Effects of chemical structure on gas transport properties of polyethersulfone polymers

Accepted Manuscript Effects of chemical structure on gas transport properties of polyethersulfone polymers Ali Naderi, Wai Fen Yong, Youchang Xiao, Tai-Shung Chung, Martin Weber, Christian Maletzko PII:

S0032-3861(17)31169-2

DOI:

10.1016/j.polymer.2017.12.014

Reference:

JPOL 20203

To appear in:

Polymer

Received Date: 11 August 2017 Revised Date:

26 October 2017

Accepted Date: 2 December 2017

Please cite this article as: Naderi A, Yong WF, Xiao Y, Chung T-S, Weber M, Maletzko C, Effects of chemical structure on gas transport properties of polyethersulfone polymers, Polymer (2018), doi: 10.1016/j.polymer.2017.12.014. 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 proof before it is published in its final 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.

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PPSU 10

PESU

CO2 O2

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7

TPESU

5 4

N2 CH4

HPESU

2 1 0

PESU

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3

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6

TPESU

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Permeability (Barrer)

8

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9

Chemical Structure

PPSU

HPESU

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Effects of chemical structure on gas transport properties of polyethersulfone polymers

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Ali Naderi1, Wai Fen Yong†, Youchang Xiao† and Tai-Shung Chung*,†

†

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Martin Weber‡, Christian Maletzkoǁ

Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117585, Singapore

‡

Advanced Materials and Systems Research, BASF SE, RAP/OUB - B1, 67056 Ludwigshafen,

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Germany

ǁ

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Performance Materials, BASF SE, G-PMFSU-F206, 67056 Ludwigshafen, Germany

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Keywords: polyethersulfone, gas separation, γ transition, d-space, pore size, fractional free volume (FFV)

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Abstract Compared to polysulfone (PSF), the effects of chemical structure on gas separation performance of polyethersulfone (PES) are rarely in-depth studied. To explore the potential of various PES

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polymers as gas separation membranes, PES containing four different backbone structures have been synthesized. They are polyphenylsulfone (PPSU), poly trimethyl benzene ethersulfone (TPESU), polyethersulfone (PESU) and hydrophilic polyethersulfone (HPESU). Both wide-

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angle X-ray diffraction (XRD) and positron annihilation life-time spectroscopy (PALS) data indicate that PPSU and TPESU have bigger d-space, pore size and fractional free volume (FFV)

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values than PESU and HPESU. Among the four PES, PPSU stands out with the highest O2 and CO2 permeability of 1.61 and 9.13 Barrer at 35 °C, respectively, because the two additional aryl groups in PPSU contributes to high segmental motions. HPESU has the lowest gas permeability but shows the highest CO2/N2 and CO2/CH4 selectivity of 34.9 and 34.6, respectively, possibly

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because of the affinity between its polyethylene oxide units and CO2. The addition of trimethyl benzene groups into TPESU not only increases all gas permeability but also increases selectivity of some gas pairs. It has the highest O2/N2 selectivity of 6.0 and the 2nd highest CO2/CH4

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selectivity of 33.8. Its O2 and CO2 permeability are 1.33 and 5.74 Barrer at 35 °C, respectively. In contrast, PESU has low gas permeability and average selectivity because of its linear

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ethersulfone chain structure. In addition, the diffusivity ratio between Henry and Langmuir modes as a function of chain structure has been investigated. The diffusivity ratio is always smaller than 3.2%. This study may provide useful insights to design better PES polymers for gas separation.

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Key words: polyethersulfone, gas separation, γ transition, d-space, pore size, fractional free

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volume (FFV)

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1. Introduction Polymeric membranes for gas separation have been commercialized for more than 3 decades and are continuously growing especially in the areas of hydrogen recovery from nitrogen and

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methane, oxygen enrichment, propylene/propane purification and CO2 capture from natural gas [1-7]. In general, the permeability and selectivity of a polymeric membrane is related to the

and polarity, as well as their interactions [8, 9].

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physicochemical properties of the polymer material and the nature of gas penetrants such as size

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Polysulfone (PSF) and polyethersulfone (PES) have been used in various membrane applications due to their good thermal stability, chemical resistance, processability and gas separation properties [10]. The effects of PSF chemical structure on its permeation and sorption properties have been well studied. For instance, the effects of different substituents such as arylamine,

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benzylamine and phthalimide groups on gas performance of PSF were studied and showed that the PSF containing benzylamine has more affinity toward CO2 molecules. [11-16]. In contrast, the effects of PES chemical structure on its gas separation performance are rarely in-depth

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studied. To explore the potential of various PES materials as gas separation membranes, PES containing four different backbone structures have been studied in this work. They are

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polyphenylsulfone (PPSU), poly trimethyl benzene ethersulfone (TPESU), polyethersulfone (PESU) and hydrophilic polyethersulfone (HPESU). Figure 1 illustrates their chemical structures. PPSU and PESU are commercially available, while TPESU were synthesized to understand the effect of the methyl substituent on chain mobility, packing and gas transport properties. HPESU were prepared to investigate if the addition of ethylene oxide (EO) group

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could affect the overall gas permeability and selectivity because it has a high affinity towards CO2 molecules through quadrupolar interactions [17-20].

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The gas transport properties of O2, N2, CH4 and CO2 of the solution-cast dense films were firstly examined. Then their sorption properties were measured as a function of pressure in order to calculate the diffusion coefficients as a function of backbone structure based on the solution-

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diffusion mechanism. In order to have an in-depth fundamental understanding of diffusion mechanisms, the diffusion coefficients of these glassy polymers in Henry and Langmuir modes

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were analyzed and compared [21-23]. Wide angle X-ray diffraction (XRD), positron annihilation life time spectroscopy (PALS) and dynamic mechanical analyzer (DMA) were also employed to elucidate the microstructural behavior of different backbone structures of PES. This work may

2. Experimental 2.1. Materials

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provide useful insights to molecularly design PES with better gas separation performance.

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Polyphenylsulfone (PPSU), poly trimethyl benzene ethersulfone (TPESU), polyethersulfone (PESU) and hydrophilic polyethersulfone (HPESU) were synthesized by BASF SE, Germany.

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These polymers were dried at 120oC under vacuum overnight to remove the moisture. Figure 1 and Table 1 show their chemical structures and physical properties, respectively. An analytical grade of N- methyl-2-pyrrolidone (NMP) was purchased from Merck and was used as received.

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Figure 1: Schematic of the PES chemical structures (a) PPSU (b) TPESU (c) PESU (d) HPESU

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Table 1: Physical properties of the PES polymers.

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Membranes

Polyphenylsulfone (Ultrason ® P3010) Trimethyl polyethersulfone (GM1324-0016) Polyethersulfone (Ultrason® E6020P) Hydrophilic polyethersulfone (GM584/P-

Glass

Melting

transition

temperature of

temperature

the Pluronic part

Weight average Molecular weight

Acronym

-1

o

o

(g.mol )

( C)

( C)

PPSU

220

_

69000

TPESU

248

_

72300

PESU

230

_

77500

HPESU

201

37.5

71600

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0302/1)

2.2. Fabrication of membranes

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Figure 2: The appearance of the fabricated membranes

The PPSU, TPESU, PESU and HPESU dense flat membranes were prepared by a solution casting method. A 2 wt% solution of each polymer was prepared by using NMP as a solvent. For example, a 0.5 g polymer was dissolved in 25 g NMP and the solution was stirred overnight.

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Thereafter, the polymer solutions were filtered through a 5µm polytetrafluoroethylene (PTFE) filter and cast in petri dishes. Then, the petri dishes were placed in a vacuum oven at 60oC for 24 h to allow the solvent to evaporate gradually.

The temperature of the vacuum oven was

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increased with a heating rate of 30 oC/30min to reach 200 oC and held for 8 h to fully remove the remaining residual solvent, followed by naturally cooling down to ambient temperature. A

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Digimatic indicator (IDC-112B-5) was used to measure the membrane thickness. The thickness of membranes was around 55±5 µm. Figure 2 shows the appearance of fabricated membranes. All glassy membranes are transparent except HPESU which has –PEO-PPO-PEO- inside its chemical structure.

2.3. Characterizations

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Wide angle X-ray diffraction (WAXD) was carried out to measure the inter-chain spacing of various membranes. This assessment was performed by using a Bruker D8 Advance X-ray diffractometer with a Cu K α as the X-ray radiation source having the wavelength of 1.54 Å. The

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d-spacing was evaluated from the Bragg’s law as follows: nλ=2d sinϴ, where d represents the dimension spacing, θ shows the X-ray diffraction angle, λ stands for the X-ray wavelength and n

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is an integer number (1,2, 3...).

The measurement of sub-Tg transition temperature was carried out by a dynamic mechanical

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thermal analyzer (Q800 DMA). The apparatus was operated in the extension mode at a frequency of 1 Hz and a heating rate of 0.5 oC min-1 in the temperature range of -140 oC to 100 o

C. Positron annihilation lifetime spectroscopy (PALS) was employed to evaluate the pore size

and fractional free volume (FFV) of polymeric membranes. The tests were performed by using a

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variable monoenergy positron beam with a counting rate of 200 to 500 counts per second in which each of the spectrum consists of two million counts. Flat sheet membranes with a dimension of 1x1cm and a thickness of around 1mm were stack at both sides of the 22Na positron

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source, thereby the polymeric membranes was sandwiched by the positron source. The quantitative information of the pore size and free volume content is assigned to the pick-off

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annihilation of the ortho-positronium (o-Ps) which is the triplet bound state of a positron and an electron.

According to the semi-empirical spherical-cavity model, there is a correlation between the mean free volume radius R (Å) and annihilation lifetime of o-Ps (τ3 in nano-second) as

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follows24,25: = [ 1 −

∆

+

sin(

∆

)]

(1)

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where ∆R is an empirical parameter (= 1.66 Å). The PATFIT program was employed to analyze the obtained PALS data. The fitted life time components were assumed to have a Gaussian distribution. Based on the Williams–Landel–Ferry (WFL) Equation (Eq. (2)), the

FFV = 0.0018 〈 ( )〉 = 0.0018

!

"# $

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relative fractional free volume (FFV) was calculated as follows.26

(2)

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Where I3 is the ortho-positronium intensity (%) for the estimated pick-off lifetime τ3 and νf is the mean free volume (Å3) of a cavity in the polymer matrix calculated from the mean free volume radius R. A detailed description of PALS can be obtained elsewhere [27-30]. The FFV of the polymeric membranes were also calculated based on the Bondi’s method using Eq. (3): % %& %&

(3)

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FFV =

Here V is the specific volume of the polymer which is calculated from the measured density (i.e., the reciprocal polymer density). Vo is the volume occupied by the polymer molecules at 0 °K.

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Vo equals to 1.3 times of the polymer’s van der Waals volume (Vw). The calculation of van der Waals volume was based on the group contribution method of Bondi [31]. Density was measured

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according to the Archimedean principle using a Mettler Toledo balance (Singapore) with a density kit. Three density measurement were recorded for each membrane film.

2.4. Measurements of gas permeation The pure gas permeability of membranes was conducted on a variable-pressure constant-volume gas permeation cell [21]. The dense membranes were placed in the permeation cell and vacuumed for 8 hours at 35oC before tests. The permeability of gases was measured in the

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sequence of O2, N2, CH4 and CO2 and each data point were repeated for three times with the average deviation of less than 5%. The rate of the downstream pressure increment (dp/dt) at the steady state was used to calculate the coefficient of gas permeability according to the Eq. (4): (× *+& (,*



-.

/0

34 12 × $ +5.3



67 68

$

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'=

(4)

where P is the gas permeability in Barrer (1 Barrer=1×10−10 cm3 (STP).cm cm−2 s−1 cmHg−1), V

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denotes to the volume of the downstream chamber (cm3), A is the effective membrane area (cm2), l refers to the membrane thickness (cm), T represent the operating temperature (K) and p2

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is defined as the upstream pressure of the permeation cell (psi). The ideal selectivity between two different gases through a polymeric membrane is the ratio of their permeability as described in Eq. (5): 9/⁄: =

1<

1=

(5)

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where PA and PB represent the gas permeability of gases A and B, respectively.

2.5. Measurements of gas sorption

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The sorption of pure gases by these PES membranes were carried out by a dual volume pressure decay method using a XEMIS microbalance apparatus [32, 33]. Prior to the assessment, each

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membrane with a weight of 50-60 mg was loaded to the sample chamber and vacuumed for 6 h. The temperature of both reference and sample chambers were kept at the desired temperature by using a water bath ((Techne FTE-10DE) from Techne Cambridge LTD (UK) with an accuracy of ± 0.01 oC.). The sorption isotherm of methane and carbon dioxide gas was conducted at 35oC up to 10 atm. The solubility of the membrane for a specific gas can be calculated using the following equation:

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?

>=

(6)

1

where C is the total concentration of absorbed gas in the membrane (cm3(STP) cm-3). The

permeability and solubility coefficients as follows: 1

(7)

A

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@=

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diffusion coefficient of each gas through membranes can be calculated after the measurement of

3. Result and discussion

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3.1 Effects of backbone structure on chain-chain distance and fractional free volume Among the four PES, PPSU has two additional aryl groups compared to PESU. TPESU consists of the trimethyl benzene group while HPESU contains a rubbery part of polyethylene oxide in the repeated units. Figure 3 and Table 2 present their XRD patterns and d-spacing values, respectively. All patterns show broad peaks which are typical for densely packed amorphous

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polymers. PPSU and TPESU have d-spacing values of about 5.1 Å corresponding to the broad peak at around 17o, while HPESU and PESU have smaller d-spacing values of around 4.7 Å corresponding to the broad peak at around 18.5o. Except HPESU, there are sharp peaks at around

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22.1o to 22.6o that correspond to d-spacing values of 3.5 Å to 4 Å for all membranes. The

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occurrence of these peaks can be attributed to the π-π stacking between the benzene rings [34] or the development of charge-transfer complexes (CTCs), which is referred to the electron transfer interactions between the electron donor and acceptor (i.e., transfer of π-electrons) [35-37] In contrast, HPESU does not have these peaks possibly because the folding of the rubbery section among the polymer chains. In other words, the rubbery portion (–PEO-PPO-PEO-) of HPESU chains may move between the benzene rings and subsequently prevents the formation of CTCs or π-π stacking between the benzene rings.

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Figure 3: XRD spectra of PES membranes Table 2: XRD data of the PES membranes o

2 theta ( )

d-spacing (Å)

for the first line

of the first line

for the second line

of the second line

17.47

5.07

22.57

3.94

16.85

5.26

22.36

3.97

18.92

4.69

22.06

4.03

HPESU

4.89

_

_

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PESU

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TPESU

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d-spacing (Å)

Membranes

PPSU

o

2 theta ( )

18.12

Table 3 shows the PALS results of the PES membranes. The fractional free volume (FFV) has been calculated from the orthopositronium lifetime (τ3) regarding to the intensity (I3) and mean free volume radius (R3) of each sample. The sequence of FFV follows the order of PESU < HPESU < TPESU < PPSU. To confirm the FFV results obtained by PALS, the Bondi’s method

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is also used to calculate the FFV based on the group contribution, as tabulated in Table 3. The same trends are observed for both FFV results. It is noteworthy that the FFV values measured from PALS are smaller than those calculated by the Bondi’s method. It is noteworthy that the

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FFV values measured from PALS are smaller than those calculated by the Bondi’s method. This may be due to the fact that PALS only considers a part of the whole size distribution of free volume (i.e., τ3 value less than 4 ns), whereas the Bondi’s method considers all accessible free

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volume.

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To interpret the PALS results, the mechanical relaxation behavior of the PES membranes is investigated. Figure 4 displays their tan δ curves and sub-Tg mechanical relaxation spectra due to the low-temperature chain motions. Table 3 tabulates the sub-Tg transitions (i.e., γ1 and γ2 transitions). PESU exhibits only one γ transition (referred to as the γ2 transition) with the peak at

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around -88oC due to the low-temperature flip motion of phenyl rings in the ethersulfone unit. In contrast, all other polymers have two transitions; namely, γ1 and γ2. In PPSU, the γ1 transition takes place at around -50 oC due to the existence of two extra aryl groups in PPSU while the γ2

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transition occurs at the same temperature of PESU owing to the flip motion of phenyl rings.

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In comparison with PPSU, HPESU exhibits both γ1 and γ2 transitions at -44 oC and -70 oC, respectively. The γ1 transition is related to the motion of the PEO portion in HPESU while the γ2 transition is due to the flip motion of phenyl rings similar to PESU and PPSU. The γ2 transition appears at a higher temperature than those of PESU and PPSU possibly because of the inhibition effect of the PEO section in HPESU which may cause the segmental motion of phenyl rings occurring at a higher temperature. This phenomenon may imply a low chain mobility and FFV in

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the HPESU polymer. Because the three methyl group in TPESU may hinder the local segmental motion of phenyl rings, but the existence of bulky methyl groups can lead to the inefficient polymer chain packing. As a consequence, the γ1 and γ2 transitions of TPESU take place at

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around -30 oC and -100 oC, respectively. The former is ascribed to the local motion of the three

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methyl groups while the latter is attributed to the flip motion of the phenyl rings.

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Figure 4: Sub-Tg mechanical relaxation spectra of PES membranes

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Table 3: PALS data and transition temperatures of the PES membranes

Membranes

PPSU

τ 3 (ns)

I3 (%)

Density R3 (Å)

FFV (%) Tγ1 (oC)

Tγ2 (oC)

1.93

23.31

2.79

3.81

-50

-88

TPESU

2.07

19.86

2.92

3.74

-30

-100

PESU

1.76

19.94

2.62

2.71

--

-88

FFV (%)[a] (g.Cm-3) 9.66

1.241

8.56

1.188

7.74

1.326

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HPESU

1.86

18.19

2.72

2.76

-44

-70

8.01

1.308

[a] Calculated based on Bondi’s method

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According to dynamic mechanical relaxation results and NMR studies reported elsewhere on PSF membranes [38-40], π flip motions of the aromatic rings and other local segmental motions lead to a more inefficient polymer chain packing and subsequently a higher FFV. Therefore,

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according to the aforementioned γ transitions, PPSU and TPESU have higher FFV than HPESU because the former pair has stronger π flip motions of aromatic rings than the latter. The FFV of

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membranes measured by PALS shown in Table 3 confirm our hypothesis. Between PPSU and TPESU, the existence of bulky methyl groups in TPESU induces greater steric hindrance for efficient chain packing than PPSU; hence, the former has a bigger pore size (2.92 vs. 2.79 Å, Table 3) and larger d-space values (Table 2) than the latter. On the other hand, the higher

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segmental motion of the two aryl groups in PPSU than the trimethyl benzene groups in TPESU leads to a slightly higher FFV of the former than the latter. (3.81vs. 3.74 %, Table 3). PESU also exhibits active π flip motions of aromatic rings but it has a linear polymer chain structure without

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any bulky side or additional aryl groups in the polymer, thus it has the lowest FFV among the four PES polymers. In contrast, the addition of PEO-PPO-PEO groups in PESU may increase the

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d-space because these groups may fill in between the aromatic rings, reduce chain packing and open up FFV. Consequently, it has a slightly higher pore size (2.72 vs. 2.62 Å, Table 3), d-space (2.48 vs. 2.38 Å, Table 2) and FFV (2.76 vs. 2.71%, Table 3) than PESU.

3.2. Effects of backbone structure on gas transport properties

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Table 4 shows the pure gas separation performance of PES membranes as a function of backbone structure. Consistent with the d-space values from XRD data and FFV data from PALS, the permeability of these PES polymers follows the order of PESU < TPESU < PPSU. However,

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HPESU is an exceptional, it has the lowest permeability. This can be attributed to the crystallinity of the –PEO-PPO-PEO- in the operating temperature (35oC), as the melting temperature of the Pluronic part is around 37.5oC (Table 1). Furthermore, the PEO-PPO-PEO

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segments of HPESU may penetrate into the aromatic segments and reduce the overall channels for gas transport. As a result, HPESU has the lower gas permeability. Clearly, the two extra aryl

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groups in PPSU and the trimethyl benzene group in TPESU induce inefficient chain packing so that they have higher permeability than PESU.

Table 4: Gas permeation properties of the PES membranes

Membrane

[a]

Ideal selectivity

O2

N2

CH4

CO2

O2/N2

CO2/N2

CO2/CH4

PPSU

1.61

0.31

0.36

9.13

5.19

29.5

25.4

TPESU

1.33

PESU

0.90

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Permeability

0.63

0.17

5.74

6.04

26.1

33.8

0.16

0.16

4.63

5.63

28.9

28.9

0.11

0.11

3.74

5.73

34.9

34.6

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HPESU

0.22

-10

[a] 1 Barrer=1×10 o C.

3

-2

-1

-1

cm (STP)cm cm s cmHg . The data were measured at 3.5 atm and 35

The diffusivity coefficients of penetrants through membranes are calculated based on the solution-diffusion model and the results are shown in Table 5. The calculation has been carried

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out based on the permeability and solubility results which have been obtained at 10atm and 35oC. The diffusivity coefficients of both CH4 and CO2 gases follow the order of PPSU > TPESU > HPESU > PESU which is exactly in accordance with the order of FFV in these polymers

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measured by PALS. Typically, the diffusion process in glassy polymer membranes rest on the nature of their fractional free volume and the flexibility of their polymer chains [39]. Hence, the highest diffusivity coefficient in PPSU can be attributed to its highest FFV and relatively high

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chain flexibility as proven by DMA (Figure 4).

Membrane

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Table 5: Diffusivity of the penetrants through the membranes

Diffusivity CH4 28.17

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PPSU

[a]

TPESU PESU

171.81

9.63

137.17

9.84

152.47

-10

2 -1

o

cm s . The data were measured at 10 atm and 35 C.

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[a] Diffusivity=1×10

256.82

11.36

EP

HPESU

CO2

In terms of selectivity, the PESU membrane has O2/N2, CO2/N2 and CO2/CH4 selectivity of 5.63, 28.9 and 28.9, respectively. Even though the incorporation of PEO-PPO-PEO crystals into PESU reduces all gas permeability significantly, HPESU shows the highest CO2/N2 and CO2/CH4 selectivity of 34.9 and 34.6, respectively among the four PES polymers possibly because of the PEO affinity to CO2. Its O2/N2 selectivity also increases slightly to 5.73. The addition of trimethyl benzene groups into TPESU not only increases all gas permeability but also increases

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selectivity of some gas pairs It has the highest O2/N2 selectivity of 6.0 and the 2nd highest CO2/CH4 selectivity of 33.8. The embedding of two additional aryl groups in PPSU contributes

selectivity because of the enlarged pore size and d-space.

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to the highest FFV and the highest gas permeability, but it has the lowest O2/N2 and CO2/CH4

3.3. Effects of backbone structure on gas transport in Henry and Langmuir sites

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Generally, PES membranes show higher CO2 sorption and diffusivity coefficients compared to CH4 (Figures 5, 6 and Table 5). This arises from the fact that the former has a higher

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condensability and a lower kinetic diameter than the latter. In order to understand the microscopic diffusion mechanism as a function of chain structure, the diffusivity in both Henry and Langmuir sites were analyzed using CO2 data. According to the dual mode sorption model, the total concentration of the sorbed gas in glassy membranes is a combination of gas

B = BC + BD = EC ' +

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concentrations in Henry and Langmuir sites as follows by Eq. (8) [9-11,21-23]: ? F G H7 H7

(8)

where C is the total penetrant concentration, CD and CH are the gas concentrations in Henry and

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Langmuir sites, respectively. The parameters KD, C’H, b and p are the Henry’s law coefficient,

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the maximum sorption capacity, the Langmuir hole affinity and the pressure, respectively.

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Figure 5: Sorption isotherms at 35 °C for polyethersulfone membranes: (a) CO2 and (b) CH4

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Figure 6: Solubility coefficient as a function of pressure for PES membranes: (a) CO2 and (b)

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CH4

Table 6 summarizes the dual-mode parameters of CO2 (e.g. C’H, kD, and b) in various PES

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membranes using the least square fitting method. The rate ratios of adsorption to desorption (i.e., b) are in the same range of about 0.6-0.65 for PESU, TPESU and PPSU because they have similar chemistry and chain structures. Although PPSU and TPESU have similar Henry’s law parameters (KD), TPESU has a greater Langmuir sorption capacity than PPSU because the former has a bigger pore size (2.92 vs. 2.79 Å, Table 3) and larger d-space values (Table 2) than the latter. Therefore, TPESU has the highest CO2 sorption coefficient (Figure 5a). The HPESU

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has the lowest dual-mode parameters (KD, C’H, b) amongst the four PES membranes because the addition of PEO-PPO-PEO into PES reduce FFV. Therefore, it has the lowest CO2 sorption

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coefficient than other membranes, as shown in Figure 5a.

Based on the partial immobilization model, the flux (N) of gas permeation through a dual-mode

I = −@C

J(?K L?G) JM

= −@C

J?K JM

− @D

J?G JM

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membrane can be calculated according to Eq. (9) [9-11,21-23,41, 42]:

(9)

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where DD and DH are the diffusion coefficients in Henry and Langmuir modes, respectively. F is the diffusivity ratio between Henry and Langmuir modes and defined as DH/DD. By integration the Eq. (9) from the high pressure side to the low pressure side across the membrane, the permeability (P) can be rewritten as follows: (Eq. (10)). LN

H72

$ @C

(10)

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' = EC 1 +

where p2 is the upstream pressure and K is a combined sorption parameter and equal to

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(C’H*b)/kD.

By plotting permeability (P) against 1/(1 + bp2), DD, F and subsequently DH can be calculated by

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least-square fitting. Table 6 tabulates the calculated diffusion coefficients in Henry and Langmuir sites for PES membranes. Consistent with previous literatures [9-11,21-23,41, 42], the Langmuir sites have a lower diffusivity than the Henry sites (DH < < DD). The order of DD (i.e., diffusivity in Henry modes) follows PPSU > TPESU > HPESU > PESU which is in agreement with the order of experimentally obtained diffusivity from the solution-diffusion model (Table 5). PPSU has the highest DH and F values. These results are in accordance with XRD, PALS, DMA analyses where PPSU shows the highest FFV and very active chain motion. In contrast,

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TPESU has the smallest DH and F values. This is possible because the bulky trimethyl benzene groups may result in the Langmuir sites with a structure more difficulties for gas to mobilize.

3

(cm ) (STP)/ cm (polymer) atm 1.153

TPESU

1.145

13.89

PESU

0.931

11.97

HPESU

0.798

10.5

37.38

300.87

0.124 7.300

0.657

57.60

242.12

0.238 8.445

0.236

64.67

254.87

0.254 3.110

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Conclusion

0.602

2

DD

− 10 2 3 b cm / s 1 × 10 cm / F K (cm ) (STP)/ 1/atm 1 × 10 3 s (Langmuir cm (polymer) (Henry mode) mode) 11.49 0.623 121.89 384.02 0.317 6.211

PPSU

− 10

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3

DH

CH’

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Membranes

KD

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Table 6: Dual mode sorption model and immobilization model parameters for CO2

In this study, a series of PESU polymers with different backbone structures were synthesized and investigated for gas separation. Both XRD and PALS data confirmed that PPSU and TPESU

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have bigger d-space, pore size and FFV values than PESU and HPESU. The following conclusions can be made:

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1. In terms of permeability, PPSU has the highest O2 and CO2 permeability because its two additional aryl groups contribute to high segmental motions. HPESU has the lowest gas permeability because its rubbery portion (–PEO-PPO-PEO-) may move between the benzene rings and prevents the formation of CTCs or π-π stacking. The trimethyl phenylene groups in TPESU increase its gas permeability, while the linear chain structure results in PESU with a low gas permeability.

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2. In terms of selectivity, the PESU membrane has average O2/N2, CO2/N2 and CO2/CH4 selectivity of 5.63, 28.9 and 28.9, respectively. HPESU shows the highest CO2/N2 and CO2/CH4 selectivity of 34.9 and 34.6 respectively possibly because of the PEO affinity to

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CO2. The addition of trimethyl phenylene groups into TPESU increases its selectivity for some gas pairs. It has the highest O2/N2 selectivity of 6.0 and the 2nd highest CO2/CH4 selectivity of 33.8. The two additional aryl groups in PPSU contribute to its highest FFV,

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but it has the lowest O2/N2 and CO2/CH4 selectivity because of the enlarged pore size and d-space.

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3. The diffusivity ratio between Henry and Langmuir modes in these four PES polymers is less than 32%. However, the chain structure affects the gas mobility in Langmuir sites.

Corresponding Author Correspondence author

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TPESU has the smallest DH and F values, while PPSU has the largest these two values.

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*E-mail address: [email protected] (T.-S. Chung)

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Tel: +65 6516 6645; fax: +65 67791936.

ACKNOWLEDGMENT

This research is supported by BASF SE, Germany for the project “The Evaluation and Characterization of Polyarylethers for Membrane Applications” (grant number R-279-000-411597), National University of Singapore, Dean’s Office of Faculty of Engineering for the project

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entitled “Natural Gas Centre” (grant number R-261-508-001-646) and Department of Chemical and Biomolecular Engineering for the project “Membrane Research for CO2 Capture” (grant

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number R-279-000-505-133).

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Highlights The effects of chemical structure on gas separation performance for a series of

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polyethersulfone have been studied. The two aryl groups in polyphenylsulfone show higher segmental motion which leads to more inefficient chain packing and fractional free volume than other polymers.

The existence of methyl bulky groups in the backbone of poly (trimethyl benzene

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high O2/N2 and CO2/CH4 selectivity.

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ethersulfone) leads to the occurrence of large pore sizes and high gas sorption as well as

The existence of PEO-PPO-PEO crystals in hydrophilic polyethersulfone results in a

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lower permeability.