Phosphonium-based poly(Ionic liquid) membranes: The effect of cation alkyl chain length on light gas separation properties and Ionic conductivity

Author’s Accepted Manuscript Phosphonium-based poly(Ionic liquid) membranes: The effect of cation alkyl chain length on light gas separation properties and Ionic conductivity Matthew G. Cowan, Miyuki Masuda, William M. McDanel, Yuki Kohno, Douglas L. Gin, Richard D. Noble www.elsevier.com/locate/memsci

PII: DOI: Reference:

S0376-7388(15)30248-9 http://dx.doi.org/10.1016/j.memsci.2015.10.019 MEMSCI14042

To appear in: Journal of Membrane Science Received date: 4 August 2015 Revised date: 22 September 2015 Accepted date: 6 October 2015 Cite this article as: Matthew G. Cowan, Miyuki Masuda, William M. McDanel, Yuki Kohno, Douglas L. Gin and Richard D. Noble, Phosphonium-based poly(Ionic liquid) membranes: The effect of cation alkyl chain length on light gas separation properties and Ionic conductivity, Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2015.10.019 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Phosphonium-based poly(ionic liquid) membranes: the effect of cation alkyl chain length on light gas separation properties and ionic conductivity

Matthew G. Cowan,§,‡ Miyuki Masuda, § William M. McDanel,§ Yuki Kohno, § Douglas L. Gin,§,‡,*and Richard D. Noble§,* §

Department of Chemical and Biological Engineering, and ‡Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309, USA

Abstract

Phosphonium poly(ionic liquid)s (PILs) have been studied as alternatives to more common ammonium and imidazolium PILs for potential transport and separation applications. This work characterizes the CO2, H2, N2, O2, CH4, and C2H4 single-gas permeability, diffusivity, solubility, and selectivity of free-standing films of poly([(tri-nalkyl)vinylbenzylphosphonium][bis(trifluoromethylsulfonyl)imide]) PILs (i.e., poly([PnnnVB][Tf2N] where n = 4, 6, 8). The gas permeability was found to increase approximately linearly with increasing alkyl chain length on the phosphonium group. To our knowledge, the CO2 permeability of 186 barrers observed for poly([P888VB][Tf2N] is the highest reported for neat PIL materials. In contrast, gas selectivity was observed to decrease with an increase in phosphonium alkyl chain length from n = 4 to n = 6, then remain approximately constant between n = 6 and n = 8. Additionally, the ionic conductivity of these materials was observed to increase from ca. 10-8 to ca. 10-5 S cm–1 as the measurement temperature was increased from 25 to 105 °C. At 25 °C, the PIL with the shortest cation alkyl chain (n = 4) was observed to have the lowest ionic conductivity. However at ca. 90 °C, the expected trend of increasing ionic conductivity in the order n = 4 > n = 6 > n = 8 was observed. Keywords: phosphonium, poly(ionic liquid)s, gas separations, ionic conductivity, alkyl chain length

1

1. Introduction Poly(ionic liquid) (PIL)-based membranes have shown promise for light gas separations1 and as ion-conductive materials.2 In the context of gas separations, many reported PIL systems have been based on the imidazolium cation,1b, 3 which shows particularly good performance for the separation of CO2 from coal-power plant flue gas, where a high CO2 permeance is preferred over high selectivity.4 Similarly, PIL materials based on imidazolium, ammonium, and phosphonium cations have been widely investigated as ion-conductive components in electrochemical devices such as fuel cells, lithium batteries, actuators, and solar cells.5 Evaluation and comparison of gas permeability and ionic conductivity often form the foundation for many academic studies on PIL-based membrane materials. However, when selecting membrane materials for specific applications, it is also important to consider other material properties such as processability, chemical and thermal stability, chemical and biological fouling resistance, electrochemical operating window, and cost.6 Some of the beneficial properties attributed to phosphonium-based ionic liquid (IL) materials, as compared to imidazolium and ammonium-based materials, include increased chemical and temperature stability, wider electrochemical operating window, and better resistance to biological fouling.5a, 7 Phosphonium-based PILs in the literature have primarily been synthesized by radical polymerization of IL monomers containing a quaternary phosphonium cation with three attached alkyl chains and a vinylbenzyl polymerizable group, along with an associated free anion (i.e., [PnnnVB][X], where n = the number of carbons in the alkyl chains and VB = the vinylbenzyl polymerizable group on the phosphonium cation). For example, controlled reversible additionfragmentation chain-transfer polymerizations of [PnnnVB][Cl] monomers have been used to produce linear PILs and block copolymers containing the phosphonium moiety.8 Separately, 2

conventional free-radical polymerization with AIBN has been used to prepare linear phosphonium-based PILs and random copolymers. Of particular relevance to this work, a series of poly([PnnnVB][X]) PILs have recently been prepared by solution polymerization of monomers with anion X = Cl and n = 1, 2, 3, 4, and 8, followed by anion exchange to produce analogues where X = BF4, TfO, and Tf2N.8 The physical properties of these phosphonium-based PILs were examined along with their bulk ionic conductivity between 95 and 135 °C. PILs with X = Tf2N were observed to have the lowest glass transition (Tg) values (30–91 °C), which generally decreased in response to increasing phosphonium group alkyl chain length (n).8 Additionally, this prior work showed that these PILs are more thermally stable and displayed higher ionic conductivities than their corresponding ammonium-based counterparts.8a In terms of light gas separations, phosphonium-based ILs and PILs have not received significant attention in the literature – in stark contrast to imidazolium-based materials in particular.3 This is perhaps due to the significantly lower CO2 permeability associated with phosphonium-based ILs compared to their imidazolium analogues.9 To our knowledge, the only specific examples of phosphonium-based IL materials examined for gas separations were in the form of neat ILs10 and as supported ionic liquid membranes (SILMs).9, 11 In particular, a 2007 report on SILMs prepared from ILs containing the [P(14)666]+ cation with Cl, [DCA], [DEP], [DBS], or [Tf2N] anions. SILMs prepared from ILs with the [P(14)666]+ cation showed similar permeabilities to SILMs prepared from imidazolium-based cations – with the exception of CO2, which was reported to be ca. 3 times higher in [EMIM][Tf2N] (1727 barrers) than in [P(14)666][Tf2N] (689 barrers).9 In terms of ionic conductivity, PILs based on a polystyrene backbone with either phosphonium or imidazolium sidegroups have been examined.

PILs with the structure 3

poly([PnnnVB][Tf2N]) (where n = 1, 2, 3, 4) displayed ionic conductivities approaching 10-3 S∙cm-1 at 135 °C.8a In contrast, polystyrene-based PILs functionalized with ethylimidazolium displayed ionic conductivities approaching 10–5 S∙cm-1 at 85 °C.12 Similarly, polystyrene-based PILs functionalized with imidazolium groups containing poly(ethylene oxide) sidechains have ionic conductivities of ca. 10-6 to ca. 10-3 S∙cm-1 at 25 to 90 °C.13 Herein, we report the preparation and characterization of free-standing PIL membranes based

on

the

free

radical

polymerization

of

a

homologous

series

of

(tri-n-

alkyl)vinylbenzylphosphonium) monomers with the bis(trifluoromethylsulfonyl)imide) anion: [PnnnVB][Tf2N]. The resulting PIL materials (i.e., poly([PnnnVB][Tf2N]) where n = 4, 6, 8) (Figure 1) were characterized for their single-gas transport properties at room temperature and for their ionic conductivity between 25 and 105 °C. To our knowledge, this is the first report of the gas transport performance of PIL materials based on phosphonium cations – which offer advantages over imidazolium- and ammonium-based PIL materials in terms of thermal, chemical, and biological resistance.

2. Experimental 2.1

Materials and Instrumentation Trialkylphosphines were purchased from Sigma-Aldrich. 2-Chloromethylstyrene was

purchased from TCI America. LiTf2N was obtained from the 3M Company. CO2, N2, CH4, O2, H2, and C2H4 gas cylinders were purchased from Airgas (Randor, PA) with purities of ≥ 99.99%. NMR spectroscopy was performed on either a Bruker Avance-III 300 MHz spectrometer or a Varian INOVA 400 MHz spectrometer operating at 300 or 400 MHz for 1H observation and 75 or 100 MHz for 13C observation. 2-D NMR spectra were obtained on a Varian INOVA 400 MHz 4

spectrometer. Elemental analysis was performed by Galbraith Laboratories, Inc. (Knoxville, TN). Mass spectrometry was performed by the Central Analytical Laboratory at the University of Colorado at Boulder. TGA was performed using a Mettler-Toledo TGA/DSC 1 STAR System. Fourier-transform infrared (FT-IR) spectra were obtained using a Thermo-Scientific Nicolet 6700 FT-IR in attenuated total reflectance (ATR) mode, and monomer conversions were calculated using a previously reported method (ESI Section 2.2).14 DSC was performed using a Mettler-Toledo DSC823e instrument. Ionic conductivity was measured via AC impedance spectroscopy using a Gamry Reference 600 potentiostat. Membrane thicknesses were measured with a digital micrometer. 2.2

Synthesis of phosphonium monomers The [P444VB][Cl],15 [P666VB][Cl],16 and [P888VB][Cl]15 monomers were prepared from the

appropriate triaklyphosphine and 2-chloromethylstyrene using methods similar to those described in the literature. The [P444VB][Tf2N] and [P888VB][Tf2N] monomers were synthesized via anion-exchange of [P444VB][Cl] and [P888VB][Cl] via adaption of a method previously described in the literature.15, 17 The [P666VB][Tf2N] monomer has not been previously reported and was synthesized from [P666VB][Cl] using the following procedure: Lithium bis(trifluoromethylsulfonyl)imide (6.1 g, 0.021 mol) was added to a solution of [P666VB][Cl] (8.5 g, 0.020 mol) in water (100 mL), and the resulting mixture was stirred for 18 h at room temperature. Dichloromethane (100 mL) was then added, and the dichloromethane layer was washed with water (50 mL) until no halide was detected by the silver nitrate test (i.e., no AgCl precipitate formation). Rotary evaporation of the dichloromethane layer, followed by evaporation in vacuo, provided the product as a clear liquid (12 g, 90%). 1H NMR (CDCl3, 300 MHz, ppm): 7.42 (d, J = 8.1 Hz, 2H), 7.19 (dd, J = 8.2 Hz, J = 2.4 Hz, 2H), 6.67 (dd, J = 17.6 5

Hz, J = 11.0 Hz, 1H), 5.77 (d, J = 17.6 Hz, 1H), 5.31 (d, J = 17.6 Hz, 1H), 3.58 (d, J = 14.4 Hz, 2H), 2.08 (m, 6H), 1.42 (m, 12H), 1.26 (m, 12H), 0.876 (t, J = 6.8 Hz, 9H). 13C NMR (CDCl3, 75 MHz, ppm): 138.3 (d, JC-P = 5 Hz), 135.7, 130.0 (d, JC-P = 7 Hz), 127.4 (d, JC-P = 4 Hz), 126.7 (d, JC-P = 12 Hz), 119.9 (q, JC-F = 427 Hz), 115.5, 30.9, 30.2 (d, JC-P = 20 Hz), 26.5 (d, JC-P = 60 Hz), 22.4, 21.4 (d, JC-P = 6 Hz), 18.5 (d, JC-P = 61 Hz), 14.0. IR (ATR, cm-1): 2959, 2931, 2867, 1630, 1512, 1470, 1459, 1410, 1348, 1328, 1225, 1179, 1134, 1112, 1054, 988, 912, 853, 836. Elemental Analysis: Found: C: 51.12, H: 6.85, N: 2.10. Calc. (C29H48F6NO4PS2): C: 50.94, H: 7.08, N: 2.05. Mass Spectrometry (+ESI, CH3CN): Found: 403.4 (100%). Calc. C27H48P: 403.4. (-ESI, CH3CN) Found: 279.9. Calc. C2F6NO4S2: 279.9. 2.3

PIL synthesis and membrane fabrication Free-standing membranes of the neat PILs were prepared using the following general

procedure: The [PnnnVB][Tf2N] monomer and 1 wt% 2-hydroxy-2-methylpropiophenone (radical photo-initiator) were combined and vortexed for 1 min. The monomer mixture was then placed under vacuum for 10–15 min to remove dissolved gases. The degassed mixture was then cast onto a quartz plate coated with Rain-X and then sandwiched with a second quartz plate. A 100-µm separator was used to control the thickness. The sample was then irradiated with 365 nm light (1.88 mW∙cm–2 at the sample surface) for 2 h at ambient temperature. The resulting, solid, flexible, free-standing membrane was then removed from the surface of the quartz plates using a razor blade. 2.4

Single-gas permeability, diffusivity, solubility, and selectivity Single-gas CO2, N2, H2, O2, CH4, and C2H4 permeability measurements were performed

using a time-lag apparatus similar to those reported previously.1b, 14 Experiments were performed at room temperature (22–24 °C), and each gas was tested in triplicate for each membrane sample. 6

Between experiments, the apparatus and membrane were evacuated for 6 h at room temperature using an Edwards RV8 vacuum pump. Data from the steady-state region was used to calculate the flux (J in cm3 (STP)∙cm-2∙s-1), permeability (P in barrers; 1 barrer = 10-10∙cm3(STP)∙cm∙cm-2∙s-1∙cmHg-1), and gas diffusivity (Di in cm2∙s–1); from Equations 1, 2, and 3.18 (

)

(1)

The steady-state flux (Ji) (cm3 (STP)∙cm-2∙s-1) was determined using Equation 1, where Δpi is the change in permeate pressure (cmHg), Δt is the change in time (s), Δpleak is the change in the permeate pressure when system is evacuated and then sealed (i.e., the ‘leak rate), V is the permeate volume (cm3); A is the membrane area (cm2), T is the absolute temperature (K), and ΔVi is the volume of gas accumulated in the permeate volume at standard temperature and pressure (cm3∙STP). (2) The permeability (Pi) (barrers) was determined using Equation 2 above, where Ji is the flux, (cm3 (STP)∙cm-2∙s-1), l is the membrane thickness (cm), and ΔPi is the trans-membrane pressure difference (psi).

(3) The diffusion coefficient (Di) (cm2∙s–1) was calculated using Equation 3, where l is the membrane thickness (cm), and θ is the time-lag (s). The time lag (θ) was determined from the xaxis intercept from a plot of the steady-state flow rate (dVi (STP)) against time (t). 7

(4) The solubility coefficient for each gas tested (Si) (in (cm3 of i at STP)∙(cm-3 of polymer)∙atm-1) was extracted from the permeability and diffusion coefficient data using Equation 4, where Pi is the permeability of the selected gas i (barrers), and Di is the diffusion coefficient of that gas (cm2∙s-1).19 (5) The ideal permeability selectivity (αi/j) was calculated using Equation 5, where Pi is the permeability of gas i, and Pj is the permeability of gas j (barrers). The membranes reported herein are free-standing films, so no tortuosity or porosity corrections were applied. Note that solubility values reported in the literature in units of (moli∙L-1polymer∙atm-1) were converted to (cm3 of i at STP ∙ cm-3 of polymer ∙ atm-1) using IUPAC defined STP.

2.5

Measurement of ionic conductivity To measure the bulk ionic conductivity () of the free-standing membranes, each

membrane was cut to an appropriate size and then sandwiched in a test cell between two stainless-steel electrodes with a PTFE spacer. A signal amplitude of 150 mV was applied to the cell at a frequency between 1 and 1 x 106 Hz. Measurements were performed at 10 °C intervals between 25 and 105 °C. The sample was equilibrated at the set temperature for 20 min between each measurement. The bulk resistance R (ohms) was calculated from the x-intercept of the generated Nyquist plot. The ionic conductivity of the sample was calculated using Equation 6:

 = L/(AR)

(6)

8

where L is the thickness of the membrane sample (cm), A is the sample area (cm2), and R is the measured resistance (ohms).

3. Results and Discussion 3.1 Materials preparation and characterization The poly([PnnnVB][Tf2N]) membrane materials were prepared from the corresponding [PnnnVB][Tf2N] monomers by radical photopolymerization in their neat states to directly form free-standing films (Figures 1 and 2). Both the [P444VB][Tf2N] and [P888VB][Tf2N] monomers have been previously reported,15,

17

and the synthetic methods used for those monomers were

adapted to prepared the previously unreported [P666VB][Tf2N] monomer. The PILs poly([P444VB][Tf2N]) and poly([P888VB][Tf2N]) have been previously reported in the literature.8a In that work, those polymers were previously prepared by solution polymerization of [PnnnVB][Cl] monomers, followed by anion-exchange of the poly([PnnnVB][Cl]) PILs.

Figure 1: Radical photopolymerization of neat [PnnnVB][Tf2N] monomers to form the poly([PnnnVB][Tf2N]) PILs and free-standing membranes tested in this work.

9

The poly([PnnnVB][Tf2N]) samples produced by neat radical photopolymerization in this work were characterized for % monomer conversion, glass transition temperature (Tg), and thermal decomposition onset temperature (ESI Sections 2.2, 2.3, and 2.4). The observed monomer conversion determined by ATR-IR was 98, 96, and 96% for poly([PnnnVB][Tf2N]) n = 4, 6, and 8, respectively (see ESI Table S1). This means that the poly([PnnnVB][Tf2N]) membranes tested in this work contain ≤4 wt% of of ‘free’ ionic liquid monomer, which will have a minor effect on their gas permeability and selectivity and ionic conductivity properties (see ESI Table S1). The measured thermophysical properties (i.e., Tg and onset of thermal decomposition values) of these PILs were consistent with previous literature reports and are presented in the ESI.

Figure 2: Image of a free-standing poly([P666VB][Tf2N]) membrane used in this study.

3.2

Single-gas permeation results The single-gas permeability, diffusivity, and solubility of CO2, N2, H2, O2, CH4, and C2H4

in the poly([PnnnVB][Tf2N]) membranes is shown in Tables 1 and 2, with additional details in the ESI (Tables S3 and S4). These results show that increasing the length of the phosphonium cation 10

alkyl chains significantly increases the permeability of each gas through the PILs, but concurrently decreases the ideal selectivity of most gas pairs (Figure 3 and ESI Table S3). Comparing the ideal selectivity results between poly([P444VB][Tf2N]) and poly([P666VB][Tf2N]) shows that the selectivity between gas pairs in which one is a diatomic molecule (e.g., H2) decreases more substantially than other gas pairs. This behavior suggests that increasing the length of the alkyl chains on the phosphonium group substantially increases the fractional free volume in the poly([PnnnVB][Tf2N]) solid matrix, thereby reducing the diffusivity selectivity between gas pairs. Additionally, we observed that further increasing the phosphonium alkyl chain length to poly([P888VB][Tf2N]) does not significantly affect the ideal gas selectivity. This result implies that the poly([P666VB][Tf2N]) polymer matrix has achieved a composition plateau where adding additional alkyl chain units has little effect on the gas selectivity.

11

Table 1. Single-gas permeability data for free-standing poly([P444VB][Tf2N]), poly([P666VB][Tf2N]), and poly([P888VB][Tf2N]) membranes. Note: the poly([PnnnVB][Tf2N]) membranes contain ≤4 wt% of ‘free’, unpolymerized IL monomer. Gas Permeability (barrers) Membrane

CO2

N2

H2

O2

CH4

C2H4

poly([P444VB][Tf2N])

51 ± 1

2.7 ± 0.2

31 ± 1

10.1 ± 0.5

4.5 ± 0.2

10.5 ± 0.5

poly([P666VB][Tf2N])

120 ± 4

8.5 ± 0.1

43 ± 1

19 ± 1

15 ± 0.7

34 ± 1

poly([P888VB][Tf2N])

186 ± 6

12.0 ± 0.3

73 ± 1

33 ± 1

25 ± 1

58 ± 2

Figure 3: (left) Single gas permeability as a function of increasing cation group alkyl chain length (n) in neat poly([PnnnVB][Tf2N]) and poly([N-alkyl-(N’vinylbenzyl)imidazolium][bis(trifluoromethylsulfanoyl)imide]) (i.e., poly([Imn][Tf2N])) membranes,1b and (right) the ideal selectivity change as a function of increasing cationic group alkyl chain length (n) in neat poly([PnnnVB][Tf2N]) membranes. Note: the poly([PnnnVB][Tf2N]) membranes contain ≤4 wt% of ‘free’, unpolymerized IL monomer.

12

Comparison to published gas transport data for the corresponding imidazolium, pyridinium, pyrrolidinium,20 and ammonium-based PILs with Tf2N anions21 (Figure 4) reveals comparatively high CO2 permeabilities for the poly([PnnnVB][Tf2N]) PILs. Indeed, to our knowledge the permeability of 186 barrers measured for the poly([P888VB][Tf2N]) membrane is the highest reported for a neat PIL membrane, surpassing even siloxane-functionalized imidazolium PILs (Figure 4) (max. 130 barrers) while maintaining comparable CO2/N2 selectivity (15 vs. 14).22 The higher CO2 permeability of poly([P888VB][Tf2N]) can be explained by comparing the gas diffusivities for CO2 and CH4 in each material (Table 2).1b Higher CO2 and CH4 diffusivity values are observed for the poly([P888VB][Tf2N]) membrane. This is expected due to the fact that phosphonium-based PILs have three alkyl chains per repeating cation unit, whereas the imidazolium-based PILs have only one. The additional alkyl chains cation moiety are expected to significantly increase the fractional free volume of the solid PIL, as reflected in the gas diffusivity data, which is up to 2 orders of magnitude higher in the phosphonium-based materials compared to PILs with other cations.21 The significantly higher CO2 solubility observed for the styrene/imidazolium-based systems is well-characterized as being due to CO2 interaction with the somewhat acidic H atom at the imidazolium C-2 position.23 However, direct comparison to other non-stryrene/non-imidazolium cation-based PILs is convoluted due to the influence of the polymer backbone on solubility. As expected, in comparison to the neat IL [P666(14)][Tf2N],9 the corresponding PILs have considerably lower gas permeabilities (689 vs. max. 182 barrers, respectively).

13

Figure 4: The general structures of the poly(phosphonium)-, poly(imidazolium), poly(pyridinium), poly(pyrrolidinium), and poly(ammonium)-based PIL materials being compared in terms of their light gas transport properties in this study. R = (CH2)3CH3, (CH2)5CH3, or (CH2)7CH3; R’ = CH3, (CH2)3CH3, (CH2)5CH3, (CH2CH2O)2CH3, CH2Si(CH3)2OSi(CH3)3; R” = CH3, (CH2)5CH3, (CH2CH2O )2CH3, or (CH2)2(CF2)2CF3.

Table 2. Comparison of permeability, diffusivity, and solubilities of gases in poly([PnnnVB][Tf2N]) and poly([ImnVB][Tf2N]) membranes and [P666(14)][Tf2N]. Permeability (P) (barrers; 1 barrer = 10-10 cm3(STP)•cm•cm-2•s•cmHg); solubility (S) (cm3gas at STP•cm3 -1 -8 2 -1 polymer•atm ); diffusivity (D) (10 cm •s ). Note: The error bars for these measurements are included in ESI Table S4. Note: the poly([PnnnVB][Tf2N]) membranes contain ≤4 wt% of ‘free’, unpolymerized IL monomer. poly([PnnnVB][Tf2N]) n

P

CO2 D

S

4

52

29

6 8

120 186 9.2 20 32

poly([ImnVB][Tf2N])1b 1 4 6 [P666(14)][Tf2N]9 666(14)

689

P

CH4 D

P

C2H4 D

S

S

1.4

4.5

10

0.4

11

6

1.3

63 145

1.2 0.9

15 25

16 26

0.7 0.7

34 58

25 64

1.1 0.7

1.7 3.5 7.7

4 4.4 3.9

0.24 0.91 2.3

0.88 1.28 3.1

0.21 0.55 0.57

0.86

169

428

1.13

A full collection of the relevant Robeson plots for each light gas tested is included in the ESI (Figures S12–S19). The selectivity vs. permeability performance of the neat 14

poly([P444VB][Tf2N]),

poly([P666VB][Tf2N]),

and

poly([P888VB][Tf2N])

membranes

was

significantly below the 2008 Robeson upper bound for all gas pairs. Interestingly, inverse selectivity was observed for the H2/CO2 (0.6–0.4) and N2/CH4 (0.5) gas pairs — a feature likely caused by a combination of the high fractional free volume of the PIL matrices, along with solubility-enhancing interactions (i.e., between the ionic constituents of the matrix and CO2, and between the longer alkyl chains and CH4). 3.3

Ionic conductivity measurements between 25 and 105 °C The effect that the nature of the cation in a PIL has on ionic conductivity has recently

been reviewed.24 However, there are few ionic conductivity studies of stryrene-based/Tf2N PILs, limiting the direct comparisons available to the current work.8a The ionic conductivities of the poly([P444VB][Tf2N]), poly([P666VB][Tf2N]), and poly([P888VB][Tf2N]) membranes were measured over a temperature range of 25 to 105 °C. For all of the membranes, the ionic conductivity was very similar irrespective of phosphonium group alkyl chain length, and generally increased from ca. 10-8 to 10-4 S cm-1 as the temperature was raised from 25 to 105 °C (Figure 5). Interestingly, at 25 °C the ionic conductivity of poly([P444VB][Tf2N]) was lower than the poly([P666VB][Tf2N]) and poly([P888VB][Tf2N]) materials. However, by ca. 90 °C, the observed ionic conductivity to the length of the alkyl chain-substituent. The latter trend agrees with the trends previously reported

for

higher-temperature

(95–135

°C)

ionic

conductivity

measurements

on

poly([PnnnVB][Tf2N]) materials.8a To explain the observed ionic conductivity behavior between 25–60 °C (i.e., where the ionic conductivity of poly([P444VB][Tf2N]) is lower than poly([P666VB][Tf2N]) and poly([P888VB][Tf2N])), we speculate that the shorter alkyl chain length permits a stronger cation-anion attractive interaction in poly([P444VB][Tf2N]), which limits ion mobility at lower temperatures. Once the temperature is increased, the strength of the cation15

anion interaction would play a less important role, and ion mobility would be more limited by the size of the polymer-bound cationic species (i.e., as a function of increasing cation alkyl chain length) – matching the correlation observed at temperatures ≥90 °C.

Figure 5: Ionic conductivity behavior of neat poly([P444VB][Tf2N]), poly([P666VB][Tf2N]), and poly([P888VB][Tf2N]) films between 25 and 105 °C. The error bars shown are the standard deviations of measurements between 3 different samples. Note: the poly([PnnnVB][Tf2N]) membranes contain ≤4 wt% of ‘free’, unpolymerized IL monomer.

16

4.

Conclusions Free-standing

membranes

prepared

from

the

phosphonium-based

PILs

poly([P444VB][Tf2N], poly([P666VB][Tf2N], and poly([P888VB][Tf2N] were found to be substantially more permeable to light gases and generally less selective than their corresponding imidazoliumbased PILs. To our knowledge, the poly([P888VB][Tf2N]) membrane exhibits the highest CO2 permeability (186 barrers) of any reported neat PIL membrane, potentially making phosphonium-based materials attractive candidates for developing economically-viable, highthroughput membranes for CO2/N2 separations. Additionally, the ionic conductivity values of these free-standing poly([PnnnVB][Tf2N]) membranes between 25 and 105 °C was measured to be 10-8 to 10–4 S∙cm-1, respectively. These values are lower than that of the corresponding imidazolium-based PILs at the same temperature, and are consistent with a previous report on the ionic conductivity of poly([PnnnVB][Tf2N]) materials measured at higher temperatures (95 and 135 °C). However, given their improved thermal stability resistance vs. their ammonium and imidazolium analogues, these phosphonium-based PIL membranes may be more suitable for specific applications that require more robust materials. Future experiments will incorporate ‘free’ IL into these poly([PnnnVB][Tf2N]) membrane materials and examine the effect that this has on gas permeability and ionic conductivity.

5.

Author Information

(R.D.N)* Corresponding author at: Department of Chemical and Biological Engineering, University of Colorado, University of Colorado, CO 80309, United States. Tel.: +1 303 492 6100. E-mail address: [email protected] (D.L.G)* Corresponding author at: Department of Chemical and Biological Engineering, and Department of Chemistry and Biochemistry, University of Colorado, University of Colorado, CO 80309, United States. Tel.: +1 303 492 7640. E-mail address: [email protected]

17

6.

Acknowledgments The authors gratefully acknowledge financial support for this work from the U.S. Dept.

of Energy ARPA-E program (grant:

DE-AR0000343). We also thank Professor R. K.

Shoemaker at CU Boulder for collection of the 2-D NMR data on the [P666VB][Tf2N] monomer.

7.

Supporting information Adapted general procedure for the synthesis of the [PnnnVB][Cl] precursor monomers;

NMR, IR, and DSC spectra on the IL monomers and PILs; TGA data, light gas permeation data and Robeson plots.

8.

References

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Graphical Abstract