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Effect of conditioning techniques of perfluorinated sulphocationic membranes on their hydrophylic and electrotransport properties
Journal of Membrane Science 209 (2002) 509–518
Effect of conditioning techniques of perfluorinated sulphocationic membranes on their hydrophylic and electrotransport properties N.P. Berezina a,∗ , S.V. Timofeev b , N.A. Kononenko a a
Department of Physical Chemistry, Kuban State University, Stavropolskaya Street 149, 350040 Krasnodar, Russia b Plastpolymer, Polyustrovsky Prospect 32, 195108 St. Petersburg, Russia Received 15 February 2002; received in revised form 2 August 2002; accepted 5 August 2002
Abstract The effect of perfluorinated sulphocation-exchange membrane conditioning on their hydrophilic and electrotransport properties was studied. Exchange capacity and water content of MF-4SC membranes having been exposed to conditioning were determined by different chemical techniques. The estimated values were compared to the same characteristics after thermal pre-treatment by sequent boiling in hydrogen peroxide, water, acid and water. Dependences of electrical conductivity, diffusion and electro-osmotic permeability upon sodium chloride concentration were determined for MF-4SC, Nafion-115 and -117 provided that the conditioning techniques varied. Consistent increase of the electrotransport membrane characteristics due to the water content increase and water energy state alteration after the membranes were thermally treated was established. It was shown that MF-4SC, Nafion-115 and -117 conditioning by this method leads to formation of ion-conducting polymeric membranes identical in the structure and transport properties. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Conditioning techniques; Electrotransport; Membranes
1. Introduction The selection of the conditioning techniques for perfluorinated polymeric membranes is of critical importance to estimate of the synthesis effect on their physicochemical properties. As known, the perfluorinated homogeneous membranes refer to uncrosslinked polymers with cluster structure. That is why their structure and properties are extra sensitive to the conditions of pre-treatment, storage and service. The perfluorinated membranes may be presented in several states: normal state typical for end product; shrinked ∗ Corresponding author. Tel.: +7-8612-699517; fax: +7-8612-699517. E-mail address: [email protected] (N.P. Berezina).
state achieved by boiling in salt solutions; extended state after sequential membranes boiling in acids and distilled water. The perfluorinated membranes achieve the super-extended state by heating in ethylene glycol to 110 ◦ C after the alkaline hydrolysis stage [1]. In the latter case the membrane structure with high water content (up to 36 mole H2 O/mole SO3 − ) and water cluster diameter up to 10 nm is formed. The samples produced by this techniques were studied in [1–3]. The alkali nature and concentration change under saponification of fluorosulphonic groups in a copolymer profoundly affects the hydrophilic characteristics of the perfluorinated membranes also changing their electrodiffusion properties [4]. The analysis of the reference data on the perfluorinated membrane conditioning techniques shows that
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the first structural analyses of these polymers, realized in the 1980s with different techniques (X-ray technique, IR-spectroscopy, NMR), were conducted using the perfluorinated Nafion membrane samples having been boiled in water within 30 min [5], 1 h [6,7] or 72 h [8]. The cluster morphology of the Nafion was studied with quasi-elastic neutron scattering, X-ray structural analysis (small angle X-ray scattering), Mossbauer spectroscopy [9,10]. Thereat, the effect of water sorption by the samples at a room temperature and at their boiling in water upon the specific structural signals [9] was established. It was shown that treating the films at 100, 220, 170 ◦ C profoundly rearranges the structure in comparison with common equilibration to water at a room temperature. Note, that the extended state is the most thermodynamically stable state. Thereat the membrane comprises two types of water: ion–dipole cluster associated water and non-associated water [5]. The content of the latter greatly depends on the pre-treatment and operating conditions of the membranes. In papers [11–15] regarding Nafion-117 membranes, the chemico-thermal method was used as following treatment: sequential boiling in hydrogen peroxide, water, acids and water. For example, in [13] it is proposed to treat Nafion-117 with 1 M HCl solution (1 h), distilled water (2 h) and 1 M NaOH solution at a room temperature. This sequence was repeated before boiling the samples within 1 h in deionized water. The authors [15] recommend to condition the same type membranes by sequential boiling within 1 h in hydrogen peroxide (3%), water, sulfuric acid (0.5 M) and again in distillate. This method have been applied recently also by authors [16] to prepare Nafion-117 for the X-ray structural analysis. In [17] it is suggested to prolong the boiling in water up to 10 h. To determine the maximum water content in Nafion-117 during the conditioning, the authors [13] have applied the transferring into the form of tetrabutylammonium for membrane dehydration, and also prolonged the vacuum drying. In papers [1–4], where electrotransport properties of perfluorinated MF-4SC type membranes were investigated, the species were held in NaCl solutions (2 M), and thereafter they were equilibrated to NaCl solutions of various concentrations. The influence of the Nafion-117 membranes pre-treatment upon the complex of their structural
and diffusion properties has been studied recently [18]. It has been shown that film drying at 120 ◦ C substantially reduces the diffusion transport of solvent ions and molecules due to rearrangement of transport channels and cavities in the membrane structures. The dynamics of ions transport in the membrane structure within varied time from 10−12 to 102 s was analyzed with three independent techniques as follows: quasi-elastic neutron scattering, small angle neutron scattering, and radioactive tracer technique. Thereat, irreversible alteration of sizes of clusters and other structural components was registered in the membrane samples after dehydration on the whole surface. Such pre-treatment of the membranes before the measurements is specified by the technical production conditions of the electrode–membrane units of hydrogen–oxygen fuel cells, produced by Nafion film molding at 120 ◦ C. Thereafter the membrane has different conducting and structural characteristics to be known to estimate the fuel cell efficiency [15,18,19]. The extension of variety of the Nafion membranes is due to their application not only in chlorine alkaline electrolysis and fuel cells, but also as catalytic agents in electrochemical processes, in water electrolysis, in creation of new electrode materials with polymer films on metal surfaces [19–22]. This has originated new conditioning techniques for these materials. In the reference literature definite modifications of perfluorinated membranes are studied such as: Nafion-110, -115, -120, -125 and -292. Commercial membrane Nafion-117 is now widely applied as a solid electrolyte in fuel cells and sensor systems. Moreover, in the process of new generation ion-exchange membrane synthesis, e.g. based on rigid chain polymers [23], Nafion-117 is applied as a standard sample for the comparative analysis of conducting and hydrophilic properties. But the issue on standardization of perfluorinated membrane conditioning is still open now, because the literature references do not give any data concerning quantitative changes in the transport properties after pre-treatment with various techniques. As Nafion-117 membranes are mostly used as the standard sample compared to the physico-chemical properties of new type ion-selective synthetic membranes, the data is required concerning the conditioning effect of perfluorinated membranes on their structure and
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properties. The objective of this paper was to produce the information for a series of Russian perfluorinated sulfocationic membranes MF-4SC, and Nafion-117, -115.
2. Subjects and methods of study Commercial samples of MF-4SC produced by “Plastpolymer” (St. Petersburg) were in K+ form after saponification of sulfonylfluoride groups with KOH concentrates. These membranes are designed to serve at the minimum water transport rate. Nafion-115 membranes with high heat stability up to 250 ◦ C applied to water electrolysis, and Nafion-117 membranes of a wider range of application produced by Dupon de Nemoure (USA) were in Na+ form. All the membranes subject to study represent transparent reinforcing fiber free films. In the present paper, we have compared the metering results of the water content and electrotransport properties after the MF-4SC and Nafion membranes had been conditioned with different techniques. Let the process of keeping the membranes in 2 M NaCl solutions followed by equilibrating to NaCl solutions of lower concentration (up to 0.05 M) under isothermal conditions at 25 ◦ C be called a salt conditioning technique. The membrane conditioning technique including additional treatment with acidic and alkaline solutions at a room temperature will be called a chemical technique. At last, sequent boiling of the membranes in 3% H2 O2 , H2 O, 0.5 M HNO3 or H2 SO4 , H2 O within 1 or 3 h in each solution will be conventionally called a thermal conditioning technique. In the latter case the sample were thermally equilibrated to water at 25 ◦ C, and then transformed in Na+ form by 2 M NaCl solution at the same temperature. When studying the concentration functions of the transport properties, the membranes were equilibrated to NaCl solutions of respective concentrations. The complex of standard and approved techniques has been applied to test the membranes. Exchange capacity Q was determined for H+ form samples by titrating the H+ -ions produced during alkaline neutralization [24]. Water content W was determined by the gravimetric method as the water mass/dry sample mass ratio, and the water was extracted at 105 ◦ C.
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Swelling factor Ks was calculated as the thickness ratio of the swollen and dry membranes [25]. The membrane hydration capacity nm characterizing the H2 O mole amount per 1 mole of the functional groups was calculated by the data on the water content exchange capacity of the samples, nm was calculated accurate to 0.1 mole H2 O/mole SO3 − , as in [13]. Schematic representation of the cells, the design equations and the determination conditions for the membrane electrotransport characteristics: electroconductivity, diffusion and electro-osmotic permeability are given in Table 1. The membrane electroconductivity κ m (S/m) was determined on the basis of the membrane electrical resistivity measurements with the mercury-contact technique at 200 kHz ac [1,26]. The electroconductivity was determined for the samples washed with distilled water (κH2 O ), and for the samples of various salt forms equilibrated to 0.1 M solution of the relevant chloride (κ 0.1 ). Such concentration of the external electrolyte solution is proposed to be the standard one when testing the ion-exchange membranes of different class [27]. The electroconductivity concentration function within a wide range of NaCl concentrations was derived for the series of perfluorinated membrane samples conditioned with various techniques. Processing this function within two-phase conductivity model of the ion-exchange membranes allows for calculating the volume fraction of a free solution in the membrane phase. Diffusion permeability for the NaCl solutions was analyzed in a cell of periodic service. The electrolyte is transferred through the membrane to a distilled water filled compartment. The electrolyte buildup rate in the water containing compartment (dc/dτ ) is controlled by the conductivity measuring technique. The salt diffusion process was numerically characterized by integral coefficient of diffusion permeability P (m2 /c). Electro-osmotic permeability of the membranes in NaCl solutions was metered with the volumetric method in a cell with reversible argentic chloride electrodes. The electro-osmotic permeability was characterized by water transport number tw (mole H2 O/F). The determination error for all the characteristics does not exceed 3–5%.
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Table 1 Schematics of the experimental setup and calculation equations Cell schematics
Calculation equation
Conditions of experimental
Reference
Km = (l/RS)a
ac frequency 200 kHz
[26]
Pm = (Vl c/Sc t)b
i = 0; c = 0
[27]
Di = V /Sit; tw = DiF /18c
i = 0; c = 0
[1]
Note: 1, membrane; 2, measuring electrodes; 3, polarizing electrodes; 4, impedance meter; 5, potentiostat; 6, measuring capillaries; 7, ampermeter; 8, stirings; 9, mercury. a Here l, membrane thickness; S, area; R, resistance. b Here V, chamber volume; c, concentration; t, time. c Here i, current density; F, Faraday number.
3. Results and discussion 3.1. Chemical and thermal conditioning of samples Fig. 1 shows the techniques of chemical conditioning of MF-4SC commercial samples described in this paper. The first group of the techniques including Techniques 1–4 (Fig. 1) is distinguished by pre-treatment of the membranes with salt solutions, and then with acids and alkalis. This sequence is usually applied to treat ion-exchange resins and heterogeneous membranes [28,29]. Another group of the techniques (Techniques 5, 6, Fig. 1) implies membrane conditioning in hydrochloric acid. Such treatment is described in the standard technical documentation for the methods of heterogeneous ionexchange membranes treatment before testing [30],
and recommended for the MF-4SC membrane manufacturers. Hence, in accordance with Fig. 1 the Na+ -form of the MF-4SC membrane can be obtained by four various techniques. Table 2 presents physicochemical characteristics for different ionic forms of the MF-4SC membrane with indication of the conditioning technique in compliance with Fig. 1. The values of the membrane hydration capacity nm are indicated here also. The analysis of the physicochemical characteristics presented in Table 2 shows that the chemical conditioning of the membranes in salt, acidic and alkaline solutions at a room temperature leads to derivation of close values of W, Ks , κH2 O and κ 0.1 . The κ m growth in case of the samples H+ -form is connected with the change of the counter-ion transport mechanism. Parallel determination of each characteristic for five
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Fig. 1. Diagram of membrane pre-treatment techniques.
samples showed that the better reproducibility of the results is observed for the samples additionally treated with acid. This phenomenon is relevant for any salt form of the membrane, and it can be concluded that the membranes should be treated with acid during chemical conditioning and preparation to testing. If the thermal treatment is applied, e.g. boiling in nitric acid within 3 h as per Technique 6, all characteristics are increased: W by c, 50%, κH2 O by c, 60%, and κ 0.1 by 10–20%. The exchange capacity thermally treated is almost unchanged. If let the samples in Na+ - and K+ -form treated as per Techniques (2 and 3 (Fig. 1)) boil in distilled water within 1 h, their electric conductivity enhances on 10% average.
The results derived in this paper agree with the authors’ data [15], who observed the hydration capacity increase of Nafion-117 in H+ -form from 13 to 22 mole H2 O/mole SO3 after samples boiling, and distinguished two basic states of water such as: aw = 0.14 and aw close to 0.75–1 in case of the maximum water absorption. Values nm < 10 correspond to the hydration water content in ion–dipole associates or ion pairs. As this water is immobilized in local electrical ion fields, the H+ transport as per tunneling mechanism is complicated. Such opportunity can be implemented only when the water content is increased to let proton move quickly as in free water that was justified during the investigation of electro-osmotic factors and water molecules self-diffusion [15].
Table 2 Physicochemical characteristics of MF-4SC pre-treated with various techniques Ionic form
Pre-treatment technique (as per Fig. 1)
W (%)
Ks
κH 2 O (S/m)
κ 0.1 (S/m)
Q (mg eq/g)
nm (mole H2 O/mole SO3 )
K+ K+ Na+ Na+ Na+ Na+ H+ H+ H+ H+
1 3 1 2 4 5 2 3 5 6
12.65 – 15.97 17.36 17.08 16.29 20.06 19.40 20.74 29.21
1.07 – 1.08 1.08 1.08 1.09 1.09 1.10 1.10 1.20
0.505 0.610 0.664 0.653 0.656 0.642 1.005 1.091 1.178 1.828
0.599 0.637 0.692 0.670 0.676 0.686 2.011 2.248 2.300 2.547
– – – – – – 0.70 0.70 0.70 0.72
10.03 – 12.67 13.77 13.55 12.93 15.92 15.39 16.46 22.54
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Fig. 2. Dependence of the electric conductivity on the hydration capacity of the perfluorinated membranes in different ionic forms.
Fig. 2 shows the electrical conductivity dependence of water washed samples on the membrane hydration capacity. Parameter nm was regulated as by counter-ion nature variation, so by the thermal treatment, nm increases at the transition from K+ to H+ -form and substabtially enhances after boiling in acid and water. As shown in the figure the κH2 O –nm dependence has an increasing character that indicates at the proportionality between hydration characteristics and electrotransport properties of the ion-exchange membranes. The estimation of counter-ions kinetic characteristics carried out in agreement with the Nernst–Einstein ratio on the basis of the data on electroconductivity of the water washed membranes, on their exchange capacity and density equal to 1.6–1.7 g/cm3 . It was shown that the increase of the self-diffusion (Di ) and counter-ion mobility (ui ) factor is observed. So, Di is 24.5 × 10−11 m2 s−1 for MF-4SC membrane in H+ -form exposed to chemical conditioning at a room temperature, and 40.2 × 10−11 m2 s−1 after thermal conditioning. This is due to the reduction of activation barrier for ion transport in more hydrophilic membranes. The results for the Na+ -form
(14.6 × 10−11 m2 s−1 ) agree with the experiment data [31,32] derived with the isotope technique. So, it is shown that the thermal treatment of MF-4SC membranes is required to derive more reproducible test results and leads to the water content increase. This confirms the data concerning the unlinked homogeneous Nafion membranes. The increase of the maximum water content in the samples by 25–30% indicates that an additional path has arisen for electrical mass transport in cluster areas of such membranes.
4. Comparative analysis of the perfluorinated MF-4SC and Nafion membranes treated with various conditioning techniques A complex of electrotransport properties of perfluorinated membranes MF-4SC and Nafion was compared under comparable experimental conditions for the case of two conditioning techniques. Fig. 3 shows the membranes concentration dependences P within a wide range of NaCl concentrations. To determine the effect of thermal treatment on the structure and
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and on the charge transfer by cations and anions of the non-exchange sorbed equilibrium electrolyte solution. The volume fraction of the latter in the membrane depends on the polymer morphology and availability in the structure of any cavities and pores containing inclusions of this “free” solution. According to this two-phase model the volume fraction of this structural element (f2 ) is easily defined as the slope ratio from the bilogarithmic dependence (lg κm − lg κ s ): f2 =
Fig. 3. Concentration dependence of the integral diffusion permeability factor of Nafion-115 (1, 3) and MF-4SC (2, 4) membranes for NaCl solutions: 1, 2—the samples boiled in water; 3, 4—the samples not thermally treated.
properties of perfluiorinated membranes, one sample of each type shown in the figure was boiled in distilled water within 30 min. As shown in the figure the concentration dependences P have an increasing character for all investigated samples of the perfluorinated membranes independently on the treatment technique. This is indicative of the formation of a convex concentration profile in the membrane phase at electrolyte diffusion [33]. Thermal treatment causes of the membrane diffusion characteristics increase: P in case of MF-4SC increases 2.5–4 times in the studied range of the NaCl concentrations, in case of Nafion-115— there is a four to five times increase. The increase of the membranes diffusion permeability agrees with growth of the kinetic characteristics calculated from the Nernst–Einstein ratio. In [1,27] it is shown that the data of the membranes electrical conductivity study depending on the salt solution concentration can be applied as a specific “structural signal.” For the majority of synthetic ionexchange membranes the electrical resistance metered by different methods is the effective characteristic of the polymer conductivity that depends on the charge transfer by counter-ions along the gel or cluster phase,
dlg κm dlg κs
The volume fraction of other conducting phase f1 = 1 − f2 . This phase includes hydrophobic polymer segments and ion–dipole associates with the unipolar conducting mechanism in accordance with the three-phase model [19,34]. These structural fragments of the perfluorinated membranes are distinguished also in other model approximations, however the grouping of the structural components to define the characteristic parameters is performed in different manners [7,32,35]. We have applied this approach to estimate the given parameter which allows for revealing the morphological changes in the samples with various water contents. Fig. 4 shows the dependences κm − c for MF-4SC, Nafion-115 and -117 metered in the range from 0.05 to
Fig. 4. Concentration dependence of the electric conductivity of Nafion-117 (1, 5), Nafion-115 (3, 6, 7) and MF-4SC (2, 4, 8) membranes in NaCl solution: 1, 7, 8—salt pre-treatment; 2, 6—boiling in deionized water; 3–5—thermal pre-treatment.
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1 M of the NaCl solution under different conditioning modes. After the salt pre-treatment the electrical conductivity of MF-4SC and Nafion-115 is almost independent on the concentration (curves 7 and 8). Thereat, a great error achieving 10–15% appears in the data, and this exceeds a common measurement error. When applying the thermal conditioning, i.e. boiling in water, the dependence on the concentration appears (curves 2, 6). However, for Nafion-117 the κ m concentration dependence is observed after salt treatment (curve 1). Applying the thermal conditioning procedure, i.e. the transition to the polymer extended state, increases κ m from 0.05 to 0.5 M of the solution that results in obtaining stable values of the electrical conductivity almost independent of the concentration (with 5–7% accuracy; curves 3–5). From the simulation calculations it follows that f2 values vary from 0–0.05 (the electrical conductivity independence of the concentration) to 0.10–0.15 in case of the dependence. From all the data it follows that the thermal treatment leads to κ m increase at the average of two times for MF-4SC and Nafion-115. Measurement discrepancies for the Nafion-117 κ m in case of different conditioning techniques indicate that in solutions with concentration lower than 0.5 M a sort of “breathing” of the clusters is observed between chains of the polymer matrix. This phenomenon is due to the water state change in the ion-cluster areas and due to the availability of more mobile water in microcavitites that causes the total water content accumulation and the parameter f2 increase. So, the microstructure of the perfluorinated membranes is stabilized under the thermal conditioning. This is due to ordering of the side segments configuration and washing-out of low-molecular components and oligomer residuals, introduced into the polymer matrix grid during the multi-stage synthesis. The known effect of the phase separation in these unlinked polymers does not confine itself in decomposition on the hydrophobic fluorethylen chains and ion-cluster phase in accordance with the traditional concept of the Nafion membrane morphology. As established in this work the thermal conditioning leads to the emergence of one more microphase representing a weak linked or “peripheric” water available in microcavities among fluorcarbonic chains and ion–dipole clusters. Probably, this type of water representing a sort of secondary hydrated shell of the clusters was
discovered by Falk during the Nafion membrane analysis with IR spectroscopy [5]. The analysis of the data on the electrical conductivity and diffusion permeability change has shown that the role of the peripheric water available in the dilated samples is in the formation of an additional path for ion transport (κ m ) or diffusion salt flux (P; Figs. 3 and 4). Hence the two times increase of electrical conductivity, and the four to five times increase of the integral diffusion permeability factor. Similar effects were observed by the authors [1] during the study of a series of MF-4SC membranes with water content varied by converting them in a superextended state. The emergence of the additional charge transfer and salt stream path was confirmed during the study of the electro-osmotic membrane permeability. Fig. 5 demonstrates the dependence of water transport numbers (tw ) through the MF-4SC and Nafion-117 membranes upon the NaCl solution concentration obtained under the same conditions of the electro-osmotic permeability measurement. Thereat, equal water transport (circa 4 mole of H2 O/F) weakly dependent on the NaCl solution concentration is observed for both
Fig. 5. Concentration dependence of the water transport numbers in Nafion-117 (1, 3) and MF-4SC (2, 4) membranes: 1, 2—thermal pre-treatment; 3, 4—salt pre-treatment.
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samples in a shrinked form. For the thermally conditioned samples the electro-osmotic permeability substantially increases: in the concentrating range from 0.1 to 1 M tw it increases 1.5–2 times. At the same time the concentration dependences for MF-4SC and Nafion-117 almost match. The revealed effect of the water transport enhancement in the electrical field is consistent with the assumption on the availability of a definite fraction of peripheric water in the membranes structure. Based on the comparative analysis of the experimental results of the physico-chemical characteristics study performed for different samples of the membranes, the selection of the MF-4SC sample having identical characteristics as for the Nafion-115 membrane has been justified. Using of the thermal conditioning technique permits to transform the MF-4SC sample in the same state as Nafion-117 membrane.
5. Conclusions The reference data have been analyzed concerning the conditioning techniques for the Nafion type perfluorinated membranes for the period of 20 years of their investigation. The basic groups of the conditioning techniques applied for the perfluorinated sulfocationic membranes MF-4SC have been selected. The role of the thermal conditioning technique for the MF-4SC, Nafion-115 and -117 membranes has been revealed by annualizing the electrotransport properties of these samples. It was determined that the transition to the extended state is conditioned by the polymer morphology change due to the availability of peripheric water at the boundary between the hydrophobic phase of fluorethylen matrix chains and the ionic cluster phase. The procedure of the thermal conditioning is permitted to recommend as the standard conditioning technique for perfluorinated membranes. It ages the membrane structure and stabilizes their operational characteristics applied in chlorine alkaline electrolysis and fuel cells. The thermal conditioning allows for weakening various conditions of membrane synthesis and producing the materials close in structural and transport properties. However, in sensor devices and electric catalytic systems (Nafion polymer films on electrode surfaces) only the chemical conditioning
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at a room temperature can be applied to reduce the diffusion and electro-osmotic transport.
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[25] GOST 19180-73. Ion-Exchange Membranes: Techniques to Determine Sizes Change at Swelling, Izd-Vo Standartov, Standards Publishing House, Moscow, 1973. [26] A.I. Meshechkov, O.A. Dyomina, N.P. Gnusin, Ion-exchange membrane mercury-contact cell impedance locus, Elektrokhimiya (Russ. J. Electrochem.) 23 (1987) 1452. [27] N.P. Gnusin, N.P. Berezina, O.A. Dyomina, N.A. Kononenko, Physicochemical principles of testing ion-exchange membranes, Elektrokhimiya (Russ. J. Electrochem.) 32 (1996) 173. [28] K.M. Saldadze, A.B. Pashkov, V.S. Titov, Ion-Exchange High-Molecular Compounds, Goskomkhimizdat, Moscow, 1960. [29] N.G. Polyansky, G.V. Gorbunov, N.L. Polyanskaya, Techniques to Test Ion-Exchangers, Khimiya, Moscow, 1976. [30] GOST 17553-72. Ion-Exchange Membranes, Treatment Techniques Applied Before Testing, Izd-Vo Standardov, Standards Publishing House, Moscow, 1972. [31] S.N. Gladkikh, S.F. Timashev, G.G. Chuvileva, et al., Water and ion transport processes in perfluorinated cation-exchange membranes MF-4SC, Zhurn. Phys. Khim. (Russ. J. Phys. Chem.) 56 (1982) 916. [32] S.F. Timashev, Physical Chemistry of Membrane Processes, Khimiya, Moscow, 1988. [33] N.P. Gnusin, N.P. Berezina, A.A. Shudrenko, O.P. Ivina, Electrolyte diffusion through ion-exchange membranes, Zhurn. Phys. Khim. (Russ. J. Phys. Chem.) 68 (1994) 565. [34] R.S. Yeo, H.L. Yeager, Structural and transport properties of perfluorinated ion-exchange membranes, in: B.E. Conway, L. Butterworth, et al. (Eds.), Modern Aspects of Electrochemistry, 1985. [35] K.A. Mauritz, C.J. Hora, A.I. Hopfinger, Theoretical model for the structure of ionomers: application to Nafion materials, in: A. Eisenberg (Ed.), Ions in Polymers: Advances in Chemistry, ACS, Washington, 1980 (Chapter 187).
Effect of conditioning techniques of perfluorinated sulphocationic membranes on their hydrophylic and electrotransport properties N.P. Berezina a,∗ , S.V. Timofeev b , N.A. Kononenko a a
Department of Physical Chemistry, Kuban State University, Stavropolskaya Street 149, 350040 Krasnodar, Russia b Plastpolymer, Polyustrovsky Prospect 32, 195108 St. Petersburg, Russia Received 15 February 2002; received in revised form 2 August 2002; accepted 5 August 2002
Abstract The effect of perfluorinated sulphocation-exchange membrane conditioning on their hydrophilic and electrotransport properties was studied. Exchange capacity and water content of MF-4SC membranes having been exposed to conditioning were determined by different chemical techniques. The estimated values were compared to the same characteristics after thermal pre-treatment by sequent boiling in hydrogen peroxide, water, acid and water. Dependences of electrical conductivity, diffusion and electro-osmotic permeability upon sodium chloride concentration were determined for MF-4SC, Nafion-115 and -117 provided that the conditioning techniques varied. Consistent increase of the electrotransport membrane characteristics due to the water content increase and water energy state alteration after the membranes were thermally treated was established. It was shown that MF-4SC, Nafion-115 and -117 conditioning by this method leads to formation of ion-conducting polymeric membranes identical in the structure and transport properties. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Conditioning techniques; Electrotransport; Membranes
1. Introduction The selection of the conditioning techniques for perfluorinated polymeric membranes is of critical importance to estimate of the synthesis effect on their physicochemical properties. As known, the perfluorinated homogeneous membranes refer to uncrosslinked polymers with cluster structure. That is why their structure and properties are extra sensitive to the conditions of pre-treatment, storage and service. The perfluorinated membranes may be presented in several states: normal state typical for end product; shrinked ∗ Corresponding author. Tel.: +7-8612-699517; fax: +7-8612-699517. E-mail address: [email protected] (N.P. Berezina).
state achieved by boiling in salt solutions; extended state after sequential membranes boiling in acids and distilled water. The perfluorinated membranes achieve the super-extended state by heating in ethylene glycol to 110 ◦ C after the alkaline hydrolysis stage [1]. In the latter case the membrane structure with high water content (up to 36 mole H2 O/mole SO3 − ) and water cluster diameter up to 10 nm is formed. The samples produced by this techniques were studied in [1–3]. The alkali nature and concentration change under saponification of fluorosulphonic groups in a copolymer profoundly affects the hydrophilic characteristics of the perfluorinated membranes also changing their electrodiffusion properties [4]. The analysis of the reference data on the perfluorinated membrane conditioning techniques shows that
0376-7388/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 0 2 ) 0 0 3 6 8 - X
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the first structural analyses of these polymers, realized in the 1980s with different techniques (X-ray technique, IR-spectroscopy, NMR), were conducted using the perfluorinated Nafion membrane samples having been boiled in water within 30 min [5], 1 h [6,7] or 72 h [8]. The cluster morphology of the Nafion was studied with quasi-elastic neutron scattering, X-ray structural analysis (small angle X-ray scattering), Mossbauer spectroscopy [9,10]. Thereat, the effect of water sorption by the samples at a room temperature and at their boiling in water upon the specific structural signals [9] was established. It was shown that treating the films at 100, 220, 170 ◦ C profoundly rearranges the structure in comparison with common equilibration to water at a room temperature. Note, that the extended state is the most thermodynamically stable state. Thereat the membrane comprises two types of water: ion–dipole cluster associated water and non-associated water [5]. The content of the latter greatly depends on the pre-treatment and operating conditions of the membranes. In papers [11–15] regarding Nafion-117 membranes, the chemico-thermal method was used as following treatment: sequential boiling in hydrogen peroxide, water, acids and water. For example, in [13] it is proposed to treat Nafion-117 with 1 M HCl solution (1 h), distilled water (2 h) and 1 M NaOH solution at a room temperature. This sequence was repeated before boiling the samples within 1 h in deionized water. The authors [15] recommend to condition the same type membranes by sequential boiling within 1 h in hydrogen peroxide (3%), water, sulfuric acid (0.5 M) and again in distillate. This method have been applied recently also by authors [16] to prepare Nafion-117 for the X-ray structural analysis. In [17] it is suggested to prolong the boiling in water up to 10 h. To determine the maximum water content in Nafion-117 during the conditioning, the authors [13] have applied the transferring into the form of tetrabutylammonium for membrane dehydration, and also prolonged the vacuum drying. In papers [1–4], where electrotransport properties of perfluorinated MF-4SC type membranes were investigated, the species were held in NaCl solutions (2 M), and thereafter they were equilibrated to NaCl solutions of various concentrations. The influence of the Nafion-117 membranes pre-treatment upon the complex of their structural
and diffusion properties has been studied recently [18]. It has been shown that film drying at 120 ◦ C substantially reduces the diffusion transport of solvent ions and molecules due to rearrangement of transport channels and cavities in the membrane structures. The dynamics of ions transport in the membrane structure within varied time from 10−12 to 102 s was analyzed with three independent techniques as follows: quasi-elastic neutron scattering, small angle neutron scattering, and radioactive tracer technique. Thereat, irreversible alteration of sizes of clusters and other structural components was registered in the membrane samples after dehydration on the whole surface. Such pre-treatment of the membranes before the measurements is specified by the technical production conditions of the electrode–membrane units of hydrogen–oxygen fuel cells, produced by Nafion film molding at 120 ◦ C. Thereafter the membrane has different conducting and structural characteristics to be known to estimate the fuel cell efficiency [15,18,19]. The extension of variety of the Nafion membranes is due to their application not only in chlorine alkaline electrolysis and fuel cells, but also as catalytic agents in electrochemical processes, in water electrolysis, in creation of new electrode materials with polymer films on metal surfaces [19–22]. This has originated new conditioning techniques for these materials. In the reference literature definite modifications of perfluorinated membranes are studied such as: Nafion-110, -115, -120, -125 and -292. Commercial membrane Nafion-117 is now widely applied as a solid electrolyte in fuel cells and sensor systems. Moreover, in the process of new generation ion-exchange membrane synthesis, e.g. based on rigid chain polymers [23], Nafion-117 is applied as a standard sample for the comparative analysis of conducting and hydrophilic properties. But the issue on standardization of perfluorinated membrane conditioning is still open now, because the literature references do not give any data concerning quantitative changes in the transport properties after pre-treatment with various techniques. As Nafion-117 membranes are mostly used as the standard sample compared to the physico-chemical properties of new type ion-selective synthetic membranes, the data is required concerning the conditioning effect of perfluorinated membranes on their structure and
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properties. The objective of this paper was to produce the information for a series of Russian perfluorinated sulfocationic membranes MF-4SC, and Nafion-117, -115.
2. Subjects and methods of study Commercial samples of MF-4SC produced by “Plastpolymer” (St. Petersburg) were in K+ form after saponification of sulfonylfluoride groups with KOH concentrates. These membranes are designed to serve at the minimum water transport rate. Nafion-115 membranes with high heat stability up to 250 ◦ C applied to water electrolysis, and Nafion-117 membranes of a wider range of application produced by Dupon de Nemoure (USA) were in Na+ form. All the membranes subject to study represent transparent reinforcing fiber free films. In the present paper, we have compared the metering results of the water content and electrotransport properties after the MF-4SC and Nafion membranes had been conditioned with different techniques. Let the process of keeping the membranes in 2 M NaCl solutions followed by equilibrating to NaCl solutions of lower concentration (up to 0.05 M) under isothermal conditions at 25 ◦ C be called a salt conditioning technique. The membrane conditioning technique including additional treatment with acidic and alkaline solutions at a room temperature will be called a chemical technique. At last, sequent boiling of the membranes in 3% H2 O2 , H2 O, 0.5 M HNO3 or H2 SO4 , H2 O within 1 or 3 h in each solution will be conventionally called a thermal conditioning technique. In the latter case the sample were thermally equilibrated to water at 25 ◦ C, and then transformed in Na+ form by 2 M NaCl solution at the same temperature. When studying the concentration functions of the transport properties, the membranes were equilibrated to NaCl solutions of respective concentrations. The complex of standard and approved techniques has been applied to test the membranes. Exchange capacity Q was determined for H+ form samples by titrating the H+ -ions produced during alkaline neutralization [24]. Water content W was determined by the gravimetric method as the water mass/dry sample mass ratio, and the water was extracted at 105 ◦ C.
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Swelling factor Ks was calculated as the thickness ratio of the swollen and dry membranes [25]. The membrane hydration capacity nm characterizing the H2 O mole amount per 1 mole of the functional groups was calculated by the data on the water content exchange capacity of the samples, nm was calculated accurate to 0.1 mole H2 O/mole SO3 − , as in [13]. Schematic representation of the cells, the design equations and the determination conditions for the membrane electrotransport characteristics: electroconductivity, diffusion and electro-osmotic permeability are given in Table 1. The membrane electroconductivity κ m (S/m) was determined on the basis of the membrane electrical resistivity measurements with the mercury-contact technique at 200 kHz ac [1,26]. The electroconductivity was determined for the samples washed with distilled water (κH2 O ), and for the samples of various salt forms equilibrated to 0.1 M solution of the relevant chloride (κ 0.1 ). Such concentration of the external electrolyte solution is proposed to be the standard one when testing the ion-exchange membranes of different class [27]. The electroconductivity concentration function within a wide range of NaCl concentrations was derived for the series of perfluorinated membrane samples conditioned with various techniques. Processing this function within two-phase conductivity model of the ion-exchange membranes allows for calculating the volume fraction of a free solution in the membrane phase. Diffusion permeability for the NaCl solutions was analyzed in a cell of periodic service. The electrolyte is transferred through the membrane to a distilled water filled compartment. The electrolyte buildup rate in the water containing compartment (dc/dτ ) is controlled by the conductivity measuring technique. The salt diffusion process was numerically characterized by integral coefficient of diffusion permeability P (m2 /c). Electro-osmotic permeability of the membranes in NaCl solutions was metered with the volumetric method in a cell with reversible argentic chloride electrodes. The electro-osmotic permeability was characterized by water transport number tw (mole H2 O/F). The determination error for all the characteristics does not exceed 3–5%.
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Table 1 Schematics of the experimental setup and calculation equations Cell schematics
Calculation equation
Conditions of experimental
Reference
Km = (l/RS)a
ac frequency 200 kHz
[26]
Pm = (Vl c/Sc t)b
i = 0; c = 0
[27]
Di = V /Sit; tw = DiF /18c
i = 0; c = 0
[1]
Note: 1, membrane; 2, measuring electrodes; 3, polarizing electrodes; 4, impedance meter; 5, potentiostat; 6, measuring capillaries; 7, ampermeter; 8, stirings; 9, mercury. a Here l, membrane thickness; S, area; R, resistance. b Here V, chamber volume; c, concentration; t, time. c Here i, current density; F, Faraday number.
3. Results and discussion 3.1. Chemical and thermal conditioning of samples Fig. 1 shows the techniques of chemical conditioning of MF-4SC commercial samples described in this paper. The first group of the techniques including Techniques 1–4 (Fig. 1) is distinguished by pre-treatment of the membranes with salt solutions, and then with acids and alkalis. This sequence is usually applied to treat ion-exchange resins and heterogeneous membranes [28,29]. Another group of the techniques (Techniques 5, 6, Fig. 1) implies membrane conditioning in hydrochloric acid. Such treatment is described in the standard technical documentation for the methods of heterogeneous ionexchange membranes treatment before testing [30],
and recommended for the MF-4SC membrane manufacturers. Hence, in accordance with Fig. 1 the Na+ -form of the MF-4SC membrane can be obtained by four various techniques. Table 2 presents physicochemical characteristics for different ionic forms of the MF-4SC membrane with indication of the conditioning technique in compliance with Fig. 1. The values of the membrane hydration capacity nm are indicated here also. The analysis of the physicochemical characteristics presented in Table 2 shows that the chemical conditioning of the membranes in salt, acidic and alkaline solutions at a room temperature leads to derivation of close values of W, Ks , κH2 O and κ 0.1 . The κ m growth in case of the samples H+ -form is connected with the change of the counter-ion transport mechanism. Parallel determination of each characteristic for five
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Fig. 1. Diagram of membrane pre-treatment techniques.
samples showed that the better reproducibility of the results is observed for the samples additionally treated with acid. This phenomenon is relevant for any salt form of the membrane, and it can be concluded that the membranes should be treated with acid during chemical conditioning and preparation to testing. If the thermal treatment is applied, e.g. boiling in nitric acid within 3 h as per Technique 6, all characteristics are increased: W by c, 50%, κH2 O by c, 60%, and κ 0.1 by 10–20%. The exchange capacity thermally treated is almost unchanged. If let the samples in Na+ - and K+ -form treated as per Techniques (2 and 3 (Fig. 1)) boil in distilled water within 1 h, their electric conductivity enhances on 10% average.
The results derived in this paper agree with the authors’ data [15], who observed the hydration capacity increase of Nafion-117 in H+ -form from 13 to 22 mole H2 O/mole SO3 after samples boiling, and distinguished two basic states of water such as: aw = 0.14 and aw close to 0.75–1 in case of the maximum water absorption. Values nm < 10 correspond to the hydration water content in ion–dipole associates or ion pairs. As this water is immobilized in local electrical ion fields, the H+ transport as per tunneling mechanism is complicated. Such opportunity can be implemented only when the water content is increased to let proton move quickly as in free water that was justified during the investigation of electro-osmotic factors and water molecules self-diffusion [15].
Table 2 Physicochemical characteristics of MF-4SC pre-treated with various techniques Ionic form
Pre-treatment technique (as per Fig. 1)
W (%)
Ks
κH 2 O (S/m)
κ 0.1 (S/m)
Q (mg eq/g)
nm (mole H2 O/mole SO3 )
K+ K+ Na+ Na+ Na+ Na+ H+ H+ H+ H+
1 3 1 2 4 5 2 3 5 6
12.65 – 15.97 17.36 17.08 16.29 20.06 19.40 20.74 29.21
1.07 – 1.08 1.08 1.08 1.09 1.09 1.10 1.10 1.20
0.505 0.610 0.664 0.653 0.656 0.642 1.005 1.091 1.178 1.828
0.599 0.637 0.692 0.670 0.676 0.686 2.011 2.248 2.300 2.547
– – – – – – 0.70 0.70 0.70 0.72
10.03 – 12.67 13.77 13.55 12.93 15.92 15.39 16.46 22.54
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Fig. 2. Dependence of the electric conductivity on the hydration capacity of the perfluorinated membranes in different ionic forms.
Fig. 2 shows the electrical conductivity dependence of water washed samples on the membrane hydration capacity. Parameter nm was regulated as by counter-ion nature variation, so by the thermal treatment, nm increases at the transition from K+ to H+ -form and substabtially enhances after boiling in acid and water. As shown in the figure the κH2 O –nm dependence has an increasing character that indicates at the proportionality between hydration characteristics and electrotransport properties of the ion-exchange membranes. The estimation of counter-ions kinetic characteristics carried out in agreement with the Nernst–Einstein ratio on the basis of the data on electroconductivity of the water washed membranes, on their exchange capacity and density equal to 1.6–1.7 g/cm3 . It was shown that the increase of the self-diffusion (Di ) and counter-ion mobility (ui ) factor is observed. So, Di is 24.5 × 10−11 m2 s−1 for MF-4SC membrane in H+ -form exposed to chemical conditioning at a room temperature, and 40.2 × 10−11 m2 s−1 after thermal conditioning. This is due to the reduction of activation barrier for ion transport in more hydrophilic membranes. The results for the Na+ -form
(14.6 × 10−11 m2 s−1 ) agree with the experiment data [31,32] derived with the isotope technique. So, it is shown that the thermal treatment of MF-4SC membranes is required to derive more reproducible test results and leads to the water content increase. This confirms the data concerning the unlinked homogeneous Nafion membranes. The increase of the maximum water content in the samples by 25–30% indicates that an additional path has arisen for electrical mass transport in cluster areas of such membranes.
4. Comparative analysis of the perfluorinated MF-4SC and Nafion membranes treated with various conditioning techniques A complex of electrotransport properties of perfluorinated membranes MF-4SC and Nafion was compared under comparable experimental conditions for the case of two conditioning techniques. Fig. 3 shows the membranes concentration dependences P within a wide range of NaCl concentrations. To determine the effect of thermal treatment on the structure and
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and on the charge transfer by cations and anions of the non-exchange sorbed equilibrium electrolyte solution. The volume fraction of the latter in the membrane depends on the polymer morphology and availability in the structure of any cavities and pores containing inclusions of this “free” solution. According to this two-phase model the volume fraction of this structural element (f2 ) is easily defined as the slope ratio from the bilogarithmic dependence (lg κm − lg κ s ): f2 =
Fig. 3. Concentration dependence of the integral diffusion permeability factor of Nafion-115 (1, 3) and MF-4SC (2, 4) membranes for NaCl solutions: 1, 2—the samples boiled in water; 3, 4—the samples not thermally treated.
properties of perfluiorinated membranes, one sample of each type shown in the figure was boiled in distilled water within 30 min. As shown in the figure the concentration dependences P have an increasing character for all investigated samples of the perfluorinated membranes independently on the treatment technique. This is indicative of the formation of a convex concentration profile in the membrane phase at electrolyte diffusion [33]. Thermal treatment causes of the membrane diffusion characteristics increase: P in case of MF-4SC increases 2.5–4 times in the studied range of the NaCl concentrations, in case of Nafion-115— there is a four to five times increase. The increase of the membranes diffusion permeability agrees with growth of the kinetic characteristics calculated from the Nernst–Einstein ratio. In [1,27] it is shown that the data of the membranes electrical conductivity study depending on the salt solution concentration can be applied as a specific “structural signal.” For the majority of synthetic ionexchange membranes the electrical resistance metered by different methods is the effective characteristic of the polymer conductivity that depends on the charge transfer by counter-ions along the gel or cluster phase,
dlg κm dlg κs
The volume fraction of other conducting phase f1 = 1 − f2 . This phase includes hydrophobic polymer segments and ion–dipole associates with the unipolar conducting mechanism in accordance with the three-phase model [19,34]. These structural fragments of the perfluorinated membranes are distinguished also in other model approximations, however the grouping of the structural components to define the characteristic parameters is performed in different manners [7,32,35]. We have applied this approach to estimate the given parameter which allows for revealing the morphological changes in the samples with various water contents. Fig. 4 shows the dependences κm − c for MF-4SC, Nafion-115 and -117 metered in the range from 0.05 to
Fig. 4. Concentration dependence of the electric conductivity of Nafion-117 (1, 5), Nafion-115 (3, 6, 7) and MF-4SC (2, 4, 8) membranes in NaCl solution: 1, 7, 8—salt pre-treatment; 2, 6—boiling in deionized water; 3–5—thermal pre-treatment.
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1 M of the NaCl solution under different conditioning modes. After the salt pre-treatment the electrical conductivity of MF-4SC and Nafion-115 is almost independent on the concentration (curves 7 and 8). Thereat, a great error achieving 10–15% appears in the data, and this exceeds a common measurement error. When applying the thermal conditioning, i.e. boiling in water, the dependence on the concentration appears (curves 2, 6). However, for Nafion-117 the κ m concentration dependence is observed after salt treatment (curve 1). Applying the thermal conditioning procedure, i.e. the transition to the polymer extended state, increases κ m from 0.05 to 0.5 M of the solution that results in obtaining stable values of the electrical conductivity almost independent of the concentration (with 5–7% accuracy; curves 3–5). From the simulation calculations it follows that f2 values vary from 0–0.05 (the electrical conductivity independence of the concentration) to 0.10–0.15 in case of the dependence. From all the data it follows that the thermal treatment leads to κ m increase at the average of two times for MF-4SC and Nafion-115. Measurement discrepancies for the Nafion-117 κ m in case of different conditioning techniques indicate that in solutions with concentration lower than 0.5 M a sort of “breathing” of the clusters is observed between chains of the polymer matrix. This phenomenon is due to the water state change in the ion-cluster areas and due to the availability of more mobile water in microcavitites that causes the total water content accumulation and the parameter f2 increase. So, the microstructure of the perfluorinated membranes is stabilized under the thermal conditioning. This is due to ordering of the side segments configuration and washing-out of low-molecular components and oligomer residuals, introduced into the polymer matrix grid during the multi-stage synthesis. The known effect of the phase separation in these unlinked polymers does not confine itself in decomposition on the hydrophobic fluorethylen chains and ion-cluster phase in accordance with the traditional concept of the Nafion membrane morphology. As established in this work the thermal conditioning leads to the emergence of one more microphase representing a weak linked or “peripheric” water available in microcavities among fluorcarbonic chains and ion–dipole clusters. Probably, this type of water representing a sort of secondary hydrated shell of the clusters was
discovered by Falk during the Nafion membrane analysis with IR spectroscopy [5]. The analysis of the data on the electrical conductivity and diffusion permeability change has shown that the role of the peripheric water available in the dilated samples is in the formation of an additional path for ion transport (κ m ) or diffusion salt flux (P; Figs. 3 and 4). Hence the two times increase of electrical conductivity, and the four to five times increase of the integral diffusion permeability factor. Similar effects were observed by the authors [1] during the study of a series of MF-4SC membranes with water content varied by converting them in a superextended state. The emergence of the additional charge transfer and salt stream path was confirmed during the study of the electro-osmotic membrane permeability. Fig. 5 demonstrates the dependence of water transport numbers (tw ) through the MF-4SC and Nafion-117 membranes upon the NaCl solution concentration obtained under the same conditions of the electro-osmotic permeability measurement. Thereat, equal water transport (circa 4 mole of H2 O/F) weakly dependent on the NaCl solution concentration is observed for both
Fig. 5. Concentration dependence of the water transport numbers in Nafion-117 (1, 3) and MF-4SC (2, 4) membranes: 1, 2—thermal pre-treatment; 3, 4—salt pre-treatment.
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samples in a shrinked form. For the thermally conditioned samples the electro-osmotic permeability substantially increases: in the concentrating range from 0.1 to 1 M tw it increases 1.5–2 times. At the same time the concentration dependences for MF-4SC and Nafion-117 almost match. The revealed effect of the water transport enhancement in the electrical field is consistent with the assumption on the availability of a definite fraction of peripheric water in the membranes structure. Based on the comparative analysis of the experimental results of the physico-chemical characteristics study performed for different samples of the membranes, the selection of the MF-4SC sample having identical characteristics as for the Nafion-115 membrane has been justified. Using of the thermal conditioning technique permits to transform the MF-4SC sample in the same state as Nafion-117 membrane.
5. Conclusions The reference data have been analyzed concerning the conditioning techniques for the Nafion type perfluorinated membranes for the period of 20 years of their investigation. The basic groups of the conditioning techniques applied for the perfluorinated sulfocationic membranes MF-4SC have been selected. The role of the thermal conditioning technique for the MF-4SC, Nafion-115 and -117 membranes has been revealed by annualizing the electrotransport properties of these samples. It was determined that the transition to the extended state is conditioned by the polymer morphology change due to the availability of peripheric water at the boundary between the hydrophobic phase of fluorethylen matrix chains and the ionic cluster phase. The procedure of the thermal conditioning is permitted to recommend as the standard conditioning technique for perfluorinated membranes. It ages the membrane structure and stabilizes their operational characteristics applied in chlorine alkaline electrolysis and fuel cells. The thermal conditioning allows for weakening various conditions of membrane synthesis and producing the materials close in structural and transport properties. However, in sensor devices and electric catalytic systems (Nafion polymer films on electrode surfaces) only the chemical conditioning
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at a room temperature can be applied to reduce the diffusion and electro-osmotic transport.
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