Applications of positron annihilation spectroscopy and molecular dynamics simulation to aromatic polyamide pervaporation membranes

Journal of Membrane Science 348 (2010) 117–123

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Applications of positron annihilation spectroscopy and molecular dynamics simulation to aromatic polyamide pervaporation membranes Se-Tsung Kao a,b , Yun-Hsuan Huang a , Kuo-Sung Liao a , Wei-Song Hung a , Kai-Shiun Chang a , Manuel De Guzman a , Shu-Hsien Huang a,d , Da-Ming Wang a,c , Kuo-Lun Tung a,∗∗ , Kueir-Rarn Lee a,∗ , Juin-Yih Lai a a

R&D Center for Membrane Technology, Department of Chemical Engineering, Chung Yuan University, Chung-Li 32023, Taiwan Institute of Polymer Science and Engineering, National Taiwan University, Taipei 10617, Taiwan c Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan d Department of Chemical and Materials Engineering, National Ilan University, I-Lan 26047, Taiwan b

a r t i c l e

i n f o

Article history: Received 4 September 2009 Received in revised form 22 October 2009 Accepted 28 October 2009 Available online 10 November 2009 Keywords: Pervaporation Aromatic polyamide Positron annihilation spectroscopy (PAS) Molecular dynamics (MD) simulation

a b s t r a c t A series of aromatic polyamide membranes for the pervaporation separation of aqueous ethanol mixtures was investigated. It was found that the permeation rate could be increased by the introduction of bulky substituted groups and arylene ether groups into the polymer backbone. The influence of the substituted group structure on the free volume in and the pervaporation performance of the aromatic polyamide membranes were systematically analyzed by positron annihilation spectroscopy (PAS) and molecular dynamics (MD) simulation. The trend of the ortho-positronium (o-Ps) lifetime and the free-volume size data evaluated by the PAS measurement and the MD simulation was highly consistent with the chemical structure of the aromatic polyamide pervaporation membranes. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Pervaporation separation processes offer potentially more economical alternatives for the difficult-to-separate mixtures such as azeotropes, close-boiling point mixtures, isomers, and heatsensitive mixtures. Many researchers have focused their attention on improving the membrane separation performance. However, the efficiency of the pervaporation process depends mainly on the intrinsic properties of the polymers used to prepare the membrane. Aromatic polyamide as a polymeric membrane possesses good mechanical properties and high chemical resistance. Its fabrication for application in gas separation and pervaporation processes can be optimized if the solubility of the aromatic polymer in organic solvents can be enhanced [1,2]. The solubility of the polymer is improved by introducing flexible links to the polymer backbone. Many efforts have been made to reduce the polymer packing density and to increase the specific volume through the introduction of bulky groups and/or flexible links into the polymer matrix [3–5].

∗ Corresponding author. Tel.: +886 3 2654190; fax: +886 3 2654198. ∗∗ Corresponding author. Tel.: +886 3 2654129; fax: +886 3 2654199. E-mail addresses: [email protected] (K.-L. Tung), [email protected] (K.-R. Lee). 0376-7388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2009.10.048

In general, the free volume and the free-volume fraction in a polymeric membrane are calculated by the method of Bondi [6]. However, it is a difficult task to estimate the free volume in the polyamide membrane during the pervaporation testing process, especially on consideration of the effects of the feed solution temperature and the membrane swelling. In recent years, positron annihilation spectroscopy (PAS) has been developed as a useful tool in probing the microscopic structure of polymeric materials. Freevolume sizes, fractions, and distributions in a variety of polymeric membranes have been reported using the PAS technique [7–9], which enables the detection of “voids” in polymers at an atomic scale. For polymeric applications, ortho-positronium (o-Ps) lifetime and its probability are related to the free-volume size and fraction, respectively. The positron annihilation lifetimes of dense membranes were determined by detecting the prompt ␥-rays (1.28 MeV) from the nuclear decay that accompanies the emission of a positron from the 22 Na radioisotope and the subsequent annihilation ␥-rays (0.511 MeV). The analyzed results of o-Ps lifetime ( 3 ) from PATFIT program were on the order of 1–5 ns in polymeric materials. Besides PAS, the molecular dynamics (MD) technique is a potential method for gaining insights into the polymeric membrane characteristics, including the polymer configuration, free volume, and transport behaviors at a microscopic scale [10–12]. In recent years, the molecular dynamics (MD) simulation technique has revealed potential insights into the micro-scale analyses

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of polymeric membranes [13–17]. From the past studies, the MD technique and positron annihilation lifetime spectroscopy (PALS) have been successfully adopted in analyzing free-volume size distribution. A good coincidence of result between theoretical and experimental work was obtained. In Hofmann et al. work, the effect of various backbone stiffness and substituent types on the free-volume distribution was analyzed by two methods, namely Vconnect and R-max method [14,15]. Furthermore, they discussed the relationships between polymer structure, free volume, and gas transport behavior based from calculations, and then compared the results with those from PALS. In the study by Wang et al., the free volume (cavity size) distribution from the viewpoint of energybased algorithm was analyzed, the results of which revealed good agreement compared to the experimental data [16]. In line with the abovementioned scrutiny on characterizing polymeric materials, a probe into the free-volume variation in aromatic polyamide membranes and its correlation with their pervaporation performance by means of PAS measurement and MD simulation is discussed in this article. 2. Experimental 2.1. Materials

Table 1 Reactant pairs of aromatic polyamides. Aromatic polyamides

Diacids

Diamines

HFPBA–BAPB HFPBA–BAPTB HFPBA–BAPDTB

HFPBA HFPBA HFPBA

BAPB BAPTB BAPDTB

HFPBA: 4,4-hexafluoroisopropylidenedibenzoic acid; BAPB: 1,4-bis(4aminophenoxy)benzene; BAPTB: 1,4-bis-aminophenoxy)2-tert-butylbenzene; BAPDTB: 1,4-bis(4-aminophenoxy)2,5-di-tert-butylbenzene.

with stirring at 100 ◦ C for 3 h. After cooling, the reaction mixture was poured with constant stirring into a large amount of methanol, producing a stringy precipitate, which was washed thoroughly with methanol and hot water, collected on a filter paper, and dried under vacuum at 100 ◦ C. From this dry precipitate, a film of membrane was prepared by solvent casting. The IR spectrum of the membrane film (HFPBA–BAPB polyamide) exhibited peaks with absorptions at around 3300 cm−1 (N–H) and 1650 cm−1 (C O). Moreover, 1 H NMR and 13 C NMR spectra for the HFPBA–BAPB polyamide (shown in Figs. 1 and 2, respectively) were obtained. The signals in the spectrum can be assigned as indicated in each figure. On the basis of the results taken from the data on the IR, 1 H NMR, and 13 C NMR spectra, it can be verified that the HFPBA–BAPB polyamide was successfully synthesized.

Three types of fluorine-containing polyamides were prepared by the direct polymerization of 4,4-hexafluoro-isopropylidenedibenzoic acid (HFPBA) with various diamines, i.e., 1,4-bis(4aminophenoxy)benzene (BAPB), 1,4-bis(4-aminophenoxy)2-tertbutylbenzene (BAPTB), and 1,4-bis(4-aminophenoxy)2,5-di-tertbutylbenzene (BAPDTB). BAPB was supplied from Wakayama Seika Co. Ltd. and was used without further purification. The diamines BAPTB and BAPDTB were prepared as described previously [18]. All reagent-grade chemicals were directly used without further purification. Water was deionized and distilled.

The polyamide membrane was prepared from a casting solution containing 10 wt.% of polyamide in N,N-dimethylacetamide (DMAc). The solution was cast at room temperature onto a glass plate using a Gardner knife to form a film with a predetermined thickness. The glass plate with the film cast on it was then heated at 70 ◦ C for 1 h. The average thickness of the membranes was about 25–30 ␮m.

2.2. Polymerization

2.4. Characterization

A series of aromatic polyamides was prepared by the direct polymerization of HFPBA with various diamines (Table 1) in a mixture of triphenyl phosphite and pyridine in N-methyl-2-pyrrolidone (NMP). A mixture of 2.5 mmol of diamine, 2.5 mmol of diacid, 0.60 g of calcium chloride, 1.8 mL of triphenyl phosphite, 1.8 mL of pyridine, and 6 mL of N-methyl-2-pyrrolidinone (NMP) was heated

1 H and 13 C NMR spectra were taken on an NMR spectrometer (Bruker, NMR-300). FTIR-ATR (PerkinElmer, Model SPECTRUMONE) was used to identify the functional groups present on the aromatic polyamide membranes. To determine the variation in the free volume in the polyamide membranes, positron annihilation spectroscopy (PAS) experiments were conducted. The positron

Fig. 1.

1

2.3. Membrane preparation

H NMR spectrum for HFPBA–BAPB membrane.

S.-T. Kao et al. / Journal of Membrane Science 348 (2010) 117–123

Fig. 2.

13

C NMR spectrum for HFPBA–BAPB membrane.

annihilation lifetime (PAL) spectroscopic data for the polyamides were evaluated by detecting the prompt ␥-rays (1.28 MeV) from the nuclear decay that accompanies the emission of a positron from the 22 Na radioisotope and the subsequent annihilation ␥-rays (0.511 MeV). All of the PAL spectra were analyzed by means of a finite-term lifetime analysis method using the PATFIT program and a continuous lifetime distribution using the MELT program. These analytical techniques were reported in our previous paper [19,20]. We made use of the results of the o-Ps lifetime to obtain the free volume. 2.5. Molecular dynamic (MD) simulation In our MD simulation work, the effects of the polymeric membrane prepared from various diamines on the fractional free volume (FFV), fractional accessible volume (FAV), free-volume size, and shape distribution analyses were discussed and compared with each other. The fractional free volume (FFV) of the membrane can be obtained from the following equations: FFV =

V − V0 V

V0 = 1.3VW

119

(1)

Table 2 Model construction parameters of MD simulation. Membrane model

 (g/cm3 )

Repeating unit

Temperature (K)

MD duration (ps)

HFPBA–BAPB HFPBA–BAPTB HFPBA–BAPDTB

1.314 1.258 1.138

15 15 15

298 298 298

500 500 500

was set to 15 to get the balance between the cell dimension and the calculation time. The details of model construction parameters are summarized in Table 2. All of the models were processed for energy minimization over 1000 iterations for obtaining the more stable membrane structures. Then, the molecular dynamics (MD) calculation was carried out under an NVT ensemble (the fixed atom numbers, cell volume, and system temperature) for a duration of 500 ps at 298 K. The Compass force field (Condensed-phase Optimized Molecular Potential for Atomistic Simulation Studies) was used for the minimization and MD calculations, including the terms of the bonded energy, cross interaction terms, Coulombic electrostatic force, and van der Waals force. The details of the model building and physical property estimation were described in our previous paper [10].

(2)

where V0 is the specific volume, VW is the van der Waals volume obtained from the van der Waals surface, and V is the cell volume of our molecular model. In the analyses of fractional accessible volume (FAV), the accessible volume was obtained using a hard spherical particle that probes the available volume for a particle to penetrate. The FAV value was then calculated by dividing the accessible volume by the cell volume of the molecular model. All of the simulated molecular models were constructed by the Cerius package from Accelrys [10]. All theoretical calculations were done at the National Center for High-Performance Computing (NCHC), Taiwan. The membrane models were constructed by the repeating units of polyamide (PA) monomers, which contained different diamines, as shown in Table 1. The degree of polymerization of our membrane model

2.6. Pervaporation and sorption measurements A traditional pervaporation process [21] was used. The effective area was 10.2 cm2 . The permeation rate was evaluated by taking the ratio of the measured weight of the permeate collected to the time of permeate sampled. The compositions of the feed solution and the permeate were measured by gas chromatography (G.C. China Chromatography 8700 T). The separation factor (˛water/alcohol ) was calculated as follows: ˛water/alcohol =

Ywater /Yalcohol Xwater /Xalcohol

(3)

where Xwater , Xalcohol and Ywater , Yalcohol are the weight fractions of water and alcohol in the feed and the permeate, respectively.

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Fig. 4. Effect of molecular structure of synthesized polyamide membranes on freevolume size and o-Ps intensity.

Fig. 3. o-Ps lifetime distributions data for polyamide membranes.

During the sorption testing, the membranes were immersed in several feed mixtures at 25 ◦ C for 24 h. Then, they were blotted between two pieces of filter paper to remove the excess solution and were quickly placed in the left tube of a twin-tube setup. The system was evacuated while the left tube was heated with hot water and the right tube cooled in liquid nitrogen. GC determined the composition of the condensed liquid in the right tube. The separation factor for sorption (˛sorp ) was calculated as follows: ˛sorp =

Ywater /Yalcohol Xwater /Xalcohol

(4)

where Xwater , Xalcohol and Ywater , Yalcohol are the weight fractions of water and alcohol in the feed and the membrane, respectively. 3. Results and discussion 3.1. Effect of chemical structure on free volume of synthesized aromatic polyamides To understand the effect of the chemical structure of the synthesized aromatic polyamide membranes on their free-volume properties, PAL spectra, were obtained. The analyzed results of the positron lifetime () and intensity (I) from PAL spectra are attributed to the positron and positronium annihilation in the polymeric membrane materials. The shortest  1 ∼ 0.125 ns (analyzed using the PATFIT program) is from p-Ps annihilation,  2 ∼ 0.45 ns is from the free positron annihilation, and  3 is due to o-Ps annihilation. The o-Ps annihilation lifetime  3 is on the order of 1–5 ns in polymeric materials; it is the so-called pickoff annihilation with electrons in molecules and is used to calculate the mean freevolume radius R (Å to nm). The position lifetime distributions and the  3 results (Fig. 3) were analyzed with positron annihilation spectroscopy. We employed the results of  3 values to analyze the free volume. The effect of the molecular structure of the synthesized polyamide membranes on the  3 values, o-Ps intensity (I3 ) and average freevolume element size can be seen from Fig. 4 and Table 3. It is indicated that the free-volume element size and o-Ps annihilation lifetime increases with the specific volume of the aromatic polymer membranes. It can be inferred that the specific volume follows this order: HFPBA–BAPDTB > HFPBA–BAPTB > HFPBA–BAPB. From the viewpoint of the molecular structure, the polymer with a larger substituted group (HFPBA–BAPDTB), which causes a higher barrier

to chain rotation in the polyamide membranes, may inhibit local segmental motion more easily. Thus, it can be deduced that the polymer packing density is lower when a larger pendent group is introduced into the polymer backbone. This result indicates that the size of the free volume is strongly molecular structure dependent. From the analysis of positron annihilation spectroscopy, the average free-volume element size increases with the size of the pendent group in polyamide membranes. The average free-volume element sizes in HFPBA–BAPB, HFPBA–BAPTB, and HFPBA–BAPDTB polyamide membranes are 52.33 ± 1.54, 56.31 ± 1.36, and 62.03 ± 2.76 Å3 , respectively. On the other hand, the lifetime distribution curves shift to higher lifetime direction (Fig. 3) as the molar volume of diamine increases. These observations might be due to the fact that the 2,5-di-tert-butylbenzene group has two bulky pendent groups, which could increase the steric hindrance. The groups increase the space between the polymer chains and the free volume as well, therefore making the rotational movements of the main chain segments easier, resulting in the lifetime distribution curves shifting to higher lifetime direction. Another interesting phenomenon revealed in this positron annihilation spectroscopy is that the relative intensity of the oPs (I3 ) also increases with the specific volume of the aromatic polyamides (Fig. 4). The increase in I3 suggests that the 2-5-di-tertbutylbenzene and hexafluoropropane both have bulky pendent groups that can cause significant steric hindrance and can increase the amounts of free volume. To further understand the relationship between the free-volume variation and the molecular structure of polyamides, the molecular dynamic (MD) simulation, which provides a useful tool for the membrane structure analysis, is discussed in this paper. Fig. 5 demonstrates the evolution of the simulated FFV values for the PA membranes for time duration of 500 ps. In the course of the MD calculation, we found that the FFV values decreased slightly in the duration of the first 200 ps and then stabilized until the end. This slight decay and stability in the FFV values indicate that the simulated system approached a stable equilibrium state. On comparing the simulated FFV values for the three PA membranes, we found that the HFPBA–BAPDTB polyamide membrane gives the highest Table 3 Effect of molecular structure of synthesized polyamide membranes on o-Ps annihilation lifetime ( 3 ) and o-Ps intensity (I3 ). Aromatic polyamides

 3 (ns)

I3 (%)

HFPBA–BAPB HFPBA–BAPTB HFPBA–BAPDTB

1.48 ± 0.014 1.53 ± 0.013 1.60 ± 0.014

12.19 ± 0.083 14.14 ± 0.080 15.00 ± 0.078

S.-T. Kao et al. / Journal of Membrane Science 348 (2010) 117–123

121

Fig. 7. Free-volume cross-section images of polyamide membranes at thickness = 1.5 Å (x = a/A, where A is cell length in amorphous model and a is free-volume cross-section position in x-direction). Fig. 5. Fractional free volume in polyamide membranes as function of time for 500 ps. () HFPBA–BAPB, () HFPBA–BAPTB, and () HFPBA–BAPDTB.

FFV. It was inferred that the bulky side group in the polymer chains of the HFPBA–BAPDTB polyamide membrane lowered the packing efficiency, which led to a higher free volume. In the case of the HFPBA–BAPB and HFPBA–BAPTB polyamide membranes, the polymer chains contain side groups occupying lower volume, which reduces the steric hindrance, resulting in less free volume. Furthermore, we found that there is a great difference between the HFPBA–BAPDTB polyamide membrane and the other two polyamide membranes (HFPBA–BAPB and HFPBA–BAPTB). That is, the apparent discrepancy in the FFV values indicates that the bulkier BAPDTB group causes a much greater notable effect on the polymer configuration than the BAPB and BAPTB groups. To analyze the free-volume morphology, an image analysis method was adopted. Fig. 6 illustrates the schema in which the chosen cross-section of the molecular model is in the x-direction (x = a/A, where A is the cell length in the amorphous model and a is the free-volume cross-section position in the x-direction). Fig. 7 presents the cross-sectional images of three kinds of polyamide membranes in the x-direction. It reveals that there is more freevolume space in the HFPBA–BAPDTB polyamide membrane than

Fig. 6. Selected cross-sections of molecular model in x-direction.

in the other two polyamide membranes. The effect of the polymer chemical structure on the free-volume variation, as analyzed by the MD simulation, is highly consistent with that evaluated by the PAL measurement. However, the bulk FFV analysis might not reflect the actual effective free space in which small molecules pass through. To investigate how the polymer chain configuration affected the membrane free volume and performance, the fractional accessible volume (FAV) and the free-volume morphology analyses were analyzed in the following section. Fig. 8 describes the FAV values explored by a hard spherical probe with various diameters. The same tendency of the FAV values was observed in comparison to the FFV results (indicated in Fig. 5). This agreement validates the fact that the packing efficiency of the polymer chains was lessened due to the existence of the bulky BAPDTB. In addition, the obvious increase in the FAV in the HFPBA–BAPDTB polyamide membrane indicates that the excess free space could be regarded as the effective free volume in which species pass through.

Fig. 8. FAV in polyamide membranes explored by probes with different diameters. () HFPBA–BAPB, () HFPBA–BAPTB, and () HFPBA–BAPDTB.

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Table 4 Effect of chemical structure of polyamide membranes on FFV and FAV values obtained by MD simulation. Polyamide membranes

Fractional free volume (FFV, %)

Fractional accessible volume (FAV, %)

HFPBA–BAPB HFPBA–BAPTB HFPBA–BAPDTB

14.20 14.52 20.08

9.63 9.48 14.08

Moreover, it can be seen in Fig. 8 that the FAV values for the HFPBA–BAPB and HFPBA–BAPTB polyamide membranes approached zero with probe diameters close to 4.5–5 Å, whereas the HFPBA–BAPDTB polyamide membrane still contained partially accessible volume. This available accessible volume explored by large probe diameters (above 5 Å) suggests that the HFPBA–BAPDTB polyamide membrane contained more free-volume element (FVE) with larger diameter than the other membranes. The larger size FVEs favored the big species transport, as our experimental work illustrated in Fig. 8. We also explored the FAV values (as Table 4 summarizes) using a probe with the same diameter as the positronium (Ps) size (1.59 Å, Ps-like particle) to compare with the PAL spectra results. It was found that the FAV probed by Ps-like particle had an obvious decrease compared to the FFV value, indicating that the smaller or tiny free space would not be regarded as a useful free volume. This phenomenon points out the importance of the statistics of the free-volume size distribution. Furthermore, it was also found that the FAV values for the HFPBA–BAPB and HFPBA–BAPTB polyamide membranes were almost overlapped for all the probe radii. This convergence suggests that the effective free volume of these two membranes might be similar. 3.2. Effect of various alcohols on PAL results and pervaporation performance To investigate the effects of solubility and diffusivity on the membrane permselectivity and sorption, pervaporation experiments on HFPBA–BAPB membranes were conducted. Table 5 tabulates the pervaporation and sorption properties of the HFPBA–BAPB membranes for alcohol–water mixtures. The data indicate that an increase in the number of carbon atoms in the alcohol results in an increase in the separation factor for pervaporation and a decrease in the separation factor for sorption, but causes a decrease in the permeation rate for pervaporation and an increase in the total sorption. These results can be explained by the molecular size and the shape of the alcohol. The separation factor is found to depend on the molecular length for this linear alcohol series. It was also found that the permeation rate of t-butanol is lower than that of the n-propanol, which may be due to the steric hindrance of the former being higher than that of the latter. The effect of various alcohols on the PAL results from the HFPBA–BAPB membrane is depicted in Fig. 9. The HFPBA–BAPB membrane demonstrates bi-model position lifetime distributions Table 5 Pervaporation and sorption alcohol–water mixturesa .

properties

of

HFPBA–BAPB

membranes

for

Alcohols

Total sorption (g/g)

˛pv

˛sop

Permeation rate (g/m2 h)

Methanol Ethanol n-Propanol t-Butanol

0.10 0.15 0.22 0.28

6.1 10.7 46.2 1799.6

24.4 16.2 9.8 7.8

586 474 271 195

a Alcohol in feed: 90 wt.%; ˛pv : selectivity for pervaporation; ˛sop : selectivity for sorption.

Fig. 9. Effect of various alcohols on PAL results from HFPBA–BAPB membrane.

as a function of the membrane submergence in aqueous alcohol mixtures. It shows that the large o-Ps annihilation lifetime (around 3.1–3.3 ns) increases with the difference in the solubility parameters between the HFPBA–BAPB membrane and the alcohol. The magnitude of each solubility parameter difference directs one’s attention to the following order: t-butanol < npropanol < ethanol < methanol (as shown in Table 6). Thus, the degree of swelling of the larger size alcohol (DSt-butanol = 28.1 wt.%) is higher than that of the smaller size alcohol (DSmethanol = 9.9 wt.%). These results completely support the data demonstrated in Fig. 9; that is, the larger size alcohol has a higher affinity for the membrane than the smaller size alcohol. This tendency results in the polymer packing density decreasing and the lifetime distribution curves shifting to a higher lifetime direction, as the degree of swelling of the HFPBA–BAPB membrane increases. Moreover, a small position lifetime distribution peak appeared at around 1.0 ns (Fig. 9); it was not found in the dry state (Fig. 3). These phenomena might be due to the fact that the high packing density region of the HFPBA–BAPB membrane was swelled by the aqueous alcohol solution, resulting in the formation of small size free volume. From the other viewpoint, the aqueous alcohol solution contacts the membrane and plasticization occurs. Plasticization can expand the existing free volumes and it can also create newer free volumes. This may enhance the o-Ps annihilation lifetime ( 3 ) and o-Ps intensity (I3 ). However, the probability of positronium formation in the free volumes containing water or alcohol may be smaller compared to empty free volumes, resulting in the o-Ps annihilation lifetime ( 3 ) decrease [22]. Thus, a small position lifetime distribution peak appeared. Table 5 indicates that the sorption selectivity of the alcohol decreases in going from methanol to t-butanol. The PAL results (Fig. 9) and the interaction Table 6 Effect of difference between solubility parameter of membrane and of alcohol on degree of swelling and composition of solution adsorbed on HFPBA–BAPB membrane at 25 ◦ C. Alcohol solutions (90 wt.%)

ımembr. − ıalcohol a

Degree of swelling (%)

Alcohol in membrane (wt.%)

Methanol Ethanol n-Propanol t-Butanol

6.5 4.7 3.9 2.6

9.9 14.6 22.5 28.1

26.9 35.7 47.8 53.3

a

ımembr. = 7.9, as predicted by Hoftyzer and Van Krevalen method.

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between the alcohols and the HFPBA–BAPB membranes (Table 6) can be used to further explain such phenomena. These results completely support the data listed in Table 5. 4. Conclusion Aromatic polyamide membranes with higher specific volumes could be prepared by the introduction of a bulky group into the polymer backbone. The o-Ps annihilation lifetime increased with the specific volume of the aromatic polymer membranes. The HFPBA–BAPB membrane exhibited bi-model position lifetime distributions as a function of the membrane submergence in aqueous alcohol mixtures. The swollen aromatic polyamide membrane showed a larger o-Ps lifetime than the dry membrane. The larger size alcohol demonstrated a higher affinity for the membrane than the smaller size alcohol, resulting in the polymer packing density decreasing and the lifetime distribution curves shifting to a higher lifetime direction. The trend of the ortho-positronium (o-Ps) lifetime and free-volume size data evaluated by the PAS measurement and MD simulation was highly consistent with the chemical structure of the aromatic polyamide pervaporation membranes.

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