Investigation of fine-structure of polyamide thin-film composite membrane under swelling effect by positron annihilation lifetime spectroscopy and molecular dynamics simulation

Journal of Membrane Science 417–418 (2012) 201–209

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Investigation of fine-structure of polyamide thin-film composite membrane under swelling effect by positron annihilation lifetime spectroscopy and molecular dynamics simulation Yun-Hsuan Huang a, Wei-Chi Chao a, Wei-Song Hung a,n, Quan-Fu An a,c, Kai-Shiun Chang a, Shu-Hsien Huang a,b, Kuo-Lun Tung a, Kueir-Rarn Lee a,n, Juin-Yih Lai a a b c

R&D Center for Membrane Technology, Department of Chemical Engineering, Chung Yuan University, 200, Chung Pei Rd.,Chung-Li 32023, Taiwan Department of Chemical and Materials Engineering, National Ilan University, I-Lan 26047, Taiwan MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science & Engineering, Zhejiang University, Hangzhou 310027, China

a r t i c l e i n f o

abstract

Article history: Received 8 December 2011 Received in revised form 17 June 2012 Accepted 22 June 2012 Available online 4 July 2012

Positron annihilation lifetime spectroscopy (PALS) and molecular dynamics (MD) simulation analyses were adopted for an in-depth understanding at the molecular scale of the variation in the fine-structure of polyamide active layers of thin-film composite (TFC) membranes in the dry or the wet condition. The interfacial polymerization reaction between 1,3-diaminopropane (DAPE) and succinyl chloride (SCC) or between DAPE and trans-3,6-endomethylene-1,2,3,6-tetrahydrophthaloyl chloride (tNBDC) on the surface of a modified polyacrylonitrile (mPAN) membrane was carried out to fabricate DAPE-SCC/ mPAN or DAPE-tNBDC/mPAN TFC membranes. PALS and MD simulation experimental results were highly consistent with each other. & 2012 Elsevier B.V. All rights reserved.

Keywords: TFC membrane Interfacial polymerization Positron annihilation lifetime spectroscopy (PALS) Molecular dynamics (MD) simulation Swelling effect

1. Introduction Pervaporation separation processes are economical alternatives to conventional ones for difficult-to-separate mixtures such as azeotropes, close-boiling-point mixtures, isomers, and heat-sensitive mixtures [1–3]. For this purpose, different types of membranes are commonly used. A composite membrane with a thin selective top layer, known as a thin-film composite (TFC) membrane [4–7], can be fabricated through a useful technique of interfacial polymerization. Polyamide has been regarded as a suitable membrane material because of its good thermal stability, excellent mechanical strength, and high resistance to organic solvents. When it is applied to pervaporation, however, the permeation rate is low as it is a dense membrane [8–10]. To increase the permeation rate without sacrificing the selectivity, converting the morphology from a dense membrane to a composite structure might be necessary. In applications of polyamide TFC membranes to pervaporation, free volume elements are regarded as important factors. They provide understanding as to how the feed solution affects the active layer during the pervaporation process, which is a key

concern to improving the membrane performance [11]. However, it is a difficult task to estimate the free volume in the polyamide active layer subjected to pervaporation conditions. Positron annihilation lifetime spectroscopy (PALS) has been developed as a useful tool for probing the microscopic structure of a variety of polymeric materials. Free volume size, fractional free volume, and size distribution have been reported by many research groups using the PALS technique [12–21]. Molecular dynamics (MD) simulation techniques also provide an in-depth understanding of the polymeric membrane structure at the molecular scale [22–31]. From past studies, PALS and MD techniques have been successfully used in analyzing free volume size distribution. A coincidence of result between theoretical and experimental work has been obtained [26,27,32]. This work adopted PALS and MD simulation techniques to analyze the effect of swelling on the free volume size and distribution in polyamide TFC membranes applied to pervaporation.

2. Experimental 2.1. Materials

n

Corresponding authors. Tel.: þ 886 3 2654190; fax: þ 886 3 2654198. E-mail addresses: [email protected] (W.-S. Hung), [email protected] (K.-R. Lee). 0376-7388/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.memsci.2012.06.036

Polyacrylonitrile (PAN) polymer was supplied by Tong-Hua Synthesis Fiber Co., Ltd., Taiwan. N-Methyl-2-pyrrolidone (NMP) used as solvent was of reagent grade. Succinyl chloride (SCC),

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O

Cl

O Cl

NH 2

Cl

NH 2

Cl

O

O Fig. 1. Chemical structures of monomers used in interfacial polymerization. (a) SCC, (b) tNBDC and (c) DAPE.

trans-3,6-endomethylene-1,2,3,6-tetrahydrophtgaloyl chloride (tNBDC), and 1,3-diaminopropane (DAPE) were purchased from Aldrich Co. Diamine DAPE and the two types of acyl chloride (SCC and tNBDC) were used as monomers for the polyamide active layer formation by interfacial polymerization. The chemical structures of the monomers are shown in Fig. 1. 2.2. Preparation of modified PAN porous membrane support The procedure we followed for preparing the modified PAN (mPAN) porous membrane support was taken from our previous study [7]. For a continuous preparation, a polymer solution containing 15 wt% of PAN in an NMP solvent was cast with a 200-mm gap knife onto a non-woven polyester fabric. The cast membrane was precipitated by immersion in a water bath. To improve the hydrophilicity of the PAN membrane for an effective sorption of the amine monomer in it, the PAN membrane was hydrolyzed in a 2-M NaOH solution at 50 1C. When PAN supports undergo hydrolysis in an NaOH solution, their –CN groups can be converted into –COOH and –CONH2 groups [30]. The mPAN porous membrane supports were washed in a water bath for several hours, and were then dried at room temperature. 2.3. Preparation of polyamide TFC membranes A polyamide active layer was synthesized through interfacial polymerization. The mPAN support surface was immersed in 1 wt% aqueous amine solution for 3 min. The excess amount of the aqueous amine solution on the mPAN support was removed. The mPAN membrane soaked with amine solution was contacted with a toluene solution containing 0.5 wt% acyl chloride (SCC or tNBDC) for 3 min to carry out the interfacial polymerization process. Finally, the resulting polyamide TFC membrane was washed in methanol and then dried at room temperature. 2.4. Membrane characterization The chemical structures of the active layers of polyamide TFC membranes were determined by using attenuated total reflectance Fourier transform infrared (ATR-FTIR) (Perkin Elmer Spectrum One) spectroscopy. The cross-sectional morphologies of the TFC membranes were observed with scanning electron microscopy (SEM) (HITACHI S-4800). The surface roughness of the membranes was measured with atomic force microscopy (AFM) (Digital Instruments, DI-NS3a USA). To determine the affinity between the feed of 90 wt% aqueous ethanol solution and the membrane surface, the contact angle using the feed solution as the test liquid was estimated with an automatic interfacial tensiometer (FACE, Mode1 PD-VP). 2.5. Positron annihilation lifetime spectroscopy Experiments on positron annihilation spectroscopy coupled to a variable monoenergy slow positron beam (VMSPB) were carried out. The purpose was to probe the variation in the free volume in the multilayer structure of the polyamide TFC membrane as a

function of positron incident energy in the range of 0–30 keV under a vacuum of  10  8 Pa at room temperature. The radioisotope beam used 50 mCi of 22Na as the positron source. Two systems of positron annihilation spectrometry were connected to the beam: PALS and Doppler broadening energy spectroscopy (DBES). The DBES spectra were measured using an HP Ge detector at a counting rate of approximately 2000 cps. The energy resolution of the solid-state detector was 1.5 keV at 0.511 MeV (corresponding to the positron 2g annihilation peak). The total number of counts for a DBES spectrum was 1.0 million. The PALS data were obtained by taking the coincident events between the start signals detected by a multichannel plate from the secondary electrons and the stop signals given by a BaF2 detector from the annihilation photons at a counting rate of 200–300 cps. A PALS spectrum contained 2.0 million counts. These techniques provide new information about the free volume properties of polymeric membrane systems, the very origins of physical structure of polymers, i.e., from several A˚ to the 1 nm level or in the range of 10  10 s to longer time scales of molecular motion. PALS has been developed also to be a quantitative probe of free volume for polymers. Not only does it probe the free volume size and the fraction of free volume, but it also gives detailed information on the distribution of free volume hole sizes in the range from 1 to ˚ In this study, we focused on the free volume variation in the 10 A. polyamide TFC membrane not only in the dry state but also in the wet state. For the sample preparation, we followed the procedure given in our previous study [11]. By depositing an inorganic SiOxCyHz layer on the polyamide TFC membrane through plasma polymerization, the TFC membrane was maintained in the wet state in the VMSPB operating at a high-vacuum condition. The layer of the glass-like plasma polymer thin film (SiOxCyHz) of several hundred nm was also for protecting the active layer of the TFC membrane and for sealing the high vacuum well. A 90 wt% aqueous ethanol solution was used to wet the membrane. All positron annihilation lifetime spectra were analyzed by a finite-term lifetime analysis method using PATFIT and MELT programs [13,15–17].

2.6. Pervaporation and sorption measurements Polyamide TFC membranes were utilized in the pervaporation separation of a 90 wt% aqueous ethanol solution. The feed was kept at 25 1C throughout the entire experiment. It was in direct contact with the active layer of the polyamide TFC membrane. The effective membrane area for pervaporation was 9.89 cm2. A vacuum pump maintained the downstream pressure at 13– 15 mmHg. The permeate was collected in a trap cooled by liquid nitrogen, and it was analyzed by gas chromatography (GC China Chromatography 8700 T). To determine the permeation rate, the measured weight of permeate was divided by the sampling time. Using Eq. (1), the separation factor of water/alcohol, aw/A, was calculated.

aW=A ¼

Y W =Y A X W =X A

ð1Þ

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203

swelling effect, however, the membrane model was affected by the feed composition. The membrane model was first constructed assuming dry-state conditions. Next, feed molecules of water and ethanol were added into the dry-state membrane model. We then considered the membrane model to be in the wet state, containing both polymer chains and feed molecules. The number of feed molecules was calculated based on the weight fractions of the components in the 90 wt% aqueous ethanol solution used in the experiments. Thus, the number of ethanol and water molecules was 94 and 27, respectively. Details of the model construction parameters are summarized in Table 1. All membrane models were processed for energy minimization consisting of over 2000 interactions to relax unfavorable overlaps for obtaining more stable membrane structures. Then, MD calculation was carried out under an NPT ensemble for 300 ps duration at 303 K and 1 atm and under an NVT ensemble for 500 ps duration at 303 K to equilibrate the system.

Xw, XA were respective weight fractions of water and alcohol in the feed, and Yw, YA were those in the permeate. Sorption of a 90 wt% aqueous ethanol solution by the polyamide composite membrane was measured at 25 1C. The description of the sorption apparatus and the procedure for sorption measurements can be found in our previous study [33]. A specially designed device was used in the membrane sorption process, which was totally different from the traditional method. With the sorption process used in this study, only the polyamide active layer side of the TFC membrane was in contact with the feed solution at 25 1C.

3. Theoretical method Molecular models were built using a Cerius2 package for Accelrys. All simulation work was done at the National Center for High-Performance Computing (NCHC), Taiwan. Details of the model construction and the physical property estimation are described below.

4. Results and discussion

3.1. Membrane model construction 4.1. Characterization

Membrane models were constructed with polyamide active layers (chemical structures are shown in Fig. 1), which contained different types of acyl chloride. For a membrane model, 5 chains of 30 repeat units each were packed in a cubic box at an initial density of 0.5 g/cm3 at periodic boundary conditions of 303 K and 1 atm. COMPASS (Condensed-phase Optimized Molecular Potential for Atomistic Simulation Studies) force field was employed [23,31]. In this study, we regarded the membrane model content as consisting of only polymers in the dry state. Based on the

Polyamide TFC membranes were characterized by ATR-FTIR for their surface chemical composition (Fig. 2). With the spectrum for mPAN as reference, the spectra for DAPE-SCC/mPAN and DAPE-tNBDC/mPAN TFC membranes indicate an increase in the intensity of the peaks at wavenumbers of 1638 and 1536 cm  1, corresponding to C ¼ O (Amide I) and N–H (Amide II), respectively. The cross-sectional SEM images of mPAN and polyamide composite membranes are exhibited in Fig. 3. Prior to interfacial polymerization, the mPAN cross-section appears as shown in Fig. 3(a). After interfacial polymerization, it is evident that thin polyamide active layers are formed onto the mPAN membrane, as illustrated in Fig. 3(b, c). The average polyamide active layer thickness for

Table 1 Dry- and wet-state model construction parameters for MD simulation. Membrane model

Initial density (g/cm3)

Repeating units/chain

Number of chains

Number of EtOH

Number of H2O

Temperature (K)

Pressure (atm)

DAPE-SCC-dry DAPE-tNBDC-dry DAPE-SCC-wet DAPE-tNBDC-wet

0.5 0.5 0.5 0.5

30 30 30 30

5 5 5 5

– – 94 94

– – 27 27

303 303 303 303

1 1 1 1

mPAN (a)

Transmittance (%)

DAPE-SCC/mPAN (b)

DAPE-tNBDC/mPAN (c)

N-H

C=O

1536 cm-1

1638 cm-1

4000.0

3600

3200

2800

2400

2000

1800

1600

1400

1200

1000

800 650.0

-1

Wavenumbers (cm ) Fig. 2. ATR-FTIR spectra for membrane support and polyamide TFC membranes. (a) mPAN, (b) DAPE-SCC/mPAN and (c) DAPE-tNBDC/mPAN.

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Average thickness (nm) 49.8 ± 2.3

Average thickness (nm) 48.2 ± 2.3

Fig. 3. SEM images (  100 k) of cross-sectional morphologies of membrane support and polyamide TFC membranes. (a) mPAN, (b) DAPE-SCC/mPAN and (c) DAPE-tNBDC/ mPAN.

Table 2 Effect of chemical structure of acyl chloride monomer on pervaporation performance of polyamide/mPAN TFC membrane for separating 90 wt% aqueous ethanol solution at 25 1C. Composite membrane

Membrane performance

Table 3 Effects of chemical structure of acyl chloride monomer on polyamide/mPAN TFC membrane surface roughness and affinity for feed solution component. Composite membrane

Contact angle (1)a

Roughness Rrms (nm) Ra (nm)

DAPE-SCC/mPAN DAPE-tNBDC/mPAN

Permeation flux (g/m2h)

Water concentration in permeate (wt%)

555 7 75 580 7 48

96.3 7 1.0 91.2 7 0.7

Polymerization conditions: immersion in 1.0 wt% DAPE aqueous solution for 3 min, and subsequent contact with 0.5 wt% acyl chloride solution for 3 min.

DAPE-SCC/mPAN DAPE-tNBDC/ mPAN

4.2. Pervaporation performance Table 2 provides the data on the effect of the chemical structure of acyl chloride monomer on the pervaporation performance of polyamide TFC membranes for separating a 90 wt% aqueous ethanol solution. The concentration of water in the permeate obtained from the DAPE-tNBDC/mPAN TFC membrane is lower than that from the DAPE-SCC/mPAN TFC membrane. But the permeation fluxes for these two kinds of polyamide TFC membranes are similar. From the molecular structure viewpoint, the tNBDC monomer with a bulky pendent group (norbornylene) inhibits local segmental motion in the DAPE-tNBDC polyamide backbone. The polymer packing density is lower when a larger pendent group is introduced into the polymer backbone. Therefore, the free volume concentration and size in the DAPE-tNBDC/mPAN TFC membrane are higher than those in the DAPE-SCC/mPAN TFC membrane. The physicochemical properties of the polyamide active layers, such as sorption, surface roughness, and affinity for the feed solution component, would likewise affect the pervaporation separation performance. For the effect of these properties on the pervaporation performance, the relevant data are given in Table 3. The contact angle of a 90 wt% aqueous ethanol solution for the DAPEtNBDC/mPAN TFC membrane is lower than that for the DAPE-SCC/mPAN TFC membrane. It means that the affinity between the test solution and the former membrane is stronger than that between the test solution and the latter membrane. As illustrated by AFM images in Fig. 4, the surface of the DAPEtNBDC/mPAN membrane is rougher than that of the DAPE-SCC/mPAN membrane. This AFM analysis corresponds well with the contact angle data. Sorption experiments of a 90 wt% aqueous ethanol solution on the polyamide TFC membranes were investigated at 25 1C. Sorption results are also indicated in Table 3, from which we can discern that the concentration of ethanol (30.7 wt%) in the DAPE-tNBDC/mPAN TFC membrane is higher than that (21.7 wt%) in the DAPESCC/mPAN TFC membrane. This result indicates that the former membrane has a higher affinity for ethanol molecules, which causes a higher plasticizing effect, than the latter membrane. Therefore, the DAPE-tNBDC polymer chain mobility increases, and the energy barrier for the diffusion of ethanol molecules declines, leading to an increased ethanol concentration in the permeate. This explains why the concentration of water in the permeate obtained from the DAPE-tNBDC/mPAN membrane is lower than that from the DAPE-SCC/mPAN membrane. The above results are further studied by PALS and MD.

24.9 7 5.5 20.6 73.6 21.77 6.1 30.5 7 8.2 22.8 75.0 30.77 4.9

Polymerization conditions: immersion in 1.0 wt% amine monomer solution for 3 min, and subsequent contact with 0.5 wt% acyl chloride monomer solution for 3 min. a

DAPE-SCC/mPAN and DAPE-tNBDC/mPAN TFC membranes is 49.8 7 2.3 and 48.2 72.3 nm, respectively. Based on ATR-FTIR spectra and SEM images, it was confirmed that the polyamide active layer of the TFC membranes was successfully synthesized.

15.5 7 1.8 10.17 1.9

Ethanol concentration in membraneb (wt%)

b

Test solution: 90 wt% aqueous ethanol solution. Feed solution: 90 wt% aqueous ethanol solution.

4.3. Correlation between free volume in polyamide active layer and pervaporation performance 4.3.1. Positron annihilation lifetime spectroscopic analysis Free volume properties of polymeric membranes provide insights into the pervaporation separation performance. The free volume size may be the factor that dominates effective separation, and the free volume intensity may be the one that controls the permeation flux. For TFC membranes, the active layer offers the main resistance during the separation process; that is, the active layer is considered as the separation layer. Pervaporation membranes are always in direct contact with the feed solution whose components are to be separated. As such, the membrane experiences a certain degree of swelling. There is a strong interaction between the feed solution components and the pervaporation membrane. To understand such an interaction when the actual wet condition of the polyamide TFC membrane is considered, it is essential to characterize the membrane’s swelling behavior. Experiments on positron annihilation spectroscopy coupled to a VMSPB were performed as a function of positron energies from 100 to 30 keV for polyamide TFC membranes both in the dry and the wet condition. In our previous study [11], we combined the techniques of plasma and interfacial polymerization to fabricate a plasma-polymerized TFC membrane, so as to probe the TFC membrane in its wet state by positron annihilation spectroscopy. The function of the resulting glasslike SiOxCyHz layer is as a protective layer to keep the TFC membrane wet during the high-vacuum operation of the VMSPB. The technique of developing a plasmapolymerized TFC membrane for VMSPB has been demonstrated to be applicable for asymmetric membrane systems maintained in the wet condition. A SiOxCyHz/ polyamide/mPAN TFC membrane was first prepared. Next, the densest part of the polyamide TFC membrane was determined by DBES (Fig. 5). We found that the densest polyamide layer is located at the positron incident energy of 7.2 and 8.0 keV for the DAPE-SCC/mPAN and the DAPE-tNBDC/mPAN TFC membrane, respectively. The free volume properties of the densest polyamide layer were then analyzed by PALS. This technique gives quantitative information on the free volume size based on the positron lifetime t and on the free volume concentration based on the positron intensity I. The positron lifetime t3 is due to o-Ps annihilation. In polymeric materials, the annihilation lifetime is on the order of 1–5 ns, which is the result of the so-called pickoff annihilation with electrons in molecules and is used to calculate the mean free volume radius R (A˚ to nm). The data on the effect of the molecular structure of the polyamide active layer in dry and wet states on t3 and I3 values are given in Table 4. A 90 wt% aqueous ethanol solution was used as the liquid to wet the membrane. In the dry state, the

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205

Fig. 4. AFM images of polyamide TFC membranes. (a) DAPE-SCC/mPAN and (b) DAPE-tNBDC/mPAN.

Fig. 5. S parameter as function of positron incident energy for plasma-polymerized SiOxCyHz/polyamide/mPAN TFC membranes in dry and wet states. Liquid used to wet membrane: 90 wt% aqueous ethanol solution.

Table 4 Data on o-Ps lifetime (t3), relative intensity (I3), and free volume radius (R) for polyamide/mPAN TFC membranes. polyamide/mPAN TFC membrane (positron incident energy)

t3 (ns)

˚ R (A)

DAPE-SCC/mPAN-dry (7.2 keV) DAPE-SCC/mPAN-wet (7.2 keV) DAPE-tNBDC/mPAN-dry (8.0 keV) DAPE-tNBDC/mPAN-wet (8.0 keV)

1.96 2.94 1.83 1.03 3.13

2.82 3.87 3.59 9.36 2.69 10.44 1.71 3.71 2.79 10.67

I3 (%)

Polymerization conditions: 1 wt% DAPE/H2O solution reacted with 0.5 wt% acyl chloride/toluene solution for 24 h.

t3 of the polyamide active layer of DAPE-SCC is longer than that of DAPE-tNBDC; the latter has a smaller free volume size than the former. This result might be due to the bulky group (norbornylene) in the DAPE-tNBDC polyamide active layer, which occupies part of the free volume space. In the wet state, however, a very interesting result of the o-Ps annihilation lifetime of the polyamide active layers was observed. The t3 and R are longer in the case of the swollen polyamide active layers of DAPE-SCC and DAPE-tNBDC than those for the dry polyamide active layers. These results demonstrate the swelling effect on the variation in the fine-structure of the polyamide active layers of the TFC membranes in the wet condition. The swelling of the packed region of the polyamide active layers may be induced by the aqueous ethanol solution, resulting in longer t3. The DAPE-tNBDC polyamide active layer in the wet state

gives bi-modal positron lifetime distributions, but the DAPE-SCC polyamide active layer in the wet state gives only mono-modal distributions. The behavior in the wet state may be analyzed as follows. The feed molecules are efficiently dissolved in the polyamide active layer, resulting in the plasticization of the layer. The swelling of the packed region of the polyamide active layer is induced by the aqueous ethanol solution. This leads to extrusion, from which a new free volume is formed, or to the creation of a new free volume (t3  1.03 ns). Another viewpoint is that due to the swelling of the polymer chains by the aqueous ethanol solution, the feed molecules occupy the newly formed free volume, and they may also contribute to the positron lifetime [34,35]. The wet polyamide active layer in DAPE-tNBDC has greater free volume size and intensity compared to the wet polyamide active layer in DAPE-SCC. Because of the bigger free volume and the higher ethanol sorption in the former active layer than in the latter, there is more opportunity for the ethanol molecules to go through the membrane, resulting in a lower concentration of water in permeate, as shown in Table 2.

4.3.2. Molecular dynamics simulation analysis MD simulation provides a useful tool for the membrane structural analysis at a microscopic level. It was adopted in our present work to have an in-depth understanding of the relationship between the free volume variation in the polyamide active layer and its molecular structure. The morphological representation of the fractional accessible volume in DAPE-SCC and DAPE-tNBDC polyamide active layers in dry and wet states is shown in Fig. 6. The content in the dry state depicted in Fig. 6(a, c) consists of polymer chains only, whereas the content in the wet state illustrated in Fig. 6(b, d) is composed of polymer chains and water and ethanol molecules. All these content components were removed, leaving empty

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Fig. 6. Three-dimensional representation of fractional accessible volume in DAPE-SCC (a, b) and DAPE-tNBDC (c, d) polyamide active layers: (a, c) dry state, (b, d) wet state. ˚ (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.). (Probe radius is 0.8 A).

respectively. The polymer chains in both active layers are linear. As such, they are flexible and have more freedom to move and rearrange or pack. For both polyamide active layers, there are three sharp peaks with high ˚ which is attributed normalized RDF intensity in the short distance region (0–3 A), to the bonding distance or the more orderly part of C–C pairs on the main chain. ˚ shows broad and low normalized RDF intensity, The long distance region ( 43 A) which is contributed by the non-bonding distance of C–C pairs between two polymer chains. The inset in Fig. 7 shows a blowup rescale of the long distance region. Compared to the DAPE-SCC polyamide active layer, the DAPE-tNBDC polyamide active layer has a lower normalized RDF intensity. This observation shows that the

20

1.5

16

Normalized RDF

spaces in the cube, to show only the free accessible volume (blue and gray colors) in the unit cell. The blue color denotes the Connolly surface, and the gray color represents the inside of the Connolly surface. Compared to the polyamide active layers in the wet state, those in the dry state show more compact structure with less free volume between the surrounding polymer chains. Apparently, the polyamide active layers are swollen by the aqueous ethanol solution. The theoretical results of the image analysis from molecular dynamics simulation are in good agreement with the experimental results on the positron annihilation lifetime from PALS. From MD simulation, the mobility of polymeric chains based on radial distribution function (RDF) analyses can be studied. This can provide information as to how polymer chain mobility affects the free volume variation. RDF analyses entail intermolecular and intramolecular behavior of the main chain and/or side chain, and the RDF spectra indicate the probability density of finding atoms at a specific/given distance. In general, a higher intensity peak shows better ordering of atoms in the molecular model, whereas a lower intensity peak indicates a higher chain mobility or flexibility [36]. Fig. 7 illustrates the normalized RDF of C–C atom pairs on the main chains of different polyamide membranes in the wet state. Peaks appeared at different positions corresponding to different locations or distances of C–C atom pairs in the polymer structures. For amorphous polymers, there are generally no peaks appearing in the RDF diagram [37–39]. We can find that some peaks appear in the RDF pattern for DAPE-SCC and DAPE-tNBDC polyamide active layers. It is inferred that these active layers might be semi-crystalline polymers. With an x-ray diffraction (XRD) instrument, the crystallinity of the active layers was analyzed and the d-spacing for DAPE˚ SCC and DAPE-tNBDC polyamide active layers were measured as 3.76 and 4.56 A,

1.0

12

DAPE-SCC-wet

8

DAPE-tNBDC-wet

0.5

3

6

9

12

15

18

4

0

0

3

6

9

12

15

18

Radius (Å) Fig. 7. Radial distribution function of C–C atom pairs on main chains of DAPE-SCC and DAPE-tNBDC polyamide active layers in wet state.

latter active layer has much higher chain mobility and flexibility, hence, is much more easily swollen by the feed solution, compared to the former active layer. These results of the molecular simulation in Fig. 7 demonstrate a good relationship with the PAS data in Table 4. The DAPE-tNBDC polyamide active layer was further analyzed. The RDF of norbornylene on the side chains of this active layer in dry and wet states is shown in Fig. 8, along with blowup rescale of the short distance region in inset (a) and of the long distance region in inset (b). There are few sharp peaks formed from the bonding C–C pairs or the ordered site of the DAPE-tNBDC polyamide active layer in the short distance region. Compared to the dry state, the wet state shows higher

Y.-H. Huang et al. / Journal of Membrane Science 417–418 (2012) 201–209

35

207

1.5

30

30

Normalized RDF

24

25

1.0

18 12

20

0.5

Dry

1.2 1.3 1.4 1.5 1.6 1.7 1.8

15

Wet 0.0

3

6

9

12

15

18

10 5 0 0

3

6

9

12

15

18

Radius (Å) Fig. 8. Radial distribution function (RDF) of C–C pairs of norbornylene on side chains of DAPE-tNBDC polyamide active layers in dry and wet states: (a) rescale range from 1 to 2 radius and (b) rescale range from 3 to 18 radius.

100

100 3

DAPE-SCC DAPE-tNBDC

DAPE-SCC-wet

80

80

DAPE-tNBDC-wet

Probability (%)

FVSD (%)

2

60 1

40

20

40

20

0

4~5 0

60

5~6

6~7

7~8

8~9 0

0~1

1~2

2~3

3~4

4~5

5~6

6~7

7~8

8~9

Deq of free volume element (Å) Fig. 9. Size distribution of free volume equivalent diameters for DAPE-SCC and DAPE-tNBDC polyamide active layers in wet state. intensity peaks in the short distance region. The RDF pattern illustrates that the structure of the polyamide active layer is semi-crystalline, as was confirmed by using XRD. In the wet state, the polymer chains of the DAPE-tNBDC polyamide active layer would be swollen by the feed solution, with the probability that the low ordered or loose area is swollen first. Then, the high ordered area might be compressed by the loose area, resulting in a lower flexibility of the polymer chain in the wet state, corresponding to a higher normalized RDF intensity, compared in the dry state. From the RDF diagram in the long distance region, we found that the normalized RDF intensity of the DAPE-tNBDC polyamide active layer in the wet state is lower than that in the dry state and that there is no apparent peak. Based on Fig. 8, it can be deduced that the mobility and flexibility of the bulky side chains in the wet state are much higher than those in the dry state. These results are in agreement with the PALS results indicated in Table 4. We suppose that the vibration and perturbation of the bulky side chains in the wet state might form a new free volume with a small size (t3  1.03 ns). Another way we can analyze the free volume size variation in polymeric membranes is by molecular simulation image analysis. We adopted an image analysis method based on a free volume morphology analysis to estimate the free volume size and shape distribution. To obtain the free volume size distribution (FVSD), the free space number with the same equivalent diameter (Deq) values was counted. In the FVSD analysis, both quantitative and qualitative analyses were used to examine the free volume size.

0.0~0.2

0.2~0.4

0.4~0.6 Eccentricity (-)

0.6~0.8

0.8~1.0

Fig. 10. Distribution of free volume equivalent eccentricity for DAPE-SCC and DAPE-tNBDC polyamide active layers. Fig. 9 illustrates FVSD results for DAPE-SCC and DAPE-tNBDC polyamide active layers in the wet state. We can find that more than 90% free volume have a Deq ˚ It means that the two polyamide active layers are dense value of less than 3–4 A. enough to offer high selectivity, and a large Deq value might be the factor that dominates transport properties. The inset in Fig. 9 is a blowup rescale plot of FVSD as a function of the higher values of Deq. We can observe from this inset that at Deq ˚ both the percentages of the free volume in the polyamide values larger than 4 A, active layers first increase then decrease. And when Deq values increase from 4 to ˚ the Deq value of the DAPE-SCC polyamide active layer is larger than that of the 7 A, DAPE-tNBDC polyamide active layer. Interestingly, we find that the percentage of the free volume in the DAPE-SCC polyamide active layers decreases to almost zero ˚ when Deq values are larger than 7 A. On correlating the FVSD analysis with the separation performance, we can find that the two polyamide TFC membranes show over 90 wt% water in permeate. However, the DAPE-tNBDC/mPAN polyamide TFC membrane indicates a lower concentration of water in permeate compared to the DAPE-SCC/mPAN polyamide TFC membrane. This is due to the DAPE-tNBDC polyamide active layer having a larger Deq value than the DAPE-SCC polyamide active layer, resulting in the easy penetration of permeants through the membrane. These results completely support the data listed in Table 2. The shape of free volume is difficult to estimate by experimental method. In this study, the free volume shape can effectively be analyzed through molecular dynamics simulation image analysis. The shape factor of the free space was analyzed by transforming the free volume area in the image into an equivalent

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ellipse (Eeq). As the shape of free volume approaches a circular structure, the Eeq value approaches zero, and it approaches a linear structure, when the Eeq value approaches one. The shape factor distribution for DAPE-SCC and DAPE-tNBDC polyamide active layers is shown in Fig. 10, which compares Eeq values for the polyamide active layers. For Eeq values in the range of 0.8–1.0, results suggest that the shape of free volume in the DAPE-SCC membrane probably approaches a linear structure. From the viewpoint of transport phenomena, more linear, connected, and extended free volume shape provides more opportunity for the penetrant to diffuse and penetrate. Based on the correlation of the Eeq analysis with the separation performance of the two TFC membranes, we can find that the DAPE-SCC polyamide active layer has a larger Eeq value (range in 0.8–1.0) than the DAPE-tNBDC polyamide active layer. It can be deduced that the shape of free volume is too linear or the shape is that of a highly elongated or prolated ellipsoid, which causes difficulty for ethanol molecules to penetrate, resulting in a higher concentration of water in the permeate obtained from the DAPE-SCC/mPAN polyamide TFC membrane than that from the DAPE- tNBDC/mPAN polyamide TFC membrane.

5. Conclusions The apparent feasibility and potential ability for conducting a microscale fine-structure analysis of polyamide membranes in the wet condition was demonstrated by PALS and MD techniques. Results from PALS and MD corresponded well with each other, as well as showed good correlation with the pervaporation performance for dehydrating an aqueous ethanol solution. The swollen polyamide active layers indicated a longer o-Ps lifetime compared to the dry polyamide active layers. The RDF of atom pairs suggested that the side chain fluctuation in the swollen polyamide active layer of DAPE-tNBDC was greater than that of DAPESCC. This greater fluctuation led to the formation of a more effective free volume in the wet DAPE-tNBDC polyamide active layer than in the wet DAPE-SCC polyamide active layer. The FVSD analysis suggested that the DAPE-tNBDC polyamide active layer in the wet state had a larger free volume size compared to the DAPE-SCC polyamide active layer in the wet state. From the analysis of the shape of free volume, the DAPE-SCC polyamide active layer had a larger Eeq value (0.8–1.0) than the DAPE-tNBDC polyamide active layer. It can be deduced that the shape of free volume is too linear or the shape is that of a highly elongated or prolated ellipsoid, which causes difficulty for ethanol molecules to penetrate, resulting in a higher concentration of water in the permeate obtained from the DAPE-SCC/mPAN polyamide TFC membrane than that from the DAPE- tNBDC/mPAN polyamide TFC membrane.

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