Applications of positron annihilation spectroscopy to polymeric membranes

Desalination 234 (2008) 89–98

Applications of positron annihilation spectroscopy to polymeric membranes Y.C. Jeana,b,c*, Wei-Song Hunga,b, Chia-Hao Loa,b, Hongmin Chenc, Guang Liuc, Lakshmi Chakkac, Mei-Ling Chengc,d, D. Nandaa,b, Kuo-Lun Tunga,b, Shu-Hsien Huanga,b, Kueir-Rarn Leea,b, Juin-Yih Laia,b, Yi-Ming Suna,d, Chien-Chieh Hua,f, Chang-Cheng Yua,e a

R&D Center for Membrane Technology, Chung Yuan Christian University, Chung-Li 32023, Taiwan b Department of Chemical Engineering, Chung Yuan Christian University, Chung-Li 32023, Taiwan c Department of Chemistry, University of Missouri–Kansas City, Kansas City, MO 64110, USA Tel. þ886(3)2654173; Fax þ886(3)2654198; email: [email protected] d Department of Chemical Engineering and Materials Science, Yuan Ze University, Chung-Li 32003, Taiwan e Department of Physics, Chung Yuan Christian University, Chung-Li 32023, Taiwan f Department of Chemical Engineering, Nanya Institute of Technology, Chung-Li 32023, Taiwan Received 31 July 2007; accepted revised 25 September 2007

Abstract Positron annihilation spectroscopy (PAS) coupled with a slow positron beam has been used to study the freevolume depth profile (0–10 mm) in polymeric membrane systems prepared by the interfacial polymerization method. Doppler broadening energy parameters of annihilation radiation vary as a function of the depth from the surface through multi-layers in polyamide asymmetric membranes prepared under different experimental parameters, such as the time, the temperature, and pH of membrane preparation process of interfacial polymerization. Three layer structures including a skin, a transition, and the porous layers in those asymmetric membrane systems are obtained. Positron annihilation lifetime results provide additional information about free-volume size and distribution in nano- and micro-scale film membrane systems. PAS results are correlated with the pervaporation separation, flux and water wt% in permeate of 70% isopropanol aqueous solution and compared with the SEM images. Keywords: Positron annihilation spectroscopy; Slow positron beam; Free volume; Polymeric membrane; Interfacial polymerization

*Corresponding author. Presented at the Fourth Conference of Aseanian Membrane Society (AMS 4), 16–18 August 2007, Taipei, Taiwan. 0011-9164/08/$– See front matter # 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2007.09.074

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1. Introduction Positron annihilation spectroscopy (PAS) was developed as a useful tool to probe the nano-scopic properties of polymeric materials during the last two decades [1]. One of the great successes in this line of research is the direct determination of polymer free-volume and hole properties at an atomic scale (0.2–2 nm). Recent investigations in membrane and thin film systems using positron annihilation spectroscopy have been reported mainly in the bulk using the conventional positron annihilation lifetime technique [2–12]. Current positron spectroscopy uses a variable mono-energy positron beam (from kT to several ten keV) coupled with positron annihilation lifetime and momentum density measurements, and is capable of probing defect profiles from the surface, interfaces, and to the bulk [13–15]. In this paper, we report the results of applying two of PAS methods, Doppler broadening energy spectroscopy (DBES) and positron annihilation lifetime (PAL) spectroscopy coupled with a variable mono-energy positron beam to one of important membrane systems for pervaporation application, i.e. polyamide on modified porous polyacrylonitrile membrane [16–19]. In addition to the use of PAS technique, we also employed conventional techniques, SEM, ATR-FTIR, and pervaporation measurements to provide more complete physical and chemical information and to compare with the obtained depth profiles and free-volume properties.

was synthesized using an interfacial polymerization technique on the modified PAN (m-PAN) membrane support (hydrolysis of PAN) between 2 wt% aqueous solution of TETA (triethylenetetraamine, purchased from Merck Co. USA) and 1 wt% TMC (trimesoyl chloride, purchased form Aldrich Chemical Co. USA) toluene solution at r.t. for 3 min of interfacial polymerization time. The resulting membranes were washed in methanol overnight to remove any un-reacted residual chemicals. The layer thickness and surface morphology of the prepared PA on m-PAN and the m-PAN membrane were measured by AFM (Digital Instruments Nanoscope IIa tapping mode) and SEM (Hitachi model S-4800). The thickness of the skin polyamide was varied by changing the doping time of TETA (0–30 min) and the doping temperature (25, 50, and 70 C). The performance of synthesized polyamide composite membranes was measured in the pervaporation of 70 wt% isopropanol aqueous solution at r.t. The feed solution was in direct contact with polyamide membrane and the flux was determined by the weight of permeate through pumping on the nonwoven side of the membrane. The composition of the feed solution

2. Experiments 2.1. Polymeric membrane preparations Polyamide membrane is a composite system with an asymmetric layer structure: a polyester non-woven base (ca. 150 mm thick), a porous supporting PAN (polyacrylonitrile, ca. 50 mm thick), and the top skin layer of interfacial polymerized0 polyamide (PA) as schematically shown in Fig. 1. The polyamide active skin layer

Fig. 1. A schematic diagram for a multi-layer membrane system for pervaporation and for positron annihilation study with the skin polyamide (PA) on modified PAN porous membrane (m-PAN), which is supported by non-woven PET substrate.

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and the concentration of permeate were determined using gas chromatography (GC Chromatography 8700 T, China). Detailed description of pervaporation measurements could be found elsewhere [17]. The chemical structure of interfacial polymerized polyamide (PA) on the m-PAN supporting membrane and the functional groups of the prepared PA and m-PAN support were characterized by ATR-FTIR (Perkin Elmer Spectrum One). 2.2. Positron annihilation spectroscopy Slow positron beam technique with variable mono energy was used to define the mean depth of the interested membrane between 0 and about 10 mm (the mean depth is converted by using an established equation from the positron incident energy from kT to 30 keV and density) [1,13]. A schematic diagram of newly built slow positron beam for this study at Center for Membrane Technology, Chung Yuan Christian University is shown in Fig. 2. This new radioisotope beam uses 50 mCi of 22Na as the positron source (purchased from ithemba Labs, South Africa) and the design is similar to the slow positron beam at the University of Missouri-Kansas City [14,15]. Two positron annihilation spectrometers were installed in this beam for this study: Doppler energy spectroscopy (DBES) and positron annihilation lifetime (PAL) spectroscopy, which uses the secondary electrons emitted from the sample surface as the starting signal. The performance of this newly built beam is as good as or better than existing radioisotope beams with the following specifications: beam diameter (<5 mm between kT and 30 keV positron energy) through a computer controlling technique on magnetic coils’ currents and voltages; slow positron conversion efficiency 3  104; and optimal positron lifetime resolution 400 ps. The S parameters (which are defined as the ratio of low momentum part of the peak region to the total 2 annihilation near 511 keV

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Fig. 2. A schematic diagram of variable mono-energy (0–30 keV) slow positron beam at the Center of Membrane Technology, Chung Yuan Christian University. This beam includes Doppler broadening energy (DBES) and positron annihilation lifetime (PAL) spectrometers for the free-volume depth profile analysis from the surface to about 10 mm. Legends: A: 50 mCi 22Na positron source, B: W-mesh moderator, C: magnetic field (60 G) coils, D: ExB filter, E: positron accelerator, F: correcting magnets, G: gas inlet, H: positron lifetime detector (MCP) for PAL, I: turbo molecular pump, J: samples, K: sample manipulator, L: ion pump, M: Ge solid state detector, N: lifetime detector (BaF2).

energy) of Doppler broadening energy spectra (DBES) were measured as a function of depth using the slow positron beam (0–30 keV). The S parameter vs depth represents the relative value of the free volume depth profile in polymeric systems [20]. The S parameter data as a function of the mean depth were fitted in a multi-layer model using a computer program VEPFIT [21]. We have tried to fit 2-, 3-, and 4-layer models in the VEPFIT analysis. The 2-layer model could not give good enough chisquares for m-PAN and PA/m-PAN membrane systems, thus is not an acceptable model while the 4-layer model gives unstable results and the resultant error bars are larger than the fitted layer and diffusion lengths. Therefore, we report here the results of 3-layer fits for m-PAN and PA/m-PAN membranes and discuss the layer structures based on 3-layer model results. On the other hand, for S data in PA, the 2-layer model gives good and reasonable results. The other parameter R, which is the 3 to 2 annihilation

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ratio and it contains information about the existence of large pores (nm to mm), where o-Ps undergoes 3 annihilation while S is from p-Ps and o-Ps 2 radiation (pick-off annihilation) in ˚ to nm). free volumes (A The PAL data contain quantitative information on the free-volume properties, size, distribution, and fraction in polymeric systems [1]. Analyzed results of ortho-Positronium (o-Ps) lifetime  3 (using the PATFIT program) and its distribution (using the MELT program) are on the order of 1–5 ns in polymeric materials [3–12], the so-called pickoff annihilation with electrons in molecules and are used to calculate ˚ to nm) based the mean free-volume radius (A on an established semi-empirical correlation equation in a spherical-infinite potential wall model [22–24].

3. Results and discussions 3.1. Free-volume size distribution We performed PAL experiments in the base materials, PAN membrane, m-PAN membrane, and polyamide (PA) resin in both dry and wet (after soaking in 70% isopropanol aqueous solution for 4 h) states. The base materials of PA, PAN, and m-PAN were prepared in the similar methods as membranes except on the glass substrate instead of nonwoven polyester base substrate. We peeled off the prepared materials, packed the samples at a thickness of >1 mm, which is sufficient to absorb all positrons emitted from 22Na source. For porous membranes PAN and m-PAN, which contain high porosity (82%) with a pore size on the order of mm, we observed additional small fraction of very long o-Ps lifetime (120–140 ns) with intensity of 1.0, and 1.5 + 0.4%, respectively from long gated PAL spectra (500 ns). The very long o-Ps lifetime is contributed from those large pores (mm) and their lifetime is beyond the sensitivity of current PAL to distinguish their

sizes. Our focus is on the third component, i.e.  3 and I3, to calculate the free-volume hole properties according to the established equations for the pick-off annihilation [1,22–24]. We observed that the o-Ps lifetime ( 3) from of those six polymeric materials are in the values of similar polymers [2–12] as o-Ps pick-off annihilation and the corresponding free-volume radius ranges from 0.25 to 0.40 nm. It is interesting to observe that polyamide has a smaller free volume (radius ¼ 0.28 + 0.1 nm and ffv ¼ 1.5 + 0.1%) than both PAN (radius ¼ 0.33 + 0.1 nm, ffv ¼ 1.9 + 0.1%) and m-PAN (radius ¼ 0.31 + 0.1 nm, ffv ¼ 2.1 + 0.1%). Fig. 3 shows the o-Ps lifetime distributions and corresponding free-volume radius distributions for dry samples. The shapes of o-Ps lifetime distributions are similar while both the lifetime values and widths of the distributions are in the order of PAN > m-PAN > PA. In the same graph, we also show the kinetic radii of water [25] and isopropanol [26] to compare with the free-volume radius distributions in those base polymers obtained by PAS. It is interesting

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Fig. 3. Free-volume radius and o-Ps lifetime distributions in polyamide (PA), porous PAN, modified PAN porous (m-PAN) membranes and compared with water and isopropanol kinetic radii.

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to observe that PA radius lies apart from and ˚ ) and between the kinetic radius of water (1.3 A ˚ ). On the other hand, the of isopropanol (2.9 A free-volume radius distributions of both PAN and m-PAN are all larger than both water and isopropanol. This could explain the effective selectivity of polyamide used in pervaporation applications (water concentration in permeate >95%) for water/isopropanol separation and the low water concentration in permeate observed in m-PAN membranes without PA (<50%). Therefore we expect that pervaporation performance, i.e. flux and water concentration in permeate, for membranes with an interfacial polymerized PA layer may be correlated with the free-volume data. In the wet states, we found that wet m-PAN and PAN swell about 30% of free volume radius while the soaking a few hours has no significant effect on PA polymer. Furthermore, we pumped those swelled samples and measured PAL again and we found that PAL results return to those in the dry states. This indicates the stability and durability of this type of PA/m-PAN membranes for pervaporation applications.

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3.2. Multi-layer depth profiles and structures We performed DBES experiments as a function of the depth in the plain m-PAN membrane, PA polymer, and PA/m-PAN membrane systems as a function of positron incident energy (or depth). Fig. 4 shows the results of S (top) and the R (bottom) parameters for m-PAN, PA, and PA/ m-PAN membranes (5 min of TETA doping time at 50 C). From those S data, we have the following observations: (1) S near the surface increases sharply as the positron energy increases; (2) S reaches a maximum and then decreases for m-PAN before plateau, while for PA there is no peak; (3) S values for m-PAN are all larger than those with PA; and (4) S values for PA/m-PAN are in between PA and m-PAN and its variation has a deep or flat region after reaching the peak. While these observations are typical for

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Fig. 4. The S (top) and R parameters (3 /2 ratio, bottom) for polyamide (PA), porous PAN membrane (m-PAN), and PA on m-PAN membrane. Three layers are resolved: skin, transition from skin to porous m-PAN, and porous m-PAN as indicated vertical lines as boundaries from the VEPFIT analysis.

polymeric systems in slow positron beam experiments, the detailed variations contain multilayer structural and chemical composition information. Fitted lines for S data from VEPFIT analysis are also plotted in Fig. 4, which shows the good fits of the 3-layer model for m-PAN and PA/m-PAN membranes and the 2-layer model for PA resin, respectively. This 3-layer structure is also seen in the R parameter vs depth (lower plot of Fig. 4). The observed increase of R parameter from the transition layer to the

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porous layer is due to 3 annihilation radiation where o-Ps is localized the porous m-PAN, which has the pore size from 1 to 10 mm (SEM cross section images) with a porosity of 82%. From S and R parameters of DBES, we obtained the depth profiles in three layers: (1) for the plain m-PAN membrane, a dense skin of m-PAN with a thickness (200 + 117 nm), the transition layer (2.2 + 1.0 mm) of m-PAN, and the porous m-PAN support, and (2) for PA/m-PAN membrane, a skin PA (162 + 59 nm), the transition layer (2.5 +1.1 mm) from the dense skin to m-PAN, and the inner porous m-PAN. We also performed PAL experiments at selected depths and the resolved  3 and I3 vs the depth also confirm that the chemical compositions of each of three layers in m-PAN and PA/m-PAN membranes. The measured layer structure and thickness are also consistent with the images from SEM experiments as shown in Fig. 5. We observe a decrease of top layer PA thickness as a function of TETA doping time because interfacial polymerization is a very fast reaction and the product, polyamide with a small free volume, will resist the contact between TETA and TMC for further reaction. In the case of longer doping TETA time, which has a higher concentration of TETA in the dense skin region

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Fig. 5. The skin layer thickness vs TETA doping time in for interfacial polymerization of PA on m-PAN membranes at 50 C from positron annihilation spectroscopy (PAS) and from SEM images.

of m-PAN, the formed impermeable layer of PA resists further polymerization and results a thinner PA skin layer in the final membrane formation. 3.3. Correlations between PAS data and pervaporation performance We have performed parallel experiments of DBES and pervaporation measurements, flux and wt% of water in permeate for 70% isopropanol aqueous solution for different temperatures (25, 50, and 70 C) which vary the pervaporation significantly at the same TETA doing time of 10 min For DBES spectra, we analyzed the S parameters vs the depth similar to those described above in a 3-layer model. Fig. 6 shows the S parameters and the layer thickness as a function of TETA doping temperatures. As shown in Fig. 6 that the S parameters for both the skin (PA) and the transition layer decrease as a function of doping temperature. Since S is an indication of free-volume quantity, a raised temperature allows the TETA penetrates further into m-PAN and results a thicker PA layer, which subsequently decreases the flux. We plot the resolved S vs flux and water concentration in permeate in Fig. 7. It is interesting that we observe a correlation between S and flux and an anti-correlation between S and water concentration in permeate. According to the free-volume theory [27], diffusivity (D) has an exponential relationship with the free volume (Fv) as, D ¼ A exp(B/Fv), where A and B are characteristic parameters. We attempted to fit the flux with S as they relate to D, and Fv quantities, respectively. We obtained reasonable good fits into an exponential function as lines in Fig. 7. While it is reasonably to expect a good correlation between free-volume parameter and the flux for the PA skin, similar correlation also existed in the transition layer is interesting. This indicates that the transition layer also contains the dense skin

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Fig. 6. Analyzed S parameters and the layer thickness (top and bottom are for the skin, and the transition layer, respectively) PA/m-PAN membranes vs a function of TETA doping temperature. Lines connected data points were drawn for eye-guide purpose only.

part of m-PAN and also possibly PA and they play a secondary role in the pervaporation separation. On the other hand, the water concentration in permeate is a quantity of selectivity and it shows an anti-correlation with the S free-volume parameter. We further plotted the skin and transition layer thickness, L1, and L2 as resolved from VEPFIT analysis of S data vs pervaporation performance, flux and water concentration in permeate in Fig. 8. As shown in Fig. 8, the PA skin thickness (L1) has an anti-correlation with the flux (permeability) which could be understood due to the resistance of transporting a

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Fig. 7. Flux and water concentration in permeate vs S parameters (top and bottom are for the skin, and the transition layer, respectively) in PA/m-PAN membranes. The lines for flux were fitted in an exponential function while lines connected between water concentration in permeate were drawn for eye-guide purpose only.

relatively large isopropanol molecule through the PA polymer, which has a smaller free volume than m-PAN. There also exists a correlation between the PA skin thickness (L1) and the water concentration in permeate (selectivity). For the transition layer thickness (L2), its relationship with flux and water concentration in permeate is not as clear as the skin PA layer thickness L1. This indicates the secondary role of the transition layer in the pervaporation separation. The less relationship between L than between S and flux and water concentration in permeate indicates that the free volume size

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Fig. 8. Flux and water concentration in permeate vs layer thickness (top and bottom are for the skin, and the transition layer, respectively) in interfacial polymerized PA/m-PAN membranes. Lines were connected between data points for eye-guide purpose only.

secondary role in pervaporation separation, is first time observed from PAS studies. The PAS results in the PA skin and porous layers are consistent with that from SEM images. Good correlations between the S parameters and flux of the skin and the transition layers are observed. Positron annihilation lifetime spectroscopy is a useful tool for quantitative analysis of free volume size and distributions, which are important information to select the size of transporting molecules in pervaporation and to provide guidance for the design of multi-layer thin film polymers for membrane separation applications. PAS appears to be a novel and valuable tool for the research and development of membrane science and technology.

Acknowledgements The authors wish to express their sincere gratitude to the National Science Council (NSC) and the Center-of-Excellence (COE) Program on Membrane Technology from the Ministry of Education (MOE), R.O.C. for the financial supports.

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4. Conclusion

[2]

Positron annihilation spectroscopy coupled with a variable energy slow positron beam has been used to determine the layer structures and free-volume properties in composite polyamide membrane systems. A transition layer from the skin to the porous m-PAN layer, which plays the

[3]

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