PRIORITY CLAIM

The present application claims priority to U.S. Provisional Patent Application Ser. No. 62/318043, filed April 4, 2016, titled "COMPOSITE ELASTOMER AND MEMBRANES AND MEMBRANE SYSTEMS CONTAINING A COMPOSITE ELASTOMER," which is incorporated by reference herein in its entirety.

TECHNICAL FIELD

The present disclosure relates to a method of forming a composite elastomer and to articles, such as membranes, containing the composite elastomer. It also relates to devices containing such a membrane as a gas barrier or as a gas separation membrane.

BACKGROUND

Materials containing crosslinked polymers are used in a wide variety of applications, including wearable devices, flexible displays, and devices for monitoring physiological signals. Crosslinked polymers provide flexibility, mechanical robustness, and comfort to such devices.

Some polymers used in composite materials, poly (dimethyl siloxane) (PDMS) are commercially available, but recipes and methods of crosslinking them are often complex and the processing and application of such materials can be limited by the as-supplied material properties, such as the high viscosity of the precursors.

Additionally, although polymers are inexpensive and lightweight, they often have other properties that are undesirable in some applications. For instance, membranes formed from crosslinked polymers are typically highly gas permeable, rendering them unsuitable for many uses, such as gas barrier membranes.

SUMMARY

The present disclosure relates to a composite elastomer including a plurality of polymers covalently bound to a plurality of platelet-like fillers via ethanolamine bonds, ethanolamide bonds, or a combination thereof, to form the crosslinked composite elastomer. The disclosure further includes other composite elastomer features, which may be combined with one another unless clearly mutually exclusive. The features include: i) the plurality of polymers may include a single type of polymer; ii-a) the plurality of polymers may include at least two types of polymers, all types of which are covalently bound to the plurality of platelet-like fillers via ethanolamine bonds, ethanolamide bonds, or a combination thereof; ii-b) the plurality of polymers may include at least two types of polymers, a first type which is covalently bound to the plurality of platelet like fillers via ethanolamine bonds, ethanolamide bonds, or a combination thereof, and a second type which is covalently bound to the first type of polymer, but not to the plurality of platelet like fillers; iii) the plurality of polymers may be covalently bound to the plurality of platelet-like fillers via ethanolamine bonds, ethanolamide bonds, or a combination thereof located on at least one end of the plurality of polymers; iv) the plurality of polymers may be covalently bound to the plurality of platelet-like fillers via ethanolamine bonds, ethanolamide bonds, or a combination thereof located at both ends of the plurality of polymers; v) the same type of ethanolamine bonds or ethanolamide bonds may be at both ends of the plurality of polymers; vi) the plurality of polymers include at least one of the following types of polymers: PDMS, poly(acrylic acid), poly(vinyl alcohol), polyamide, polyarylamide, poly(acrylamide), poly(acrylic anhydride), poly(vinyl naphthalene), poly(styrene), cellulose acetate, polyacrylonitrile, polysulfone, poly(vinylidene fluoride), poly(ethylene terephthalate), poly(ethylene-co-vinyl acetate), poly(vinyl chloride); vii) the platelet-like fillers may include graphene oxide (GO); viii) the platelet-like fillers may have an average aspect ratio between 100 and 2000; ix) the total weight of the plurality of polymers may be at least 70% by weight of the total weight of only the plurality of polymers and the plurality of platelet-like fillers; x) at least 75% of the plurality of polymers and plurality of platelet-like fillers, by weight, may be crosslinked; xi) the plurality of platelet-like fillers may be aligned approximately parallel to a bottom surface of the composite elastomer; xii) the plurality of platelet-like fillers may be homogenously distributed in the composite elastomer; xiii) the composite elastomer may have sufficient tensile strength to be stretched up to 100% of its length before failure; xiv) the composite elastomer may be in the form of a membrane has a gas permeability for a single gas that is reduced by at least 45% as compared to a similar material comprising only the plurality of polymers and not the plurality of platelet-like fillers; and xv) the composite elastomer may be in the form of a membrane and has a gas selectivity of at least 10 for C0 ₂ in a gas mixture.

The present disclosure further provides a method of forming a composite elastomer. The method includes reacting amine or amide functional groups, or a combination thereof, on a plurality of polymers with epoxide functional groups on a plurality of platelet-like fillers to form an ethanolamine or ethanolamide bond and thereby crosslink the plurality of polymers via the plurality of platelet-like fillers.

The disclosure further includes other method features, which may be combined with one another unless clearly mutually exclusive. The features include: i) the plurality of polymers may be flexible at 21 °C; ii) the amine or amide functional groups may include primary amines, secondary amines, secondary amides, and combinations thereof; iii) the amine or amide functional groups may include between 0.05% by weight and 4.00% by weight of the plurality of polymers; iv) the plurality of polymers may be telechelic for the amine or amide functional groups, of a combination thereof; v) the amine or amide functional groups on both ends of the polymers may react with the epoxide functional groups on the plurality of platelet-like fillers; vi) the method may include reacting a plurality of additional polymers with the plurality of polymers to form covalent bonds there between, where the plurality of additional polymers do not react with the plurality of platelet-like fillers; vii) the method may further include forming a solution of the plurality of polymers and plurality of platelet-like fillers in a solvent; vii-i) the solution may include at least 1% plurality of polymers by weight of the total weight of plurality of polymers and solvent; vii-ii) the solution may have a viscosity of at least 1 cSt; vii-iii) the method may further include solution casting the solution to form the composite elastomer; vii- iv) the method may further include spin-coating the solution to form the composite elastomer.

Composite elastomers described above may be formed using the methods described above. The disclosure further provides a gas separation membrane including the composite elastomer described above or a composite elastomer formed using a method above.

The disclosure also provides a gas separation system including such a gas separation membrane.

The disclosure further provides a gas barrier membrane including the composite elastomer described above or a composite elastomer formed using a method above.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the disclosure may be better understood through reference to the following figures, in which:

FIG. 1 A is a chemical structural diagram of GO with epoxide functional groups;

FIG. IB is a chemical structure formula of an amine-terminated telechelic

PDMS;

FIG. 1C is a diagram of the reaction of a primary amine on a polymer with an epoxide on the surface of GO to form an ethanolamine bond;

FIG. ID is a chemical structural formula of a secondary amide-terminated telechelic PDMS;

FIG. IE is a diagram of the reaction of a secondary amide on a polymer with an epoxide on the surface of GO to form an ethanolamide bond;

FIG. 2A is a cross sectional scanning electron microscope (SEM) image of a 2% by weight GO composite elastomer; GO platelets are indicated by arrows;

FIG. 2B is a cross sectional scanning electron microscope (SEM) image of a 5% by weight GO composite elastomer; GO platelets are indicated by arrows;

FIG. 2C is a cross sectional scanning electron microscope (SEM) image of a 8% by weight GO composite elastomer;

FIG. 3 A is a photograph of a PDMS-GO composite elastomer sheet measuring 3.8 centimeters (cm) in diameter;

FIG. 3B is a photograph of the composite elastomer of FIG. 2A when freestanding;

FIG. 3C is a photograph of the composite elastomer of FIG. 2A when bent; FIG. 3D is a photograph of the composite elastomer of FIG. 2A when flexed;

FIG. 3E is a photograph of the composite elastomer of FIG. 2A when knotted;

FIG. 3F is a photograph of a PDMS-GO composite elastomer sheet measuring 10 centimeters (cm) in diameter;

FIG. 4A is a schematic diagram of a gas separation system including a composite elastomer;

FIG. 4B is a schematic diagram of another gas separation system including a composite elastomer;

FIG. 5A is a graph of a Fourier transform infrared spectra of a composite elastomer containing an ethanolamine bond before and after thermal annealing;

FIG. 5B is a graph of a selected region of Fourier transform infrared spectra of a composite elastomer containing an ethanolamine bond before and after thermal annealing;

FIG. 5C is two graphs of a Fourier transform infrared spectra of a composite elastomer containing and ethanolamide bond before and after thermal annealing;

FIG. 6A is a graph of the oscillatory strain sweep of a PDMS-GO sol and PDMS-GO composite elastomer;

FIG. 6B is graph of the oscillatory frequency sweep of a PDMS-GO sol and PDMS-GO composite elastomer;

FIG. 6C is a graph of the oscillatory temperature sweep of PDMS-GO sol and PDMS-GO composite elastomer;

FIG. 6D is a graph of the stress relaxation results of a PDMS-GO sol and PDMS-GO composite elastomer;

FIG. 7 is a graph of the stress-strain curves of PDMS-GO composite elastomers;

FIG. 8 A is a photograph of PDMS-GO elastomer after annealing;

FIG. 8B is a photograph of PDMS-GO sol after solution casting;

FIG. 9A is a graph of the single gas permeability of a PDMS-GO composite elastomer as a function of upstream pressure expressed as delta P;

FIG. 9B is a graph of the single gas permeability of neat PDMS a function of upstream pressure expressed as delta P; FIG. 1 OA is a graph of nitrogen gas (N ₂ ) single gas permeability of neat PDMS and PDMS-GO composite elastomers at 35 °C as a function of upstream pressure;

FIG. 1 OB is a graph of oxygen gas (0 ₂ ) single gas permeability of neat PDMS and PDMS-GO composite elastomers at 35 °C as a function of upstream pressure;

FIG. IOC is a graph of hydrogen gas (H ₂ ) single gas permeability of neat PDMS and PDMS-GO composite elastomers at 35 °C as a function of upstream pressure;

FIG. 10D is a graph of methane gas (CH ₄ ) single gas permeability of neat PDMS and PDMS-GO composite elastomers at 35 °C as a function of upstream pressure;

FIG. 10E is a graph of carbon dioxide (C0 ₂ ) single gas permeability of neat PDMS and PDMS-GO composite elastomers at 35 °C as a function of upstream pressure;

FIG. 1 OF is a graph of C0 ₂ single gas hysteresis permeability of neat PDMS and PDMS-GO composite elastomers at 35 °C as a function of upstream pressure;

FIG. 11 is a differential scanning calorimetry thermogram of neat telechelic PDMS and PDMS-GO composite elastomers.

FIG. 12 is a graph of the relative permeability coefficient as a function of GO volume fraction depending on the state of GO alignment and concentration of GO;

FIG. 13 A is a graph of the C0 ₂ /N ₂ gas permeation selectivity of a composite elastomer at 10 atm, 35 °C as a function of GO content;

FIG. 13B is a graph of the C0 ₂ / CH ₄ gas permeation selectivity of a composite elastomer at 10 atm, 35 °C as a function of GO content.

FIG. 13C is a graph of the 0 ₂ /N ₂ gas permeation selectivity of a composite elastomer at 10 atm, 35 °C as a function of GO content;

FIG. 13D is a graph of the C0 ₂ / H ₂ gas permeation selectivity of a composite elastomer at 10 atm, 35 °C as a function of GO content;

FIG. 14A is a graph of the C0 ₂ /N ₂ gas permeation selectivity of a composite elastomer at 35 °C as a function of pressure and GO content;

FIG. 14B is a graph of the C0 ₂ /CH ₄ gas permeation selectivity of a composite elastomer at 35 °C as a function of pressure and GO content; FIG. 14C is a graph of the O ₂ /N ₂ gas permeation selectivity of a composite elastomer at 35 °C as a function of pressure and GO content;

FIG. 14D is a graph of the CO ₂ /H ₂ gas permeation selectivity of a composite elastomer at 35 °C as a function of pressure and GO content;

FIG. 15 is a relative ideal selectivity plot based on CO ₂ gas for PDMS/GO composite elastomers based on GO content; and

FIG. 16 is a CO ₂ /N ₂ gas selectivity comparison based on the Robeson Limit

2008.

DETAILED DESCRIPTION

The present disclosure relates to a method of forming a composite elastomer by reacting an amine functional group, an amide functional group, or a combination thereof, on a plurality of polymers with an epoxide functional group on a plurality of platelet-like fillers to form an ethanolamine or ethanolamide bond and crosslink the polymers via the fillers. An ethanolamine bond forms when an amine functional group reacts with an epoxide functional group to form a covalent bond. An ethanolamide bond forms when an amide functional group reacts with an epoxide functional group to form a covalent bond.

The present disclosure also relates to devices containing the composite elastomer. Such devices may include a membrane, particularly a gas barrier or gas separation membrane. Such a device may be superior to existing devices that include platelet-like materials that are merely coated on the surface of crosslinked polymers and bound by non-ethanolamine or ethanolamide adhesion forces. For instance, membranes including a composite elastomer described herein are not susceptible to delamination of a GO layer because they contain no such layer. In addition, devices containing a composite elastomer described herein may be less susceptible to adsorption of environmental contaminations whereas devices in which a platelet-like material is merely coated on crosslinked polymers will be.

Composite elastomer and Methods of Forming a composite elastomer

Using methods described herein, an amine functional group, an amide functional group, or a combination thereof on a plurality of polymers is reacted with an epoxide functional group on a plurality of platelet-like fillers to form an ethanolamine or ethanolamide bond. Whenever at least two individual polymers react with the same platelet-like filler, they are crosslinked via the filler. Typically, many individual polymers will react with the same individual platelet-like filler and any one individual polymer will also react with two separate individual platelet-like fillers. However, particularly if a combination of types of polymers is used, one type of polymer may react with the platelet-like filler, while another type of polymer reacts with and further crosslinks the first type of polymer.

Platelet-Like Filler

Suitable platelet-like fillers include any material with sufficient epoxide functional groups to form an elastomeric composite when reacted with the plurality of polymers having amine or amide functional groups. For instance, the platelet-like filler may include GO or reduced graphene oxide (RGO), or a combination thereof.

Although GO is often used herein to illustrate the principles of the present disclosure, those principles may be applied to RGO or other platelet-like fillers with epoxide functional groups.

The chemical structure of GO is shown in FIG. 1A. GO may be produced by chemical functionalization and physical exfoliation of high purity, pre-oxidized graphite through stirring or sonication, such as using Hummer's method or a modification thereof. GO contains numerous oxidative functional groups, including hydroxyls, carboxylic acids, and epoxides, that are covalently bonded to the carbon- based benzene-like ring structure. The resulting material is a sheet of carbon-based rings continuously arranged into a sheet as shown in FIG. 1 A, with oxidative functional groups external to the sheet.

The hydroxyl and epoxide functional groups are typically present in a larger proportion than other functional groups, particularly when GO is produced using Hummer's method. Although the oxidative functional groups typically include a variety of different functional groups, GO may also contain only one type of oxidative functional group, the epoxide functional group. In FIG. 1 A the epoxide functional group is shown in the color green.

RGO is typically formed by treating GO with a chemical reducing agent, such as hydrazine, hydroiodic acid, and ascorbic acid, or by other methods which may include heat, sunlight, microwave, external energy or a combination of these. All of these methods reduce oxidative functional groups, which may include hydroxyl, epoxide, and carboxylic acid. However, even RGO retains some epoxide functional groups, they are just more limited in number than before reduction.

The platelet-like filler may have an average aspect ratio between 100 and 2000, between 500 and 1600, or between 900 and 1200.

Polymer

The polymer used in the composite elastomer may include at least one type of polymer with at least two amine or amide functional groups, or a combination of amine or amide functional groups. These amine or amide functional groups may include primary amines, secondary amines, secondary amides, and combinations thereof.

The polymer may include types of polymers that are amorphous or

semicrystalline. It may be including types of polymers that are hydrophobic or hydrophilic. The polymer may be a homopolymer or a copolymer. The polymer may further be linear or branched. The polymer may be flexible at 21 °C.

Typically, the type of polymer with two amine or amide functional groups, or a combination thereof is telechelic for the functional group. A telechelic polymer is a linear polymer that contains identical functional groups at both ends of the polymer. However, it is also suitable for at least one type of polymer to have different amine or amide groups on its ends. Suitable types of polymers may also have only internal {i.e., mid-chain as pendant groups or in the backbone but not at the ends) amine or amide functional groups, or a combination thereof, which may be the same or different from one another. These internal amine or amide functional groups may be in addition to or in the absence of end amine or amide functional groups. If the polymer has both internal and end amine or amide functional groups, some or all of the internal functional groups may be the same as one or both end amine or amide functional groups. If the polymer has both internal and end amine or amide functional groups, the end functional groups may be different from one another and the internal ones may be different from the end functional groups.

Different locations and types of amine or amide functional groups may affect the method of forming the composite elastomer as well as properties of the material formed, but typically at least two amine or amide groups, or a combination thereof are present somewhere on at least one type of polymer so that it may react with epoxide functional groups on two separate individual platelet-like fillers.

Many types of polymers, such as the PDMS used illustratively herein, normally contain no amines. These polymers may be modified to add an amine or amide functional group at the appropriate location.

If a type of a polymer containing an amine- or amide-terminated monomer is used, such that the polymer already has an amine or amide functional group on at least one end, then that type of polymer may be used without further modification of the end(s) with an amine or amide functional group, or it may be modified to include one or more additional amine or amide functional groups, or a combination thereof, on one or both ends.

The amine or amide functional group, the combination thereof may be present in the polymer in the range of 0.05% by weight to 2.0% by weight, or in the range of 0.05 % by weight to 4.00 % by weight. This weight percentage may be calculated using the chemical formula of the polymer.

The neat polymers may have a viscosity of at least 1 cSt, 100 cSt, or 500 cSt. The polymers may have a viscosity of no greater than 8,000 cSt or 10,000 cSt. The viscosity may also range between any combinations of these lower and upper endpoints.

The viscosity of the polymer, which is directly related to the molecular weight

(M _n ) of the polymer, may affect the ability to form a composite elastomer in which the platelet-like filler is homogenously distributed. When the platelet-like filler is homogenously distributed, less than 0.05% of the individual platelet-like fillers are in agglomerates, or there are no observable agglomerates of the platelet-like filler in then composite elastomer when representative samples of the composite elastomer are examined by a microscope.

When a low viscosity polymer is used, the platelet-like filler may settle, particularly during solution casting to form a membrane. This settling typically produces agglomerates that may be observed with a microscope. A polymer with a higher molecular weight or viscosity may reduce or eliminate this settling problem.

The polymer may include silicon-based polymer or a carbon-based polymer or a silicon-carbon copolymer. The polymer may include one type of polymer or a combination of at least two different types of polymers.

In particular, the polymer may be one or a combination of any of the following:

i. PDMS,

ii. poly(acrylic acid),

iii. poly(vinyl alcohol),

iv. polyamide,

v. polyarylamide,

vi. poly(acrylamide),

vii. poly(acrylic anhydride),

viii. poly(vinyl naphthalene),

ix. poly(styrene),

x. cellulose acetate,

xi. polyacrylonitrile,

xii. polysulfone,

xiii. poly(vinylidene fluoride),

xiv. poly(ethylene terephthalate),

xv. poly(ethylene-co-vinyl acetate),

xvi. poly(vinyl chloride)

xvii. a polymer with aromatic rings in the polymer backbone or the pendent group which form strong pi-pi interactions,

xviii. a polymer with a maleic anhydride functional group,

xix. a thermoplastic elastomer, which is processable at a temperature above at least 21 °C, but which contains some crosslinks and is typically not processable at 21°C or below, and which may include

a. an ethylene oxide and propylene oxide block copolymer, such as Pluronic® (BASF, US),

b. a polystyrene and rubber block copolymer, such as a polystyrene and polybutadiene, polyisoprene, or their hydrogenated equivalent block copolymer, particularly a triblock copolymer with polystyrene at the extremities such as Kraton ® polymers (Kraton Polymers, US), xx. a general purpose polymer, such as polypropylene, polyethylene, and poly(methyl methacrylate),

xxi. a engineering or super-engineering plastic, such as polyimide, polyamide polytetrafluoroethylene, and polyethylene oxide, xxii. any copolymer containing at least two different monomer units with end or internal amine or amide functional groups.

If a combination of types of polymers is used, any type of polymer in the combination may be present in an amount of at least 0.01% by total polymer weight, at least 0.05%> by total polymer weight, 0.1%> by total polymer weight, 1%> by total polymer weight, 5% by total polymer weight, 10%> by total polymer weight, 25% by total polymer weight, 50%> by total polymer weight, 75% by total polymer weight, 90%) by total polymer weight, 95% by total polymer weight, or 99% by total polymer weight.

In the example polymer of FIG. IB, telechelic primary amine functional groups are shown in the color red.

FIG. 1C is a diagram of the reaction of a primary amine functional group on the end of a polymer with an epoxide on the surface of GO to form an ethanolamine bond. The reaction proceeds by the epoxide ring in the epoxide functional group opening to bond to the primary amine in the amine functional group via a stable C-N ethanolamine bond. This reaction produces no by-products, making it readily scalable to mass produce the composite elastomer in a cost-effective manner.

This reaction, repeated between a plurality of PDMS polymers and a plurality of GO platelet-like fillers, crosslinks the PDMS to form an ethanolamine macromolecular network in the composite elastomer.

In the example polymer of FIG. ID, telechelic secondary amide functional groups are circled in red.

FIG. IE is a diagram of the reaction of a secondary amide functional group on the end of a polymer with an epoxide on the surface of GO to form an ethanolamide bond. The reaction proceeds by the epoxide ring in the epoxide functional group opening to bond to the secondary amide in the amide functional group via a stable C- N ethanol amide bond. This reaction also produces no by-products, making it also readily scalable to mass produce the composite elastomer in a cost-effective manner.

In general, ethanolamide bonds may form more slowly than ethanolamine bonds, but certain additional molecules bound to a secondary amide, such as the pyrene molecules of FIG. ID, may promote more homogenous distribution of the platelet-like filler, making the use of secondary amides worthwhile overall.

In addition to the ethanolamine reaction mechanism of FIG. 1C or the ethanolamide reaction mechanism of FIG. IE, the plurality of polymers may be further crosslinked, for instance by an additional chemical treatment, heat treatment, or UV treatment. Thus further crosslinking may increase the mechanical integrity of the composite elastomer.

In addition, when the polymer is a combination of at least two types of polymers, the first type may react with the platelet-like filler, while the second type may react with the first type of polymer to further crosslink it. This reaction between the first and second type of polymers typically also results in a covalent bond. For instance, the polymer may include poly(vinyl alcohol) covalently bonded to graphene oxide and glutaraldehyde covalently bonded to poly(vinyl alcohol).

Composite elastomers

To form a composite elastomer, the plurality of polymers and the plurality of platelet-like fillers may be homogeneously dispersed in a solution. The solution is dried, for example by solution casting or spin coating, to form a sol, which is unreacted polymer and platelet-like filler. The sol is a gel precursor to the composite elastomer. The platelet-like filler is homogenously distributed in the sol. The sol is then annealed, for instance in a vacuum oven.

The reaction between the amine or amide functional group, or combination thereof, on the polymer and the epoxide functional group on the platelet-like filler takes place during or after the solution is cast or spin-coated. Typically, the reaction proceeds very slowly or not at all in the absence of an external energy source, such as heat. So the degree to which the reaction occurs in the solution prior to casting or spin-coating is very limited or reaction prior to casting or spin-coating is avoided by simply not supplying an external energy source until during or after solution casting or spin-coating. The polymer may be present in the solution an amount of at least 1 % by weight, at least 2% by weight, at least 5% by weight, at least 10% by weight, or at least 20%) by weight based on total weight of polymer and solvent. The polymer may be present in an amount of no more than 25% by weight, 30%> by weight, 40% by weight, or 50%> by weight based on total weight of polymer and solvent. The amount of polymer present may also be in a range between any of 1%> by weight, 2% by weight, 5 %> by weight, 10%> by weight, or 20% by weight and any of 25% by weight, 30%) by weight, 40% by weight, or 50% by weight, based on total weight of polymer and solvent.

The solution may have a viscosity of at least 1 cSt, no greater than 8,000 cSt, or between 1 cSt and 8,000 cSt.

Solution-casting may be useful to form composite elastomer membranes. In solution casting, a solution of the plurality of polymers and plurality of platelet-like fillers is formed, then it may be sonicated. The solution is then placed in a casting dish. In general, solvents are evaporated in ambient conditions. However, an energy source can also be supplied, which evaporates the solvent even faster, so that a sol is formed in the casting dish. Finally, the sol is annealed, typically in a vacuum, to produce a composite elastomer with dimensions that in part conform to the casting dish by crosslinking reaction. The composite elastomer may then be removed from the casting dish. Solution-casting is particularly useful to form freestanding, thick, flat composite elastomers of roughly uniform thickness.

In spin-coating, a solution of the plurality of polymers and plurality of platelet-like fillers is formed, then sprayed onto another article, such as a membrane, silicon wafer, or filer, via a spin-coater. Energy may be supplied shortly before, during, or shortly after to allow the crosslinking reaction to proceed before the solution flows off the article. The solvent tends to evaporate during the spraying, such that a sol is formed on the article without the need for further evaporation. The sol is then annealed, typically in a vacuum, to produce a composite elastomer with dimensions that conform to the article. Although in some instances the composite elastomer may be removable from the article, particularly if the article is sacrificed in the process, typically the composite elastomer remains on the article. Spin-coating is useful to form irregularly-shaped sub-micrometer or nanometer thick composite elastomers and composite elastomers that are in close contact with the article.

The total polymer content of the composite elastomer (which is the weight of only one polymer if a single polymer is present, or the combined weight of two or more polymers if a polymer combination is present) may be at least 70 % by weight, at least 80% by weight, at least 90% by weight, at least 95% by weight, at least 99% by weight, or at least 99.99% by weight. The total polymer content may also be between any of 70% by weight, 80% by weight, 90% by weight, 95% by weight, 99% by weight, or 99.9% by weight.

The total platelet-like filler content of the composite elastomer (which is the weight of only one platelet-like filler if a single filler is present, or the combined weight of two or more platelet-like fillers if a filler combination is present may be no more than 30% by weight, no more than 20% by weight, nor more than 10% by weight, no more than 5% by weight, no more than 1% by weight, or no more than 0.01%) by weight. The total platelet-like filler content may also be between any of 30%) by weight, 20% by weight, 10% by weight, 5% by weight, 1% by weight, or 0.01%) by weight. Greater than 30% by weight platelet-like filler may impair the mechanical integrity of the composite elastomer to an unacceptable degree.

Different proportions of the polymer and platelet-like filler may give rise to different properties in the elastomeric composite, such that relative proportions, often expressed concentration of platelet-like filler, may be selected to confer a given property.

The % by weight of polymer or platelet-like filler is calculated based on the total weight of the polymer and platelet-like filler in the composite elastomer. Other materials, including polymer not expected to participate in a crosslinking reaction or a filler not expected to form an ethanolamine or ethanolamide bond with a crosslinked polymer, are excluded from the total weight of the composite elastomer for purposes of this calculation, even if they are present in the composite elastomer.

At least 75% by weight, at least 80% by weight, at least 90% by weight, or at least 95% by weight of the total polymer and platelet-like filler in a composite elastomer described herein may be crosslinked. The % by weight of crosslinked polymer and platelet-like filler may be determined by weighing a dried composite elastomer, then washing it with a variety of solvents, such as water, then alcohol, then acetone, followed by removal of residual solvent and drying. The weight of the composite elastomer after this solvent-treatment is at least 75% of the weight before the solvent treatment if at least 75% by weight of the total polymer and platelet-like filler are crosslinked. Solvents may be selected based on the materials used to form the composite elastomer, but, in general, crosslinked polymer and platelet-like filler should not be removable by any solvent.

Remaining sol, in some processes, may be removed from the composite elastomer. It may also be retained, particularly if the unreacted polymer or platelet- like filler serve a useful purpose in the intended use of the composite elastomer.

Because the platelet-like filler contains the epoxide functional groups that react with the amine or amide functional groups on the polymer, increasing the proportional amount of platelet-like filler in the composite elastomer results in increased crosslinking. In contrast, as the M _n of the polymer increases, the mol fraction of the amine or amide functional groups is reduced, decreasing crosslinking.

SEM images of example elastomer compositions with different GO content may be seen in FIG. 2A, FIG. 2B, and FIG. 2C.

In addition, the alignment of individual platelet-like fillers may be controlled in elastomer compositions formed by solution casting. Because the solution used for solution casting is not very viscous and the amine or amide functional group-epoxide functional group reaction largely does not occur until during or after solution casting, when an external energy source is provided, individual platelet-like fillers may move around in the solution during casting. Typically, gravity aligns the platelet-like fillers parallel to the plate or other bottom material on which the solution is cast. The renders them parallel to a bottom side of the composite elastomer as well. The platelet-like fillers tend to align so they are not agglomerated. Increased platelet-like filler concentration tends to increase alignment, as they orient collectively since they cannot occupy the same space or rotate through one another.

The alignment of platelet-like fillers may also be controlled in elastomer compositions formed by spin-coating. In spin-coating, centrifugal spinning force drives the alignment of the platelet-like fillers, such that they are typically aligned to be parallel to the bottom substrate or a bottom side of the elastomer composite. Regardless of how alignment is achieved, it may be assessed using microscopy, such as SEM. FIG. 2A, FIG. 2B, and FIG. 2C illustrate increasing alignment with increasing GO concentration. In FIG. 2A, the curved GO platelets are not parallel to the bottom plate. In contrast, in FIG. 2C, the curved GO platelets are nearly parallel to be bottom plate.

Alignment in a composite elastomer membrane for use as a gas barrier or in gas separation may be perpendicular to the gas transport direction.

Mechanical Properties

A composite elastomer as disclosed herein may have sufficient mechanical integrity to be freestanding and to be formed in a variety of sizes. See the examples of FIG. 3A, FIG. 3B, and FIG. 3F. The composite elastomer may also be bendable and flexible. See the examples of FIG. 3C and FIG. 3D. Furthermore, it may be sufficient elastic for form a knot, as shown in the example of FIG. 3F. The composite elastomer may have a high tensile strength, allowing it to be stretched up to 100%, up to 300%), or even up to 500%) of its original length before failure (described in ASTM D638, ASTM International, PA, US).

The bendability, flexibility, elasticity, and tensile strength of the composite elastomer may vary based on the type(s) of the polymer, molecular weight of the polymer(s), the type(s) of platelet-like filler, the concentration of platelet-like filler, and the amount of crosslinking in terms of the % by weight of crosslinked material or as determined by another measure of crosslinking.

Gas Barrier Properties and Systems

Gas permeability of a membrane is determined by two components. One component is the solubility of molecules of the gas in the membrane, which allows the molecules to dissolve into the membrane, then exit the other side. Solubility links the concentration of the molecule of gas in the gas phase to the concentration of the molecules of gas in the membrane. The other component is the diffusivity of a molecule of the gas through the membrane. Diffusivity is a kinetic property that reflects the effects of the surrounding membrane environment, such as small channels, on the motion of molecules of gas through the membrane. Solubility and diffusivity tend to be influenced by different properties of the membrane or gas as a result. Gas permeability is a product of both solubility and diffusivity, such that a significant decrease in either factor lowers gas permeability overall.

Generally, in a membrane formed from rubbery polymers, such as PDMS, gas permeability depends much more on the relative solubility of a specific gas molecule, than on the diffusivity of the gas. Therefore, PDMS is more permeable to large, soluble and condensable molecules than to small penetrants. However, as the GO content increases, diffusivity is more strongly affected than the solubility. GO fillers introduce tortuous pathways and modify the microstructure of the composite elastomer rather than its interactions with the gas, and as a result, its size-selective capability increases.

A composite elastomer disclosed herein, when formed into a membrane, may significantly inhibit gas permeability as compared to a similar composite containing only the polymer and lacking the platelet-like filler. For instance, gas permeability for single gasses may be reduced by at least 45 %, at least 50%, at least 75%, at least 80%, at least 90 %, at least 95 %, at least 99 %, or at least 99.9 % as compared to a membrane formed from neat polymer elastomer of the same polymers. Thus, such a composite elastomer membrane may be used as a gas barrier. Although gas permeability for only a set of single gasses, H ₂ , N ₂ , 0 ₂ , CH ₄ , and C0 ₂ is tested in the examples herein, this set of gasses represents some of the smallest gasses possible. Other gasses are larger and, therefore, have even lower permeability.

The single gas permeability of a composite elastomer membrane may vary based on factors such as the concentration of or the alignment of the platelet-like filler. It may also be affected by the thickness of the membrane. A gas barrier may have a thickness of between 0.01 and 1 mm.

Gas barrier membranes may be formed as freestanding membranes of one or more layers of composite elastomer, or they may include supporting materials, such as metal foil or other tough or malleable material. The composite elastomer may be used in place of the gas barrier component for any given gas in any given gas barrier membrane or other gas barrier device.

Although composite elastomers as described herein may be used as any type of gas barrier in any application, they may have particular uses as well. For instance, they may be used in building materials, for example as a water barrier or radon gas barrier. They may be used as protective barriers, for instance in manufacturing and industrial processes where they contain poisonous or corrosive gasses, flammable gasses, or gasses, such as hydrocarbon gasses, that may have deleterious effects to human health or the environment after long-term exposure. They may also be used to contain harmful gasses present in the environment or released during environmental remediation. In addition, they may be used to contain nuisance gasses, such as sulfide gasses produced by sewage processing and treatment.

Gas Separation Properties and Systems

A composite elastomer disclosed herein, when formed into a membrane, may also be used as a gas separation membrane due to its different permeabilities to different gasses. The gas separation capacity of the composite elastomer may be particularly influenced by the platelet-like filler. Increased concentration of the platelet-like filler increases the tortuosity of the path through which a gas may diffuse, slowing passage of gasses with larger kinetic diameters much more than passage of gasses with smaller kinetic diameters. Aspect ratio of the platelet-like filler may also decrease diffusivity of gasses, affecting those with larger kinetic diameters more than those with smaller kinetic diameters.

Composite elastomer membranes described herein may have a gas selectivity of at least 10, at least 15, or at least 20 for C0 ₂ in a gas mixture, which may include C0 ₂ /N ₂ , C0 ₂ /0 ₂ , C0 ₂ /H ₂ , or C0 ₂ /mixed gasses. Gasses separated using composite elastomer membranes described herein may be between 2.6 Angstroms and 5.0 Angstroms in size. Selectivity for a given gas combination may be improved as compared to neat polymer by at least 50%, 100%, at least 125%, at least 140%, or at least 150%.

Gas separation membranes may also be formed as freestanding membranes of one or more layers of composite elastomer. They may also include support materials, so long as the support materials do not impede gas separation. For instance, gas separation membrane 20a in FIG. 4 A includes composite elastomer 30a surrounded by a frame 40, which may facilitate air-tight seal and also to make the gas separation membrane 20a easier to install, inspect, and replace. Gas separation membrane 20b in FIG. 4B includes composite elastomer 30b formed upon a gas-permeable support 140. Referring now to FIG. 4 A and FIG. 4B a gas separation system 10 may include a gas source 60, such as waste gas or exhaust, that includes at least one separable gas 70a and at least one non-separable gas 70b. Gas from gas source 60 flows through compressor 80, which may be optional in some gas separation systems. Compressed gas from compressor 80 enters intake chamber 100 or gas separation chamber 90 where molecules of separable gas 70a are separated from molecules of non-separable gas 70b. Molecules of non-separable gas 70b may be returned via return pathway 190 to a flow pathway from gas source 60 either before compressor 80, as shown, or after compressor 80 (not shown). In other embodiments (not shown), particularly those where separable gas 70a is harmful, while non-separable gas 70b is benign, molecules of non-separable gas 70a removed from intake chamber 100 may simply be released into the air.

Molecules of separable gas 70a pass through gas separation membrane 20 and enter separated gas output chamber 110. Molecules of separable gas 70a then exit gas separation chamber 90 and enter output container 120.

In gas separation system 10a of FIG. 4 A, gas separation membrane 20a divides intake chamber 100a from separated gas output chamber 110a. Frame 40 of gas separation membrane 20a may form an air-tight seal with other elements of gas separation chamber 90a between intake chamber 100a and separated gas output chamber 110a so that gas in intake chamber 100a may only enter separated gas output chamber 110a by passing through gas separation membrane 20a.

Separable gas 70a in separated gas output chamber 110a may be moved to output container 120 via a vacuum or partial vacuum, via air flow, or due to concentration differences.

Non-separable gas 70b and any remaining separable gas 70a in intake chamber

100a may be removed via an air flow, such as an air flow between intake vent 150 and outtake vent 160. Gas in intake chamber 100a may also be removed by other mechanisms, such as a vacuum or partial vacuum or due to concentration differences.

In gas separation system 10b of FIG. 4A, gas separation membrane 20b is formed on gas-permeable support 140 and located inside separated gas output chamber 110b. Intake chamber 100b is located inside gas separation membrane 20b. Intake chamber 100b may also extend beyond gas separation membrane 20b and separated gas output chamber 110b, as shown, or it may be located entirely within separated gas output chamber 110b, or entirely within gas separation membrane 20b (not shown). Gas separation membrane 20b may also form an air-tight seal between intake chamber 100b and separated gas output chamber 110b so that gas in intake chamber 100b may only enter separated gas output chamber 110b by passing through gas separation membrane 20b.

Separable gas 70a in separated gas output chamber 110b may be removed and moved to output container 120 via an air flow, such as an air flow between intake vent 170 and outtake vent 180. It may also be removed by other mechanisms, such as a vacuum or partial vacuum or due to concentration differences.

Non-separable gas 70b and any remaining separable gas 70a in intake chamber 100b may be removed via air flow through the chamber, or by a vacuum or partial vacuum or due to concentration differences. It travels via

A mixture of separable gas 70a and non-separable gas 70b may be separated using gas separation system 10 in a continuous fashion or in an iterative fashion.

Using the gas separation system including a composite elastomer as described herein, such as gas separation system 10 at least 25%, at least 50%, at least 75%, at least 90%), at least 95%, at least 99%, or at least 99.9% of separable gas may be removed from a mixture of separable gas and non-separable gas. The separable gas thus obtained may contain no more than 10% non-separable gas, no more than 5% non-separable gas, no more than 1% non-separable gas, no more than 0.5% non- separable gas, or no more than 0.1% non-separable gas.

Although FIG. 4A and FIG. 4B provide example gas separation systems, the composite elastomer may be used in place of the gas separation component for any given gas in any given gas separation membrane or other gas separation system or device, such as hollow fiber gas separator and modules using hollow fiber gas separators.

Other Devices

The composite elastomer may be formed in a variety of shapes and sizes, such as membrane or coatings on other articles. A composite elastomer of this disclosure may also be used as a desalination membrane, or in wearable devices, flexible displays, and devices for monitoring physiological signals. EXAMPLES

The following examples illustrate aspects of the invention; no example is intended to encompass the invention as a whole. Furthermore, although some examples may present discrete embodiments of the invention, aspects of such examples may be combined with other variations of the invention as described above or in different examples unless such combinations would be clearly inoperable to one of skill in the art. For instance, the polymer in a given example may be exchanged with a different polymer.

The composite elastomers discussed in these examples may reference the materials used to form them, even if such materials have been modified in the final composite elastomer. For instance, an amine-terminated telechelic PDMS/GO composite elastomer refers to a material formed from amine-terminated telechemic PDMS, even though most or all of the PDMS is no longer amine-terminated, but rather terminates in an ethanolamine bond with GO.

Example 1 - Composite Elastomer Formation and Basic Characterization

A composite elastomer was formed by combining amine terminated telechelic PDMS polymer, GO platelet-like filler, and tetrahydrofuran (TFIF) solvent.

Aminopropyl terminated telechelic PDMS with a viscosity of 1,000 cSt (M _n = 25,000 g/mol, DMS-A31) (N1000) and 2,000 cSt (M _n = 30,000 g/mol, DMS-A32) (N2000) were purchased from Gelest and used without any further purification.

GO was synthesized according to the Hummers' s method with slight modifications as described in Ha, et. al, "Mechanically Stable Thermally Crosslinked Poly(acrylic acid)/Reduced Graphene Oxide Aerogels," ACS Appl, Mater. Interfaces 7(l l):6220-9 (March, 2015). In particular, preoxidized graphite (5 g) was added to concentrated sulfuric acid (98%, 125 mL in an ice bath. Potassium permanganate (15 g) was added slowly using a spatula with vigorous stirring of the solution. The mixture was stirred at 35 °C for 2 h. Then, deionized water (DI water, 230 mL) was carefully added using a pipet followed by terminating with DI water (700 mL) and 30%) hydrogen peroxide solution (12.5 mL). Dilute hydrochloric acid solution with DI water in a volume ratio of 1 : 10 were used to remove residual manganese salt and excess acid products. Subsequently, the solution was washed with DI water until the pH of the rinsed water reached neutral. Finally, this solution was filtered using a vacuum assisted Biichner funnel and filtrate converted into a thick slurry of GO in water. The aqueous dispersion of GO was lyophilized under vacuum.

A known amount of amine terminated telechelic PDMS (for 1 wt % GO composites, 990 mg) was dissolved and stirred in THF (6 milliliters (ml)). Then dried GO (10 mg) was suspended in the solution with vigorous stirring. Homogenous dispersion of GO in this solution was promoted by sonication using a 400 W probe sonicator with 10% amplitude for 10 min (24 kJ) in an ice bath. Although THF is not a good solvent for GOs, amine terminated telechelic PDMS can effectively act as a surfactant to assist in dispersing GOs homogenously in THF though hydrogen bonding between amine ends and oxygen containing groups on GO surfaces.

Dispersion after sonication was confirmed by examining the residual liquid film in the vial while rolling the vial containing the solution and after pouring the majority of the solution into a PTFE evaporating dish for solution casting. In both cases, the liquid film in the vial was essentially transparent for all solution concentrations. However, the liquid film was greyish-black from the GO content. No aggregates were identified by the naked eye. This solution was immediately poured into a Teflon dish and covered to slowly evaporate the solvent, thereby solution casting a film at room temperature. The solvent was evaporated for at least 2 days and then the sample was vacuum dried at room temperature for one additional day to make a homogenous amine terminated telechelic PDMS-GO sol. To form a crosslinked composite elastomer, the sol was heated to 160 °C in a vacuum oven for 24 hours and cooled slowly to room temperature before use. The resulting composite elastomer is shown in FIG. 3A. The composite elastomer could be bent, flexed, tied in a knot, and was made at various diameters as shown in FIGS. 3B-3F.

The morphology of composite elastomers was also analyzed using scanning electron microscopy (SEM; Zeiss Supra 40 V). 12 nm of Pt/Pd were sputter coated on the sample before taking measurements. Due to the low glass transition temperature of the PDMS/GO composites, it was difficult to microtome or cryo-fracture the sample to observe the cross-section of the material. Instead, each sample was torn by hand to obtain a relatively undamaged cross-sectional surface for SEM imaging. The SEM images of FIG. 2A, FIG. 2B and FIG. 2C show the internal structures of the composite elastomers, such as the increased alignment of GO platelets as GO concentration increases.

Composite elastomers with ethanolamide bonds were formed from telechelic pyrene-terminated PDMS and GO solubilized in THF. The pyrene end groups are linked to the PDMS via amide bonds, this providing amide functional groups at the ends of the polymers. In addition to successfully reacting with GO to form a composite elastomer, pyrene-terminated PDMS, via the pyrene groups, additionally promoted homogenous distribution of GO in THF by forming strong π-π interactions. Crosslinking of this composite elastomer proceeded more slowly than that observed with anime-terminated PDMS and GO. The resulting composite elastomer was freestanding and elastic.

Composite elastomers formed using telechelic anime-terminated PDMS and GO as described above were used in all of the following Examples, with the exception of the portion of Example 2 relating to FIG. 5C, in which a composite elastomer formed using telechelic pyren-terminated PDMS and GO was used.

A neat PDMS elastomer was synthesized for comparison to the composite elastomers by reacting N1000 and epoxypropoxypropyl-terminated telechelic PDMS (diepoxy) in THF with a 5 fold molar excess of diepoxy. The molecular weight of N1000 was 25,000 g/mol and the molecular weight of diepoxy was 550 g/mol.

Depending on the molecular weight of the telechelic PDMS material, the weight ratio between the two components were adjusted accordingly. N1000 has 4 active sites, which include one H atom each. Each active site is capable of reacting with an epoxide. Diepoxy has 2 active sites, which include one epoxide each. Calculating the extent of reaction at the gel point by Flory 's method gives a value very near to 1. After solution casting a film in a PTFE dish, the mixture was annealed at 160 °C.

Example 2 - Composite Elastomer Characterization with FTIR Spectra

The chemical crosslinking reaction between amine terminated telechelic PDMS and GO was investigated using Fourier transform infrared spectroscopy (FTIR; Thermo Nicolett, 6700) with a scan size (resolution) of 2 cm ^"1 collected over 256 scans per sample. FIG. 5 A shows the FTIR spectra before thermal annealing (referred to as "GO Sol") and after thermal annealing (referred to as "GO Elastomer") of NlOOO/1 wt % GO with amine ends at approximately 0.6 mol % (i.e., 0.12 wt %). More specifically, FIG. 5B shows the 1500-1700 cm ^"1 region of the FTIR spectra. Before annealing, the peak associated with N-H bending was assigned at 1600 cm ^"1 for both N1000 PDMS and N1000 PDMS/1 wt % GO sol samples. However, after annealing the sol to crosslink the sample producing a composite elastomer, a significant fraction of the N- H bending peak disappeared, indicating that the majority of the primary amine end groups were reacted during this step. In addition, due to a known thermal annealing induced reduction reaction of GO to reduced GO, a stronger C=C ring stretching peak at 1580 cm ^"1 was formed for the N1000 PDMS/1 wt % GO elastomer sample.

Table 1 below shows detailed peak assignments for the FTIR spectra of FIG.

5A.

Table 1

Wavenumber (cm ^"1 ) Mode

3380 N-H asymmetric stretching 3326 N-H symmetric stretching 2962 CH ₃ asymmetric stretching 2905 CH ₃ symmetric stretching 1600 N-H bending (broad) 1445 CH ₃ asymmetric bending shoulder 1412 CH ₃ asymmetric bending 1258 CH ₃ symmetric bending 1020-1250 C-N stretching (weak)

1080 Si-O-Si asymmetric stretching 1009 Si-O-Si symmetric stretching

CH ₃ asymmetric rocking

786

Si-C asymmetric stretching

702 Si-C symmetric stretching Analogously to the primary amine-containing telechelic PDMS/GO composite elastomers, the crosslinking reaction between secondary amide functional groups and epoxide functional groups was confirmed by FTIR analysis (FIG. 5C). For secondary amides, signals for N-H stretching at 3300 cm ^"1 and N-H bending at 1550 cm ^"1 clearly disappeared after the crosslinking reaction occurred, while the C=0 stretch coupled with a hydrogen bonded amide group at 1640 cm ^"1 shifted to 1700 cm ^"1 after the secondary amide was reacted. This spectroscopic evidence clearly demonstrates that the secondary amide functional groups successfully reacted with the epoxide groups on the GO surface.

Example 3 - Composite Elastomer Crosslinking Evaluation

Another method to confirm the formation of chemically crosslinked composite elastomers is to measure the swelling ratio and gel content of the composite elastomer. Solvent uptake and gel content of each composite elastomer were calculated using Equation 1 and 2 below where 'w' indicates weight, according to ASTM D2765 test method C. TFIF was used as the solvent to dissolve away unreacted soluble polymers.

Solvent Uptake (%) = ^{Wa ter swe} " ^{"Wfee ore swe} " x 100 (1)

w before swell

fn _{/ \} ^w after swell and dried in vacuum _/ /- _{> \}

Gel Content (%) = — x 100 (2)

w before swell

By controlling the content of GO and the M _n of the amine terminated telechelic PDMS polymer used to form the composite elastomer, the gel crosslink density can be altered effectively as shown in Table 2 below. For example, increasing the GO content resulted in the introduction of more crosslinking sites and, as a result, dramatically increased the gel content and decreased the solvent uptake of the elastomers. On the other hand, as the M _n of the amine terminated telechelic PDMS was increased from 25 kg/mol to 30 kg/mol, the mol fraction of the amine end groups was reduced from 0.6 % to 0.5 %. Table 2 shows the solvent uptake and gel content for composite elastomer samples measured as a function of amine terminated telechelic PDMS molecular weight and GO content. The indicated error is standard deviation of at least 5 separate samples. The results in Table 2 show that the degree of swelling expectedly increased and gel fraction decreased; the likelihood of amines functional groups reacting with the epoxide functional groups of GO would be expected to decrease in this situation reducing crosslinking density. An associated factor is that the mobility of a higher M _n polymer is slightly lower due to increased entanglements and viscosity, which also decreases the crosslink density.

Table 2

Solvent uptake Gel content Soluble fraction

Sample

(g/mol) (%) (%) (%)

NlOOO/1 wt % GO 25,000 711 ± 26 77.0 ± 1.7 -23

N1000/2 wt % GO 25,000 334 ± 17 89.2 ± 0.8 -11

N2000/1 wt % GO 30,000 836 ± 31 72.1 ± 1.8 -28

N2000/2 wt % GO 30,000 421 ± 4 84.4 ± 1.3 -16

Example 4 - Physical Properties Composite Elastomers

All rheological experiments were conducted on a shear rheometer (TA Instruments, AR-2000EX) using an 8 millimeter (mm) parallel upper plate and a Peltier lower plate fixture for temperature control with a gap of 500 μπι. First, a strain sweep was conducted to identify the linear viscoelastic regime at 25 °C in the range of 0.01-10 % strain at a frequency of 1 Hz. A frequency sweep was then conducted at 25 °C in the range of 0.01-10 Hz with 0.1 % strain. Temperature sweeps were conducted from 25 °C to 100 °C with 5 °C step changes using both 0.1 % strain and a frequency of 1 Hz. Stress relaxation tests were conducted by subjecting the sample to an initial strain of 10% and 20%.

Comparison in rheological properties between NlOOO/1 wt % GO sol and composite elastomer samples is shown in FIG. 6. FIG. 6A shows an oscillatory strain sweep based on % strain, FIG. 6B shows an oscillatory frequency sweep based on frequency, FIG. 6C shows an oscillatory temperature sweep, and FIG. 6D shows stress relaxation results. During the oscillatory frequency sweep in FIG. 6B, the NlOOO/1 wt % sol sample's raw phase reached the instrument limit (> 150°). Thus, data in the region of uncertainty is not shown, but is expected to continue in a similar trend. As shown in FIGS. 6A-6C, the amine terminated telechelic PDMS/GO composite elastomer exhibited a storage modulus (G') that was approximately an order of magnitude higher than the loss modulus (G") over a wide range of strains, frequencies, and temperatures, which is indicative of stable, solid-like viscoelastic behavior. In contrast, the amine terminated telechelic PDMS/GO sol exhibited liquidlike behavior (G" being higher than G') over the range of interest and showed considerable changes across both the frequency and temperature sweep, which is distinguishable from that of the amine terminated telechelic PDMS/GO composite elastomer sample. A stress relaxation test also confirmed the amine terminated telechelic PDMS/GO composite elastomer as a viscoelastic solid, which is in agreement with the aforementioned rheological observations shown in FIGS. 6A-6C. After relatively short relaxation times, the stress relaxation curve reached an asymptotic equilibrium stress of approximately 20 kPa. However, the uncrosslinked amine terminated telechelic PDMS/GO sol exhibited complete stress relaxation within a few seconds.

Example 5 - Mechanical Integrity of Composite Elastomers

The mechanical integrity of the amine terminated telechelic PDMS/GO composite elastomer was investigated using a universal tensile test. To measure the mechanical properties of the amine terminated telechelic PDMS/GO composite elastomers, microtensile specimens were prepared by solution casting on a mold to produce a dog bone sample with a gauge length of 22 mm, width of 4.8 mm, and thickness of about 0.7 mm (FIG. 8 A) satisfying ASTM D 1708- 13 standards. An Instron (model 5966) equipped with a 1 kN load cell was used with a strain rate of 10 mm/min at a gauge length of 22 mm, which is a nominal strain rate equivalent to 50 mm/min for ASTM D638 standard samples and samples were tested in triplicate. Increasing the GO concentration resulted in a higher tensile strength while reducing the elongation at break as shown by the stress-strain curve in FIG. 7 and the data in Table 3. The data in Table 3 show the tensile properties of amine terminated telechelic PDMS/GO composite elastomers depending on the M _n of the amine terminated telechelic PDMS and GO content. The indicated error is standard deviation of at least 3 separate samples. This result shows the effect of chemical crosslinks between amine terminated telechelic PDMS and GO. Although the gel content for high M _n amine terminated telechelic PDMS (N2000) composites was slightly lower than that of low M _n amine terminated telechelic PDMS (N1000, related data shown in Table 2), the N2000 samples exhibited moderately higher tensile strength. Thus, a major contributing factor is trapped entanglements between the reacted amine terminated telechelic PDMS chain ends that act as physical crosslinks within the network, which increases the tensile strength.

Table 3

Sample Tensile strength at break (MPa) Elongation at break (%)

NlOOO/1 wt % GO 0.15 ± 0.02 280 ± 12

N1000/2 wt % GO 2.20 ± 0.17 136 ± 17

N2000/1 wt % GO 0.77 ± 0.02 232 ± 8

N2000/2 wt % GO 2.33 ± 0.40 98 ± 16

For comparison to the composite elastomer, silanol terminated telechelic PDMS with a viscosity of 1,000 cSt (M _n = 26,000 g/mol, DMS-S31) and epoxypropoxypropyl terminated telechelic PDMS with a viscosity of 15 cSt (M _n = 550 g/mol, DMS-E11) were purchased from Gelest and used as received. GO was synthesized by the modified Hummer's method as described in Example 1.

Microtensile testing samples were prepared by reacting silanol terminated telechelic PDMS with 1 wt % GO and amine terminated telechelic N1000 with 1 wt % GO. While the viscosity of both materials were identical, only amine terminated telechelic PDMS was capable of forming a freestanding composite elastomer as shown in FIG. 8A. Silanol terminated telechelic PDMS samples were liquid sol after annealing as shown in FIG. 8B.

Example 6 - Single Gas Permeability of Composite Elastomers

Single gas, including H ₂ , 0 ₂ , N ₂ , CH ₄ and C0 ₂ , permeabilities of composite elastomers were measured using the constant volume, variable pressure method at 35 °C with ultra-high purity grade gases from Airgas. A 1000 psig pressure transducer (Honeywell Sensotec, Model STJE) was used to measure the upstream pressure in the system. A 10 Torr capacitance manometer (MKS, Baratron 626 A) was used to measure the downstream pressure, and the downstream pressure was kept below 10 Torr using a vacuum pump. All data were recorded using National Instruments Lab- VIEW software. Permeability, which is an intrinsic property of a specific material to a specific permeate, was expressed in barrer units, where 1 barrer equals 10 ^"10 cm ³ (STP) cm/cm ² s cmHg. The average thickness of the sample was measured by a dial gauge. Thickness for the amine terminated telechelic PDMS NlOOO/1 wt % GO composite elastomer was 0.354 ± 0.010 mm. FIG. 9A shows the single gas permeability of amine terminated telechelic PDMS NlOOO/1 wt % GO composite elastomer. For the various gases tested, the gas permeability for the amine terminated telechelic PDMS/GO composite elastomer increased in the order of N ₂ , 0 ₂ , H ₂ , CH ₄ , and C0 ₂ . Reacting 1 wt % GO (0.43 vol %) with amine terminated telechelic PDMS to form the composite elastomer resulted in an average reduction of - 45 % in N ₂ , 0 ₂ , CH ₄ , and C0 ₂ single gas permeabilities.

Single gas, including H ₂ , 0 ₂ , N ₂ , CH ₄ and C0 ₂ , permeabilities were measured in a neat PDMS elastomer for comparison. The average thickness of the neat PDMS elastomer was 0.998 ± 0.021 mm. For the various gases tested, the gas permeability for neat PDMS elastomer increased in the order of N ₂ , 0 ₂ , H ₂ , CH ₄ , and C0 ₂ as shown in FIG. 9B.

The gas selectivity values of neat PDMS elastomer and of the amine terminated telechelic PDMS/GO composite elastomer are shown in Table 4. The selectivity values are calculated based on 10 atm permeability values. The gas selectivity values for the amine terminated telechelic PDMS/GO composite elastomers having 1 wt % GO were more or less similar to that of the neat PDMS elastomer. Table 4

Material C0 ₂ /N ₂ C0 ₂ /CH ₄ 0 ₂ /N ₂

Neat PDMS 10 3 2

NlOOO/1 wt % GO 10 4 2

Single gas permeability results for elastomer composites having higher concentration of GOs are presented in FIG. 10A to FIG.10F. In general, the gas permeability declined by more than three orders of magnitude upon increasing the concentration of GOs incorporated into the composite elastomer. Strongly sorbing penetrants, such as C0 ₂ (FIG. 10E), had the tendency to plasticize the composite elastomer and thereby slightly increase the permeability with increasing pressure. Other low-sorbing small molecules (FIG. 10A to FIG. 10D) were less affected by the increase in gas pressure. Minimal changes in the C0 ₂ hysteresis loop shown in FIG. 10F illustrates that all of the composite elastomers tested behaved as rubbery materials.

The gas permeabilities for single gasses in neat PDMS and 1-2 wt % PDMS/GO composite elastomers increased in the order of N ₂ , 0 ₂ , H ₂ , CH ₄ , and C0 ₂ . However, as the concentration of GO in the composite elastomer was increased, the order changed to N ₂ , 0 ₂ , CH ₄ , H ₂ , and C0 ₂ for 3-5 wt % PDMS/GO composite elastomers and N ₂ , CH ₄ , 0 ₂ , H ₂ , and C0 ₂ for 8 wt % PDMS/GO composite elastomers. The more significant decrease in the CH ₄ gas permeability is due to the smaller gas channels formed as the concentration of GOs increased. The kinetic diameter of the CH ₄ gases is approximately 3.8 A, which indicates that the gas channels are small enough to size-sieve large penetrants. This is directly reflected to the dramatic decline in the relative permeability coefficient (R).

Telechelic PDMS/GO elastomers often showed a greater reduction in permeability even with a smaller amount of filler content when compared to other polymer/GO composites (Table 5). For instance, all of the gases tested showed a greater than 99.9% reduction in gas permeability by incorporating 3.55 vol % unmodified GOs. This may be due to efficient dispersion of GO throughout the composite elastomer, or to the high aspect ratio of GO. Table 5 presents the reported relative single gas permeability data for known polymer/GO composites and for a telechelic amine-terminated PDMS/GO composite elastomer according to the present disclosure. Relative permeability coefficients (R) are a ratio of the composite permeability to the neat polymer permeability.

Table 5

Filler Loading Operation

Polymer Methods Rii Ro ₂ R Rc¾, Rco ₂ RH Re

Type (vol %) conditions

35 °C

PEN TRG 2.4 Melt mix 0.45 - - - - - 1 atm

PC G 6.7 35 °C Melt mix - - 0.64 - - -

PC TRG 1.6 1 atm Melt mix - - 0.56 - - -

25 °C

PP GNP 3.0 Melt mix - 0.80 - - - - 1 atm

0.93*

PS fGO N/A In situ - 0.75 0.41 - - - (2 wt %)

35 °C

TPU fGO 1.6 In situ - - 0.40 - - - 1 atm

35 °C

TPU iGO 1.6 Solution - - 0.10 - - - 1 atm

PS iGO 2.27 23 °C Solution - 0.39 - - - -

25 °C

PVA GO 0.72 Solution - 0.01 - - - 0.32

1 atm

25 °C

PLA GO 1.37 Solution - 0.55 - - 0.32 - 1 atm

21.1* 25 °C

PI rGO Solution - 0.07 - - - - (30 wt %) 1 atm

5.52* 30 °C

Matrimid GO Solution - - 0.37 0.35 0.73 - (10 wt %) 1.97 atm 1.29 0.37 0.28 0.26 0.25 0.35

Telechelic 35 °C

GO 2.18 Solution 0.21 0.15 0.13 0.12 0.21 PDMS 10 atm

3.55 0.01 0.01 0.01 0..01 0.01

The following abbreviations are used in Table 5 - PP: polypropylene, PVA: poly(vinyl alcohol), PLA: poly(lactic acid), TPU: thermoplastic polyurethane, PEN: poly(ethylene-2,6-naphthalate), PDMS: poly(dimethylsiloxane), Matrimid:

5 thermoplastic polyimide copolymer, G: graphite, GNP: graphite nanoplatelets, GO: graphene oxide prepared by modified Hummer's method, TRG: thermally reduced graphene oxide, fGO: functionalized graphene oxides for in situ polymerization, iGO: isocyanate treated graphite oxide. Volume percent was calculated using the following densities: PS = 1.05 g/cm ³ , PI = 1.42 g/cm ³ , Matrimid = 1.2 g/cm ³ , fGO = rGO = 2.280 g/cm ³ .

Example 7 - Thermal Properties of Composite Elastomers

Thermal properties of composite elastomers were characterized by differential scanning calorimetry (DSC; Mettler Toledo DSC1). Heating and cooling rates of 10 °C min ^"1 were used for all experiments, and the second heating curve was used for the5 glass transition temperature (T _g ) analysis to erase thermal history.

FIG. 11 shows a differential scanning calorimetry thermogram comparison between neat telechelic PDMS elastomer and amine terminated telechelic PDMS/GO composite elastomer. The glass transition temperature (T _g ) and melting temperature (T _m ) of all of the materials are essentially identical. The decrease in cold

0 crystallization temperature (T _cc ) is due to GOs and chemical crosslinks acting as an effective nucleating agent in the system. FIG. 11 shows that the significant decrease in gas permeability upon the addition of GO is mostly due to the impermeable GO dispersed phase, which constrains gas molecules to migrate through irregular amorphous regions of the composite elastomer and not by the reduction in chain5 segment mobility in the interstitial amorphous phase.

Example 8 - Effects of Platelet-Like Filler Alignment in Composite Elastomers

The SEM images of FIG. 2A, FIG. 2B, and FIG. 2C show that GOs are wrinkled and not flat. This wrinkled structure may allow GOs to align rather than restack. Flat sheets with a high aspect ratio will have a tendency of restacking because of the high surface area driven by the attractive van der Waals force between sheets. Wide-angle X-ray diffraction (WAXD) was performed to confirm any presence of large agglomerates and only an amorphous halo peak was observed for all of the samples.

Furthermore, as their concentration increased, GOs tended to align themselves in a manner parallel to a bottom side of the composite elastomer. The comparison in FIG. 12 of alignments observed in FIG. 2A, FIG. 2B, and FIG. 2C to various possible models shows that alignment behavior most closely followed a modified Nielsen random model at low concentration of GO and a modified Cussler model at high concentration of GO.

Example 9 - Gas Separation by Composite Elastomers

Gas selectivity was also tested to confirm that the composite elastomers may be used in gas separation membranes. Gas selectivity was determined using methods similar to those used to determine single gas permeability. Combinations of gas permeabilities and ideal selectivities for telechelic PDMS/GO composite elastomers with varying GO concentration are compared to those of the neat PDMS elastomer in FIG. 13 A to FIG. 13D. For comparison to actual test data, the A/B ideal selectivity or a.A/B for gas A (more permeable) and B (less permeable) was calculated as the ratio of pure gas permeabilities of two different gases:

A/B Ideal Selectivity = a _A / _B ≡ P _A /P _B ■

The inherently different behaviors of composite elastomers with different GO concentrations become more obvious when comparing the gas selectivities of the materials. Significant increases in the ideal selectivities were observed for C0 ₂ /N ₂ , C0 ₂ /CH ₄ , and 0 ₂ /N ₂ . The increase in the ideal selectivity is likely due to the size and shape differences between two penetrants. Fillers having a high aspect ratio increase the length of the tortuous path for gas to diffuse through in the polymer matrix, and this restricts the diffusion of large molecules but favor small molecules with less resistance. The kinetic diameters of N ₂ and CH ₄ are considerably larger than that of C0 ₂ , and the kinetic diameter of N ₂ is larger than 0 ₂ . As shown in FIG. 13A to FIG. 13C, greater reductions in gas permeation with increasing GO content occurred for larger gas molecules (N ₂ and CH ₄ ) as compared to smaller gas molecules (C0 ₂ and 0 ₂ ), and as a result, the ideal selectivities were increased substantially. In contrast, as shown in FIG. 13D, the CO ₂ /H ₂ selectivity decreased as GO content increased because the kinetic diameter of the CO ₂ gas molecule is much greater than that of H ₂ , thus presenting the opposite situation as compared to FIG. 13A to FIG. 13C. These results were consistent across different pressures as well as shown in FIG. 14A to FIG. 14D, although pressure did exhibit some differential effects among different gas combinations as GO concentration increased.

The effects of kinetic diameter differences and the size-sieving effect were further observed by comparing the relative selectivities (ratio of composite selectivity to neat polymer selectivity) for different gasses. FIG. 15. compares the relative selectivity increases of various gas mixtures for selected concentrations of GOs. CO ₂ /O ₂ , CO ₂ /N ₂ and CO ₂ /CH ₄ have kinetic diameter differences of 0.16, 0.34, and 0.50 A, respectively. It is evident that the relative selectivity increased dramatically as the kinetic diameter difference increases, reflecting that the size-sieving effect plays an important role in gas separation for telechelic PDMS/GO elastomers.

Furthermore, the C0 ₂ /N ₂ selectivity increased by a factor of approximately 2 for 8 wt % GO (3.55 vol %), while the CO ₂ /CH ₄ selectivity increases by a factor of approximately 3. CO ₂ /N ₂ showed one of the most pronounced increments in selectivity as the GO concentration increased. This clearly demonstrates that composite elastomers formed according to this disclosure may be used in gas separation membranes, particularly in a C0 ₂ sweetening process system or a post- combustion carbon capture system.

Robeson plot 2008 in FIG. 16 depicts the CO ₂ /N ₂ gas selectivities of various polymers used to form membranes. (Polymers include liquid crystal polymer (LCP), polysulfone (PSf), thermally rearranged polymer (TR polymer), polyimide (PI).) The plot of data obtained using the telechelic amine-terminated PDMS/GO composite elastomer exhibits a behavior that would be normal for "tightening" the polymer by adding more impermeable filler, in this case, GO. As noted above, increasing the GO concentration constricts the bottlenecks that govern selectivity. Such movements nearly parallel to the upper bound are expected if the permeability in a polymer were being systematically decreased by copolymerization or crosslinking. The gas selectivity of the telechelic PDMS/GO elastomer is comparable to that of polysulfone (PSf) and polyimide (PI), as shown in FIG. 16. Furthermore, a 143% improvement in selectivity as compared to neat PDMS was observed.

Although the present disclosure and its advantages have been described in detail, it should be understood that various changes, substitutions, and alterations can be made without departing from the spirit and scope of the disclosure.