CROSSLINKED POLYPHOSPHAZENE BLENDS FOR GAS SEPARATION MEMBRANES

Abstract

The disclosed invention relates to a composition comprising a crosslinked blend of polyphosphazene polymers. The composition comprises a first polyphosphazene and a second polyphosphazene, where the first polyphosphazene and the second polyphosphazene being bound by a thiol bearing crosslinking agent. Such compositions are useful as a membrane material for the separation of gasses in a gaseous mixture.

Claims

1. A composition comprising at least a first polyphosphazene and a second polyphosphazene, wherein the first polyphosphazene has the formula ##STR00005## where R comprises 50% to 95% ethyloxylate and 5% to 50% aryloxy, and where at least a portion of R comprises a crosslinked functional group; wherein the second polyphosphazene has the formula ##STR00006## where R′ are aryloxy groups, and where at least a portion of R′ comprises a crosslinked functional group; and wherein the first polyphosphazene and the second polyphosphazene are bound by a crosslinking agent where the crosslinking agent comprised two or more thiol groups prior to undergoing crosslinking.

2. The composition of claim 1, where a portion of R is a methoxyphenoxy.

3. The composition of claim 1, where a portion of R is 4-methoxyphenoxy.

4. The composition of claim 1, where the crosslinked functional group of the first polyphosphazene was an allylphenol.

5. The composition of claim 1, where the crosslinked functional group of the first polyphosphazene was 2-allylphenoxy.

6. The composition of claim 1, where a portion of R is 2-(2-methoxyethoxy) ethanol.

7. The composition of claim 1 where a portion of R′ is a phenoxy group.

8. The composition of claim 1 where a portion of R′ is phenoxy.

9. The composition of claim 1 where the crosslinked functional group of the second polyphosphazene was an allylphenol.

10. The composition according to claim 1 where the first polyphosphazene is present in a wt % ranging from about 75 wt % to about 99 wt % and the second polyphosphazene is present in a wt % ranging from about 1 wt % to about 25 wt %.

11. The composition according to claim 1, where the crosslinking agent is selected from the group consisting of pentaerythritol tetrakis(3-mercaptopropionate), poly(3-mercaptopropyl methyl)siloxane, or a combination thereof.

12. The composition according to claim 1 further comprising a photo-initiating agent.

13. The composition according to claim 12 where the photo-initiating agent is a small organic peroxide.

14. The composition according to claim 13 where the photo-initiating agent is 2,2-dimethoxy-2-phenylacetophenone.

15. The composition of claim 1 where a portion of R is 2-(2-methoxyethoxy) ethanol, where a portion of R′ is phenoxy, where the crosslinking agent is selected from the group consisting of pentaerythritol tetrakis(3-mercaptopropionate), poly(3-mercaptopropyl methyl)siloxane, or a combination thereof, and where the first polyphosphazene is present in a wt % ranging from about 75 wt % to about 99 wt % and the second polyphosphazene is present in a wt % ranging from about 1 wt % to about 25 wt %.

16. The composition of claim 1 where a portion of R is a 4-methoxyphenoxy and a second portion of R is 2-(2-methoxyethoxy) ethanol, where a portion of R′ is phenoxy, where the crosslinking agent is selected from the group consisting of pentaerythritol tetrakis(3-mercaptopropionate), poly(3-mercaptopropyl methyl)siloxane, or a combination thereof and where the first polyphosphazene is present in a wt % ranging from about 75 wt % to about 99 wt % and the second polyphosphazene is present in a wt % ranging from about 1 wt % to about 25 wt %.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIG. 1: Illustrates a chemical structure of a first polyphosphazene.

[0014] FIG. 2: Illustrates a chemical structure of a first polyphosphazene.

[0015] FIG. 3: Illustrates a chemical structure of a first polyphosphazene wherein a copolymer polyphosphazene backbone is substituted with 4-methoxyphenol (R.sub.1), 2-allylphenol (R.sub.2), and methoxyethoxy ethanol (R.sub.3 & R.sub.4).

[0016] FIG. 4: Illustrates a chemical structure of a second polyphosphazene wherein R.sub.1′ and R.sub.2′ are substituted with phenoxy.

[0017] FIG. 5: Depicts a graph of Stress (%) vs. Strain (%) of two crosslinked polyphosphazene blend membranes and a second polyphosphazene membrane.

[0018] FIG. 6: Illustrates a chemical structure of the tetrathiol crosslinking agent pentaerythritol tetrakis(mercaptopropionate).

[0019] FIG. 7: Illustrates a chemical structure of the thiosiloxane crosslinking agent poly((mercaptopropyl)methylsiloxane).

DETAILED DESCRIPTION OF THE INVENTION

[0020] Glassy type polymers are typically used in material applications because of transport properties, structural support, and electrostatic properties. These polymers have at least two states of matter a solid, glassy state, and a liquid, viscous state, that is determined by the transition temperature, Tg. Below Tg, the polymer behaves as solid-like while, above Tg, will adopt a liquid-like state. The Tg dictates many physical properties of polymers important for various applications. Some polymer may crystallize, rendering solid-like properties to an otherwise liquid-like material. For example, polyethylene has very low Tg but is solid at room temperature due to crystallinity. Crosslinking also renders an otherwise liquid-like material solid, for example silicone rubber. With respect to the polyphosphazenes, because both components of the polyphosphazenes are above Tg at room temperature, but a combination of crosslinking and crystallinity render the product solid.

[0021] Barrer is the measure of gas permeability through a membrane. The higher the Barrer unit the greater amount of gas will permeate the membrane.

[0022] Polyphosphazenes are hybrid polymers composed of an inorganic backbone with alternating nitrogen phosphorus bonds as can be seen in FIG. 1. The phosphorus atom(s) may be substituted with two or more R groups, typically through nucleophile attack reactions. The substitution of various organic groups of the monomer unit provides a diversity of possible structures and unique functions. Post polymerization, the various R group substitutions may be scattered randomly across the polymer. Thus, a single monomer may have four different R groups (R.sub.1, R.sub.2, R.sub.3, and R.sub.4) or any one or a combination of the various R groups.

[0023] The substituted R groups are nucleophiles and will influence both mechanical and chemical properties. In CO.sub.2 separation operations, ethoxy groups interact with CO.sub.2. Increasing the amount of ethoxylates substituted on phosphazene enhances CO.sub.2 selectivity, but reduces the Tg and mechanical durability. The phenoxy groups have moderate CO.sub.2 selectivity but increased Tg and allowable mechanical durability. With a substituted phenoxy group, for example adding an allyl or methoxy group increased CO.sub.2 selectivity without loss in mechanical durability. A hydrocarbon either branched or straight chain configuration with an allyl or alkyne group attached may be substituted on the phenoxy. Heteroatoms, for example, nitrogen, sulfur, oxygen, phosphorous, fluorine, chlorine, bromine, and iodine are also possible substituents.

[0024] Accordingly, polyphosphazenes substituted with 2-allylphenol (AP), 4-methoxyphenol (MEOP), methoxyethoxy ethanol (MEE), and phenoxy were evaluated for gas separation applications. Methoxyethoxy ethanol substituted polyphosphazenes have excellent gas separation, but poor mechanical stability with a low Tg forming a liquid-like membrane. 4-methoxyphenol and phenoxy have improved mechanical stability with a high Tg, but have poor gas separation properties. However, the crosslinkable allyl groups present (and any alkyne substitutions) may be crosslinked by thiol-ene reactions to further enhance mechanical durability.

[0025] The present disclosure provides a composition comprising a crosslinked blend of at least two dissimilarly substituted polyphosphazenes to provide a membrane with both excellent gas separation qualities and high mechanical stability. The composition comprises a first polyphosphazene featuring a heterofunctional polyphosphazene [N—PR.sub.1R.sub.2—N—PR.sub.3R.sub.4].sub.n where R.sub.1, R.sub.2, R.sub.3 and R.sub.4 can be combinations of aryloxy (—O—(C.sub.6-C.sub.12) phenolic) functional groups and ethoxylate groups (having the general formula —O(CH.sub.2CH.sub.2O).sub.NR wherein N is 1 to 10 and R any hydrocarbon), a second polyphosphazene (FIG. 2) where R.sub.1′ and R.sub.2′ groups are aryloxy groups, and where at least a portion of the first polyphosphazene and the second polyphosphazene are bound by a crosslinking agent where the crosslinking agent binds to the polyphosphazenes through thiol-ene reaction with the crosslinkable functional groups. In turn, crosslinkable functional groups present in the polyphosphazenes prior to crosslinking are crosslinked functional groups after undergoing crosslinking via thiol-ene reaction with the crosslinking agent. Where the crosslinkable functional group is 2-allylphenol prior to crosslinking, for example, the functional group bonds to the crosslinking agent and the now crosslinked functional group was the 2-allylphenol.

[0026] With respect to the first polyphosphazene, R groups may be homogenous or heterogeneous on any single monomer. In one embodiment, a portion of R may be the aryloxy methoxyphenoxy. In a preferred embodiment, a portion of R is 4-methoxyphenoxy (MEOP). Another portion of R may be allylphenol. In a preferred embodiment, a portion of R is the allylphenol 2-allylphenoxy (2-AP)(prior to crosslinking). In one preferred embodiment, a portion of Ris the ethoxylate group 2-(2-methoxyethoxy) ethanol (MEE).

[0027] With respect to the first polyphosphazene, R groups comprise 50% to 95% ethyloxylate, 5% to 50% aryloxy, and where at least a portion of R comprises a crosslinkable functional group. In one embodiment, the crosslinkable functional group comprises 2-AP (prior to crosslinking). As noted, any monomer across the first polyphosphazene may have any one or combination of the R constituents, thus, the constituents are randomly distributed.

[0028] With respect to the second polyphosphazene, R′ groups comprise primarily aryloxy groups and up to about 5% crosslinkable functional group (crosslinked after undergoing crosslinking with the crosslinking agent). As with the first polyphosphazene any monomer across the first polyphosphazene may have any one or combination of the R constituents, thus, the constituents are randomly distributed

Membranes comprising the first polyphosphazene alone (80% MEE, 15% MEOP, and 5% 2-AP as illustrated in FIG. 3) demonstrated excellent CO.sub.2 permeation of around 530 Barrer and good selectivity 33 CO.sub.2/N.sub.2, but poor mechanical properties and is a semi-solid with low Tg, making any industrial membrane use difficult. Conventional thermal-initiated free-radical polymerization increased mechanical rigidity and reduced CO.sub.2 permeability to 340 Barrer but the membrane became too brittle for practical application.

[0029] The second polyphosphazene comprises a single unit where R.sub.1′ and R.sub.2′ groups are both aryloxy groups. R.sub.1′ and R.sub.2′ may be same or different. The phenolic group may be substituted. However, at least a portion of R must comprise a crosslinkable functional group capable of facilitating crosslinking via the thiol-ene reaction with the crosslinking agent. In a preferred embodiment, R.sub.1′ is aryloxy. In another embodiment, R.sub.1′ and R.sub.2′ are aryloxy groups (FIG. 4). In a preferred embodiment, the second polyphosphazene is PPOP composed of 97% phenoxy and 3% 2-AP. PPOP alone displayed good mechanical strength, but provided low CO.sub.2 permeability of about 16 Barrer. Exemplary possible substitution schemes of the first polyphosphazene and the second polyphosphazene are depicted in Table 1.

TABLE-US-00001 TABLE 2 A substitution scheme of first polyphosphazene (MEEP-PPZ) and second polyphosphazene (PPOP)                     Polymer/Blend     [00001]           [00002]           [00003] [00004] 80% 15%  9% 5%  0%  0% 5% PPOP-PPZ  0%  0% 97% indicates data missing or illegible when filed

[0030] Polymers may be blended together to combine physical and chemical characteristics. With limitation in chemical modifications, blending of polymers expands on innovative physical properties. The successfulness of a polymer blend will depend on molecular interactions, hydrogen bonding, dipole-dipole, and hydrophobic interactions, between the polymers. If molecular interactions are unfavorable the blend will separate or form a semi-separated polymer with highly variable properties.

[0031] Blending of the first and second polyphosphazenes may be performed by dissolving the polymers in solvent, usually benzene, tetrahydrofuran, acetone, or any combinations thereof. Non-covalent interactions, if favorable, will hold the polymers together; however, crosslinking and blending is another method to combine the polymers to maximize the benefit of their different properties.

[0032] Crosslinking may be accomplished by three methods: thermal, high energy radiation, or enzymatic reaction. With thermal reaction, the advantageous properties of the starter materials may degrade due to the reactive nature of allyl groups, such as chain transfer and could damage the product. Enzymatic reactions require an amino acid as the R group and need to be further purified.

[0033] In contrast, crosslinking via thiol-ene reaction using low intensity UV light and a sensitizer is highly efficient. Thiols may be used to crosslink polymers via the allyl functional groups under exposure to UV light. The covalent crosslinked bonds are stronger than non-covalent interactions therein helping the blended polymers form a uniform membrane. Meanwhile, crosslinking increases mechanical durability. As shown in FIG. 5, crosslinking of the blend (XL 75/25% MEEP80/PPOP-PPZ and XL 90/10% MEEP80/PPOP-PPX) provides increased strain at failure in comparison to the second polyphosphazene PPOP-PPZ, indicative of better flexibility, while the first polyphosphazene MEEP80-PPZ alone was too weak for testing when performed using a TA Instruments Q800 DMA at 40° C. As a result, the crosslinked blend materials imparts better mechanical toughness of PPOP-PPZ on to the flexible MEEP80-PPZ. crosslink

[0034] While photopolymerization is an effective method to initiate crosslinking, the thiol-ene reaction may be initiated through a thermal method, or a base catalyst can be used. Further, initiators such as DMPA may be incorporated to facilitate photocuring.

[0035] Crosslinking agents as disclosed herein include compounds with two or bearing two or more thiol-groups capable of undergoing thiol-ene polymerization. After undergoing a crosslinking reaction, the crosslinking agents are bound via the thiol-ene reaction to the first and second polyphosphazenes to form a crosslinked polymer blend. A preferred crosslinking agent is a tetrathiol. A preferred tetrathiol crosslinking agent is pentaerythritol tetrakis(mercaptopropionate) (FIG. 6). Another preferred crosslinking agent are the thiosiloxanes. A preferred thiosiloxane are poly((mercaptopropyl)methylsiloxane) (FIG. 7). having a MW ranging from about 4000 to about 7000.

[0036] Composition Blends:

[0037] Blending the first polyphosphazene and the second polyphosphazene and crosslinking them provides a composition which when used as a gas separation membrane provides both acceptable CO.sub.2 permeation and mechanical durability. Membranes described in Table 2 were fabricated for evaluation. Tetrahydrofuran was used to dissolve the polymers, followed by mixing them together along with the crosslinking agent in the specified ratios. The blend was cast onto a smooth glass surface preferable. The solvent was removed and a polymer film, 100 micrometers, was left. Films less than one micrometer thick may be cast on a porous polymeric support Ultraviolet light was then used to initiate crosslinking of the polymers.

[0038] Within the disclosure, the first polyphosphazene is present in a range wt % ranging from about 75 wt % to about 99 wt % and the second polyphosphazene is present in a wt % ranging from about 1 wt % to about 25 wt %, and the crosslinking agent is present in an amount less than about 5 wt %. In a preferred blend. A most preferred blend is present about 90 wt % first polyphosphazene, where the first polyphosphazene comprises 95 wt % methoxyethoxy-ethanol and 5 wt % 2-allylphenol, about 10 wt % second polyphosphazene where the second polyphosphazene comprises of about 97 wt % phenoxy and about 3 wt % 2-allylphenol, and where the crosslinking agent is a thiosiloxane. Table 2 provides presents compositions providing membranes with excellent selectivity and permeability.

TABLE-US-00002 Composition CO.sub.2 perm. N.sub.2 perm. CH.sub.4 perm. MEEP-PPZ Cross-linker (MEEP/PPOP) wt % (barrer) (barrer) (barrer) CO.sub.2/N.sub.2 CO.sub.2/CH.sub.4 MEEP80 Tetrathiol 100/0  475 14.5 — 33 — 90/10 320 10 — 32 — 75/25 200 5.9 — 34 — Thiosiloxane 100/0  500 14.5 — 34 — MEEP95 Tetrathiol 90/10 430 14.6 36 30 12 75/25 330 11.2 28 29 12 Thiosiloxane 90/10 610 17.7 63 15 9.7 75/25 390 11.5 40 14 9.7
Table 2 providing CO.sub.2/N.sub.2/Ch.sub.4 permeability and CO.sub.2/N.sub.2&CO.sub.2/CH.sub.4selectivity by blend %.

Gas Separation Membranes:

[0039] The composition has use as a material to form membranes for gas separation membranes, for example to separate CO.sub.2 from the flue gas of fossil-fueled power plants. An embodiment of this membrane comprises the crosslinked polyphosphazene polymer blend. The composition comprises a first polyphosphazene and a second polyphosphazene, where the first polyphosphazene and the second polyphosphazene were bound by a crosslinking agent where the crosslinking agent comprised two or more thiol groups for binding the polyphosphazenes through a thiol-ene reaction. The first polyphosphazene is a polyphosphazene where R1 and R2 are aryloxy groups and R.sub.3 and R.sub.4 are ethoxylate groups. The second polyphosphazene [N—PR.sub.1′R.sub.2′].sub.n has R.sub.1′ and R.sub.2′ aryloxy groups. In one embodiment, the first polyphosphazene is present in a wt % ranging from about 75 wt % to about 99 wt %, the second polyphosphazene is present in a wt % ranging from about 1 wt % to about 25 wt %, and the crosslinking agent is present in a wt % ranging less than 5 wt %.

[0040] The blended crosslinked polyphosphazene membrane offers a great advantage in efficient and economical method to remove CO.sub.2 from flue gas. Polyphosphazene membranes are highly gas permeable and the present introduction of ethoxylate groups greatly improves their CO.sub.2/N.sub.2 selectivity as compared to other polyphosphazenes. The method of synthesis for polyphosphazene also offers unprecedented control over their properties just by varying the compositions of the functional groups. High permeability allowed advanced membranes, suitable for the demands of economical carbon capture from flue gas, to be fabricated.

[0041] In an exemplary gas separation operation, the product flue gas from combusting hydrocarbon based fuel, which is at high temperature, is cooled and filtered to remove large particles and excessive gaseous contaminants such as mercury, sulfur oxides and nitrogen oxides. The flue gas is passed over a polyphosphazene membrane arranged in different shapes, preferably planar or a tubular hollow fiber, to maximize surface area contact. The membrane must be resistant enough from breaking and the chemical composition must selectively permeate CO.sub.2 gas. A vacuum may be used on the filtered side to induce lower pressure. When CO.sub.2 contacts the membrane there is a concentration and pressure gradient with surface interactions that drives it through the membrane. An effective membrane can mechanically withstand process conditions while maintaining its gas separation performance. This process may be repeated two or more times to increase CO.sub.2 permeation. A cross sweep setup may be used wherein the filtered gas flows the opposite way to increase CO.sub.2 permeation. The filtered CO.sub.2 is then condensed and removed. After membrane contact the impermeable gas may be filtered by additional membranes or exhausted into the environment.

[0042] The disclosed crosslinked polyphosphazene blend membranes provide CO.sub.2 permeability greater than about 200 barrer and an N.sub.2 permeability less than about 15.0 barrer. In a preferred embodiment, the membrane provides a CO.sub.2 permeability greater than about 500 barrer. Additionally, the crosslinked polyphosphazene blend membranes provide CO.sub.2/N.sub.2 selectivity greater than about 29. In a preferred embodiment, the membrane provides a CO.sub.2/N.sub.2 selectivity greater than about 35. The CO.sub.2/CH 4 selectivity of the membranes is greater than about 9.0.

[0043] Having described the basic concept of the embodiments, it will be apparent to those skilled in the art that the foregoing detailed disclosure is intended to be presented by way of example. Accordingly, these terms should be interpreted as indicating that insubstantial or inconsequential modifications or alterations and various improvements of the subject matter described and claimed are considered to be within the scope of the spirited embodiments as recited in the appended claims. Additionally, the recited order of the elements or sequences, or the use of numbers, letters or other designations therefor, is not intended to limit the claimed processes to any order except as may be specified. All ranges disclosed herein also encompass any and all possible sub-ranges and combinations of sub-ranges thereof. Any listed range is easily recognized as sufficiently describing and enabling the same range being broken down into at least equal halves, thirds, quarters, fifths, tenths, etc. As a non-limiting example, each range discussed herein can be readily broken down into a lower third, middle third and upper third, etc. As will also be understood by one skilled in the art all language such as up to, at least, greater than, less than, and the like refer to ranges which are subsequently broken down into sub-ranges as discussed above. As utilized herein, the terms “about,” “substantially,” and other similar terms are intended to have a broad meaning in conjunction with the common and accepted usage by those having ordinary skill in the art to which the subject matter of this disclosure pertains. As utilized herein, the term “approximately equal to” shall carry the meaning of being within 15, 10, 4, 3, 2, or 1 percent of the subject measurement, item, unit, or concentration, with preference given to the percent variance. It should be understood by those of skill in the art who review this disclosure that these terms are intended to allow a description of certain features described and claimed without restricting the scope of these features to the exact numerical ranges provided. Accordingly, the embodiments are limited only by the following claims and equivalents thereto. All publications and patent documents cited in this application are incorporated by reference in their entirety for all purposes to the same extent as if each individual publication or patent document were so individually denoted.