollow fiber membranes described herein can be manufactured using a variety of known manufacturing methods, depending on the desired asymmetric hollow fiber structure and the desired asymmetric hollow fiber composition. Microporous membranes can be manufactured according to a variety of production techniques, including the wet process, the particle stretching process, and the dry stretching process (also known as the CELGARD process).

[0027] Generally, in wet processes (also known as phase inversion, extraction, or TIPS processes), polymeric materials are mixed with oil, solvent, and / or other materials. This mixture is extruded, and pores are formed when the oil, solvent, and / or other materials are removed. These films can be stretched before or after removal of the oil, solvent, and / or other materials.

[0028] Generally, in particle stretching, a polymeric material is mixed with particulates, the mixture is extruded, and during stretching, the interface between the polymer and the particulates is broken by the stretching force, forming pores.

[0029] The dry-laid process differs from the wet-laid and particle-stretched processes by typically producing porous asymmetric hollow fibers without the addition of processing oils, oils, solvents, plasticizers, etc., or particulate materials. Thus, although other processes may be used, the substrate and skin materials of the present disclosure can be used in the dry-laid process to produce asymmetric hollow fiber membranes with the desired porosity without the need for liquid or particulate additives. Thus, preferably, microporous membranes are formed by the CELGARD® process, also referred to as the "extrusion, annealing, stretching" or "dry-stretching" process, whereby a semi-crystalline polymer is extruded to provide an asymmetric hollow fiber precursor, and porosity is induced in the microporous substrate by stretching the extruded precursor.

[0030] In a typical dry-laid process, the resins of the skin and substrate layers (along with any additional layers) are coextruded through an annular coextrusion die to form an asymmetric fiber precursor, which is then stretched to form an asymmetric hollow fiber comprising a porous substrate layer and a skin layer surrounding a hollow lumen.

[0031] Typically, the asymmetric fiber precursor is stretched to form an open porous structure by uniaxial stretching of at least 10% and up to 500%, for example, 50% to 300%, or 100% to 200%. Stretching can be performed using one or multiple cold stretching stages, followed optionally by one or multiple hot stretching stages. Generally, the cold stretching temperature can be, for example, 20°C to 90°C, for example, 30°C to 70°C. The hot stretching temperature can be, for example, 100°C to 200°C, for example, 120°C to 170°C.

[0032] In some embodiments, the asymmetric fiber precursor can be annealed before stretching, for example, by exposing the asymmetric fiber precursor to a temperature of, for example, 100° C. to 150° C. for, for example, 5 minutes to 30 minutes.

[0033] In some embodiments, the asymmetric hollow fiber membranes can be heat treated or heat set after stretching to reduce stress within the fibers. The heat setting temperature is typically selected to be at least 5°C, at least 10°C, or even at least 15°C higher than the heat stretching temperature. The heat setting time is typically selected to be at least 30 seconds, at least 1 minute, or at least 90 seconds. Alternatively or additionally, the asymmetric hollow fiber membranes may be subjected to a stress relaxation step that causes some shrinkage of the fiber length, for example, by at least 2%, or even by at least 5%.

[0034] In some embodiments, when a skin layer is adjacent to a porous substrate, the polymer of the skin layer can diffuse into the pores of the porous substrate. Such diffusion can improve adhesion of the intermediate layer, but can also reduce gas transport through the porous layer. [Example]

[0035] The materials used in the examples are summarized in Table 1.

[0036] [Table 1]

[0037] Skin Resin Analysis. The two skin resins (PMP-B and PMP-C) were analyzed after high-temperature dissolution in trichloroethylene (TCE) using a BRIKER 600 MHz spectrometer (Bruker Company) equipped with an inverse cryoprobe at 400 kJ. 13C NMR analysis was performed. The spectra of the polyethylene and polypropylene were compared. Each component was quantitatively analyzed, as summarized in Table 2. As shown, these resins have the highest levels of comonomer of any PMP grade, containing much more propylene than PMP-A, which is typically used for the skin layer in coextruded hollow fiber membranes.

[0038] [Table 2]

[0039] Differential scanning calorimetry was performed on each of the PMP resins using a heating ramp from 10.0°C to 200°C per minute and a cooling ramp from 10.0°C to -20°C per minute. PMP-A exhibited a melting peak at 224°C and a crystallization peak at 211°C. In contrast, PMP-B exhibited neither a melting nor a crystallization peak. PMP-C exhibited a melting peak at 130°C but no crystallization peak. Thus, PMP-B and PMP clearly exhibit different behavior from semicrystalline PMPs such as PMP-A.

[0040] Asymmetric fiber precursors (AFPs) were prepared as follows: The skin and substrate resins were extruded through two separate single-screw extruders. The two melts were delivered to a fiber-spinning die by a melt gear pump. The die had an annular orifice for the primary air supply and one central hole. The coextruded asymmetric hollow fiber precursor was quenched by an air loop, which provided the fiber with a uniform environment. The molten fiber was drawn through three sets of godet rolls at a fiber draw rate of 100 meters per minute. The materials, extruder temperatures, and melt pump speed (cubic centimeters per minute) for the substrate and skin, along with the die temperature, are listed in Table 3.

[0041] [Table 3]

[0042] Asymmetric hollow fiber membranes were prepared from asymmetric fiber precursors by annealing followed by dry stretching. For annealing, asymmetric fiber precursor bundles (approximately 25 cm long) were prepared by taping one end together. Each bundle was then annealed in a convection oven set to the temperature shown in Tables 4-6. The annealing time for each asymmetric fiber precursor bundle was 15 minutes at the set temperature. For dry (hot / cold) stretching, the bundles (annealed or unannealed) were clamped in the temperature-controlled environmental chamber of an Instron Mechanical Tester (Model 5969, Norwood, MA). Fibers 127 mm (5 inches) long were cold stretched at 25°C at a stretch rate of 600 mm per minute, followed by hot stretching at 120°C at a stretch rate of 30 mm per minute. The total stretch ratios after 10% relaxation are shown in Tables 4-6.

[0043] The resulting CO2 flux (GPU) and CO2 / N2 selectivity were measured as follows.

[0044] The gas permeability test is used as an integrity test for the nonporous skin layer as well as a performance test for the hollow fiber membrane. Loop modules were prepared by sealing hollow fibers together in 0.65 cm (1 / 4 inch) OD nylon tubing with epoxy adhesive. The lumen of each fiber was exposed by cutting the sealed tubing with a razor blade. Each loop module contained 10 fibers with an effective length of approximately 10.2 cm (4 inches).

[0045] Gas permeability tests were performed using a custom-designed test stand. The stand was equipped with cylinders of pure gas (CO2 and N2), a pressure gauge, and an in-line gas flow meter. The principle of this test is to supply pure gas to the fiber lumen and measure the rate of gas leakage through the fiber wall to the ambient environment. Each fiber loop module was tested using CO2 and N2, respectively. The gas pressure within the fiber lumen was typically set at approximately 207 kPa (30 psi). Both the gas pressure and gas flow rate were monitored by data acquisition software, and data were acquired when both the pressure and gas flow rate were stable.

[0046] The gas permeability (GPU) of each fiber membrane was calculated as follows:

[0047]

number

[0048] The gas selectivity of a fiber is used as an indicator of the integrity of the skin. The selectivity of PMP is typically in the range of 11 to 13, according to literature reports (Polymer, 1989, 30, p. 1357). Any fiber with a gas selectivity below 8 was considered to have a defective skin. The CO2 / N2 selectivity of the fiber was calculated from the gas transmission rates of each gas as follows:

[0049]

number

[0050] Samples annealed at 25° C. were prepared using PMP-C (AFP-2 and AFP-3) and PMP-B (AFP-4) as the skin resin. The process conditions and results are summarized in Table 4.

[0051] [Table 4] * Not measured, calculated based on fiber diameter of 300 microns.

[0052] Additional samples were prepared using PMP-C modified with additional polypropylenes (AFP-5 and AFP-6) as the skin resin. PP-1 was used as the base resin in all samples. These samples were annealed at 25°C, and the results are summarized in Table 5 and compared with Sample 1, in which the PMP-C skin resin was not modified.

[0053] [Table 5]

[0054] Samples were prepared using PMP-C and modified PMP-C as the skin resin. The base resin was PP-1 in all samples. The samples were annealed at temperatures above 100°C. The results are shown in Table 6.

[0055] [Table 6] * Not measured, calculated based on fiber diameter of 300 microns.

[0056] A three-layer asymmetric fiber precursor (AFP) was prepared as follows: The skin and substrate resins were extruded through two separate single-screw extruders. The two melts were delivered to separate fiber-spinning dies by a melt gear pump. The die had an annular orifice for the main air supply and a central hole. Within the die, the substrate resin melt was split into two streams, sandwiching the skin melt and then extruding through the die face. The coextruded asymmetric hollow fiber precursor was quenched by an air loop, providing a uniform fiber environment. The molten fiber was drawn through three sets of godet rolls at a fiber draw rate of 100 meters per minute. The materials, extruder temperatures, and melt pump speed (cubic centimeters per minute) for the substrate and skin materials, along with the die temperature, are listed in Table 7.

[0057] A cross section of AFP-10 was analyzed by scanning electron microscopy (SEM, Model Hitachi TM4000 plus II, obtained from Hitachi High-Tech Corporation, Japan). The three-layer structure was clearly shown as a thick inner layer (44.3 micrometers), a thin middle layer (1.21 micrometers), and a thin outer layer (2.59 micrometers). The middle layer was formed from a skin material, and both the inner and outer layers were formed from a substrate material.

[0058] [Table 7]

[0059] Three-layer asymmetric hollow fiber membranes were prepared from these asymmetric fiber precursors by annealing followed by dry stretching. For annealing, bundles of asymmetric fiber precursors (approximately 25 cm long) were prepared by taping together one end. Because the skin layer was protected by the second substrate layer, processing these bundles at high temperatures without causing defects in the skin layer was more practical.

[0060] Each bundle was annealed in a convection oven set at a temperature of 140°C. The annealing time for each asymmetric fiber precursor bundle was 15 minutes at the set temperature. For dry (hot / cold) drawing, the bundles were clamped in the temperature-controlled environmental chamber of an Instron Mechanical Tester (Model 5969, Norwood, MA). 127 mm (5 inch) long fibers were cold drawn at 25°C with a draw rate of 600 mm per minute, followed by hot drawing at 120°C with a draw rate of 30 mm per minute. The total stretch ratio after 10% relaxation is shown in Table 8.

[0061] The resulting CO2 flux (GPU) and CO2 / N2 selectivity were measured according to the methods described above, and these results are also summarized in Table 8.

[0062] [Table 8]

[0063] Samples 14-17 were derived from the same three-ply asymmetric fiber precursor but with different draw ratios. As can be seen, gas flux increases as the draw ratio increases up to 150%; then, it decreases as the draw ratio increases further. However, gas selectivity remains constant. Increasing the draw ratio without creating any micropores or defects in the protected skin layer appeared to result in the formation of a thinner skin layer. However, when stretched up to 180%, the resistance of the porous substrate layer to gas flow may become significant, thereby reducing flux.

[0064] Sample 14 was further characterized by examining its exterior and interior surfaces with a high-resolution scanning electron microscope (Field Emission-SEM, Model Hitachi S-4700, obtained from Hitachi High-Tech Corporation, Japan), which revealed that both the exterior and interior surfaces had a porous microstructure.

[0065] Separation components can be prepared from the asymmetric hollow fiber membranes of the present disclosure using known methods, such as those described in WO 2021 / 105838 A1. For example, multiple asymmetric hollow fiber membranes may be arranged substantially parallel in an array pattern and secured together, for example, by braiding or tying the individual hollow fiber membranes together using string, thread, yarn, or the like. In some embodiments, the array may be pleated, folded, or wound into a cylinder or cassette for storage.

Claims

1. 1. An asymmetric hollow fiber membrane comprising: a first porous substrate comprising a polyolefin polymer and having a plurality of pores surrounding an interior lumen; and a skin layer overlying the porous substrate, wherein the skin layer comprises a copolymer comprising at least 80 weight percent 4-methyl-1-pentene units and at least 5 weight percent propylene units, the total weight percent of 4-methyl-1-pentene units and propylene units being at least 98%, based on the total weight of monomer units in the copolymer.

2. 3. The asymmetric hollow fiber membrane of claim 2, wherein the copolymer of the skin layer comprises 84 to 94 weight percent 4-methyl-1-pentene units and 6 to 16 weight percent propylene units, and optionally, the copolymer of the skin layer comprises 90 to 94 weight percent 4-methyl-1-pentene units and 6 to 8 weight percent propylene units.

3. 3. The asymmetric hollow fiber membrane of claim 1, wherein the copolymer of the skin layer further comprises from 0.1 weight percent to 0.5 weight percent of at least one additional α-olefin comonomer.

4. The asymmetric hollow fiber membrane of any one of claims 1 to 3, wherein the polyolefin polymer comprises polypropylene.

5. 5. The asymmetric hollow fiber membrane according to claim 1, wherein the pore diameter is from 0.01 micrometer to 1 micrometer.

6. 6. The asymmetric hollow fiber membrane according to any one of claims 1 to 5, wherein the porosity of the asymmetric hollow fiber membrane is between 5% and 80%, and optionally the porosity is at least 20%.

7. The asymmetric hollow fiber membrane according to any one of claims 1 to 6, wherein the skin layer is non-porous.

8. 8. The asymmetric hollow fiber membrane of claim 1, wherein the skin layer is from 0.1 micrometer to 20 micrometers thick, and optionally from 0.5 micrometer to 10 micrometers thick.

9. The asymmetric hollow fiber membrane of any one of claims 1 to 10, wherein the first porous substrate is from 5 micrometers to 200 micrometers thick.

10. The asymmetric hollow fiber membrane of any one of claims 1 to 9, wherein the first porous substrate comprises a surface of the lumen.

11. The asymmetric hollow fiber membrane according to any one of claims 1 to 10, wherein the skin layer comprises an outer surface of the asymmetric hollow fiber membrane.

12. The asymmetric hollow fiber membrane of any one of claims 1 to 9, wherein the skin layer comprises the surface of the lumen.

13. 10. The asymmetric hollow fiber membrane of claim 1, further comprising a second porous substrate comprising a polyolefin polymer and having a plurality of pores, wherein the first porous substrate comprises a surface of the lumen, the second porous substrate comprises an outer surface of the asymmetric hollow fiber membrane, and the skin layer is sandwiched between the first porous substrate and the second porous substrate.

14. 14. The asymmetric hollow fiber membrane of claim 13, wherein the thickness of the second porous substrate is from 0.5 micrometers to 10 micrometers.

15. A separation component comprising a plurality of asymmetric hollow fiber membranes according to any one of claims 1 to 14 arranged substantially parallel in an array pattern and secured together.

16. 16. The separation component of claim 15, wherein the plurality of asymmetric hollow fiber membranes are arranged in an array, the array being pleated, folded, or wound into a cylinder or cassette.

17. The separation component comprises at least 10 ２ / N ２ and optionally the separation component has a selectivity of up to 20 CO ２ / N ２ 17. A separation element according to claim 15 or 16, which has selectivity.

18. The separation component is configured to generate at least 15 GPU of CO ２ and optionally the separation component has a CO2 flux of up to 65 GPU. ２ A separation component according to any one of claims 15 to 17, having a gas flux.

19. A method for producing the asymmetric hollow fiber membrane of any one of claims 1 to 14, comprising the steps of: co-extruding a base resin comprising polypropylene and a skin layer resin comprising said copolymer comprising at least 80 weight percent 4-methyl-1-pentene units and at least 5 weight percent propylene units to form an asymmetric fiber precursor; stretching the asymmetric fiber precursor to form the asymmetric hollow fiber membrane having a skin layer made of the skin layer resin overlying a porous substrate made of the substrate resin surrounding the inner lumen, the porous substrate comprising the plurality of pores.

20. The method of claim 19 , further comprising annealing the asymmetric fiber precursor.

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