Perfluorodioxolane Polymers for Gas Separation Membrane Applications

membranes

Review Perﬂuorodioxolane Polymers for Gas Separation Membrane Applications

Yoshiyuki Okamoto 1,*, Hao-Chun Chiang 1 , Minfeng Fang 1, Michele Galizia 2, Tim Merkel 3, Milad Yavari 4, Hien Nguyen 4 and Haiqing Lin 4

1 Department of Chemical and Biomolecular Engineering, New York University, 6 MetroTech Center, Brooklyn, NY 11201, USA; [email protected] (H.-C.C.); [email protected] (M.F.) 2 School of Chemical, Biological and Materials Engineering, University of Oklahoma, Norman, OK 73019, USA; [email protected] 3 Membrane Technology and Research, Inc., 39630 Eureka Drive, Newark, CA 94560, USA; [email protected] 4 Department of Chemical and Biological Engineering, University of Buffalo, The State University of New York, Buffalo, NY 14260, USA; miladyav@buffalo.edu (M.Y.); hn26@buffalo.edu (H.N.); haiqingl@buffalo.edu (H.L.) * Correspondence: [email protected]; Tel.: +1-646-997-3638

Received: 24 September 2020; Accepted: 23 November 2020; Published: 4 December 2020

Abstract: Since the discovery of polytetrafluoroethylene (PTFE) in 1938, fluorinated polymers have drawn attention in the chemical and pharmaceutical field, as well as in optical and microelectronics applications. The reasons for this attention are their high thermal and oxidative stability, excellent chemical resistance, superior electrical insulating ability, and optical transmission properties. Despite their unprecedented combination of desirable attributes, PTFE and copolymers of tetrafluoroethylene (TFE) with hexafluoropropylene and perfluoropropylvinylether are crystalline and exhibit poor solubility in solvents, which makes their processability very challenging. Since the 1980s, several classes of solvent-soluble amorphous perfluorinated polymers showing even better optical and gas transport properties were developed and commercialized. Amorphous perfluoropolymers exhibit, however, moderate selectivity in gas and liquid separations. Recently, we have synthesized various new perfluorodioxolane polymers which are amorphous, soluble, chemically and thermally stable, while exhibiting much enhanced selectivity. In this article, we review state-of-the-art and recent progress in these perfluorodioxolane polymers for gas separation membrane applications.

Keywords: amorphous perﬂuorodioxolane polymers; gas separation; membrane

1. Introduction Fluorinated polymers have been used extensively due to their high thermal and oxidative stability, excellent chemical resistance, superior electrical insulating ability, and unique optical properties [1–3]. Polytetrafluoroethylene (PTFE) and its copolymers with hexafluoropropylene and perfluoropropylvinyl compounds are crystalline and have poor optical transparency and solubility. Since 1989, several classes of amorphous perfluorinated polymers with extraordinary optical and gas transport properties were developed and commercialized by Dupont (as TeflonTM AF), Solvay (as Hyflon® AD), and Asahi Glass (as CytopTM)[4–6]. Over the past 30 years, gas transport properties of these amorphous perfluoropolymers have also been investigated. These perfluoropolymers usually have high gas permeability, significant gas selectivities for some gas pairs (such as He/CO2 and He/CH4), and resistance to hydrocarbon-induced plasticization [7–17]. They also exhibit better resistance to thin-film physical aging than hydrocarbon

Membranes 2020, 10, 394; doi:10.3390/membranes10120394 www.mdpi.com/journal/membranes Membranes 2020, 10, 394 2 of 14 polymers [18,19]. Due to these attributes, they have been used in membrane separation applications for some gases.

VariousMembranes new perfluorodioxolane 2020, 10, x FOR PEER REVIEW polymers have been synthesized over the past2 of 17 two decades. These perfluoropolymers are not only amorphous and soluble in fluorinated solvents, but also form plasticization [7–17]. They also exhibit better resistance to thin-film physical aging than hydrocarbon thin and continuouspolymers films.[18,19]. Due Recently, to these weattributes, have they discovered have been thatused thesein membrane perfluorodioxolane separation applications films also show extraordinaryfor gas some transport gases. properties and low swelling behavior in the presence of vapors and liquids, which make themVarious appealing new perfluorodioxolane as gas separation polymers membrane have been materialssynthesized [over20– the29 ].past two decades. These perfluoropolymers are not only amorphous and soluble in fluorinated solvents, but also form thin and continuous films. Recently, we have discovered that these perfluorodioxolane films also 2. Synthesisshow and extraordinary Physical-Chemical gas transport Properties properties and of low Perfluoropolymers swelling behavior in the presence of vapors and liquids, which make them appealing as gas separation membrane materials [20–29]. Teflon AF and Hyflon AD are a family of copolymers of tetrafluoroethylene (TFE) with (I) 2,2- bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole2. Synthesis and Physical-Chemical Properties of Perfluoropolymers and (II) 2,2-bis(trifluoromethyl)-4-fluoro-5- trifluoromethoxy-1,3-dioxole,Teflon AF and Hyflon respectively AD are a family (Figure of copolymers1). The homopolymers of tetrafluoroethylene of (TFE) (I) and with (II) (I) 2,2- are di fficult to bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole and (II) 2,2-bis(trifluoromethyl)-4-fluoro-5- process as evidenced by their high glass transition temperature (Tg). As a result, copolymerization of trifluoromethoxy-1,3-dioxole, respectively (Figure 1). The homopolymers of (I) and (II) are difficult these monomers with other olefins is used to improve the processability. In the case of copolymers of to process as evidenced by their high glass transition temperature (Tg). As a result, copolymerization TFE with (I) andof these (II), monomers Tg decreases with other drastically olefins is used as to theimprove molar the processability. percentage In of the TFE case increases.of copolymers For example, of TFE with (I) and (II), Tg decreases drastically as the molar percentage of TFE increases. For example, Tgs of Teflon AF 1600 containing 65 mol% dioxole (I) and Teflon AF 2400 containing 87 mol% dioxole Tgs of Teflon AF 1600 containing 65 mol% dioxole (I) and Teflon AF 2400 containing 87 mol% dioxole become 160 C and 240 C, respectively. The T s of Hyflon AD 60 containing 60 mol% dioxole (II) ◦become 160 °C and◦ 240 °C, respectively. The Tgsg of Hyflon AD 60 containing 60 mol% dioxole (II) and and Hyflon ADHyflon 80 AD containing 80 containing 85 85 mol% mol% dioxoledioxole are are 100 100 °C ◦andC and135 °C, 135 respectively,◦C, respectively, while the T whileg of the Tg of homopolymer (II) reaches 170 °C [8]. homopolymer (II) reaches 170 ◦C[8].

Figure 1. ChemicalFigure 1. structuresChemical structures of monomers of monomers (I) (I) and and (II) and and their their copolymers copolymers with tetrafluoroethylene with tetrafluoroethylene (TFE): Teflon(TFE): AF and Teflon Hyflon AF and AD.Hyflon AD.

In addition, Cytop is a homopolymer obtained by cyclopolymerization of a perfluorodiene, perfluoro-(4-vinyloxy-1-butene) specifically. Cytop is formed by penta- and hexa-cyclic structures, and the five-membered ring structure is predominant [8] (Figure2). Table1 summarizes the physical and optical properties of these commercially available perfluoropolymers. These perfluoropolymers are completely amorphous, contain no hydrogen atoms, show excellent chemical and thermal stability, and are soluble in fluorinated solvents, such as hexafluorobenzene and perfluorohexane. The dielectric constants of these fluoropolymers are low, and they are almost unaffected by humidity. Membranes 2020, 10, x FOR PEER REVIEW 3 of 17

Membranes 2020, 10, x FOR PEER REVIEW 3 of 17 In addition, Cytop is a homopolymer obtained by cyclopolymerization of a perfluorodiene, perfluoro-(4-vinyloxy-1-butene) specifically. Cytop is formed by penta- and hexa-cyclic structures, Inand addition, the five-membered Cytop is aring homopolymer structure is predominant obtained [8]by (Figurecyclopolymerization 2). of a perfluorodiene, perfluoro-(4-vinyloxy-1-butene) specifically. Cytop is formed by penta- and hexa-cyclic structures, Membranes 2020, 10, 394 3 of 14 and the five-membered ring structure is predominant [8] (Figure 2).

Figure 2. Chemical structure of the cyclic perfluoropolymer, CytopTM.

Table 1 summarizes the physical and optical properties of these commercially available TM perfluoropolymers.FigureFigure 2. Thes 2.Chemical Chemicale perfluoropolymers structure structure of thethe are cyclic cyclic completely perfluoropolymer, perfluoropolymer, amorphous, Cytop contain CytopTM. no. hydrogen atoms, show excellent chemical and thermal stability, and are soluble in fluorinated solvents, such as Table 1. Physical Properties of Teflon AF, Hyflon AD, and Cytop. Tablehexafluorobenzene 1 summarizes and theperfluorohexane. physical and The optical dielectr icproperties constants ofof these these fluoropolymers commercially are availablelow, perfluoropolymers.and they are almost Thes unaffectede perfluoropolymers by humidity.Teflon are AFcompletely Hyflon amorphous, AD contain no hydrogen atoms, show excellent Propertieschemical and thermal stability, and are soluble in fluorinatedCytop solvents, such as Table 1. Physical Properties1600 of Teflon 2400 AF, Hyflon 60 AD, and Cytop. 80 hexafluorobenzene and perfluorohexane. The dielectric constants of these fluoropolymers are low, T ( C) Teflon160 AF 240 100 Hyflon AD 135 108 and they are almostProperties unaffected g ◦ by humidity. Cytop Refractive index1600 1.312400 1.29 60 1.33 1.32 80 1.34 3 Tg (°C)DensityTable 1. (g Physical/cm 160) Properties 1.78 of 240 Teflon 1.67 AF, Hyflon 100 1.93 AD, and 1.80 135 Cytop. 1.84 108 RefractiveDielectric index constant 1.31 (ε) 1.93 1.29 1.90 1.33 1.87 2.00 1.32 2.1–2.2 1.34 Density (g/cm3) 1.78Teflon AF 1.67 1.93 Hyflon AD 1.80 1.84 Properties Cytop Dielectric constant (ε) 16001.93 2400 1.90 1.87 60 2.00 80 2.1–2.2 The commercial amorphous perfluoropolymers exhibit excellent properties in many aspects. Tg (°C) 160 240 100 135 108 The commercial amorphous perfluoropolymers exhibit excellent properties in many aspects. However,Refractive the preparation index methods 1.31 of these fluorinated 1.29 polymers 1.33 are usually 1.32 complex 1.34 and expensive, However, the preparation methods of these fluorinated polymers are usually complex and expensive, and the TDensityg of Cytop (g/cm is3) relatively 1.78 low. In order 1.67 to overcome 1.93 these limitations, 1.80 we have 1.84synthesized and the Tg of Cytop is relatively low. In order to overcome these limitations, we have synthesized variousDielectric perfluoro-2-methylene-1,3-dioxolanes,various constantperfluoro-2-methylen (ε) 1.93e-1,3-dioxolanes, 1.90 the the structuresstructures 1.87and and corresponding corresponding 2.00 T g of their Tg polymersof 2.1–2.2 their polymers are shownare in shown Figure in3 Figure[22]. 3 [22]. The commercial amorphous perfluoropolymers exhibit excellent properties in many aspects. However, the preparation methods of these fluorinated polymers are usually complex and expensive, and the Tg of Cytop is relatively low. In order to overcome these limitations, we have synthesized various perfluoro-2-methylene-1,3-dioxolanes, the structures and corresponding Tg of their polymers are shown in Figure 3 [22].

Figure 3.FigureChemical 3. Chemical structures structures of perﬂuorodioxolaneof perfluorodioxolane monomers monomers and T andg of their Tg of polymer. their polymer.

These perfluorodioxolaneThese perfluorodioxolane monomers monomers were were preparedprepared by by direct direct fluorination fluorination of the corresponding of the corresponding hydrocarbonhydrocarbon monomers. monomers. Figure Figure4 illustrates 4 illustrates aa typicaltypical synthetic synthetic scheme scheme for a forperfluorodixolane a perfluorodixolane monomer and its polymer. The monomer is polymerized using a free radical initiator, such as a monomerperfluorobenzyl and its polymer. peroxide. The The monomer peroxide was is polymerizeddecomposed at 60–8 using0 °C a by free a homolysis radical mechanism initiator, such as a perfluorobenzylresulting peroxide.in the formation The of peroxide pentafluorophenyl was decomposed radicals, which at initiate 60–80 polymerization◦C by a homolysis and end up mechanism resulting inas thestructuralFigure formation 3. Chemicalunits ofat pentafluorophenyl structuresthe polymer of perfluorodioxolane chain radicals,end. This whichmonomerspolyperfluoro(2-methylene-4-methyl-1,3- initiate and Tg polymerization of their polymer. and end up as structuraldioxolane), units at the poly(PFMMD), polymer chain was end.also synthesized This polyperfluoro(2-methylene-4-methyl-1,3-dioxolane), using a perfluorinated precursor (perfluoro- propyleneoxide) as shown in Figure 5 [23]. poly(PFMMD),These perfluorodioxolane was also synthesized monomers using were a perfluorinated prepared by direct precursor fluorination (perfluoro-propyleneoxide) of the corresponding as hydrocarbon monomers. Figure 4 illustrates a typical synthetic scheme for a perfluorodixolane shown in FigureMembranes5[23 2020]. , 10, x FOR PEER REVIEW 4 of 17 monomer and its polymer. The monomer is polymerized using a free radical initiator, such as a perfluorobenzyl peroxide. The peroxide was decomposed at 60–80 °C by a homolysis mechanism resulting in the formation of pentafluorophenyl radicals, which initiate polymerization and end up as structural units at the polymer chain end. This polyperfluoro(2-methylene-4-methyl-1,3- dioxolane), poly(PFMMD), was also synthesized using a perfluorinated precursor (perfluoro- propyleneoxide) as shown in Figure 5 [23].

Figure 4. A typicalFigure 4. syntheticA typical synthetic route route for afor perﬂuorodioxolane a perfluorodioxolane monomer monomer and the and polymer. the polymer.The case of The case of poly(PFMMD)poly(PFMMD) is shown. is shown.

Figure 5. Synthesis of perfluoro-2-methylene 4-methyl 1,3-dioxolane and poly(PFMMD).

These perfluorodioxolane monomers can be copolymerized with other existed fluorovinyl monomers, as well as with each other (A-H) to form amorphous copolymers that are soluble in fluorinated solvents. The composition determines the Tg of these copolymers. For example, when monomers D and H in Figure 3 are copolymerized, as the relative amount of monomer D increases from 20 to 75 mol%, the Tg varies from 105 to 155 °C accordingly. The perfluorodioxolane monomers can also be copolymerized with other fluorovinyl monomers such as chlorotrifluoroethylene (CTFE). Figure 6 shows typical examples of the copolymers [24,27].

Membranes 2020, 10, x FOR PEER REVIEW 4 of 17

Membranes 2020, 10,Figure 394 4. A typical synthetic route for a perfluorodioxolane monomer and the polymer. The case of 4 of 14 poly(PFMMD) is shown.

Figure 5. SynthesisFigure 5. Synthesis of perﬂuoro-2-methylene of perfluoro-2-methylene 4-me 4-methylthyl 1,3-dioxolane 1,3-dioxolane and poly(PFMMD). and poly(PFMMD).

These perfluorodioxolaneThese perfluorodioxolane monomers monomers can can bebe copolymerized copolymerized with other with existed other fluorovinyl existed fluorovinyl monomers,monomers, as well asas well with as eachwith each other other (A-H) (A-H) to to form form amorphous amorphous copolymers copolymers that are soluble that in are soluble fluorinated solvents. The composition determines the Tg of these copolymers. For example, when in fluorinatedmonomers solvents. D and TheH in Figure composition 3 are copolymerized, determines as the therelative Tg amountof these of monomer copolymers. D increases For example, when monomersfrom 20 D to 75 and mol%, H the in T Figureg varies 3from are 105 copolymerized, to 155 °C accordingly. as The the perfluorodioxolane relative amount monomers of monomer D can also be copolymerized with other fluorovinyl monomers such as chlorotrifluoroethylene (CTFE). increases from 20 to 75 mol%, the Tg varies from 105 to 155 C accordingly. The perfluorodioxolane Figure 6 shows typical examples of the copolymers [24,27]. ◦ monomers can also be copolymerized with other fluorovinyl monomers such as chlorotrifluoroethylene

(CTFE). Figure6 showsMembranes 2020 typical, 10, x FOR examples PEER REVIEW of the copolymers [24,27]. 5 of 17

Figure 6. Chemical structures of the monomers for PFMDD, PFMMD, and CTFE, and the copolymers Figure 6. Chemical structures of the monomers for PFMDD, PFMMD, and CTFE, and the copolymers of poly(PFMDD-coof -PFMD),poly(PFMDD- poly(PFMDD-co-PFMD), poly(PFMDD-co-CTFE),co-CTFE), and and poly(PFMMD-co-CTFE)co-CTFE) [24,27]. [24,27].

These copolymersThese are copolymers also amorphous are also amorphous and soluble and soluble in only in only ﬂuorinated fluorinated solvents, solvents, and andtheir their physical physical properties are summarized in Table 2. When PFMMD was copolymerized with properties are summarizedpentafluorostyrene in (PFSt), Table the2. copolymers When PFMMD obtained were was soluble copolymerized in common organic with solvents pentaﬂuorostyrene such (PFSt), the copolymersas acetone and obtained THF (Figure were 7). soluble in common organic solvents such as acetone and THF (Figure7).

Table 2. Physical Properties of Various Perﬂuorodioxolane Copolymers.

Copolymers Samples Tg (◦C) Name Composition (mol:mol) 1 74:26 155 2PFMDD-co-PFMD PFMDD:PFMD = 58:42 145 3 43:57 125 4 80:20 152 PFMDD-co-CTFE PFMDD:CTFE = 5 65:35 132 6 63:37 113 7PFMMD-co-CTFE PFMMD:CTFE = 51:49 99 8 43:57 89 9 0:100 110 10PFMMD-co-PFSt PFMMD:PFSt = 18:82 94 11 35:65 90

Figure 7. Chemical structures of polyPFSt and poly(PFMMD-co-PFSt).

Membranes 2020, 10, x FOR PEER REVIEW 5 of 17

Figure 6. Chemical structures of the monomers for PFMDD, PFMMD, and CTFE, and the copolymers of poly(PFMDD-co-PFMD), poly(PFMDD-co-CTFE), and poly(PFMMD-co-CTFE) [24,27].

These copolymers are also amorphous and soluble in only fluorinated solvents, and their physical properties are summarized in Table 2. When PFMMD was copolymerized with Membranes 2020,pentafluorostyrene10, 394 (PFSt), the copolymers obtained were soluble in common organic solvents such 5 of 14 as acetone and THF (Figure 7).

Figure 7.FigureChemical 7. Chemical structures structures of of polyPFStpolyPFSt and and poly(PFMMD- poly(PFMMD-co-PFSt). co-PFSt).

3. Gas Transport Properties The performance of non-porous polymers is evaluated in terms of permeability and selectivity.

Permeability (P) is defined in Equation (1) with the pressure difference (∆p), thickness (l), and normalized flux (n)[28,30,31]: nl P = (1) ∆P 10 3 2 Permeability has units of Barrer, where 1 Barrer = 10− cm (STP) cm/(cm s cmHg). For asymmetric membranes with an unknown selective layer thickness, the gas permeation property is characterized using 6 3 2 the term of permeance (P/l), which is expressed in units of GPU, where 1 GPU = 10− cm (STP)/(cm s cmHg). In the solution-diffusion model, permeability can be expressed as the product of the concentration- 2 3 3 1 averaged diffusion coefficient (D, cm /s) and the sorption coefficient (S, cm (STP) cm− atm− ), which represent the kinetic and thermodynamic components of the transport process, respectively [30]:

P = D S (2) ×

The selectivity, αA/B, is defined as the ratio of the permeabilities of two permeating species, A and B: PA DA SA αA/B = = (3) PB DB × SB where DA/DB is the diffusivity selectivity, and SA/SB is the solubility selectivity. Modern polymeric membrane materials are designed and engineered by tailoring the sorption and diffusion behavior for selectivity improvement [31,32].

4. Gas Separation Properties of Perfluorodioxolane Polymers Two perfluorodioxolane polymers, poly(PFMD) and poly(PFMMD) (as shown in Figure6), were fabricated into 10–50 µm thin films by solvent casting or melt pressing. The films were completely dried by heating at 10 ◦C below the Tg of the polymers in air before the gas permeation tests. Figure8 shows the gas permeability of CH 4,N2, Ar, CO2,H2, and He in these two perfluorodioxolane films as a function of feed pressure at 35 ◦C. Gas permeability is independent of the feed pressure, suggesting that the films were defect-free [28]. Table3 summarizes the gas separation properties of the perfluoropolymers. Poly(PFMD) and Cytop have similar fractional free volume (FFV) and Tg. However, poly(PFMD) is less permeable (except for He) and much more selective than Cytop. Similarly, poly(PFMMD) has lower N2 and CO2 permeability than Hyflon AD 80 and much higher selectivity, while their FFV and Tg are similar. Membranes 2020 10 Membranes, 2020, 394, 10, x FOR PEER REVIEW 7 of 176 of 14

Gas solubility of these perﬂuorodioxolane polymers is generally low and depends on the applied Membranespressure, 2020 as, shown10, x FOR in PEER Figure REVIEW4[28]. 7 of 17

Figure 8. Effect of feed pressure on the pure-gas permeability of the freestanding films at 35 °C for (a) Poly(PFMD) and (b) Poly(PFMMD).

Table 3 summarizes the gas separation properties of the perfluoropolymers. Poly(PFMD) and Cytop have similar fractional free volume (FFV) and Tg. However, poly(PFMD) is less permeable

(except for He) and much more selective than Cytop. Similarly, poly(PFMMD) has lower N2 and CO2 Figure 8. Effect of feed pressure on the pure-gas permeability of the freestanding films at 35 ◦C for Figurepermeability 8. Effect thanof feed Hyflon pressure AD on80 andthe pure-gasmuch higher permea selectivity,bility of whilethe freestanding their FFV and films Tg atare 35 similar. °C for ( Gasa) Poly(PFMD)(asolubility) Poly(PFMD) ofand these ( andb) Poly(PFMMD). (perfluorodioxolaneb) Poly(PFMMD). polymers is generally low and depends on the applied pressure, as shown in Figure 9 [28]. Table 3. Comparison of Physical Properties and Gas Separation Properties (35 ◦C) of Poly(PFMD), Table 3 summarizes the gas separation properties of the perfluoropolymers. Poly(PFMD) and Poly(PFMMD), and Commercial Glassy Perfluoropolymers [9,28]. Cytop haveTable similar 3. Comparison fractional of freePhysical volume Properties (FFV) and and Gas TSegparation. However, Properties poly(PFMD) (35 °C) of Poly(PFMD),is less permeable Poly(PFMMD), and Commercial Glassy Perfluoropolymers [9,28]. (except for He) and much more selective thanPermeability Cytop. Similarly, (Barrer) poly(PFMMD) Selectivity has lower N2 and CO2 Polymers Tg (◦C) FFV permeability than Hyflon ADTg 80 and much higherPermeability selectivity, (Barrer) while their FFV Selectivity and Tg are similar. Gas Polymers FFV He H2 N2 CO2 He/CH4 H2/CO2 CO2/CH4 (°C) He H2 N2 CO2 He/CH4 H2/CO2 CO2/CH4 solubilityPoly(PFMD) of these perfluorodioxolane 111 0.21 210polymers 50 is 0.71generally 5.9 low and 1650 depends 8.4 on the 46applied Poly(PFMD) 111 0.21 210 50 0.71 5.9 1650 8.4 46 pressure,Poly(PFMMD) as shown in Figure 135 9 [28]. 0.23 560 240 7.7 58 280 4.1 29 Poly(PFMMD) 135 0.23 560 240 7.7 58 280 4.1 29 TeflonTeflon AF AF 1600 1600 162 162 0.31 0.31 - - 550 550 110 110 520 520 - - 1.1 1.1 6.5 6.5 TableHyflon Hyflon3. Comparison AD AD 80 80 of 134 Physical 134 0.23 0.23Properties 430 430 and 210 210 Gas Se 24paration 150 150 Properties 36 36 (35 °C) 1.4 1.4of Poly(PFMD), 13 13 Poly(PFMMD),CytopCytop and Commercial 108108 0.21 Glassy0.21 170Perfluoropolymers170 5959 5.05.0 [9,28].35 35 84 841.7 1.718 18

Tg Permeability (Barrer) Selectivity Polymers FFV (°C) He H2 N2 CO2 He/CH4 H2/CO2 CO2/CH4 Poly(PFMD) 111 0.21 210 50 0.71 5.9 1650 8.4 46 Poly(PFMMD) 135 0.23 560 240 7.7 58 280 4.1 29 Teflon AF 1600 162 0.31 - 550 110 520 - 1.1 6.5 Hyflon AD 80 134 0.23 430 210 24 150 36 1.4 13 Cytop 108 0.21 170 59 5.0 35 84 1.7 18

FigureFigure 9. Pure-gas 9. Pure-gas sorption sorption isotherms isotherm fors variousfor various gases gases in (ina) ( poly(PFMD)a) poly(PFMD) and and (b )(b poly(PFMMD)) poly(PFMMD) ﬁlms at 35 ◦filmsC[28 at]. 35 °C [28].

Poly(PFMMD) was directly compared with its analog hydrocarbon polymer, poly(4-methyl- 2-methylene-1,3-dioxolane) (poly(MMD)) [16]. Poly(PFMMD) with an FFV of 0.23 exhibits much higher gas permeability than poly(MMD) with an FFV of 0.095. For example, the CO2 permeabilities of poly(PFMMD) and poly(MMD) at 35 ◦C are 58 Barrer and 1.3 Barrer, respectively. The bulky –CF3 and

Figure 9. Pure-gas sorption isotherms for various gases in (a) poly(PFMD) and (b) poly(PFMMD) films at 35 °C [28].

Membranes 2020, 10, 394 7 of 14

–CF2 groups hinder perfluoropolymers chain packing and result in much higher FFV, and therefore increased permeability. Table4 compares the solubility and di ffusivity of CO2 and CH4 in the perfluorodioxolane polymers, Hyflon AD 80, and Cytop [28]. Gas diffusivity can be calculated using Equation (2), and it is usually influenced by the polymer FFV and chain rigidity. Poly(PFMD) exhibits CO2 diffusivity one order of magnitude lower and CO2/CH4 diffusivity ratio 80% higher than poly(PFMMD) because poly(PFMMD) has a bulky CF3 substituent on the dioxolane ring and thus higher Tg and FFV.

Table 4. Comparison of Solubility Parameter (δP), Gas Solubility, and Diﬀusivity in Poly(PFMD), Poly(PFMMD), Cytop, and Hyﬂon AD80 at 10 atm and 35 ◦C[28].

3 3 S 7 2 D 0.5 SA (cm (STP)/(cm atm)) CO2 DA (10− cm /s) CO2 Polymers δP (MPa ) D SCH4 CH4 CO2 CH4 CO2 CH4 Poly(PFMD) 10.3 1.3 0.39 3.3 0.36 0.026 14 Cytop 1.3 0.30 4.3 2.0 0.48 4.2 Poly(PFMMD) 10.3 1.4 0.48 2.9 3.2 0.15 9.9 Hyﬂon AD80 10.8 1.5 0.75 2.0 8.2 1.1 7.7

Interestingly, poly(PFMD) shows lower gas diffusivity than its counterpart commercial perfluoropolymer (Cytop) despite their similar Tg and FFV, and poly(PFMMD) exhibits much lower diffusivity than Hyflon AD 80. On the other hand, the perfluorodioxolane polymers show higher diffusivity selectivity (stronger size-sieving ability) than the commercial perfluoropolymers with similar free volume. These results can be ascribed to the narrower pore connections and larger microcavities in perfluorodioxolane polymers compared to their commercial counterparts, as evidenced by the d-spacing values from the WAXD measurements [28]. Poly(PFMD) and poly(PFMMD) also exhibit a solubility selectivity up to 65% larger relative to commercial perfluoropolymers, especially for separations involving CO2. The molecular origin of this behavior, which has been investigated using the lattice fluid theory [29], lies in the more favorable CO2 interaction with poly(PFMD) and poly(PFMMD) than commercial perfluoropolymers. This favorable interaction stems, in turn, from the higher oxygen/carbon ratio exhibited by poly(PFMD) and poly(PFMMD) relative to Teflon AF. Specifically, the oxygen/carbon ratio, which is 2:4 for poly(PFMD), 2:5 for poly(PFMMD), and 2:5.3 for Teflon AF 2400. Polar C–O–C bonds are known to interact favorably with CO2 molecules, which is consistent with the solubility selectivity increasing in the order: poly(PFMD) > poly(PFMMD) > Teflon AF 2400. Based on lattice fluid modeling, the CO2-polymer binary interaction becomes more favorable in the order: poly(PFMD) > poly(PFMMD) > Teflon AF 2400. These results mirror the orders of the oxygen/carbon ratio values, as well as of the Hildebrand solubility parameters. Specifically, the Hildebrand solubility parameter exhibited by 0.5 poly(PFMD) is larger relative to poly(PFMMD), that is, it is closer to the CO2 value (i.e., 21.8 MPa ). Equally important, the polymer cohesive energy density, which is represented by the characteristic pressure, p*, in the lattice fluid framework, is the key parameter to tailor solubility selectivity, all other factors (i.e., interactional pattern, free volume) being equal. Typically, solubility selectivity increases with increasing polymer cohesive energy density [29]. This conclusion is consistent with the lattice fluid characteristic pressure, p*, increasing in the order: poly(PFMD) > poly(PFMMD) > Teflon AF.

5. Gas Separation Properties of Copolymers of Perfluorodioxolane The gas permeability of these perfluorodioxolane polymers were investigated and summarized in Table5. PFMDD (Figure6) has two trifluoromethyl groups on the 4 0 and 50 position with trans and cis isomers. In this case, the ratio of trans to cis isomer is 73 and 27 mol%, and the polymer obtained was completely amorphous and soluble in fluorinated solvent with a Tg of 165 ◦C. PFMD was also polymerized by a free radical initiator and the obtained polymer is semicrystalline. The Tg and melting point are 106 and 228 ◦C, respectively. The copolymers of PFDMM and PFMMD with PFMD were Membranes 2020, 10, 394 8 of 14 obtained (Figure6). The monomer reactivity ratios are almost equal and thus the resulting copolymers form an ideal random structure.

Table 5. Gas Separation Properties of Thin-Film Composite Membranes Based on PFMMD-co-PFMD

and PFMDD-co-PFMD at 22 ◦C[24,27].

Copolymers Permeance (GPU) Gas/CH4 Selectivity Tg (◦C) Name PFMD (mol%) CH4 He N2 H2 He CO2 0 135 20 2160 3.4 57 108 26 25 132 18.7 2270 4.3 80 122 34 PFMMD-co-PFMD 40 126 7 2320 5.8 157 332 49 60 123 7.34 2970 6.0 162 405 55 26 155 17 1400 32 48 82 23 PFMDD-co-PFMD 42 145 11.5 1250 42 72 130 34 80 125 8.3 1400 53 130 200 47

Table5 shows that increasing the PFMD content in the copolymers decreases the gas permeances and increases the gas/CH4 selectivity due to the more efficient polymer chain packing for the less hindered PFMD, which results in stronger sieving ability [24]. Table6 summarizes the gas permeation properties of perfluorodioxolane copolymers with CTFE [25,27]. The Tgs of the copolymers decreased as the amount of CTFE was increased. The molecular weights of the copolymers are over 200,000, and they are still amorphous and soluble in fluorinated solvents (up to 50 mol% CTFE). The flexibility and fracture due to elongation of the films increases with increasing CTFE content in the copolymers. Increasing the CTFE content in the copolymers also increases the gas selectivity, especially for some gas pairs, such as He/CH4 and H2/CH4. The introduction of small polar monomers, such as CTFE, increases the efficiency of chain packing, leading to enhanced size sieving ability. Additionally, polychlorotrifluoroethylene (PCTFE) exhibits higher solubility parameter and, thus, higher CO2/gas solubility selectivity than PTFE [33], as well as better separation performance for He/H2 and He/CH4.

Table 6. Physical Properties of PFMMD-co-CTFE and PFMMD-co-CTFE and Gas Transport Properties

of Their Thin Film Composite Membranes at 22 ◦C[24,27].

Copolymers Permeance (GPU) Gas/CH4 Selectivity Tg (◦C) Name CTFE (mol%) N2 H2 He CO2 N2 H2 He CO2 20 152 42 580 1010 320 2.5 34 60 19 PFMDD-co-CTFE 35 132 9.8 450 610 345 6.1 210 480 48 60 120 8.1 340 770 78 3.1 82 201 28 37 113 20 633 1360 181 4.5 144 309 41 PFMMD-co-CTFE 49 99 12 457 1120 108 5 194 473 46 57 89 5 254 804 44.3 5.5 284 900 49

Gas transport properties of various polystyrene polymers have also been investigated. The permeability of gases such as CO2, CH4, and N2 is much higher in pentafluorostyrene (PFSt) than in other substituted polystyrenes [34]. Pentafluorostyrene is soluble in common organic solvents such as acetone and THF. The Tg is 110 ◦C and it is thermally stable. The copolymers of PFMMD and PFSt were prepared (Figure7). When PFSt content is more than 20 mol% in the copolymer, the polymer was soluble in acetone, and the polymer films obtained were clear and transparent. The gas permeability and selectivity of the polymer are measured. The typical physical data for the copolymer PFMMD and PFSt are shown in Table7[35]. Membranes 2020, 10, x FOR PEER REVIEW 10 of 17 Membranes 2020, 10, 394 9 of 14 permeability and selectivity of the polymer are measured. The typical physical data for the copolymer PFMMD and PFSt are shown in Table 7 [35]. Table 7. Physical data of PFMMD and PFSt. Table 7. Physical data of PFMMD and PFSt.

PFSt mol % Tg (◦C) Solubility in Acetone PFSt mol % Tg (°C) Solubility in Acetone 100100 110 110 Soluble Soluble 80 8094 94Soluble Soluble 50 90 Soluble 50 90 Soluble 20 110 Soluble 20 110 Soluble 0 135 Insoluble 0 135 Insoluble

Figure 10Figure shows 10 shows the gas the permeability gas permeability of theof the PFMMD- PFMMD-coco-PFSt-PFSt ﬁlmfilm as as function function of of feed feed pressure pressure and Table8 summarizesand Table 8 summarizes the typical the gas typical permeability gas permeability of the of ﬁlm. the PFMMD-film. PFMMD-co-PFStco-PFSt exhibits exhibits muchmuch higher gas selectivityhigher gas as selectivity compared as withcompared the neatwith the PFSt neat polymer, PFSt polymer, but it but shows it shows lower lower gas gas selectivity selectivity than than other other PFMMD copolymers. PFMMD copolymers.

Figure 10. Eﬀect of feed pressure on the pure gas permeability of the ﬁlm (50 mol% PFSt). Figure 10. Effect of feed pressure on the pure gas permeability of the film (50 mol% PFSt).

Table 8. Gas transport properties of PFMMD-co-PFSt (50mol%) at 50 psig and 35 ◦C[34,36]. Table 8. Gas transport properties of PFMMD-co-PFSt (50mol%) at 50 psig and 35 °C [34,36].

Permeance Permeance (GFU)(GFU) Gas Gas Pair Pair Selectivity Selectivity (Gas/CH (Gas/4CH) 4)

NN2 2 HH22 HeHe CO CO2 2 CH44 NN2 2 H2 H2He HeCO2 CO2 PFMMD-PFMMD-co-PFStco-PFSt (50 mol%)(50 mol%) 2.3 2.3 48 48 79 79 29 29 1.4 1.4 2.0 2.0 35 35 57 57 21 21 PFStPFSt 5.95.9 6565 88 88 50 50 5.1 5.1 1.1 1.1 12 1216 1610 10

6. Mechanical Properties of Polyperfluorodioxolane Polymers 6. Mechanical Properties of Polyperfluorodioxolane Polymers Polyperfluorodioxolane polymers show superior gas separation properties. However, the Polyperfluorodioxolanepolymer films show some polymers brittleness show and superiorcan crack gasupon separation bending. Polymer properties. mechanical However, properties the polymer films showcan be some improved brittleness by the and addition can crackof plasticizers upon bending.. Perfluoropolyether Polymer (PFPE) mechanical is chemically properties and can be improvedthermally by the stable addition and used of plasticizers.as a lubricant in Perfluoropolyether the aerospace and automotive (PFPE) industries. is chemically Poly(PFMMD) and thermally is relatively compatible with PFPE, and the blended films of poly(PFMMD) and PFPE are clear and stable and used as a lubricant in the aerospace and automotive industries. Poly(PFMMD) is relatively transparent. Table 9 records the mechanical and gas transport properties of the blends of compatiblepoly(PFMMD) with PFPE, and andPFPE, the including blended tensile films strength of poly(PFMMD) (σ, MPa) and Young’s and PFPE modulus are clear (E, MPa) and [37]. transparent. Table9 records the mechanical and gas transport properties of the blends of poly(PFMMD) and PFPE, including tensile strength (σ, MPa) and Young’s modulus (E, MPa) [37].

Table 9. Physical and Gas Transport Properties of the Blends of PFMMD and PFPE [34].

Permeance (GPU) Selectivity PFPE Content (wt%) Tg (◦C) σ (MPa) E (MPa) CH4 N2 CO2 N2/CH4 H2/O2 H2/CH4 CO2/CH4 0 135 12.1 8.5 20 68 520 5.7 5.0 57 26 5 131 18 6.1 26 86 621 3.3 3.9 46 24 10 125 17.4 4.2 29 91 657 3.2 3.9 42 23 ® PFPE (CF2-CF2O)n Fomblin Z60 molecular weight 13,000, m.p. = 75 ◦C and and b.p. > 270 ◦C. The Mw of PFMMD used was 1.0 106 g/mol. − × Membranes 2020, 10, 394 10 of 14

As the PFPE content increases from 0 to 5 wt% and 10 wt%, the breaking elongation of the blends increases from 1.3% to 4.1% and 7.36%, respectively. Young’s modulus decreases with increasing PFPE content, indicating reduced brittleness and increased flexibility. The gas permeance, especially N2 and O2, increases with increasing PFPE content, while gas selectivity decreases (Table9). This finding is consistent with general transport behavior in plasticized glassy polymers. The plasticizer increases polymer chain motions and thus gas diffusion coefficients, particularly for large molecules. As a result, the size-sieving ability of the plasticized polymeric membrane is reduced, resulting in a decrease in selectivity. Another way to improve the film flexibility is to copolymerize with a smaller monomer to reduce the steric crowding along the polymer chain while ensuring the amount of the smaller monomer is suitable to retain the amorphous nature. The best co-monomer choice would be TFE. However, TFE is not easy to handle in a common chemical laboratory. On the other hand, chlorotrifluoroethylene (CTFE) can be an attractive alternative because it is commercially available and safer to handle in academic laboratories than TFE. CTFE is the most widely used fluoroalkene after TFE and vinylidene fluoride (VDF) and is readily copolymerized with various vinyl monomers to yield novel copolymers [33]. We have prepared copolymers of PFMMD and CTFE (cf. Figure6). Table 10 summarizes the physical properties of PFMMD and its copolymers with CTFE. The copolymers are thermally stable near 300 ◦C under an air atmosphere. Increasing the CTFE content in the copolymers decreases the Tg and Young’s modulus, and increases both tensile strength and elongation at break of the copolymer. Figure 11 also depicts the stress–strain curves for these copolymers. These results validate that the brittleness of poly(PFMMD) films can be reduced, and film flexibility can be improved when CTFE is introduced in the polymer chains [25].

Table 10. Physical Properties of PFMMD and PFMMD-co-CTFE Copolymers.

a 6 Samples CTFE mol% Tg (◦C) Intrinsic Viscosity (dL/g) Mv (10 ) σ (MPa) Elongation at Break (%) E (MPa) 1 0 135 0.175 1 9 1.7 6.6 2 23 125 0.167 1 10 2.1 4.7 3 30 120 0.16 0.99 10.2 3.1 3.3 4 50 99 0.153 1 13 8.7 1.5 a Average molecular weight was estimated using the Mark-Houwink equation. Membranes 2020, 10, x FOR PEER REVIEW 12 of 17

Figure 11. The stress–strain curves of polymer films obtained at room temperature for samples (1) Figure 11. Thepoly(PFMMD), stress–strain (2) curvespoly(PFMMD- of polymerco-CTFE) (23 ﬁlmsmol% CTFE), obtained (3) poly(PFMMD- at roomco temperature-CTFE) (30 mol%for samples (1) poly(PFMMD),CTFE), (2) poly(PFMMD- and (4) poly(PFMMD-coco-CTFE)-CTFE) (50 (23 mol% mol% CTFE) [26]. CTFE), (3) poly(PFMMD-co-CTFE) (30 mol% co CTFE), and7. (4) Relationships poly(PFMMD- between-CTFE) Gas Permeability (50 mol% and CTFE) Selectivity [26 and]. Robeson Tradeoff Plots 7. Relationships betweenFor polymers Gas used Permeability as membrane materials and for Selectivity gas separation, and it is desirable Robeson that they Tradeo have ﬀhighPlots gas permeability combined with high selectivity for one gas over another. High permeability is For polymersimportant used to reduce as membrane the amount of materials membrane area for required gas separation, to perform a gas it separation is desirable processthat they have economically (reducing the capital costs), while high selectivity improves the efficiency of the high gas permeabilityseparation combined(decreasing operation with highcosts). However, selectivity in practice, for one there gas exists over a trade-off another. relationship High permeability between selectivity and permeability. For example, high selectivity polymers tend to possess low permeability and vice versa. The trade-off relationship between permeability and selectivity was firstly outlined empirically by Robeson in 1991 [38,39], and it was further theoretically explained by Freeman in 1999 [40]. Since then, research on gas separation membranes has focused on the discovery of polymer materials that exhibit separation properties near or above the upper bound line. Perfluoropolymers, whose properties were elucidated starting from the late 1990, helped re-define an updated upper bound in 2008. More importantly, the performance of perfluorodioxolane polymers exceeds that of commercial perfluoropolymers for the separation of light gases, such as He, N2, and CO2 (Figures 12–14).

Membranes 2020, 10, 394 11 of 14 is important to reduce the amount of membrane area required to perform a gas separation process economically (reducing the capital costs), while high selectivity improves the efficiency of the separation (decreasing operation costs). However, in practice, there exists a trade-off relationship between selectivity and permeability. For example, high selectivity polymers tend to possess low permeability and vice versa. The trade-off relationship between permeability and selectivity was firstly outlined empirically by Robeson in 1991 [38,39], and it was further theoretically explained by Freeman in 1999 [40]. Since then, research on gas separation membranes has focused on the discovery of polymer materials that exhibit separation properties near or above the upper bound line. Perfluoropolymers, whose properties were elucidated starting from the late 1990, helped re-define an updated upper bound in 2008. More importantly, the performance of perfluorodioxolane polymers exceeds that of commercial perfluoropolymers for the separation of light gases, such as He, N2, and CO2 (Figures 12–14). Membranes 2020, 10, x FOR PEER REVIEW 13 of 17 Membranes 2020, 10, x FOR PEER REVIEW 13 of 17

Figure 12. Pure-gasFigure 12. COPure-gas2/CH CO4 2selectivity/CH4 selectivity as as a a function function of of CO CO2 permeance2 permeance for various for perfluorinated various perfluorinated Figurepolymer 12. membranes. Pure-gas CO All2/CH data4 selectivity were measured as a function at 22 °C, of 50CO psig2 permeance feed pressure, for various and 0 perfluorinatedpsig permeate polymer membranes. All data were measured at 22 ◦C, 50 psig feed pressure, and 0 psig permeate pressure. polymerpressure. membranes. PD-1: PFMDD- All dataco-PFMD were measured58:42 mol%; at 22 PD-2:°C, 50 psigPFMMD- feed copressure,-PFMD and60:40 0 psigmol%; permeate PD-3: PD-1: PFMDD-pressure.poly(PFMMD);co-PFMD PD-1: PD-4: 58:42PFMDD- poly(PFM mol%;co-PFMDD); PD-2:and 58:42 PD-5: mol%; PFMMD- poly(PFMMD- PD-2:co PFMMD--PFMDco-CTFE)co 50:50 60:40-PFMD mol%. mol%;60:40 mol%; PD-3: PD-3: poly(PFMMD); PD-4: poly(PFMD);poly(PFMMD); and PD-5:PD-4: poly(PFM poly(PFMMD-D); and PD-5:co-CTFE) poly(PFMMD- 50:50co-CTFE) mol%. 50:50 mol%.

Figure 13. Pure-gas N2/CH4 selectivity as a function of N2 permeance for various perfluorinated Figure 13. Pure-gas N2/CH4 selectivity as a function of N2 permeance for various perfluorinated Figure 13. Pure-gaspolymer membranes. N2/CH4 Allselectivity data measured as at a 22 function °C, 50 psig feed of N pressure2 permeance and 0 psig forpermeate various pressure. perﬂuorinated polymerPD-1: PFMMD- membranes.co-PFMD All data40:60 measured mol%; PD-2: at 22 PFMMD- °C, 50 psigco-PFMD feed pressure 50:50 mol%; and 0 PD-3: psig permeatePFMMD- pressure.co-PFMD polymer membranes. All data measured at 22 ◦C, 50 psig feed pressure and 0 psig permeate pressure. PD-1:75:25 PFMMD-mol%; PD-4:co-PFMD poly(PFMMD- 40:60 mol%;co-CTFE) PD-2: PFMMD- 63:37 mol%;co-PFMD and 50:50PD-5: mol%; poly(PFMMD- PD-3: PFMMD-co-CTFE)co-PFMD 50:50 PD-1: PFMMD-75:25mol%.co mol%; -PFMD PD-4: 40:60 poly(PFMMD- mol%; PD-2:co-CTFE) PFMMD- 63:37 mol%;co-PFMD and PD-5: 50:50 poly(PFMMD- mol%; PD-3:co-CTFE) PFMMD- 50:50 co-PFMD 75:25 mol%; PD-4:mol%. poly(PFMMD-co-CTFE) 63:37 mol%; and PD-5: poly(PFMMD-co-CTFE) 50:50 mol%.

Membranes 2020, 10, 394 12 of 14

Membranes 2020, 10, x FOR PEER REVIEW 14 of 17

Figure 14. Pure-gas HeFigure/CH 14. Pure-gasselectivity He/CH4 asselectivity a function as a function of He of He permeance permeance for various for various perfluorinated perfluorinated polymer polymer membranes.4 All data were measured at 22 °C, 50 psig feed pressure, and 0 psig permeate membranes. All datapressure. were PD-1: measured PFMDD-co-PFMD at 22 58:42◦C, mol%; 50 psig PD-2: feedPFMMD- pressure,co-PFMD 60:40 and mol%; 0 psigPD-3: permeate pressure. poly(PFMMD-co-PFMD) 40:60 mol%; PD-4:poly(PFMDD-co-CTFE): 43:47 mol%; and PD-5: PD-1: PFMDD-co-PFMDpoly(PFMMD- 58:42co-CTFE) mol%; 50:50 mol%. PD-2: PFMMD-co-PFMD 60:40 mol%; PD-3: poly(PFMMD- co co -PFMD) 40:60 mol%;8. A relevant PD-4:poly(PFMDD-co-CTFE): Application of Perfluoropolymers: Recovery 43:47 of Helium mol%; from and Natural PD-5: Gas poly(PFMMD- -CTFE) 50:50 mol%. Helium (He) is extensively used in many industrial, medical, and scientific fields. Helium’s main source is from natural gas. The content of He in natural gas can vary from 0.01 to 4.0 mol% [41]. 8. A relevant ApplicationPresently, the of main Perfluoropolymers: industrial method of He producti Recoveryon is cryogenic of distillation, Helium which from is very energy Natural Gas consumable. Another more efficient method for recovering He is proposed by using gas separation membranes, which has attracted research in these fields for many years [36,42–46]. Helium (He) is extensivelyPerfluorodioxolaneused polymers in exhibit many much industrial, higher He/CH medical,4 selectivity compared and scientific to commercial fields. Helium’s main source is from naturalperfluoropolymers gas. The and content seem promising. of HeThe obtained in natural data points gas are shown can to varybe above fromthe Robeson 0.01 to 4.0 mol% [41]. upper bounds (Figures 12–14). Presently, the main industrialFluoropolymers method and fluorinated of He liquids production exhibit higher is He/gas cryogenic solubility distillation,selectivity than their which is very energy hydrocarbon analogs [17,38], which is consistent with perfluoropolymers having unfavorable consumable. Anotherinteraction more with effi hydrocarbonscient method including forCH4. recoveringAs a result, perfluoropol He isymers proposed are very promising by using gas separation membranes, which hasmaterials attracted for He/CH research4 separation. in these fields for many years [36,42–46]. Perfluorodioxolane polymers exhibit much9. Conclusions higher He/CH4 selectivity compared to commercial perfluoropolymers and seem promising. The obtainedPerfluorodioxolane data points polymers are shown are highly to stable be both above chemically the and Robeson thermally. These upper polymers bounds (Figures 12–14). have the features of being amorphous and soluble in fluorinated solvents. They have also shown Fluoropolymersrelatively and high fluorinated permeability and liquids selectivity exhibitfor gas pairs highersuch as N2, CO He2, H/gas2, He, with solubility CH4 compared selectivity than their with commercial perfluoropolymers, including Teflon AF, Hyflon AD, and Cytop. The gas separation hydrocarbon analogsproperties [17,38 ],of whichthese polymers is consistent tend to be near with or above perfluoropolymers the upper bound for a number having of important unfavorable interaction with hydrocarbons includinggas pairs. In addition, CH .the As preparation a result, of the perfluoropolymers perfluorodioxolane polymers areis rather very simple promising and materials for practical, and it is affordable4 at large scale. For these reasons, perfluorodioxolane polymer-based He/CH4 separation. membranes are currently being developed for a number of membrane gas separations including helium recovery and CO2 removal from natural gas resource. 9. Conclusions

Perfluorodioxolane polymers are highly stable both chemically and thermally. These polymers have the features of being amorphous and soluble in fluorinated solvents. They have also shown relatively high permeability and selectivity for gas pairs such as N2, CO2,H2, He, with CH4 compared with commercial perfluoropolymers, including Teflon AF, Hyflon AD, and Cytop. The gas separation properties of these polymers tend to be near or above the upper bound for a number of important gas pairs. In addition, the preparation of the perfluorodioxolane polymers is rather simple and practical, and it is affordable at large scale. For these reasons, perfluorodioxolane polymer-based membranes are currently being developed for a number of membrane gas separations including helium recovery and CO2 removal from natural gas resource. Funding: This research was funded by the USA National Science Foundation (NSF) through the STIR award No. 11P-1632229 and NSF career award No. 1554236. Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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