Poly(ethylene oxide-b-amide 6) blend membranes

Accepted Manuscript Preparation, morphology and gas permeation properties of carbon dioxide-selective vinyl acetate-based Polymer/Poly(ethylene oxide-b-amide 6) blend membranes Mahdi Abdollahi, Morteza Khoshbin, Hossein Biazar, Ghader Khanbabaei PII:

S0032-3861(17)30603-1

DOI:

10.1016/j.polymer.2017.06.033

Reference:

JPOL 19766

To appear in:

Polymer

Received Date: 9 April 2017 Revised Date:

13 June 2017

Accepted Date: 17 June 2017

Please cite this article as: Abdollahi M, Khoshbin M, Biazar H, Khanbabaei G, Preparation, morphology and gas permeation properties of carbon dioxide-selective vinyl acetate-based Polymer/Poly(ethylene oxide-b-amide 6) blend membranes, Polymer (2017), doi: 10.1016/j.polymer.2017.06.033. 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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Preparation, Morphology and Gas Permeation Properties of Carbon DioxideSelective Vinyl Acetate-Based Polymer/ Poly(ethylene oxide-b-amide 6) Blend

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Membranes Mahdi Abdollahi*, Morteza Khoshbin, Hossein Biazar, Ghader Khanbabaei

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New CO2-selective vinyl acetate (VAc)-based polymer/ Pebax 1657 blend membranes were prepared by solution casting method. Blending Pebax with VAc/dibutyl maleate copolymer can

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be considered as an appropriate strategy to prepare CO2/CH4 selective membranes.

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Preparation, Morphology and Gas Permeation Properties of Carbon DioxideSelective Vinyl Acetate-Based Polymer/ Poly(ethylene oxide-b-amide 6) Blend

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Membranes Mahdi Abdollahia*, Morteza Khoshbina, Hossein Biazara, Ghader Khanbabaeib

a) Polymer Reaction Engineering Department, Faculty of Chemical Engineering, Tarbiat

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Modares University, PO. Box: 14115-114, Tehran, I.R. Iran

b) Development Division of Chemical, Polymer and Petrochemical Technology, Research

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Institute of Petroleum Industry, PO. Box 18745-4163, Tehran, I.R. Iran

Abstract

Blend membranes based on the Pebax 1657 and polyvinyl acetate (PVAc) or VAc/ dibutyl maleate copolymer (P(VAc-co-DBM)) were prepared via solution casting. SEM micrographs did

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not show any phase separation in the macro-scale. FT-IR results showed that hydrogen bonding of NH groups with carbonyl groups in the polyamide (PA) microphases or ether groups in the

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poly(ethylene oxide) (PEO) microphases changes towards hydrogen bonding of NH groups with the VAc-based polymers in the blends. Melting temperature and crystallinity of the both PA and

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PEO microphases as well as the glass transition temperature of the PEO microphase decreased by adding VAc-based polymers. It was found that P(VAc-co-DBM) copolymer chains in the blend membrane containing 50 wt% copolymer have almost a similar tendency towards both PA and PEO microphases, while those in the blend membranes containing copolymer content lower than 50 wt% are mostly located between PA microphases. It was also deduced from DSC thermograms that a separate phase of VAc-based copolymer can be formed in the membranes *

Corresponding author: Tel: +98 21 8288 4959, Email: [email protected]

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containing copolymer content more than 20 wt%. Despite decreased CO2 permeability, selectivity of CO2/CH4 improved significantly from 17.6 for pure Pebax to 37.5 for a blend with 30 wt% copolymer. Results obtained from permeability test revealed that in the blends

more than that of the P(VAc-co-DBM) copolymer.

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containing similar amounts of VAc-based polymers, the effect of the PVAc homopolymer is far

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permeability and selectivity; carbon dioxide and methane

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Key words: Vinyl acetate-based (co)polymer; blend membrane; structure and morphology;

Introduction

Natural gas is one of the most commonly used energy sources which, in addition to its low price, it has lower pollution in comparison with the other energy sources. Methane (CH4) is the major

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component present in the natural gas, however, low but significant amounts of heavy hydrocarbons, acid gases (i.e. H2S and CO2), steam and lighter gases such as nitrogen (N2) and Helium (He) are present in the natural gas [1,2]. Presence of acid gases of SO2 and CO2 in the

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natural gas not only decreases energy content of the transported gas but also causes transport pipeline corrosions. Moreover, these acid gases are released to the environment when natural gas

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is consumed; therefore, concentration of these gases increases in atmosphere, resulting in the warming of Earth [3]. Due to the significant amount of CO2 relative to the other acid gases, it is important to remove CO2 gas. According to the common standards, amount of CO2 in the natural gas should be less than 2 v% [4]. Different methods are used in the industry to remove CO2. Chemical absorption by reactive solvent and cryogenic separation are most commonly used methods. In addition to operational

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problems, these methods are expensive, complex and energy consuming [5]. Hence in the past two decades, researches in the gas separation have focused on the membranes. Gas separation by the membrane is not a complex process and consumes a little energy. There are different types of

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membranes, however, due to their low cost, widely available mechanical properties and easy preparation methods, polymer membranes have attracted a lot of attention [6].

Block copolymers of polyamide (PA) and polyether under the trade name of Pebax® with the

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microphase- separated morphology have frequently been used as a thermoplastic elastomer in the preparation of gas separation membranes [7-12]. To investigate effect of block type and amount

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of Pebax on the CO2 permeability, four different types of this block copolymer with trade names of Pebax 2533, Pebax 4033, Pebax 1074 and Pebax 4011 have been used [10]. These copolymers have

different

structures

in

the

polyether

blocks

(polyethylene

oxide

(PEO)

or

polytetramethylene oxide (PTMO)) and PA blocks (PA6 or PA12) with different weight

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percentage (wt%) of components in the copolymer chains. It has been observed that by increasing the polyether content, not only CO2 permeability but also its selectivity over nonpolar gases of N2 and hydrogen (H2) increases [10]. It has been attributed to the increased CO2

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solubility of the membranes due to the increased quadrupolar interactions between the ether groups in the block copolymer and CO2 gas. Blending polymers with inorganic (nano)particles or

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with other polymers is a simple way to improve desired properties. There are reports on the improvement in the separation properties of Pebax via blending with inorganic particles and various polymers [13-18]. Blend membranes based on the Pebax 1657 and 50 wt% PEO with a molecular weight of 200 g/mol has been prepared and subjected to the permeability test. In comparison with pure Pebax membranes, CO2 permeability increased two fold and CO2/CH4 selectivity increased 20% [11,12]. Moreover, order of crystalline structure in the polyether and

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PA microphases ruptured for blends with 20 wt% or higher PEO, resulting in the significant increase of the free volume between the chains. Therefore, permeability increased with a higher rate for membranes with higher PEO content.

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In another work, effect of blending PEO/polypropylene oxide (PPO) random or block copolymer with Pebax 1657 on the gas permeability of the blend membranes have been studied [19]. It has been observed that PPO units randomly distributed in the PEO chain are more effective in

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decreasing crystallinity; therefore, membranes prepared by random copolymer have a better performance in comparison with those prepared with PEO/ PPO block copolymer.

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With the aim of replacing tricholorofluoromethane (CFCl3) with another suitable chemical in the preparation of the polymer foams, solubility of CO2 under different pressures and temperatures in PVAc has been obtained to be between 5-300 g/Kg of polymer, indicating that there is a good interaction between CO2 and PVAc [20]. On the other hand, alternative copolymer of VAc and

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dibutyl maleate (DBM) with xanthate end groups has been synthetized by controlled radical polymerization mediated by the reversible addition-fragmentation transfer polymerization (RAFT) [21]. Solubility results showed that these alternative copolymers are soluble well in

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supercritical carbon dioxide (Sc-CO2) at a temperature range of 25-75 °C and a pressure higher than 2000 psi [21]. It was found that solubility of alternative copolymer is higher than PVAc approaching

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homopolymer,

that

of

perfluoropolymer

and

poly(dimethyl

siloxane)

monomethacrylate (PDMS-mMA) [21]. It has been reported that xanthate end groups present in the VAc-based polymers decrease CO2 solubility of the polymers [21, 22], while incorporating halogen atoms into the VAc-based polymers increase CO2 solubility of polymers [23-25]. More recently, Sc-CO2 solubility of both alternative and random copolymers of VAc and DBM has also been investigated [26]. It has been found that under same molecular weight, solubility of

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random copolymers in the Sc-CO2 is higher than alternative one. On the other hand, CO2philicities of the PVAc homopolymer and P(VAc-co-DBM) copolymers with different alkyl groups including methyl, ethyl and butyl were evaluated by multiscale molecular modeling and

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dissolution behavior measurements [27]. It has been found that although copolymerization of dialkyl maleate with VAc decreases the polymer-CO2 interactions, however the weakened polymer-polymer interactions increase CO2-philicity of the copolymers. Results of cloud point

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pressure measurement showed that P(VAc-co-DBM) has the most CO2-philicity among the four polymers studied.

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Gas permeability properties of the blend membranes prepared from high molecular weight PVAc (MW= 1.7 × 105 g/mol) and Pebax 1074 have been studied [18]. All of the prepared membranes were homogenous on the macro-scale; however, DSC and SEM results showed the microphase separation. PVAc/ Pebax 1074 blend membrane with a high PVAc content has been reported to

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be a suitable membrane for separation of CO2 from CH4.

To our knowledge, there is no report on the preparation and gas permeation evaluation of the blend membranes from low molecular weight PVAc and Pebax 1657. Moreover preparation of

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the blend membranes based on the VAc/ DBM copolymer and Pebax has also not been reported. Hence, blend membranes from Pebax 1657 and P(VAc-co-DBM) copolymer (0-50 wt%) or

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PVAc (0-10 wt%) are prepared in the present study by solution casting method. PVAc and P(VAc-co-DBM) are synthetized via controlled radical polymerization based on the (reverse) iodine transfer polymerization ((R)ITP) technique. Then structure, morphology and thermal properties of the blend membranes are investigated by FT-IR, SEM and DSC analyses respectively. Thermodynamic tendency of VAc-based polymer towards the PA6 and PEO microphases of the Pebax was also evaluated theoretically. Then, gas permeation properties of

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the membranes with the aim of the more efficient separation of CO2 from CH4 are evaluated by constant volume/ variable pressure method. Finally, morphological change in the blends by adding P(VAc-co-DBM) and its effect on the permeation properties is investigated. Based on the

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results obtained and by considering membrane plasticization by CO2, it will be conclude in the present work that P(VAc-co-DBM) copolymer, instead of PVAc, can be introduced as a suitable

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candidate for a selective separation of CO2 from CH4 with a good aging properties.

Experimental

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Materials

Pebax 1657, comprising 60 wt% PEO and 40 wt% PA6 (Fig. 1), was received from Arekma. Monomers, VAc (99% purity, Merck), DBM (synthesis grade, Merck) were dried by anhydrous calcium hydride under magnetic stirring for 12 hours and then distilled under reduced pressure

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and were kept at -4 °C before use. 2,2ʹ-Azobis (isobutyronitrile) (AIBN) (98% purity, Fluka) as an initiator and hexane as a nonsolvent of the product in both ITP and RITP techniques, ethyl iodoacetate (EtIAc) (Merck) as a chain transfer agent (CTA) in the ITP and molecular Iodine (I2)

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(Merck) as an in situ generator of CTA in the RITP were used without further purification. nbutanol and n-propanol as the solvents, both from Sigma-Aldrich, were used for preparation of

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the membrane solutions.

Fig. 1

Synthesis of PVAc and P(VAc-co-DBM) PVAc and P(VAc-co-DBM) (Fig.1) were synthetized using similar procedures reported in the literature by ITP [28] and RITP [29] techniques, respectively. Molar ratios of [VAc]0: [EtIAc]0: [AIBN]0 in ITP and [VAc+DBM]0: [AIBN]0: [I2]0 in RITP were chosen to be 100:1:1 and

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60:1:0.2, respectively, where molar ratio of [VAc]0: [DBM]0 in the latter case was adjusted to be 10:1 [28,29]. After 15h in the case of ITP and 4h in the case of RITP, reactions were stopped and products were precipitated by an excess amount of the n-hexane. Precipitates were then

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redissolved in the solvent and precipitated again in n-hexane. Purified products were dried under vacuum at 60°C for 24h. PVAc and P(VAc-co-DBM) products were subjected to NMR and GPC analysis and then used in the preparation of the blend membranes with Pebax 1657.

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Preparation of blend membranes

A solution (3wt%) of Pebax 1657 in the n-propanol/ n-butanol mixture (3/1 V/V) was prepared

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under stirring and refluxed at about 100°C for 24h. Solution was cooled to 60°C and required amount of PVAc or P(VAc-co-DBM) was then added to the solution and stirred for additional 2h to obtain homogenous solution. The obtained solution was poured into the glass petri dish and dried in the vacuum oven under reduced pressure for 48 h at 40°C. Drying of the blend

Characterization

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membrane was continued until its weight remained constant.

To obtain copolymer composition of P(VAc-co-DBM), 500 MHz 1H-NMR spectrometer (Bruker

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Avance) at the ambient temperature was used. Chloroform-d (CDCl3) was used as a solvent. Gel permeation chromatography (GPC) (1100 Agilent) was used to determine molecular weight and

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its distribution of the VAc- based homo- and copolymers. GPC columns were calibrated with narrow molecular weight distribution polystyrene standards in the molecular weight range of polymers under analysis. THF was used as an eluent solvent with a flow rate of 1ml/min at 30°C. To evaluate thermal behavior and to study phase state in the blend membranes, differential scanning calorimeter (DSC) (Perkin Elmer 8000) was used under N2 atmosphere at a temperature range from -100°C to 250°C with a heating rate of 10°C. To further study morphology of the

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membranes, scanning electron microscope (SEM) model TESCAN SEM-VEGA was used. To increase contrast of the micrographs, samples were coated under vacuum with a thin film of gold. Samples were broken in liquid nitrogen and cross sections of the membranes were

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subjected to SEM observation. Structure and intermolecular interactions in the membrane films were studied by Fourier transform infrared (FT-IR) spectrometer (model Perkin Elmer frontier) in the mode of attenuated total reflectance (ATR) in the wave number range of 500-4000 cm-1.

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Gas permeation measurement

CO2 and CH4 permeabilities of the blend membranes were measured at the ambient temperature

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with a volume constant/ variable pressure method (time-lag method) at the constant upstream feed pressure of 3 bar while change in pressure in the downstream of membrane as a function of time was recorded. All tests were repeated three times and average values were reported. The experimental error in the permeability measurements relative to its average value was observed

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to be lower than ± 1.5%. To remove air before test, both upstream and downstream volume of the permeability test were evacuated. Permeability (P) (based on the barrer unit) was calculated via solution-diffusion model under steady state condition by equation (1).

1 V 273.15 l dP 76 A T P dt

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P = D ×S =

(1)

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in which D (cm2/s) and S (cm3/cm3.bar) indicate gas diffusion and solubility coefficients, respectively, l (cm) is the membrane thickness, V (cm3) indicates the chamber volume, T (K) is the temperature, A (cm2) indicate an area of the membrane which gas permeate from it, P (cmHg) is the pressure of feeding gas (3 bar in the present work) and dP/dt (cmHg/s) indicates slope of the gas permeated from membrane versus time under steady state condition which was obtained from experimental data of downstream pressure versus time curves for both CO2 and CH4 gases. 8

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Then, diffusion coefficient (D) can also be calculated with time-lag method using equation (2).

D=

l2 6θ

(2)

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in which θ (s) indicates time lag of permeate, i.e. intercept on the time axis in the experimental downstream pressure versus time curves. It should be noted that due to very low permeability of the CH4 from mmbranes, time-lag method for calculation of D was used in the present work with

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a high accuracy only for CO2 permeation. From P and D data at hand, solubility coefficient (S) can now be estimated using Eq. (1).

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Ideal selectivity of the membrane for CO2 over CH4 (

/

) was calculated from

permeabilites of the pure CO2 and CH4 gases using equation (3).

αCO

2 /CH 4

=

PCO 2 PCH 4

=

S CO 2 .DCO 2 S CH 4 .DCH 4

= α S .α D

(3)

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in which αS and αD indicate solubility and diffusion selectivity, respectively.

Results and Discussion

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As a comonomer with a suitable solubility parameter, DBM has been used in a synthesis of VAcbased polymers to increase ability of the copolymer for interaction with the CO2 [21, 26, 27, 30].

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Due to the proper solubility parameter of PVAc relative to the blocks present in the Pebax [18,30-32], although PVAc has been used previously in the preparation of gas separation membrane based on the Pebax 1074 [18]; however, cloud point pressure results of PVAc and P(VAc-co-DBM) have indicated that P(VAc-co-DBM) has the most CO2-philicity [27]. Solubility parameters of the components present in the prepared blends membranes along with the CO2 gas are shown in Table 1. Weakened polymer-polymer interactions, instead of its

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solubility parameter, in the P(VAc-co-DBM) copolymer has been reported to increase CO2philicity of the copolymer [27]. Therefore, investigating effect of the blending P(VAc-co-DBM) copolymer with Pebax and comparing their gas permeation results with those reported for PVAc/

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Pebax blend membrane could be interesting. Table 1 Characterization of VAc-based polymers

) and polydispersity index (PDI) of the molecular weight

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Number average molecular weight (

were obtained by GPC analysis to be 7720 g/mol and 1.40, respectively, for PVAc and 8010

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g/mol and 1.64, respectively, for P(VAc-co-DBM). 1H-NMR spectrum of the P(VAc-co-DBM) is shown in Fig. 2. All peaks were assigned to the corresponding protons [29]. Mole fraction of VAc in the copolymer (FVAc) was calculated from 1H-NMR spectrum by using Eq (4). 4I b 4I b + I e

(4)

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FV A c =

in which Ib and Ie indicate intensities of peaks “b” and “e” shown in Fig. 2. FVAc was calculated to be 0.844 for P(VAc-co-DBM) synthetized in the present work.

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Fig. 2

Structure and morphology of the blend membranes

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Two PEO and PA blocks in the Pebax have been reported to show phase separation in the microscale [18,19]. In other words, membranes based on the Pebax are homogenous in the macroscale. Thermal behavior of the blend membranes in the present work were studied by DSC analysis (Fig. 3) and effect of PVAc and P(VAc-co-DBM) on the melting (Tm) and glass transition (Tg) temperature as well as on the crystallinity (Xc) of the microphases in the blend membranes was evaluated. VAc units in the VAc-based polymers have been reported to undergo degradation at a temperature higher than 200°C [33]; hence, DSC tests was recorded only in one 10

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heating stage from -100 to 250°C. It is clear from Fig. 3 that pure Pebax 1657 shows only one Tg at -54 °C corresponding to the PEO microphase. Moreover, two melting temperature at about 19.17 and 209.47 °C can be observed for pure Pebax 1657, corresponding to the PEO-rich and

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PA6-rich, respectively, microphase- separated domains. There is also a broad peak at a range of about 40-100°C which has been appeared for pure Pebax 1657 as well as for blend membrane especially for that containing 10 wt% P(VAc-co-DBM). This broad peak has been disappeared

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for pure Pebax under re-heating (Fig. 3) after cooling, indicating that it is related to the formation of defect crystals in the PA6 microphase [17]. Hence, when a new additive is added to the

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microphase- separated matrix such as Pebax 1657, homogenous distribution of additive in the matrix is not expected. In other words, chains of the additive are located mostly in a microphase with a higher interaction with the additive. PVAc chains have a higher tendency to establish the hydrogen bonding with the PA6 chains. On the other hand, Tg of the PEO microphases decreased

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by increasing the copolymer content due to the weakening of the hydrogen bonding interaction between the PEO and PA6 block chains. It means that there is also no significant interaction between the PEO chains and VAc-based polymer chains.

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Fig. 3

To distinguish the quality of carbon black dispersion in the blends of high density polyethylene

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(HDPE) and ethylene-vinyl acetate copolymer (EVA), Sumita`s approximation method (Eq. 5) [34] has been used to obtain the wetting coefficient [35].

w A − B = ( γ C − B − γ C −A ) / γ A −B

(5)

in which γA-B, γC-A and γC-B are defined as interfacial tension between polymers A and B, polymer A and additive C and polymer B and additive C, respectively. wA-B is the wetting coefficient of the two component matrix containing A and B (micro)phases. Based on the value

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of wA-B, three situations can be considered for dispersion of additive C. when wA-B value was greater than 1, then additive C will be dispersed mostly in the phase A, while when wA-B was less than -1, then additive C will be dispersed mostly in phase B. on the other hand, additive C will be

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dispersed mostly between the interface of the polymers A and B when -1 < wA-B < 1. To estimate interfacial tension of the two phases i and j, equations (6) and (7) can be used [36].

 γ id γ dj γ ip γ jp +  γ id + γ dj γ ip + γ jp 

=

  

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γ ij = γ i + γ j − 4  +

(6)

(7)

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in which γi is the interfacial tension of phase i, γid and γip indicate dispersion and polar components of the interfacial tension, respectively. Values of interfacial tensions of the materials used in the present work are given in Table 2. Due to the low amount of DBM comonomer (15.6 mol %) in the copolymer, only interfacial tension of PVAc homopolymer was considered in the

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calculations. By data reported in Table 2, wetting coefficient of PEO/PA6 matrix for PVAc was obtained to be -2.21 which is less than -1, meaning that PVAc and P(VAc-co-DBM) have a thermodynamic tendency to locate in the PA6 microphase. On the other hand, due to the

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preparation of blend membranes at 40°C and softening temperature of PA6 blocks at about 50°C [37], viscosity of the PA6 microphase seems to be high enough to not surround the additive

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added to the matrix. On the other hand, PEO microphase melts at a temperature lower than 20°C. Therefore, thermodynamic tendency of the VAc-based polymers towards the PA6 microphase seems to have a slow kinetic depending on the operation condition and amount of VAc-based polymers added to the Pebax matrix under a such condition. Hence, locating VAc-based polymers between the low viscous PEO chains is also possible. Table 2

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DSC analysis was used (Fig. 3) to further investigate microphase separation of the blend membranes. It is clear from this figure and Table 3 that normalized area of the melting peaks of PEO and PA6 decrease by increasing P(VAc-co-DBM) content, indicating that crystallinity

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decrease in both microphases by adding copolymer. This decrease can also be observed for area of the defect crystals appeared at about 40-100°C. Crystallinity percentage in microphase i (Xc,i(%)) can be calculated using Eq. (8).

∆H m ,i

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X c ,i =

∆H mo ,i

(8)

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in which ∆Hm,i (J/g of microphase i, i.e. J/gi) indicates melting enthalpy of the crystals of microphase i and is obtained after normalization of the corresponding melting peak area relative to the weight fraction of the microphase i present in the blend membrane. ∆H°m,i is defined as a melting enthalpy of the microphase i at a 100% crystalline state where its values for PEO and

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PA6 have been reported to be 166.4 and 230.0 J/g, respectively [19]. Tg and Tm values along with Xc(%) values for each microphase are given in Table 3. Table 3

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By increasing the copolymer content in the blend membrane, both Tm and Xc values of both PEO and PA6 microphases decreased, indicating the partial compatibility and interactions between the

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copolymer chains with PEO and PA6 microphases. Decrease in the Xc (%) value of the PA6 microphase is almost equal with that of the PEO microphase, meaning that almost equal amounts of copolymer chains enters into the Pebax microphases. However, more detailed investigation of the decrease trend in Tm and Xc(%) with copolymer content can be more important. Tm of the PA6 crystals decreases significantly from 209.47°C for pure Pebax to 203.13 °C for a blend membrane with 20 wt% copolymer, while with further increasing the copolymer content, Tm of the PA6 microphase decrease only to 200.80 °C for a blend membrane with 50 wt% copolymer 13

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(Table 3). The similar trend can also be observed for the PEO crystals (Table 3) except that decrease in the Tm value is not significant for the blend membrane containing 30 wt% and higher copolymer content. There is a proven relationship between the compatibility and change in the

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Tm value of crystals in the polymer blends [32,38]. Therefore, one can conclude that PA6 and PEO microphases in blend membranes containing 30 and 20 wt%, respectively, copolymer cannot form further effective interactions with the added copolymer chains. Then, it is expected

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that peak of Tg for a separate phase of VAc-based copolymer can also be observed in the DSC thermograms; however, due to the broad peak of PA6 defect crystals in the temperature range of

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about 30-100°C, observing this peak is not possible [18]. Another important point in DSC results is its decreasing trend in the Xc(%) values of PEO and PA6 microphases of the blend membranes as a function of copolymer content. Xc,PEO and Xc,PA6 values decreased from 21.67 and 25.75%, respectively, for the pure Pebax to 15.17 and 18.25%, respectively, for the blend membranes

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containing 30 wt% copolymer. With further increasing the copolymer content to 50 wt%, Xc,PEO decreased significantly to 10.75%, while Xc,PA6 decreased only to the 16.50%. It means that in the blend membranes containing low copolymer content up to 30 wt%, most of copolymer chains

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enter into the PA6 microphases but in those containing copolymer content as high as 50 wt%, copolymer chains tend to enter into the PEO microphases. It can be attributed to the kinetically

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easy entrance of copolymer chains between the PEO chains due to the lower Tg and viscosity of the PEO microphases in comparison with those of PA6 microphases. These results are in good agreement with those obtained theoretically. DSC analysis was also done for PVAc/ Pebax 1657 blend membrane containing 10 wt% PVAc homopolymer (Fig. 4). Comparing thermograms of the blend membranes containing similar 10 wt% homo- and copolymer (Figs. 3 and 4 and Table 3) indicates that interactions and

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compatibility between the PVAc and Pebax are more effective and stronger than those between the P(VAc-co-DBM) and Pebax. It can be attributed to the presence of bulky DBM units in the

Fig. 4

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copolymer where interaction between the copolymer and Pebax decreases [27].

Intermolecular interactions between the ingredients in the blend membranes were further studied using FT-IR spectroscopy (Fig. 5) (please see full spectra in the wave number range of 400-4000

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cm-1 in the supplementary data as Figs. S1 and S2). Characteristic absorption bands of Pebax 1657 appear at 3295 (strong peak, stretching vibration of the amide NH group), 1638 (strong

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peak, stretching vibration of the amide carbonyl group), 1734 (weak peak, stretching vibration of the carbonyl groups of ester linkage between the PA6 and PEO blocks) and 1094 cm-1 (strong peak, stretching vibration of the ether C-O group) (Fig. 1). Results given in Fig. 5 and Table S1 (given in the supplementary data) for pure Pebax and blend membranes showed that wave

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number of characteristic peaks of the Pebax change by adding VAc-based homo- and copolymer, indicating that there are significant intermolecular interactions between the VAc-based polymers and Pebax [39]. In other words, hydrogen bonding between the Pebax chains shift towards those

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between the Pebax chains and VAc-based chains, resulting in the partial compatibility between the dispersed and matrix phases in the blend membranes [17]. The results are consistent with

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those obtained from DSC thermograms. Fig. 5

FT-IR results (Table S1) for blend membranes containing the same homo- or copolymer content (i.e. 10 wt%) indicated again more effective and stronger interaction between the ingredients of the PVAc/Pebax membrane in comparison with the P(VAc-co-DBM)/ Pebax membrane. By considering the FVAc in the copolymer (0.844) and this fact that each DBM contains two carbonyl

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groups, mmol of the carbonyl groups per gram of the VAc-based homo- and copolymer were calculated to be 11.6 and 10.7, respectively, indicating that under same weight content of the VAc-based polymers, PVAc chains have a more sites to interact with the Pebax. Moreover,

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bulky DBM groups in the P(VAc-co-DBM) has been reported to weaken the polymer-polymer interactions [27]. These results are in a good agreement with those of the DSC analysis.

SEM micrographs obtained from surface and cross-section of the pure Pebax, 10 wt% PVAc/

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Pebax and 30 wt% P(VAc-co-DBM)/ Pebax membranes are shown in Fig. 6. There is no phase separation in the macro-scale, indicating a good mixing of VAc-based additives with Pebax.

Gas permeation properties

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There is also no significant defect and/or crack in the prepared membranes.

As mentioned previously, more attentions have focused on the CO2 separation from low polar gases by Pebax due to its good CO2 permeability and selectivity. Solubility parameter of the CO2

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is close to the PEO in comparison with PA6 (Table 1), indicating higher solubility and interaction of CO2 with the PEO phase. Moreover, Tg and Tm of the PEO microphase are below room temperature (25°C). Therefore, it is expected that CO2 permeates mostly from PEO

EP

microphases which PA6 microphases act as an improver of the mechanical properties of the membranes. Due to its linear structure, PEO chains have a higher ability to crystallize,

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decreasing the free volume between the chains and thereby decreasing the membrane permeability at the temperature about lower than 25°C [40]. Therefore, efforts have been made to increase permeability parameter of Pebax by reduction of the order structure of the Pebax via blending with various materials. These efforts have been resulted in successful results where CO2 permeability has been increased significantly without remarkable decrease in the selectivity [12,17-19]. It is clear from Table 1 that in comparison with PEO and PA6, solubility parameters

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of VAc and DBM is much closer to that of CO2. Therefore, PVAc and P(VAc-co-DBM) were used in this work as additives in the preparation of Pebax-based membranes in order to increase

and selectivity can be improved. Fig. 6

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interactions between the ingredients of the membrane and polar CO2 gas from which solubility

Results of permeability test performed on the blend membranes are given in Table 4.

DBM) are plotted in Fig. 7.

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Table 4

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Permeabilities of CO2 and CH4 from blend membranes containing various wt% of P(VAc-co-

Fig. 7

It is clear from Fig. 7 that CO2 permeability decreases significantly by increasing the copolymer content up to 40 wt% and then remains almost constant with further increasing the copolymer

TE D

content. On the other hand, CH4 permeability decreases significantly by increasing copolymer amount up to 40 wt% and then increases by further increasing the copolymer content. Maximum decrease in the CO2 and CH4 permeability of the blend membranes relative to the pure Pebax

EP

membrane was observed to be 51.5% and 73.0%, respectively, for membranes containing 50 and 40%, respectively, P(VAc-co-DBM). It means that CO2/CH4 selectivity would be increased by

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the copolymer content up to 30 wt% (Table 4). Based on Eq. (1), permeability is a product of two parameters, i.e. diffusivity and solubility of gas in the membrane. The diffusivity and solubility coefficients of the CO2 calculated from permeation test are given in Table 4. As expected, CO2 solubility in the membranes increased gradually by increasing the copolymer content up to 40% and then increased significantly by further increasing the polymer content to 50 wt% (Fig. 8). Remarkable increase in the solubility coefficient (S) of CO2 for a blend

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membrane containing 50 wt% copolymer may be attributed probably to the formation of continuous phase comprising P(VAc-co-DBM) and PEO block. In other words, change in the morphology of the blend membrane is occurred when Pebax is blended with more than 30 wt%

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copolymer which is noticed in the DSC results. On the other hand, decreased FFV value may reduce diffusion rate of the CO2 gas across the membrane film, resulting in the decreased permeability of CO2 (Table 4) by increasing the copolymer content in the blend membranes.

copolymer probably due to the morphology change.

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Fig. 8

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Simultaneously, CH4 permeability increases significantly for a blend membrane with 50 wt%

By increasing the copolymer content, quadropole interactions between polar CO2 gas and membrane increases, resulting in the increased CO2 solubility especially when the copolymer content increases to 50 wt%. on the other hand, CO2 diffusivity decreases significantly by

TE D

increasing the polymer content in a range of 0-50 wt% (Fig. 9), meaning that CO2 permeability decreases by increasing VAc-based copolymer due to the significant decrease in the diffusion coefficient. It can be attributed to the increased density and thereby decreased fractional free

EP

volume (FFV) of the blend membranes by introducing VAc-based polymers into the Pebax

AC C

matrix (Table 5). Diffusion coefficient is affected by the FFV value of membrane [6] (Eq. (9)). Fig. 9

B   D = A exp  −   FFV 

(9)

in which A and B are constants of the equation which are dependent on the type and pressure of the permeated gas. It is clear from equation (9) that at constant A and B values, D values are only affected by the FFV values of the membrane. Decreased CO2 diffusion coefficient in the presence of additives (Table 4) can be explained firstly by the higher densities of the VAc-based

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polymers in comparison with the pure Pebax (Table 5) and secondly by effective and strong interactions between the Pebax and VAc-based chains which can result in the packing of chains

Table 5

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in the blend membranes [39].

Densities of pure ingredients constituting the blend membranes are given in Table 5. Theoretical density of blend membranes can be estimated by the mixture rule via Eq. (10) [41].

ρb

=

w1 w 2 +

ρ1

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1

ρ2

(10)

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in which ρb indicates the blend density, w1 and w2 indicate weight fraction of component 1 and 2 (Pebax and VAc-based polymer, respectively, in the present study) in the blend membrane and ρ1 and ρ2 are corresponding densities of the components 1 and 2. Theoretical density of blend membranes was calculated by Eq. (10) using data given in Table 5. Results are given in Fig. S3

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in the supplementary data, indicating that the blend density increase by increasing the P(VAc-coDBM) content from which decrease in the FFV values and thereby in the D values are expected. Effect of the P(VAc-co-DBM) content in the blend membranes on the CO2/CH4 selectivity is

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shown in Fig. 10. It is clear from this figure that selectivity increases significantly by increasing the P(VAc-co-DBM) content up to 30 wt% and then decreases by further increasing the

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copolymer content. Based on the above explanations, increase in the selectivity can be attributed to the increased solubility of CO2 gas without any significant solubility change in the CH4 solubility [18], as well as to the significant decrease in the CH4 diffusivity in comparison with that in the CO2 one (Fig. 11, see the following section for more details about this figure) because kinetic diameter of CH4 (3.80 A°) is greater than that of the CO2 (3.30 A°) [42].

Fig. 10

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According to Eq. (1), solubility coefficient of any gas in a membrane can be calculated by dividing permeability to the diffusion coefficient of the corresponding gas. CH4 permeability (PCH4) and diffusion coefficient (DCH4) for pure Pebax and Pebax 1657/ PEO blend membranes

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have been reported in the literature [12], from which CH4 solubility coefficient (SCH4) can be obtained via Eq. (1). Then, Eq. (11) can be used to calculate normalized CH4 solubility coefficient in the blend relative to that in the pure Pebax, i.e. Sblend/Spure

Pebax.

Results of the

SC

calculation have been given in the supplementary material as Fig. S4, indicating that SCH4 has not been affected by adding PEO to the Pebax matrix, indicating that interaction between the

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nonpolar CH4 and membrane is not changed by adding PEO as an polar additive to the Pebax . In other words, it means that changes in the permeability of these membranes can be attributed to changes in the SCO2, DCO2 and/or DCH4.

S pure Pebax

=

Dblend

Ppure Pebax D pure Pebax

(11)

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Pblend S blend

Same as above-mentioned work [12], polar additive, i.e. VAc-based polymer, has been added to

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the Pebax 1657 in the present work. Therefore, Sblend/Spure Pebax for CH4 can be assumed to be almost equal to 1 for all membranes prepared in the present work. Then, Dblend/Dpure Pebax ratio for

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CH4 will be equal to the Pblend/Ppure Pebax ratio for CH4. Calculations results for CH4 and CO2 are shown in Fig. 11. It is clear from this figure that by adding P(VAc-co-DBM) to the Pebax up to 30 wt%, Dblend/Dpure Pebax ratio for CH4 (slope = - 2.37) decreases with a higher rate than that for CO2 (slope = - 1.98); however, by further adding copolymer to the Pebax, this ratio for CH4 increase significantly (Fig. 11). As observed from DSC results, this behavior for blend membranes having more than 30 wt% copolymer can be attributed to change in the morphology of the membranes where PEO/P(VAc-co-DBM) mixture forms a discrete continuous CO2-phile 20

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(Fig. 8) and low crystalline (Table 3) micro-separated phase, resulting in the significantly increased CO2 solubility and CH4 diffusivity. On the other hand, Due to the stronger interactions between the CO2 and this discrete continuous CO2-phile phase, CO2 diffusivity decreases for

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blend membranes having high content P(VAc-co-DBM). Therefore, CO2/CH4 selectivity decreases by further increasing the P(VAc-co-DBM) content in the blends (Fig. 10). More detailed investigation as a separate work is under study to experimentally determine density and

present work.

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Fig. 11

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FFV values of the blend membranes and then fully explain permeability results observed in the

CO2 and CH4 permeability results are also given for the 10 wt% PVAc/ Pebax membrane in Table 4. Compared with P(VAc-co-DBM)/ Pebax membrane under same additive content, PVAc/ Pebax membrane shows much higher CO2 solubility and very low CO2 diffusivity,

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resulting in the significant decrease of the CO2 permeability. On the other hand, under same conditions, CH4 permeability decreases with a higher rate, resulting in the increased CO2/CH4 selectivity of the PVAc/ Pebax. It was attributed to the higher density of the PVAc chains (Table

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5) as well as to more packing of chains due to the stronger interaction between the components in the blend, which results in the significant decrease in the FFV value of the membranes. It

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should be mentioned that further increase in the amount of the low molecular weight PVAc in the preparation of the membranes resulted in membranes with low mechanical properties against the CO2 gas where CO2 permeability test was not possible. It can be attributed to the very high tendency of the CO2 gas to be dissolved in the membrane. However, under almost same selectivity values, PVAc-based membrane has a CO2 permeability higher than that of P(VAc-coDBM)-based copolymer (Table 4). It means that simultaneous higher selectivity and

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permeability can be achieved by PVAc homopolymer; however, aging (plasticization effect) of this membrane does not seem to be suitable due to the higher CO2-philicity of the corresponding blend membranes. In other words, higher selectivity along with the acceptable permeability and

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aging property can be achieved when the P(VAc-co-DBM) is used to prepare blend membrane. Although CO2-philicity of the P(VAc-alt-DBM) alternating copolymer (FVAc = 0.5) has been reported to be better than that of PVAc homopolymer [27]; however, results given in Table 4

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showed that under the 10 wt% of the VAc- based polymers, solubility of CO2 in the blend membrane with the P(VAc-co-DBM) random copolymer (FVAc = 0.844) is lower than that with

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PVAc homopolymer. It can be attributed to different microstructure of VAc/DBM copolymer (random copolymer in the present study versus alternating copolymer in [27]) and different αend groups and molecular weight and its distribution of PVAc and P(VAc-co-DBM) [21, 22, 27] as well as to different morphologies of the blend membranes with PVAc and P(VAc-co-DBM)

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which was discussed in the previous section.

There is a tradeoff relation between the permeability and selectivity which is shown by Robeson`s upper bond curve [43]. This upper bond curve for the CO2/CH4 gas pair is shown in

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Fig. 12 along with some results reported in the literature and those obtained in the present work. This curve shows that the 30wt% P(VAc-co-DBM)/ Pebax blend membrane has the best

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performance, approaching the present upper bond. Also, 10 wt% PVAc/ Pebax membrane and then 40 wt% P(VAc-co-DBM) are also evaluated to have a better performance in comparison with the pure Pebax membrane.

Fig. 12

Conclusion

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Blend membranes from Pebax 1657 and different weight percent of VAc-based polymers in the range of 0-50 wt% were successfully prepared by solution casting method. Then, effect of PVAc and P(VAc-co-DBM) content on the structure, phase behavior, morphology and permeability

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properties was studied. SEM, FT-IR and DSC results showed the blend membranes are homogenous in the macro-scale while are phase separated in the micro-scale. Partial compatibility between the VAc-based polymers and two PEO and PA6 microphases of the Pebax

SC

matrix was observed. Theoretical calculations revealed that VAc-based polymers have a thermodynamic tendency toward the PA6 chains, while results showed that partial compatibility

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is controlled kinetically for the PEO microphase due to the its low viscosity and thermodynamically for the PA6 microphase due to the strong hydrogen bonding between the amide group from PA6 chains and carbonyl esters groups from VAc-based polymer chains. Up to about 30 wt% VAc-based polymers in the blend membranes, P(VAc-co-DBM) chains were

TE D

located mostly between the PA6 microphases, while with further increasing the copolymer content, additional copolymer chains were located in the PEO microphases and a separate additive microphase was also formed. It was also found that under same copolymer content, Tm

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and Xc decreases in both PEO and PA6 microphases are higher for PVAc in comparison with P(VAc-co-DBM). It was attributed to higher thermodynamic and kinetic tendencies of PVAc

chains

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chains towrads PA6 and PEO microphases, respectively, in comparison with the copolymer

Permeability results showed that permeabilities of both CO2 and CH4 decrease with different rates by increasing the copolymer content up to the 30 wt%, resulting in the enhancement of CO2/CH4 selectivity. It was attributed to the weakened interactions between the VAc-based chains by incorporating the DBM units, resulting in the stronger polymer-CO2 interactions and

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higher solubility of the CO2 without any significant change in CH4 solubility as well as to the decreased FFV values in the blend membranes which in turn diffusion of the CH4 gas with the kinetic diameter of 3.80 A° can be reduced with a higher rate in comparison with the CO2 gas

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with the kinetic diameter of 3.30 A°. Highest selectivity of 37.5 with a CO2 permeability of 103 barrer was found for 30 wt% P(VAc-co-DBM)/ Pebax blend membrane as a best membrane, however, blend membranes containing 10 wt% PVAc and 40 wt% P(VAc-co-DBM) showed also

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a better performance. By considering membrane plasticization by CO2, one can conclude that P(VAc-co-DBM) copolymer, instead of PVAc, can be introduced as a suitable candidate for a

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selective separation of CO2 from CH4 with a good aging properties. One can conclude from results obtained in the present work that a reasonable but not very great enhancement in the CO2 solubility without any change (or probably decrease) in the CH4 solubility along with a rational decrease in the FFV value of the Pebax membranes via its blending with polymer such as

Acknowledgment

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P(VAc-co-DBM) can be an appropriate strategy to prepare CO2/CH4-selective membranes.

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Author, M. Abdollahi, thank Mr. M. Farrokhi for his helping in the synthesis of VAc-based

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polymers as a part of his MSc project.

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EP

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EP

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Figures’ captions Fig. 1. Schematic representation of the chemical structures of (a) Pebax 1657 with a given composition [7], (b) PVAc homopolymer and (c) P(VAc-co-DBM) copolymer

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Fig. 2. 1H-NMR spectrum of P(VAc-co-DBM) copolymer in CDCl3 along with peaks assignment to the corresponding protons

Fig. 3. DSC thermograms (exo: upward) recorded for pure Pebax membrane, pure P(VAc-co-

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DBM) copolymer and Pebax/ copolymer membranes with different compositions

Fig. 4. DSC thermograms (exo: upward) recorded for pure PVAc and 10 wt% PVAc/ Pebax

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membrane

Fig. 5. FTIR spectra with ATR mode recorded for (a) Pebax/P(VAc-co-DBM) and (b) Pebax/PVAc membranes with various compositions expanded in three different wave number regions

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Fig. 6. SEM micrographs of the surface (left side pictures) and cross section (right side pictures) of (a) pure Pebax, (b) 10 wt% PVAc/ Pebax and (c) 30 wt% P(VAc-co-DBM)/ Pebax membranes

copolymer content

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Fig. 7. CO2 and CH4 permeabilities of Pebax/ P(VAc-co-DBM) membranes as a function of the

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Fig. 8. Effect of P(VAc-co-DBM) content on the CO2 solubility in the Pebax/ P(VAc-co-DBM) blend membranes

Fig. 9. Effect of P(VAc-co-DBM) content on the CO2 diffusivity in the Pebax/ P(VAc-co-DBM) blend membranes Fig.10. Effect of P(VAc-co-DBM) copolymer content on the CO2/CH4 selectivity of blend membranes from Pebax and VAc-based copolymer

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Fig. 11. Effect of P(VAc-co-DBM) content on the normalized CO2 and CH4 diffusivity coefficients of Pebax/ P(VAc-co-DBM) blend membranes Fig. 12. CO2/CH4 selectivity versus CO2 permeability data obtained for various blend

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membranes in the present work along with present Robeson`s upper bond curve

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Fig. 1. Schematic representation of the chemical structures of (a) Pebax 1657 with a given

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composition [7], (b) PVAc homopolymer and (c) P(VAc-co-DBM) copolymer

32

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Fig. 2. 1H-NMR spectrum of P(VAc-co-DBM) copolymer in CDCl3 along with peaks assignment

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to the corresponding protons

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Fig. 3. DSC thermograms (exo: upward) recorded for pure Pebax, pure P(VAc-co-DBM) copolymer and Pebax/ copolymer membranes, with different compositions

34

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Fig. 4. DSC thermograms (exo: upward) recorded for pure PVAc and 10 wt% PVAc/ Pebax membranes

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Fig. 5. FTIR spectra with ATR mode recorded for (a) Pebax/P(VAc-co-DBM) and (b)

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regions

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Pebax/PVAc membranes with various compositions expanded in three different wave number

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Fig. 6. SEM micrographs of the surface (left side pictures) and cross section (right side pictures) of (a) pure Pebax, (b) 10 wt% PVAc/ Pebax and (c) 30 wt% P(VAc-co-DBM)/ Pebax membranes

37

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Fig. 7. CO2 and CH4 permeabilities of Pebax/ P(VAc-co-DBM) membranes as a function of the copolymer content

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blend membranes

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Fig. 8. Effect of P(VAc-co-DBM) content on the CO2 solubility in the Pebax/ P(VAc-co-DBM)

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AC C

blend membranes

EP

Fig. 9. Effect of P(VAc-co-DBM) content on the CO2 diffusivity in the Pebax/ P(VAc-co-DBM)

40

TE D

M AN U

SC

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EP

Fig.10. Effect of P(VAc-co-DBM) copolymer content on the CO2/CH4 selectivity of blend

AC C

membranes from Pebax and VAc-based copolymer

41

TE D

M AN U

SC

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ACCEPTED MANUSCRIPT

EP

Fig. 11. Effect of P(VAc-co-DBM) content on the normalized CO2 and CH4 diffusivity

AC C

coefficients of Pebax/ P(VAc-co-DBM) blend membranes

42

EP

TE D

M AN U

SC

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AC C

Fig. 12. CO2/CH4 selectivity versus CO2 permeability data obtained for various blend membranes in the present work along with present Robson’s upper bond curve

43

δ [(MPa)1/2]

material (repeating unit)

19.4

M AN U

VAc

SC

Table 1. Solubility parameters of materials used in this work

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ACCEPTED MANUSCRIPT

DBM Ethylene Oxide (EO) Amide6 (A6)

[30]

18.4

[27]

20.2

[30]

22.9

[30]

13.2

[27]

AC C

EP

TE D

CO2

Reference

44

RI PT

ACCEPTED MANUSCRIPT

SC

Table 2. Interfacial tension data for materials used in the preparation of the blend membranes

γd (N/m)

PEO: phase A

29

PA6: phase B

16.65

PVAc: phase C

20.93

AC C

EP

TE D

Material

γp (N/m)

γ (N/m)

19

48

21.75

38.4

15.57

36.51

M AN U

[30]

45

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Table 3. Thermal behavior, Tg, Tm and Xc values of various microphases in the blend membrane obtained from DSC thermograms Tm,PEO

∆HPEOa

Xc,PEOb

Tm,PA6

∆HPA6a

Xc,PA6b

(°C)

(°C)

(J/gPebax)

(%)

(°C)

(J/gPebax)

(%)

-54.0

19.2

22.3

21.7

209.5

23.8

25.8

10 wt%

-56.1

15.9

18.8

18.7

206.1

20.9

22.8

20 wt%

-56.7

11.3

18.6

18.5

203.1

19.1

20.7

30 wt%

-60.4

10.8

15.2

15.17

201.1

16.8

18.3

50 wt%

-61.7

Pebax

PVAc

a

-57.3

10.7

10.7

200.8

15.1

16.5

8.6

15.9

15.8

203.0

17.4

19.0

Normalized melting peak area relative to the weight fraction of the corresponding microphases in the

AC C

blend membranes b

10.9

EP

10%

TE D

P(VAc-co-DBM)

M AN U

Blend

SC

Tg,PEO

Calculated by Eq. (8)

46

ACCEPTED MANUSCRIPT

and various blend membranes SCO2

DCO2

PCO2

(cm3/cm3.bar)

(10-6 cm2/s)

(barrer)

0.127

9.87

167

10 wt%

0.179

6.48

20 wt%

0.187

30 wt%

PCH4

αCO2/CH4

(barrer)

SC

Blend

Pebax

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Table 4. CO2 and CH4 permeabilities and CO2/CH4 selectivity values obtained for pure Pebax

9.51

17.6

155

7.93

22.6

5.42

136

5.36

25.1

0.193

4.01

103

2.76

37.5

40 wt%

0.214

3.03

86.6

2.57

31.4

50 wt%

0.392

1.55

81.0

5.63

14.4

1.50

117

3.58

32.6

0.585

AC C

EP

10 wt%

TE D

PVAc

M AN U

P(VAc-co-DBM)

47

ρ (g.cm-3)

Material

1.14

M AN U

Pure Pebax

SC

Table 5. Densities of materials used in this work

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PVAc

[19]

1.19

[28]

1.16

[29]

AC C

EP

TE D

P(VAc-co-DBM)

Reference

48

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Preparation, Morphology and Gas Permeation Properties of Carbon DioxideSelective Vinyl Acetate-Based Polymer/ Poly(ethylene oxide-b-amide 6) Blend

RI PT

Membranes Mahdi Abdollahi*, Morteza Khoshbin, Hossein Biazar, Ghader Khanbabaei

SC

Research Highlights:

- VAc- based (co)polymers were synthesized by controlled radical polymerization.

M AN U

- New CO2-selective VAc-based polymer/ Pebax blend membranes were prepared. - Membranes were homogeneous in macro-scale with a micro-phase separated morphology. - CO2 solubility increased by addition of VAc-based polymer in blend membranes.

AC C

EP

TE D

- CO2/CH4 selectivity of 37.5 was achieved for a 30 wt% copolymer/Pebax membrane.