Microstructural characterization and evaluation of pervaporation performance of thin-film composite membranes fabricated through interfacial polymerization on hydrolyzed polyacrylonitrile substrate

Accepted Manuscript Microstructural characterization and evaluation of pervaporation performance of thin-film composite membranes fabricated through interfacial polymerization on hydrolyzed polyacrylonitrile substrate Quan-Fu An, Micah Belle Marie Yap Ang, Yun-Hsuan Huang, Shu-Hsien Huang, YuHsuan Chiao, Cheng-Lee Lai, Hui-An Tsai, Wei-Song Hung, Chien-Chieh Hu, YoPing Wu, Kueir-Rarn Lee PII:

S0376-7388(19)30603-9

DOI:

https://doi.org/10.1016/j.memsci.2019.04.050

Reference:

MEMSCI 17046

To appear in:

Journal of Membrane Science

Received Date: 27 February 2019 Revised Date:

13 April 2019

Accepted Date: 20 April 2019

Please cite this article as: Q.-F. An, M.B.M.Y. Ang, Y.-H. Huang, S.-H. Huang, Y.-H. Chiao, C.-L. Lai, H.-A. Tsai, W.-S. Hung, C.-C. Hu, Y.-P. Wu, K.-R. Lee, Microstructural characterization and evaluation of pervaporation performance of thin-film composite membranes fabricated through interfacial polymerization on hydrolyzed polyacrylonitrile substrate, Journal of Membrane Science (2019), doi: https://doi.org/10.1016/j.memsci.2019.04.050. 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.

ACCEPTED MANUSCRIPT 1

Microstructural characterization and evaluation of pervaporation performance of

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thin-film composite membranes fabricated through interfacial polymerization on

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hydrolyzed polyacrylonitrile substrate

4 Quan-Fu An1, Micah Belle Marie Yap Ang2, Yun-Hsuan Huang2, Shu-Hsien Huang3,2*,

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Yu-Hsuan Chiao4,2, Cheng-Lee Lai5, Hui-An Tsai2, Wei-Song Hung6,2, Chien-Chieh Hu6,2,

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Yo-Ping Wu3, Kueir-Rarn Lee2*

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Beijing Key Laboratory for Green Catalysis and Separation, College of Environmental and

Energy Engineering, Beijing University of Technology, Beijing 100124, China

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University, Chung-Li 32023, Taiwan

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Taiwan

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United States

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and Science, Tainan 717, Taiwan

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Science and Technology, Taipei 10607, Taiwan

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R&D Center for Membrane Technology, Department of Chemical Engineering, Chung Yuan

Department of Chemical and Materials Engineering, National Ilan University, Yilan 26047,

Department of Chemical Engineering, University of Arkansas, Fayetteville, AR 72701,

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Department of Environmental Engineering and Science, Chia-Nan University of Pharmacy

Graduate Institute of Applied Science and Technology, National Taiwan University of

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*To whom all correspondence should be addressed

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Kueir-Rarn Lee

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Tel.: +886-3-2654190. Fax: +886-3-2654198.

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E-mail address: [email protected] (K. R. Lee)

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Shu-Hsien Huang

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Tel.: +886-3-9317505. Fax: +886-3-9357025.

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E-mail address: [email protected] (S. H. Huang)

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Abstract

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A series of thin-film composite (TFC) polyamide membranes was fabricated through

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interfacial polymerization on a modified polyacrylonitrile (mPAN) substrate, where an

4

aqueous solution of diamine was reacted with an organic solution of diacyl chloride. Various

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monomers were used: two different diamines [1,3-diamino-2-propanol (DAPL) and hydrazine]

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and

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trans-5-norbornene-2,3-dicarbonyl chloride]. Combining these monomers resulted in a new

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polyamide layer. We investigated the effect of the monomer chemical structure and interfacial

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polymerization conditions on the membrane pervaporation performance in dehydrating an

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aqueous solution of ethanol. According to field emission scanning electron microscopy,

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DAPL-SCC provided the thinnest polyamide layer, which was also confirmed through

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positron annihilation lifetime spectroscopy. The results were used to correlate the variation in

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the membrane microstructure with the pervaporation performance of the fabricated TFC

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polyamide membranes. Both microstructural characteristics and surface properties affected

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the pervaporation performance. DAPL-SCC/mPAN membranes with lower free-volume sizes

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and suitable hydrophilicity were evaluated to deliver the highest water concentration in

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permeate and the lowest permeation flux (on the basis of the pervaporative dehydration of an

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aqueous solution of 90 wt% ethanol at 25 oC).

diacyl

chlorides

[succinyl

chloride

(SCC)

and

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Keywords: Polyamide; TFC membrane; Interfacial polymerization; Ethanol dehydration;

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Positron annihilation lifetime spectroscopy

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1. Introduction Global fuel shortage is a crucial issue. Accordingly, researchers [1, 2] have been

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investigating on renewable resources of energy, and many of them [2-5] are interested in

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bioethanol, which is a most promising clean liquid fuel. The conventional technology for

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producing pure bioethanol is distillation or a hybrid system of multistage evaporation and

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distillation. However, azeotropic mixtures form in the process of distilling ethanol at 78.15 oC,

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limiting the distillate composition to 95.6% by weight ethanol. Therefore, using distillation to

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purify mixtures containing more than 85% ethanol is energy-intensive and impractical [6].

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Membrane pervaporation has been considered as one of the most effective and

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energy-saving processes for difficult-to-separate azeotropes, close-boiling-point mixtures,

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isomers, and heat-sensitive mixtures [7-9]. The key to pervaporation processes is the

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fabrication of suitable membranes; for example, membranes designed for purifying

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azeotropic mixtures of ethanol and water should be used. Polyamide is one kind of polymeric

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materials with outstanding mechanical strength, excellent resistance to organic solvents, and

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high thermal stability. However, the problem with a dense polyamide membrane when

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applied to pervaporation is its low permeation flux [9-11]. A solution to this problem is to

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transform it into a composite membrane with a very thin active layer deposited onto a porous

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substrate or support [12, 13].

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Interfacial polymerization is useful for fabricating composite membranes [9, 14-18]. This

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technique is based on a polymerization reaction that forms a polymer film at the interface of

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two immiscible phases—an aqueous-phase containing a highly reactive monomer and an

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organic-phase with a different highly reactive monomer. Interfacial polymerization is

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associated with the following attractive features: a low requirement for reactant purity, no

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rigid requirement for stoichiometric amounts of reactants, and rapid reaction rates at ambient

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temperature. Hence, interfacial polymerization is advantageous for fabricating thin-film 3

ACCEPTED MANUSCRIPT composite (TFC) polyamide membranes, whose microstructure is affected by fabrication

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conditions and post-treatment processes [19-21]. During pervaporation, the microstructure of

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polymeric materials affects their separation performance [22, 23]. Therefore, many efforts

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have investigated the polymer microstructure and how it affects the pervaporation

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performance for separating alcohol/water mixtures [24-27]. In interfacial polymerization, two

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monomers react with each other to form an active layer. As such, the layer microstructure can

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be tailored, depending on the chemical properties of the monomers. In many studies [9, 24,

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26-28], selective layers of pervaporation membranes fabricated through interfacial

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polymerization were made of cross-linked polyamides (formed as a result of

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three-dimensional polycondensation reactions). Only one study [23] investigated and

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characterized the microstructure of TFC membranes formed from linear polycondensation

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reactions.

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We examined selective polyamide layers produced by either linear or three-dimensional

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polycondensation reactions on a porous substrate. Two diacyl chlorides with different

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structures were considered: succinyl chloride (SCC) with a linear structure and

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trans-5-norbornene-2,3-dicarbonyl chloride (tNBDC) with a bulky pendant group. Linear

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polycondensation occurred when hydrazine (HA) was reacted with either SCC or tNBDC,

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and three-dimensional polycondensation ensued from reacting 1,3-diamino-2-propanol

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(DAPL) with either SCC or tNBDC. The combination of these monomers to form polyamide

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layers has not been explored. The microstructural characteristics of the resultant polyamides

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were dependent on their physicochemical properties such as intermolecular interaction,

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cross-linking effect, and polymer chain distance. For example, hydroxyl groups of DAPL

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may react with acyl chlorides or may have hydrogen bonding with the amide groups of the

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formed polyamide, thereby decreasing the distance between polymer chains in the membrane.

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We investigated the properties of polyamide membranes formed from linear and

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ACCEPTED MANUSCRIPT three-dimensional polycondensation reactions, as well as the effects of the monomer

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chemical structure and polymerization conditions on the dehydration of ethanol by

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pervaporation. TFC polyamide membranes were characterized using positron annihilation

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lifetime spectroscopy to correlate the membrane microstructural characteristics with the

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pervaporation performance. Furthermore, we validated that both the microstructure and

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surface property affected the pervaporation performance.

2. Experimental

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2.1. Materials

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Polyacrylonitrile (PAN) polymer, supplied by Tong-Hwa Synthetic Fiber Co. Ltd.

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(Taiwan), was used as membrane support. The solvent for PAN was a reagent grade

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N-methyl-2-pyrrolidone (NMP)., whereas the solvent for organic-phase monomers was

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toluene. Both solvents were purchased from Tedia Company Inc. (USA). Aqueous-phase

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monomers DAPL and HA were bought from Tokyo Chemical Industry Co. Ltd. (Japan),

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whereas organic-phase monomers SCC and tNBDC were distributed by Sigma-Aldrich

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(USA). Figure 1 shows the chemical structures of the monomers. NaOH was used for

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hydrolysis of PAN, which was delivered by Showa Chemical Co. LTD. (Japan). Methanol

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and ethanol (Echo Chemical Co. Ltd, Taiwan) were used either for washing membranes or as

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feed for pervaporation operations. Liquid nitrogen and helium were provided by Ming Yang

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Special Gas Co. Ltd., Taiwan.

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Figure 1. Chemical structure of monomers for fabricating TFC polyamide membranes: (a)

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HA, (b) DAPL, (c) SCC, and (d) tNBDC.

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2.2. Preparation of porous modified PAN

A porous modified PAN (mPAN) membrane support was prepared by following a

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similar procedure described in our previous study [29]. A polymer solution containing 15

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wt% PAN in NMP was cast onto nonwoven polyester fabrics by using a casting knife with a

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gap of 200 µm to fabricate a flat, porous PAN membrane support. The cast film was

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precipitated by immersion in a water bath, and the resulting PAN membrane was washed in a

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water bath for more than one day and then dried at room temperature. The PAN membrane

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was hydrolyzed in a 2 M NaOH solution at 50 °C to improve its surface hydrophilicity and,

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in turn, facilitate the spread of an aqueous amine solution on the surface. This hydrolysis step

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modified the surface by converting the –CN groups of PAN into –COOH or –CONH2 groups.

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The porous mPAN membrane support was washed in a water bath for several hours and then

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dried at room temperature. During interfacial polymerization, ionic bonds were formed

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through an acid-base reaction between –NH or –NH2 groups of amine and the –COOH of

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mPAN [30].

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2.3. Preparation of TFC polyamide membranes Active polyamide layers were synthesized through interfacial polymerization [9, 31]. An

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mPAN membrane was first immersed in a 1 wt% aqueous diamine solution for 3 min. After

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the membrane was retrieved, the excess diamine solution adhering on the membrane was

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removed by using a rubber roller. Then, a 0.5 wt% organic diacyl chloride solution was

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poured onto the surface of the mPAN membrane to allow contact between the diamine and

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diacyl chloride monomers for 3 min to effect interfacial polymerization. The resulting TFC

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polyamide membrane was washed in methanol (as both the residual amines and acyl

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chlorides dissolved in methanol) and then dried at room temperature. (After the process of

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interfacial polymerization, several methods of washing or post-treatments are available

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[19-21].)

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The best combination of diamine and diacyl chloride was determined as a result of

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varying the following conditions: diacyl chloride concentration from 0.01 to 1 wt%; diamine

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concentration from 0.01 to 2 wt%; reaction time from 10 to 180 s; and immersion time from

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20 to 180 sec.

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2.4. Characterization of TFC polyamide membranes Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy

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(Perkin Elmer Spectrum One, USA) was used to characterize TFC polyamide membranes for

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their chemical structure. The membrane cross-sectional morphologies were observed with

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field emission scanning electron microscopy (FESEM) (HITACHI S-4800, Japan). The

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membrane surface roughness was probed by using atomic force microscopy (AFM) (Digital

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Instruments DI-NS3a, USA) with a scanning area of 5 µm x 5 µm. To understand the

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membrane surface hydrophilicity, water contact angle was estimated with an automatic

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interfacial tensiometer (FACE Mode 1 PD-VP, Japan).

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2.5.

Analysis of free volume in TFC polyamide membranes To determine the variation of free volume in TFC polyamide membranes with a

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multilayer structure, they were probed with a variable monoenergy slow positron beam

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(VMSPB) as a function of positron incident energy in the range of 0–30 keV under a vacuum

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of 10−8 torr at room temperature. This radioisotope beam used 50 mCi of

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source.

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Na as positron

Two positron annihilation techniques were connected to VMSPB: positron annihilation

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lifetime spectroscopy (PALS) and Doppler broadening energy spectroscopy (DBES). With

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DBES, free volume in the active polyamide layer was measured with a standard γ-ray

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spectrometer equipped with an HP Ge detector at a counting rate of approximately 2000 cps.

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The energy resolution of the solid-state detector was 1.5 keV at 0.511 MeV, corresponding to

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a positron 2γ annihilation peak. For a DBES spectrum, the total number of counts was 1.0

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million. Simple line-shaped parameters, S (shape) and W (wing), described the

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Doppler-broadened width. The S parameter was defined as the ratio of the annihilation

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spectrum central part to the total spectrum reflecting positron annihilation where

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low-momentum valence electrons were used. If the relative contribution to positron

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annihilation from low-momentum electrons increased in the open-volume defects, the the S

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value increased. The W parameter was defined as the ratio of the annihilation spectrum edge

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to the total spectrum reflecting positron annihilation where high-momentum core electrons

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were used. W was related to the material composition of the environment where annihilation

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occurred.

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PALS was a quantitative probe of free volume in polymers. Not only did it probe the

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free-volume size and fraction, but it also gave detailed information on the distribution of

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free-volume size in the range of 1 to10 Å. PALS data were obtained by taking the coincident 8

ACCEPTED MANUSCRIPT 1

events between start signals detected by a multichannel plate from secondary electrons and

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stop signals given by a BaF2 detector from annihilation photons at a counting rate of 200–300

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cps. A PALS spectrum contained 2.0 million counts. All positron annihilation lifetime spectra

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were analyzed using a finite-term lifetime analysis method in PATFIT program [32-36].

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The mean free-volume radius (r, Å) was calculated using a semi-empirical correlation

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equation:

1  1 2   1 − +  sin 

 (1) 2  + ∆ 2  + ∆

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 =

where τ3 represented the o-Ps lifetime and ∆r was an empirical constant (1.66 Å). The relative

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fractional free volume (FFV) was calculated using the following equation:

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 =  ( ) (2) 9 10

where A was the normalization constant (= 0.0018), I3 was the o-Ps intensity (%), and V3 = 4/3πr3.

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2.6.

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Pervaporation performance of TFC polyamide membranes TFC polyamide membranes were tested for their pervaporation performance in

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separating an aqueous solution of ethanol. A test method similar to that in previous studies

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[37, 38] was used. The membrane was in direct contact with the feed solution, with the

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effective membrane area for pervaporation equal to 9.89 cm2. One hour was allotted to

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stabilize the whole system. Afterward, the mass of permeate was collected every 5 min until

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two consecutive measurements were similar (i.e., flux measured was steady). The permeation

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flux was determined by dividing the permeate weight by the membrane area and then by the

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sampling time. Feed solution and permeate compositions were measured using gas

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chromatography (China Chromatography 8700T). The separation factor of water/alcohol,

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αW/A, was calculated from equation (3).

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ACCEPTED MANUSCRIPT Y /Y (3) X /X

α/ = 1

where X and Y were weight fractions in feed and permeate, respectively; subscripts W and A

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denoted water and alcohol, respectively.

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3. Results and discussion

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3.1. Characterization of TFC polyamide membranes

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The chemical structure of mPAN and TFC polyamide membranes (Figure 2) was

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analyzed using ATR-FTIR. Figure 2(a) shows the spectrum of mPAN. The spectra of

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DAPL-SCC/mPAN and DAPL-tNBDC/mPAN membranes (Figure 2b,d) indicated peaks at

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wavenumbers of 1638 and 1536 cm−1, corresponding to primary amide (amide I) C=O and

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secondary amide (amide II) N-H, respectively. For HA-SCC/mPAN and HA-tNBDC/mPAN

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membranes (Figure 2c,e), shift in the respective peaks of primary and secondary amides to

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1595 and 1489 cm−1 was caused by the intrinsic chemical structure of HA [39]. For the

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mPAN support, a peak at 1737 cm−1 corresponded to C=O of carboxylic acid. For membranes

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prepared from DAPL that reacted with either SCC or tNBDC, a peak at the same

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wavenumber of 1737 cm−1 corresponded to ester groups formed from a cross-linking reaction

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between hydroxyl groups and acyl chlorides of SCC or of tNBDC. A weak peak at 1737 cm−1,

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which probably came from the mPAN support, existed in HA membranes because of linear

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polycondensation reactions between the monomers; no other groups could interrupt the

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reaction of amines with acyl chlorides. The peak intensity in the mPAN support weakened

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because it was covered with polyamide.

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2400 2200 2000 1800 1600 1400 1200 1000

800

1737

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1638

(b)

1536

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Transmittance (%)

(a)

1489

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1489 1595

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(c)

(d)

(e) 800

Wave number (cm-1)

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Figure 2. ATR-FTIR spectra of membrane support and TFC polyamide membranes: (a)

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mPAN, (b) DAPL-SCC/mPAN, (c) HA-SCC/mPAN, (d) DAPL-tNBDC/mPAN, and (e)

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HA-tNBDC/mPAN. Polymerization conditions: 3-min immersion of mPAN in 1.0 wt%

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diamine solution; 3-min contact between diamine solution (absorbed in mPAN) and 0.5 wt%

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diacyl chloride solution.

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Figure 3 illustrates the cross-sectional FESEM images of mPAN and TFC polyamide

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membranes. Figure 3a describes the mPAN cross-section. After interfacial polymerization,

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thin active polyamide layers were formed on mPAN (Figure 3b–e). Membranes prepared 11

ACCEPTED MANUSCRIPT from HA were thicker than those prepared from DAPL (corresponding membrane thickness

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of HA-SCC/mPAN; HA-tNBDC/mPAN; DAPL-SCC/mPAN; and DAPL-tNBDC/mPAN: 72

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± 5 nm; 59 ± 19 nm; 48 ± 3 nm; and 49 ± 3 nm). The reaction between HA and SCC

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proceeded at faster reaction rates than that between DAPL and SCC. Despite the three

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functional groups of DAPL (one hydroxyl and two amines), which had potential to engage in

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three-dimensional polycondensation reactions, the hydroxyl hindered the amines from

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reacting with acyl chlorides (i.e., lower cross-linking effect), because hydroxyl groups had

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slower reaction rates than amines and they were more polar, so they had stronger affinity with

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water in the solution. Hence, membranes prepared from HA were thicker. In terms of the

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chemical structure, HA had shorter chains than DAPL and, as such, exhibited less steric

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hindrance. At the same interfacial polymerization reaction time, the reaction rate of HA with

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acyl chlorides was faster compared with that of DAPL with acyl chlorides, leading to the

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formation of a thicker polyamide layer.

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Figure 3. Cross-sectional FESEM images (×100k): (a) mPAN, (b) HA-SCC/mPAN, (c)

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DAPL-SCC/mPAN, (d) HA-tNBDC/mPAN, and (e) DAPL-tNBDC/mPAN. Different combinations of diamines and acyl chlorides to produce polyamides had an

4

effect on the membrane surface morphology, roughness, and hydrophilicity (Figure 4 and

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Table 1). The membrane surface with nodules or ridge and valley structures (Figure 4d)

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resulted in high surface roughness (large surface area), which increased permeation flux.

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High hydrophilicity favored both high concentration of water in permeate and high

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permeation flux. Because of its bulky norbornylene group, tNBDC monomer inhibited the

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mobility of polyamide chains in the layer that was formed from the reaction between tNBDC

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and HA with a very short molecular length. This restricted mobility in the HA-tNBDC

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polyamide layer was associated with a low packing efficiency, leading to high surface

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roughness. Such a condition explained why the HA-tNBDC/mPAN membrane had the

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highest surface roughness or the lowest water contact angle. However, the reaction between

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SCC and HA, which were both linear monomers, formed a polyamide layer with a linear

15

molecular structure that promoted polymer chain mobility and enhanced the molecular chain

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packing efficiency. As such, the HA-SCC/mPAN membrane exhibited the smoothest surface

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and the lowest roughness (the highest water contact angle).

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Hydroxyl or pendent groups of DAPL in polyamide layers formed from the reaction

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between SCC and DAPL served not only to enhance the hydrophilicity but also to restrain the

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polyamide chain mobility. Therefore, a comparison between HA-SCC and DAPL-SCC

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indicated that the water contact angle for DAPL-SCC was lower (higher hydrophilicity) and

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the surface roughness higher than for HA-SCC. For the DAPL-tNBDC polyamide layer, the

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hydroxyl groups of DAPL formed hydrogen bonds or cross-linked with the acyl chloride

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groups of tNBDC, leading to enhanced packing efficiency; thus, the DAPL-tNBDC layer had

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ACCEPTED MANUSCRIPT lower surface roughness and higher water contact angle (lower hydrophilicity) than the

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HA-tNBDC layer.

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Figure 4. Surface FESEM images (×5k): (a) mPAN, (b) HA-SCC/mPAN, (c)

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DAPL-SCC/mPAN, (d) HA-tNBDC/mPAN, and (e) DAPL-tNBDC/mPAN.

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Table 1. Water contact angle and surface roughness of polyamide membranes. Water contact angle (o)

Membrane HA-SCC/mPAN

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DAPL-SCC/mPAN

HA-tNBDC/mPAN

Roughness

79 ± 2

Rrms a) (nm) 19.1 ± 5.1

Ra b) (nm) 14.4 ± 4.0

62 ± 2

26.6 ± 6.5

20.6 ± 5.2

57 ± 3

55.1 ± 14.6

42.3 ± 12.8

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18.9 ± 2.8

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DAPL-tNBDC/mPAN 63 ± 2 23.7 ±3.1 b) Rrms: root-mean-square roughness; Ra: average roughness.

a)

3.2. Microstructural analysis of TFC polyamide membranes

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In addition to discussing surface characteristics (Figures 2 and 4 and Table 1), we

10

analyzed the variation of free volume in polyamide/mPAN membranes probed with VMSPB.

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Figures 5 and 6 present qualitative free-volume data from DBES. On the basis of the data on

12

S parameter as a function of positron incident energies for TFC polyamide/mPAN membranes

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(Figure 5), the polyamide layer and mPAN membrane support were determined to 14

ACCEPTED MANUSCRIPT correspond to positron incident energies in the low range of 0.5–2.5 keV and those higher

2

than 2.5 keV, respectively. The densest part of the polyamide layer was at 2–2.5 keV [34].

3

Plateau regions denoted the polyamide layers [40, 41]. From Figure 5, the DAPL-SCC/mPAN

4

membrane had the thinnest polyamide layer. This result agrees with the FESEM analysis.

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Apart from the S vs. keV data, another useful approach to present the free-volume data was

6

based on S-W plots (Figure 6). These plots provided information on which region of TFC

7

polyamide membranes referred to the polyamide layer or the mPAN membrane support. From

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the analysis of positron trapping, W vs. S data were plotted in Figure 6a–d. Two straight

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lines were drawn through the data, and the intersection between these lines revealed the

10

transition from the polyamide layer to the mPAN membrane support. In the polyamide layer,

11

the value of W was high, whereas that of S was low. In the polyamide + mPAN skin layer, W

12

was low, whereas S was high. The intersection point in the W vs. S data for the

13

HA-SCC/mPAN membrane (Figure 6a) was at S = 0.478; for DAPL-SCC/mPAN (Figure 5b),

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S = 0.494; for HA-tNBDC/mPAN (Figure 6c), S = 0.497; for DAPL-tNBDC/mPAN (Figure

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5d), S = 0.493. All these S data agree with those in Figure 5.

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ACCEPTED MANUSCRIPT

1 2

Figure 5. S parameter as function of positron incident energy for TFC polyamide/mPAN

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membranes: (-■-) HA-SCC/mPAN

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(-▼-) DAPL-tNBDC/mPAN.

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(-▲-) HA-tNBDC/mPAN

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(-●-) DAPL-SCC/mPAN

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ACCEPTED MANUSCRIPT 0.152

0.156

HA-SCC/mPAN

DAPL-SCC/mPAN 0.148

0.148

W parameter

0.144

0.140 0.136

0.132

0.140

0.136

(a) 0.475

0.144

(b) 0.480

0.485

0.490

0.495

0.132

0.500

0.480

0.485

S parameter

1

W parameter

0.138 0.136 0.134

0.144

0.141

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W parameter

0.140

0.138

(c) 0.132 0.494

0.496

0.498

0.500

S parameter

2

0.500

SC

0.147

0.492

0.495

DAPL-tNBDC/mPAN

HA-tNBDC/mPAN

0.142

0.490

0.490

S parameter

0.150 0.144

RI PT

W parameter

0.152

3

0.502

0.504

(d)

0.135

0.484

0.486

0.488

0.490

0.492

0.494

0.496

0.498

0.500

S parameter

Figure 6. W parameter as function of S parameter for TFC polyamide/mPAN membranes: (a)

5

HA-SCC/mPAN;

6

DAPL-tNBDC/mPAN.

DAPL-SCC/mPAN;

(c)

HA-tNBDC/mPAN;

(d)

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(b)

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PALS provides quantitative information on free-volume sizes (based on positron

9

lifetimes τ3) and on free-volume concentrations (based on positron intensity I3). The positron

10

lifetime τ3 is due to ortho-positronium (o-Ps) annihilation. In polymeric materials, the

11

annihilation lifetime is on the order of 1–5 ns, which is the result of so-called pick-off

12

annihilation with electrons in molecules, and is used to calculate the mean free-volume radius

13

R (Å to nm). In general, longer o-Ps lifetime corresponds to larger free-volume sizes in

14

polymeric materials.

15

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Table 2 summarizes PALS data for TFC polyamide/mPAN membranes at a positron 17

ACCEPTED MANUSCRIPT incident energy of 2 keV. Polyamide membranes with tNBDC segments indicated longer

2

positron lifetimes τ3 and lower intensities I3 than those with SCC segments, because the bulky

3

norbornylene groups of tNBDC inhibited polyamide chain packing, leading to large

4

free-volume sizes and low free volume concentration. DAPL-SCC polyamide layers had

5

longer τ3 and lower I3 than HA-SCC polyamide layers, because the hydroxyl pendent groups

6

of DAPL inhibited polyamide chain packing, resulting in large free-volume sizes and low free

7

volume concentration. DAPL-tNBDC polyamide layers had shorter τ3 and higher I3 than

8

HA-tNBDC layers; the hydroxyl pendent groups of DAPL not only formed hydrogen bonds

9

or cross-linked with the acyl chloride groups of tNBDC that enhanced the polyamide chain

10

packing, resulting in small free-volume sizes, but also migrated to the space around the bulky

11

tNBDC, thereby dividing large spaces into smaller ones, leading to increased free-volume

12

concentrations.

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15

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Table 2 Data on o-Ps lifetime (τ3), relative intensity (I3), mean free-volume radius (R) and, fractional free volume (FFV) for TFC polyamide/mPAN membranes at 2 keV. τ3 (ns) I3 (%) R (Å) FFV (%) TFC Membrane HA-SCC/mPAN 1.93 17.02 2.79 2.79 DAPL-SCC/mPAN 1.98 14.98 2.84 2.59 HA-tNBDC/mPAN 2.68 5.48 3.41 1.64 DAPL-tNBDC/mPAN 2.13 8.34 2.97 1.65

16

3.3. Effect of TFC polyamide membrane structure on pervaporative

17

dehydration performance

18

The molecule structural design of TFC polyamide membranes was based on varying the

19

chemical structures of monomers for fabricating membranes, or by changing the reaction

20

which was either linear or three-dimensional polycondensation. The monomer structure or the

21

kind of reaction could change the polyamide membrane characteristics and could affect the 18

ACCEPTED MANUSCRIPT membrane separation performance. Therefore, we investigated the effect of polyamide

2

chemical structures resulting from the polycondensation of monomers on the pervaporation

3

performance of TFC polyamide/mPAN membranes in dehydrating a 90 wt% aqueous ethanol

4

solution (Table 3). Polyamide membranes consisting of DAPL (or HA) segments and tNBDC

5

indicated lower concentration of water in permeate than membranes consisting of DAPL (or

6

HA) segments and SCC, because membranes with tNBDC had larger free-volume size than

7

membranes with SCC. However, the permeation flux was similar because polyamide

8

membranes with SCC had higher fractional free volume (Table 2) than membranes with

9

tNBDC. DAPL-tNBDC/mPAN delivered higher concentration of water in permeate than

10

HA-tNBDC/mPAN. These results were attributed to the smaller free-volume size (with

11

similar fractional free volume) in membranes with DAPL segments (Table 2). For the

12

membranes where the common segment was SCC, DAPL-SCC had higher concentration of

13

water in permeate than HA-SCC. Although the free volume in DAPL-SCC was higher than

14

HA-SCC, the permeation flux was similar because the fractional free volume in HA-SCC was

15

higher than DAPL-SCC. Herein, hydrophilicity (Table 1) rather than free-volume size was

16

the dominant factor; active polyamide layers with DAPL segments had higher hydrophilicity

17

than layers with HA segments. DAPL-SCC/mPAN membrane had lower water contact angle

18

than HA-SCC/mPAN membrane; thus, it had a strong affinity with water molecules, resulting

19

in high separation factor.

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According to Tables 1–3, not only the microstructural characteristics of polyamide

21

layers had influence on the pervaporative dehydration performance, but also the surface

22

properties or the affinity of the feed with the membrane. The pervaporation results indicated

23

that DAPL-SCC/mPAN membranes delivered the highest dehydration performance and had a

24

small free-volume size, a low fractional free volume, a high hydrophilicity, and the thinnest

19

ACCEPTED MANUSCRIPT 1

polyamide layer. Hence, the performance of these TFC polyamide membranes as a function

2

of different polymerization conditions is discussed in the next section.

3

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4 5 Table 3 Pervaporation performance of TFC polyamide/mPAN membranes in dehydrating 90 wt% aqueous ethanol solution at 25 oC. Pervaporation performance Separation factor TFC membrane Permeation flux Water concentration in 2 (g/m h) permeate (wt%) 561 ± 35

95.3 ± 1.5

182

DAPL-SCC/mPAN

524 ± 46

97.3 ± 0.8

324

HA-tNBDC/mPAN

601 ± 38

90.2 ± 2.1

82

DAPL-tNBDC/mPAN

537 ± 79

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SC

HA-SCC/mPAN

6 7

94.4 ± 1.4

151

3.4. Effect of polymerization conditions on pervaporative dehydration

9

performance of TFC DAPL-SCC/mPAN membranes

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8

Figure 7 plots the effect of the concentration of organic SCC solution on the

11

pervaporation performance of TFC DAPL-SCC/mPAN membranes in dehydrating a 90 wt%

12

aqueous ethanol solution at 25oC. Three polymerization conditions were fixed: 1.0 wt%

13

DAPL solution, immersion time of 3 min, and reaction time of 3 min. The concentrations of

14

water in permeate increased when the SCC solution concentrations increased up to 0.5 wt%,

15

but at higher SCC concentrations, the water concentrations did not change anymore. When

16

the amount of SCC that reacted with a fixed amount of DAPL increased, the resulting

17

polyamide layer became increasingly dense and thick; thus, the permeation flux decreased

18

and the concentration of water in permeate increased. However, when the SCC concentration

19

was higher than 0.5 wt%, the polyamide layer was already dense and thick enough that the

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ACCEPTED MANUSCRIPT 1

concentrations of water in permeate remained unchanged, but the permeation flux slightly

2

decreased. Figure 8 describes the effect of the concentration of aqueous DAPL solution on the

4

pervaporation performance of TFC DAPL-SCC/mPAN membranes in dehydrating a 90 wt%

5

aqueous ethanol solution at 25 °C. Three polymerization conditions were held constant: 0.5

6

wt% SCC solution, immersion time of 3 min, and reaction time of 3 min. The permeation

7

flux was unaffected by the increasing DAPL solution concentration. However, the

8

concentrations of water in permeate slightly increased when the DAPL solution

9

concentrations increased up to 0.1 wt%; beyond 0.1 wt% DAPL, the water concentrations

10

remained unchanged. A 0.1 wt% DAPL concentration was enough to form a dense polyamide

11

layer (from the reaction between DAPL and SCC for a period of time); consequently, the

12

DAPL monomer was unable to penetrate through the polyamide layer, so it could not react

13

with the SCC monomer in the organic solution. This condition (so-called self-limiting effect

14

of interfacial polymerization) limited the polyamide layer growth [42, 43].

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Figures 7 and 8 establish that 0.5 wt% SCC and 0.1 wt% DAPL were the appropriate

16

monomer concentrations for fabricating TFC DAPL-SCC/mPAN membranes with optimum

17

performance.

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21

100

1200

80

900

60

RI PT

Water concentration in permeate (wt%)

1500

2

Permeation flux (g/m h)

ACCEPTED MANUSCRIPT

600

40

20

0 0.0

0.2

0.4

0.6

SC

300

0.8

0

1.0

Concentration of organic SCC solution (wt%)

2

Figure 7. Effect of SCC solution concentration on pervaporation performance of TFC

3

DAPL-SCC/mPAN membranes in dehydrating 90 wt% aqueous ethanol solution at 25 °C.

4

Polymerization conditions: 3-min immersion of mPAN in 1.0 wt% DAPL solution; 3-min

5

contact between DAPL solution (absorbed in mPAN) and different SCC solutions of varying

6

concentrations.

EP

100

800

80

AC C

2

Permeation flux (g/m h)

1000

600

60

400

40

200

20

0 0.0

8

0 0.5

1.0

1.5

Water concentration in permeate (wt%)

7

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2.0

Concentration of aqueous DAPL solution (wt%)

22

ACCEPTED MANUSCRIPT Figure 8. Effect of DAPL solution concentration on pervaporation performance of TFC

2

DAPL-SCC membranes in dehydrating 90 wt% aqueous ethanol solution at 25 °C.

3

Polymerization conditions: 3-min immersion of mPAN in different DAPL solutions of

4

varying concentrations; 3-min contact between DAPL solution (absorbed in mPAN) and 0.5

5

wt% SCC solution.

RI PT

1

6

The aforementioned appropriate monomer concentrations were used in determining the

8

effect of polymerization time on the pervaporation performance of TFC DAPL-SCC/mPAN

9

membranes for dehydrating a 90 wt% aqueous ethanol solution at 25 °C (Figure 9). Three

10

polymerization conditions were fixed: 0.1 wt% DAPL solution, 0.5 wt% SCC solution, and

11

immersion time of 3 min. Figure 8 indicates that a high concentration of water in permeate

12

was maintained. The permeation flux decreased at varying periods of polymerization time up

13

to 60 sec, but for longer lengths of time, the flux ceased to change. A polymerization time of

14

15 sec was enough to form a dense polyamide layer. However, at prolonged periods of time,

15

the polyamide layer became much denser and thicker, and consequently, the permeation flux

16

decreased and approached the same value. Therefore, we deduced that 15 sec was the

17

optimum polymerization time to form a dense polyamide layer.

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SC

7

23

100

800

80

600

60

RI PT

Water concentration in permeate (wt%)

1000

2

Permeation flux (g/m h)

ACCEPTED MANUSCRIPT

400

40

20

0 0

30

60

90

120

SC

200

150

0

180

Polymerization time (sec)

2

Figure 9. Effect of polymerization time on pervaporation performance of TFC

3

DAPL-SCC/mPAN membranes in dehydrating 90 wt% aqueous ethanol solution at 25 °C.

4

Polymerization conditions: 3-min immersion of mPAN in 0.1 wt% DAPL solution; varying

5

periods of contact time for DAPL solution (absorbed in mPAN) and 0.5 wt% SCC solution.

TE D

6

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1

Figure 10 illustrates the effect of immersion time in aqueous solution on the

8

pervaporation performance of TFC DAPL-SCC/mPAN membranes in dehydrating a 90 wt%

9

aqueous ethanol solution at 25 °C. Three polymerization conditions were maintained the

10

same: 0.1 wt% DAPL solution, 0.5 wt% SCC solution, and reaction time of 15 sec. The

11

concentration of water increased as immersion time progressed to 60 sec, but it stopped to

12

change at longer periods of time. However, the permeation flux showed almost no change.

13

These results were attributed to the self-limiting effect of polyamide layers formed through

14

interfacial polymerization. For immersion time shorter than 60 sec, the amount of DAPL

15

absorbed in mPAN was not enough to form a dense polyamide layer from its reaction with

16

SCC. But longer than 60 sec, the polymerization reaction resulted in a dense polyamide layer.

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ACCEPTED MANUSCRIPT 1

Figures 9 and 10 denote that 15 sec was the optimum polymerization time and 60 sec

2

the optimum immersion time in fabricating high-performing TFC DAPL-SCC/mPAN

3

membranes.

1200

100

80

2

Permeation flux (g/m h)

1000

SC

800

400 200 0 20

40

60

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600

80

100

120

140

160

60

40

20

180

0 200

Water concentration in permeate (wt%)

RI PT

4

Immersion time of aqueous solution (sec)

TE D

5

Figure 10. Effect of immersion time in DAPL solution on pervaporation performance of TFC

7

DAPL-SCC/mPAN membranes in dehydrating 90 wt% aqueous ethanol solution at 25 °C.

8

Polymerization conditions: varying periods of immersion time for mPAN in 0.1 wt% DAPL

9

solution; 15-sec contact between DAPL solution (absorbed in mPAN) and 0.5 wt% SCC

11

solution.

AC C

10

EP

6

12

3.5. Comparison of pervaporation performance in dehydrating aqueous

13

solutions of ethanol

14

Numerous studies have been conducted to investigate the development of materials and

15

fabrication or modification methods aimed at improving the pervaporation performance for

16

separating an aqueous solution of ethanol. Table 4 presents an interesting comparison of 25

ACCEPTED MANUSCRIPT pevaporation performance in ethanol dehydration reported by the present study and in the

2

literature[44-55]. Several homogeneously dense and composite membranes prepared from

3

hydrophilic polymers were selected for comparison with the DAPL-SCC/mPAN membrane

4

obtained from this study. From the Table 4, dense membrane based from chitosan and

5

polyvinyl alcohol had lower permeation flux than DAPL-SCC/mPAN membrane, but the

6

water concentration in permeate was comparable. These results showed that the composite

7

membrane prepared from DAPL-SCC/mPAN had an advantage over the other polymeric

8

materials. Furthermore, TFC membranes prepared using m-phenylenediamine (MPD) and

9

trimesoyl chloride exhibited better performance than membranes prepared from chitosan or

10

polyvinyl alcohol. However, as MPD monomers tend to oxidize relatively fast, DAPL

11

monomers are a better substitute. Overall, the performance of our DAPL-SCC/mPAN

12

membrane compared well with that of the membranes reported in the literature.

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1

13

17 18 19 20 21

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15

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14

22 23 24

Table 4 Comparison of pervaporation performance (reported in literature and demonstrated in 26

ACCEPTED MANUSCRIPT this study) for dehydrating aqueous solutions of ethanol. Membrane Ethanol Operating Permeation Water concentration temperature flux conc. in in feed (oC) (g/m2 h) permeate (wt%) (%) DAPL-SCC/mPAN 90 25 600 96.7

TFC polyamide j TFN polyamide k TDI cross-linked TFC polyamide l TFC polyamide (hollow fiber with PVDF) m

95 90 86 90 90 90 85

30 30 22 30 25 25 50

85

a

220 90 182 145 122 102 137

50

98.7 94.5 91.8 99.5 98.5 97.8 96.7

This study [44] [45] [46] [47]

RI PT

30 30 30 40 40 40 40

SC

PVA/TiO2–MWCNT i

95 95 85 85 85 85 90

[48]

132 39 1300 388 2425 4500 2000

98.2 93.7 95.2 98.9 62.5 77.0 95.9

[49] [50] [51] [52] [53]

1288

85.3

[55]

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CS/NaAlg blend a P-SA/PVP blend b H-ZSM5-loaded PVA c PVA-P4-80 hybrid d PVA-D4-80 hybrid d PVA-A4-80 hybrid d Cross-linked PVA-filled GO e CS-PAA/PSf f EPS-PES g (CS/PAA)7CSx h

Reference

[54]

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CS = chitosan; NaAlg = sodium alginate. Phosphoric acid cross-linked sodium alginate (SA)/poly(vinylpyrrolidone) (PVP) blend membrane. c PVA = poly(vinyl alcohol); H-ZSM5 is zeolite additive. d Organosilanes used to modify polyvinyl alcohol (PVA): phenyltriethoxysilcane (P), diethoxydiphenylsilane (D), aminopropyltriethoxysilane (A); 4 in P4 or D4 or A4: 0.4 mmol organosilane loaded per gram of PVA polymer; 80: thermal treatment at 80oC. e PVA = poly(vinyl alcohol); GO = graphene oxide f PAA = polyacrylic acid; PSf = polysulfone. g Composite membrane with thin-film of cross-linked exopolysaccharide (EPS) cast on polyethersulfone (PES) support. h Polyelectrolyte membrane was prepared by layer-by-layer self-assembly on polyamide (MPD-TMC)/PSf membrane formed through interfacial polymerization. i PVA blend with TiO2 functionalized multi-walled carbon nanotubes (MWCNTs) j TFC polyamide membrane was synthesized through interfacial polymerization involving MPD and TMC on mPAN substrate. k TFN polyamide membrane was synthesized through interfacial polymerization involving MPD and TMC with nano-NaX zeolite on mPAN substrate. l Toluene 2,4-diisocyanate was cross-linked with TFC polyamide membrane m TFC polamide was synthesized through interfacial polymerization involving MPD and TMC on polyvinylidene fluoride hollow fiber substrate. b

27

ACCEPTED MANUSCRIPT 1

4. Conclusions TFC polyamide membranes were fabricated through interfacial polymerization

3

involving two different diamines (DAPL and HA) and two different diacyl chlorides (SCC

4

and tNBDC). These membranes were applied to dehydrate ethanol by pervaporation. AFM

5

and water contact angle results indicated that surface roughness and hydrophilicity were the

6

dominant factors that affected the pervaporative dehydration performance of TFC

7

DAPL-SCC/mPAN membranes. PALS data revealed that TFC polyamide membranes with

8

tNBDC segments exhibited longer o-Ps lifetime τ3 (larger free-volume size) and lower o-Ps

9

intensity I3 (lower free-volume concentration) than membranes with SCC segments.

10

Membranes formed from three-polycondensation reactions had higher performance than

11

those from linear polycondensation reactions. The optimum conditions for fabricating TFC

12

DAPL-SCC/mPAN membranes were as follows: 60-sec immersion of mPAN in an aqueous

13

solution of 0.1 wt% DAPL and 15-sec contact between DAPL (absorbed in mPAN) and an

14

organic solution of 0.5 wt% SCC. Under these conditions, a permeation flux of 600 g/m2h

15

and a permeate water content of 97 wt% were obtained as a result of using

16

DAPL-SCC/mPAN membranes for dehydrating an aqueous solution of 90 wt% ethanol at 25

17

°C through pervaporation.

19 20 21

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22 23 24

28

ACCEPTED MANUSCRIPT 1 2

Acknowledgements The authors wish to sincerely thank the Ministry of Science and Technology of Taiwan (MOST

106-2221-E-033-062-MY3,

MOST

106-2218-E-033-010,

MOST

4

106-2221-E-197-024, and MOST 98-2221-E-197-001) for financially supporting this work.

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ACCEPTED MANUSCRIPT

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ACCEPTED MANUSCRIPT Highlights


Polyamides formed from linear and three-dimensional polycondensation reactions are compared. Variation in microstructures of different TFC polyamide membranes is analyzed.




Microstructures of TFC membranes can be tuned by varying monomer structures.




Both microstructures and surface properties of TFC membranes affect pervaporation

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performance.