Unexpectedly High CO2/N2 Permeation Selectivities from … · washers (Boker's, Inc., Allentown,...

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Unexpectedly High CO2/N2 Permeation Selectivities from

Hyperthin Polyelectrolyte Multilayers

Minghui Wang, Vaclav Janout and Steven L. Regen*

Department of Chemistry, Lehigh University, Bethlehem, PA 18015

Supporting Information

Materials and Methods. Poly(4-styrenesulfonic acid), sodium salt (PSS)

(Mw=70,000, Sigma-Aldrich Co., St. Louis, MO.), poly(diallyldimethylammonium

chloride) (Sigma-Aldrich Co., St. Louis, MO) and poly[1-(trimethylsilyl)-1-propyne]

(PTMSP, Gelest, Inc., Morrisville, PA) were used as obtained. House-deionized water

was purified using a Millipore Milli-Q-filtering system containing one carbon and two

ion-exchange stages. Experimental methods that were used to (i) synthesize

5,11,17,23,29,35-hexakis[(N,N,N-trimethylamonium)-N-methyl-37,38,39,40,41,42-

hexakis-n-hexadecyloxy-calix[6]arene chloride (i.e., 4), (ii) deposit LB monolayers of

PSS-crosslinked 4 onto poly[1-(trimethylsilyl)-1-propyne] (PTMSP) supports or silicon

wafers (silylated with n-octadecyltrichlorosilane, OTS), (iii) quantify gas permeabilities,

and (iv) measure film thicknesses by ellipsometry were similar to those that have

previously been described.1-3 Ellipsometry measurements were made using VASE

ellipsometer (J.A. Woollam Co., Inc., Lincoln, NE), equipped with variable wavelength (

from 250-1700nm) and angle of incidence (15º-90º) system, a λ of 632.8nm and an

incidence angle of 70º were chosen for measurements. The refractive indices that were

used to estimate the thickness of the OTS, the PSS-crosslinked LB layer, and the

Electronic Supplementary Material (ESI) for Chemical CommunicationsThis journal is © The Royal Society of Chemistry 2013

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polyelectrolyte multilayers were 1.46, 1.41, and 1.41, respectively. All AFM

measurements were made using a tapping mode atomic force microscopy (Solver Next,

Multimode SPM, NT-MDT America Inc., Santa Clara, CA), and a minimum of three

different locations along the surface were examined. For each sample, 5 × 5 and 10 × 10

µm2-size images were obtained. Film thicknesses were also determined via AFM by

scratching the surface of OTS-modified silicon wafers bearing the LB films with a razor

blade to remove the surfactant assembly, and measuring step heights. Specific

procedures that were used were similar to those described in the literature.4 A Nima 612D

film balance (Nima Technologies, Coventry England) was used for all LB depositions.

The aqueous polymer solutions that were used for layer-by-layer depositions were

adjusted to pH 8.0 (polycations solution) or 4.0 (PSS solution) by addition of a small

volume of either 1 M NaOH or 1 M HCl. All 1H-NMR spectra were recorded on a

Bruker 500-MHz instrument; chemical shifts are reported in ppm and were referenced to

residual solvents.

Supports. Supports that were made from PTMSP were prepared by using a casting

technique. A typical casting apparatus consisted of a Pyrex glass square (8 in. × 8 in ×

1/8 in.), an aluminum centering ring seal, 160 ISO flange size (Kurt J. Lesker Co.,

Allentown, PA), and five (2.10 in. o.d. × 1.64 in. i.d. × 0.010 in.) low carbon steel

washers (Boker's, Inc., Allentown, PA). The Pyrex glass square, ring seal, and five

washers were cleaned with chloroform, methanol, and acetone, with the aid of kimwipes.

The ring seal was then adhered to the glass square by using a 5% toluene solution (HPLC

grade) of PTMSP, which acted as "glue". The steel washers were placed symmetrically

within the ring seal/glass square casting unit. A PTMSP/toluene casting solution (ca. 480


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mg/30 mL) was then poured into the ring seal and covered with 15 large pieces of filter

paper (Whatman qualitative circles, 18.5 cm) in order to keep the casting unit dust-free,

and also to avoid the solvent evaporating too fast. The toluene was allowed to evaporate

for at least 15 h, leaving a PTMSP film across the steel washers and glass square. A

surgical blade (S/P Surgical Blades, Baxter Diagnostics) was used to cut out the

individual washers. Water was poured into the ring seal to help separate the washers. The

PTMSP cast films were placed between several large pieces of filter paper and allowed to

dry for at least 24 h. The resulting membranes, having a typical thickness of ca. 30 µm,

were placed in antistatic bags for at least 15 min prior to use in composite membrane

fabrication.

Layer-by-Layer Depositions. An OTS-modified silicon wafer (or PTMSP support),

which was modified with a Langmuir-Blodgett monolayer of PSS-crosslinked 4 on one

side, was washed with water and subjected to layer-by-layer depositions. Typically, the

substrate was immersed, directly, into 200 mL of an aqueous solution of PDADMAC (15

mM repeat unit concentration) for 15 min. The substrate was then removed and washed

twice by immersing in 200 mL of pure water (600 mL were used for PTMSP membranes)

for 5 min each, followed by air-drying. A similar treatment was then performed using a

15 mM aqueous solution of PSS, and the membranes allowed drying in the laboratory

ambient for 35 h after the depositions were completed prior to gas permeation

measurements. In addition, special set-up was made to avoid polyelectrolyte solution

contaminating the back side of the PTMSP support that was not covered with LB

monolayer. In detail, 2 PTMSP membranes on washers were always attached back-to-

back onto a home designed disk magnet holder which was wrapped with PTFE tape, so


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that only LB monolayer modified side exposures to polymer solutions. The flat surface of

the holder assures the tightness between the holder and PTMSP membranes, thus

preventing polyelectrolyte solutions from contacting the back side of PTMSP

membranes.

Gas Permeation Measurements. Gas permeation measurements were made with a

home-built stainless steel permeation apparatus.2 The gases studied were H2 (Ultra High

Purity, water <3ppm, Messer Griesheim Industries, Inc., Malvern, PA), N2 (prepurified

grade, water <3ppm, Praxair, Inc., Danbury, CT) and CO2 (“Bone-dry” grade, water

<10ppm, Airgas, Inc., Radnor, PA). A membrane to be measured was placed in the

permeation cell between two Viton rubber O-rings (3.45 cm i.d., Scientific Instrument

Services, Inc.) with a support screen (4.70 cm, Millipore Corp.) and held securely with a

quick flange clamp (Scientific Instrument Services, Inc., not shown). Membranes were

always placed in the cell such that the polyelectrolyte multilayers faced the high pressure

side of the pressure gradient. The permeant gas travelled from the gas cylinder to an

inline filter (15 m) and elastomer diaphragm regulator (Brooks Instrument, 8601D),

which was connected to a Heise gauge port. Another plastic tubing line connected from

the Heise gauge outlet to the permeation cell. The pressure gradient that was applied to

each membrane was 30 psig. After passing through the membrane, the gaseous permeant

was directed into a 40 cm long glass U-tube flowmeter (2 mm i.d.). The volumetric flow

rate of the gas was then measured by recording the time (tf - ti) that was required for a

methyl isobutyl ketone solution containing crystal violet to travel a set distance (df - di),

thereby sweeping out a defined volume. Measurements were taken until steady-state

values were achieved (typically 1 to 2 h). At least five volumetric flow rates were


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recorded for each membrane, and the mean and standard deviations were determined. The

normalized flux was calculated with use of the mean volumetric flow rate, the area that

was available for flow (9.36 cm2), and the pressure gradient (2.1 atm) that was employed.

This procedure was repeated for the next permeant gas. In general, the permeation

properties were first measured for H2, then for N2, and finally for CO2.

Prior to gas permeation measurements, all membranes were allowed to dry in the

laboratory ambient for 35 h after the layer-by-layer depositions were completed. During

such time, the approximate temperature and relative humidity in the laboratory were 22 ±

2°C and 40 ± 10%, respectively. To ensure that no damage to the membrane had

occurred while these measurements were being made, after the last permeant was

investigated (i.e., CO2), the H2 and N2 permeances were measured again and found to be

unchanged.

Synthesis of Poly(4-styryl-methylene trimethylammonium chloride, PSTAC or 3.

To 2.50 mL of an ethanolic solution of trimethylamine (11mmol) was added 6 mL of a

chloroform solution containing 0.495 g of poly(4-chloromethylbenzylchloride) (3.24

mmol) (Mw 40500, Mn 20300, Polymer Source Inc., Montreal, Canada) and the reaction

mixture was stirred in sealed flask under an argon atmosphere for 24 h. After removal of

all volatiles under reduced pressure at 45o C (10 Torr), the resulting polymer was

dissolved in 5 mL of H2O, purified by dialysis using Spectrapor membrane MWCO 1000

Da and isolated by freeze drying. The final product, 3, was obtained in 0.582 g (85%)

yield as a colorless solid having 1H-NMR (D2O, δ, ppm): 7.18(bs, 2H); 6.68(bs, 2H);

4.38(bs, 2H); 2.99(s, 9H); 1.20-2.30(bm, 3H).


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1H-NMR Spectrum of PSTAC (or 3) Table S-1 Thickness of PEMs measured by ellipsometer and AFM

PEMs Ellipsometry AFM NaCl free (32700 + 2)18 13.8 ± 0.2nm 13.3 ± 0.5nm

NaCl free (18500 + 2)18 14.2 ± 0.4nm 14.0 ± 0.6nm

NaCl free (1150000 + 2)14 13.8 ± 0.5nm 13.7 ± 0.4nm

NaCl free (1450000 + 2)13.5

13.8 ± 0.7nm 13.7 ± 0.3nm

0.1M NaCl (18500 + 2)6.5 15.1 ± 0.2nm 14.7 ± 0.7nm

0.1M NaCl (1150000 + 2)5.5

14.8 ± 0.1nm 14.3 ± 0.9nm

0.1M NaCl (1450000 + 2)5 15.4 ± 0.2nm 14.7 ± 1.0nm

1.0M NaCl (18500 + 2)3.5 18.4 ± 0.6nm --

1.0M NaCl (1150000 + 2)3 18.5 ± 1.2nm --

1.0M NaCl (1450000 + 2)2.5

17.0 ± 0.4nm 16.1 ± 1.4nm


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Figure S-1 Ellipsometric film thickness versus number of alternating layers of PSS (70K) and PDADMAC ( )8.5 K; ( ) 150 K; ( ) 450 K formed in 1.0 M NaCl.


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Figure S-2 Height image (AFM, tapping mode, 5 × 5 µm2) and Root Mean Square (RMS) Roughness of PEMs formed in pure water: a=(18500 + 2)18, b=(1150000 + 2)14 and c=(1450000 + 2)13.5; in 0.1M NaCl: d=(18500 + 2)6.5, e=(1150000 + 2)5.5, and f=(1450000 + 2)5; in 1.0M NaCl: g=(18500 + 2)3.5, h=(1150000 + 2)3, and i=(1450000 + 2)2.5; and j=(32700 + 2)18 in pure water


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Figure S-3 Height image and section profile (AFM, tapping mode, 5 × 5 µm2) showing the film thickness of PEMs formed in pure water: a=(32700 + 2)18, b=(18500 + 2)18, c=(1150000 + 2)14 and d=(1450000 + 2)13.5; in 0.1M NaCl: e=(18500 + 2)6.5, f=(1150000 + 2)5.5, and g=(1450000 + 2)5; in 1.0M NaCl: h=(1450000 + 2)2.5. (Note, the thickness of ca. 2.3nm PSS-crosslinked LB monolayer of 4 is always included in all these step height measurements)


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Table S-2 Observed permeances and selectivities (at 30psi) across hyper-thin membranes formed in the 1.0M NaCla

Membranesb H2 CO2 N2 H2/N2 CO2/N2 PTMSP 617 1440 255 2.42 5.65 PTMSP 621 1480 258 2.41 5.74 (18500+2)3.5 21.9 6.94 0.055 398 126 (1150000+2)3 17.7 5.07 0.044 402 115 (1150000+2)3 24.6 5.3 0.068 362 78 (1450000+2)2.5 20.4 5.27 0.050 408 105 (1450000+2)2.5 17.7 4.28 0.041 432 105

aPermeances at ambient temperature, 106P/l (cm3/cm2-s-cm Hg), were calculated by dividing the observed flow rate by the area of the membrane (9.36 cm2) and the pressure gradient (30 psi) employed, using ca. 30 µm thick PTMSP supports. All measurements were made at ambient temperatures. Values were obtained from 5-10 independent measurements of the same sample; the error in each case was ±5%. Duplicate membranes are reported in all cases. bThe subscript represents the total number of bilayers needed to reach a film thickness of ca. 18 nm.

Table S-3 Observed permeances and selectivities (at 10psi) across hyper-thin membranes formed in the absence of NaCla

Membranesb H2 CO2 N2 H2/N2 CO2/N2 PTMSP 615 1472 262 2.35 5.62 PTMSP 621 1460 269 2.31 5.43 (18500+2)18 40.2 25.8 0.22 183 117 (18500+2)18 37.0 16.3 0.13 285 125 (1150000+2)14 35.1 15.9 0.13 271 123 (1150000+2)14 46.9 16.8 0.12 386 139 (1450000+2)13.5 34.5 22.9 0.17 203 135 (1450000+2)13.5 34.5 17.8 0.16 217 113 (327000+2)18 93.7 48.1 0.48 195 100 (327000+2)18 108 51.9 0.50 215 104

aPermeances at ambient temperature, 106P/l (cm3/cm2-s-cm Hg), were calculated by dividing the observed flow rate by the area of the membrane (9.36 cm2) and the pressure gradient (10 psi) employed, using ca. 30 µm thick PTMSP supports. All measurements were made at ambient temperatures. Values were obtained from 5-10 independent measurements of the same sample; the error in each case was ±5%. Duplicate membranes are reported in all cases. bThe subscript represents the total number of bilayers needed to reach a film thickness of ca. 18 nm.


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Table S-4 Observed permeances and selectivities (at 10psi) across hyper-thin membranes formed in the 100 mM NaCla

Membranesb H2 CO2 N2 H2/N2 CO2/N2 PTMSP 615 1472 262 2.35 5.62 PTMSP 621 1460 269 2.31 5.43 (18500+2)6.5 28.4 16.8 0.11 251 149 (18500+2)6.5 29.3 12.7 0.11 274 119 c(1150000+2)5.5 28.8 9.27 <0.10 >288 >92.7 c(1150000+2)5.5 23.9 9.12 <0.10 >239 >91.2 (1450000+2)5 28.1 14.3 0.089 316 162 (1450000+2)5 26.8 11.7 0.12 232 101

aPermeances at ambient temperature, 106P/l (cm3/cm2-s-cm Hg), were calculated by dividing the observed flow rate by the area of the membrane (9.36 cm2) and the pressure gradient (10 psi) employed, using ca. 30 µm thick PTMSP supports. All measurements were made at ambient temperatures. Values were obtained from 5-10 independent measurements of the same sample; the error in each case was ±5%. Duplicate membranes are reported in all cases. bThe subscript represents the total number of bilayers needed to reach a film thickness of ca. 18 nm. cThe permeance of nitrogen was not detectable due to the lack of enough pressure difference, and was assumed to be less than 0.10.

References:

(1) Yan, X.; Janout, V.; Hsu, J. T.; Regen, S. L J. Am. Chem. Soc., 2003, 125, 8094-

8095 (2003). (2) Hendel, R. A.; Nomura, E, Janout, V.; Regen, S. L. J. Am. Chem. Soc. 1997, 119,

6909-6918.

(3) Li, J.; Janout, V.; Regen, S. L. Langmuir, 2006, 22, 11224-11229.

(4) (a) McAloney, R. A.; Sinyor, M.; Dudnik V.; Goh, M. C. Langmuir, 2001, 17,

6655-6663. (b) Vercelli, B.; Zotti, G.; Berlin, A.; Grimoldi, S. Chem. Mater., 2006,

18, 3754-3763. (c) Sarno, D. M.; Martin, J. J.; Hira, S. M.; Timpson, C. J.;

Gaffney, J. P.; Jones, Jr., W. E. Langmuir, 2007, 23, 879-884.


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