Some aspects of polyaniline template synthesis within and on the surface of perfluorinated cation exchange membrane

Synthetic Metals 261 (2020) 116292

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Some aspects of polyaniline template synthesis within and on the surface of perfluorinated cation exchange membrane

T

Natalia V. Loza*, Irina V. Falina, Natalia A. Kononenko, Daria S. Kudashova Kuban State University, 149 Stavropolskaya St, Krasnodar, 350040, Russia

ARTICLE INFO

ABSTRACT

Keywords: Nafion-type membrane Polyaniline Modification Sorption Chemical polymerization Kinetics

Detailed investigation has been made into the peculiarities of monomer sorption and desorption at different stages of aniline oxidative polymerization inside the perfluorinated membrane during preparation of composites with absorbance lower than 3 abs. units. The evaluation of the membrane saturation degree by monomer at every synthesis stage is performed, the mass fraction of the polyaniline in the composite membrane is estimated. The investigation into the kinetics of polyaniline synthesis with cationic and anionic oxidants in the membrane matrix has shown that the oxidant nature affects the polymerization rate and permitted to discover the limiting stage of the polymerization reaction. The limiting stage of polyaniline synthesis inside the membrane matrix is identified. The conditions to cease the polymerization reaction using oxidants of different nature are revealed. The opportunity to control the distribution of the modifier in the composite by choosing the appropriate oxidant is shown.

1. Introduction Composites based on Nafion-type membranes and polyaniline (PANI) are of great interest to the researchers due to the conjunction of the polymers properties. The perfluorinated matrix of Nafion-type membrane provides mechanical strength, chemical and thermal stability and high proton conductivity. PANI gives the perfluorinated membrane a set of unique electrochemical and optical properties [1,2]. Numerous studies discuss the aniline oxidative polymerization in solution, on the inert substrate or electrode and indicate that PANI properties depend on preparation conditions [3–11]. According to [10] the pH value of the polymerizing solution affects not only on PANI structure and properties but also on the mechanism of polymerization reaction. It is due to the different reactivity of protonated and nonprotonated aniline. Non-conducting polyaniline with irregular structure forms in neutral and alkali solutions, non-conducting oligomers form at pH 2.5–4 and regular conducting PANI could be obtained at pH < 2.5 [10,12]. The properties of composites based on ion-exchange membranes and PANI also depend on preparation conditions. The characteristics of the composite membranes depend on the quantity of PANI and the peculiarities of its distribution in the perfluorinated matrix [13,14]. The variety of preparation methods of the composites can be divided in two groups. The first group includes the dissolution of the PANI powder in

⁎

the solution of perfluorosulfonic acid and subsequent casting of the film. This method can be used to make single-layer homogeneous membrane containing PANI if the solution is casted on inert plates (e.g. glass plates) [15,16]. If the substrate is the polymer film, the bilayer composite is obtained [17]. Casting method permits to introduce the exact quantity of PANI into the polymer composition, but samples with a surface distribution of the modifier can not be obtain. In addition, it is doubtful that the PANI distribution would provide continuous proton transfer across the composite membrane. The second group includes the template PANI synthesis in the ionexchange membrane by oxidative polymerization of the monomer. Free-standing film is subsequently immersed into monomer and oxidant solutions or in their mixture in this method. The variation of the second method is adding the monomer to the solution of perfluorinated acid with its subsequent polymerization, which could occur both before and after casting the film [18]. The template synthesis of the PANI in the membrane phase is prospective method since it allows to prepare membranes with bulk and surface distribution of the modifier and to vary its quantity. Tan et al. estimated the PANI content on the surface of the Nafion membranes by the XPS analysis [14]. However, it is difficult to determine the exact quantity of PANI in the composites obtained by the template synthesis method. As a rule, the template synthesis includes saturation of the basic membrane with a monomer and subsequent immersion to the oxidant

Corresponding author. E-mail address: [email protected] (N.V. Loza).

https://doi.org/10.1016/j.synthmet.2020.116292 Received 10 July 2019; Received in revised form 28 November 2019; Accepted 3 January 2020 0379-6779/ © 2020 Published by Elsevier B.V.

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solution. It is known that the use of Fe3+ ions as the oxidizing agent leads to the formation of composites with bulk distribution of PANI [13]. This is due to the fact that anilinium (An+) and Fe3+-ions are counter-ions towards the cation-exchange membrane, so the monomer and oxidant enter the membrane phase via the ion exchange mechanism. As a result, polymerization occurs inside the membrane and composite with bulk distribution of the modifier is formed. The use of co-ions (S2O82−, Cr2O72− or MnO4-) as the oxidants leads to the formation of composites with a primarily surface distribution of PANI [19]. In some cases PANI could both forms in the surface layer and penetrates the opposite non-modified side of the membrane [13]. As a result, composites have gradient structure. The quantity of PANI is directly proportional to the monomer and oxidant concentrations and duration of the membrane exposure in the oxidant solution [13,20–22]. If composites are prepared in external electric field, the amount of PANI is also regulated by the current density and amount of electricity passed through the electromembrane system. The sequence of the membrane treatment with polymerizing solutions also affects on the polymerization reaction rate: the PANI formation rate is significantly higher in the case of preliminary saturation of the membrane with monomer than with oxidant for both S2O82− and Fe3+-ions [20,23]. Investigation into the electrotransport properties of composites has shown that an increase in PANI content inside the perfluorinated membrane leads to a decrease in electroosmotic and diffusion permeability and conductivity of the membrane due to significant changes in the structure and surface morphology of the membrane [23–25]. Understanding the mechanisms of PANI formation in the matrix of perfluorinated membranes permits to find the way to control the preparation of composites with a given set of properties most suitable to the particular process. However, this requires new research into the peculiarities of PANI synthesis within and on the surface of perfluorinated membranes. Therefore, the purpose of this work is the investigation into the sorption and desorption of the monomer at different stages of aniline oxidative polymerization inside the perfluorinated membrane and kinetics of the polymerization with different oxidants.

III membranes were immersed in the mixture of 0.01 M aniline, 0.005 M H2SO4 and 0.05 M (NH4)2S2O8. 2.2. Preparation of the composites under the oxidant diffusion The membrane sample saturated with the monomer in a solution of 0.1 M aniline in 0.5 M sulfuric acid was placed in a two-chamber cell. One chamber was filled with oxidant solution and the other with deionized water (Fig. 2). Formation of PANI in the structural cavities of the membrane occurs during the diffusion of oxidant solution through the membrane in water. The solutions of 0.025 M (NH4)2S2O8 in water and 0.01 M FeCl3 in 0.5 M H2SO4 were used as oxidants. 2.3. Determination of aniline content in solution We estimated the degree of the membrane saturation by aniline on the base of analysis of the monomer concentrations in the solution before and after contact with the sample. Aniline concentration was determined by the photometric method [26], which is based on the formation of the colored Schiff base and subsequent photometry of the sample at a wavelength of 432 nm. The detection limit of aniline in solution according to this method is 3·10−7 mol/L and the experimental error is less than 10 %. 2.4. Electronic absorption spectra and FTIR-spectra of the membranes Electronic absorption spectra were measured with a Leki SS 2109 scanning spectrometer at the wavelengths range from 250 to 1100 nm. The initial MF-4SK membrane served as the comparison sample. Measurement of the FTIR-spectra of the membrane surfaces was carried out on a Bruker Vertex 70 spectrometer fitted with a prefix of frustrated total internal reflection on diamond crystal. Before measurements, the samples were washed with deionized water to remove the sorbed electrolyte and air-dried to prevent the influence of swelling water. 3. Results and discussion

2. Experimental

3.1. Monomer sorption and desorption at different stages of modification

The Russian Nafion-type perfluorinated MF-4SK (JSC Plastpolymer, St. Petersburg, Russia) membrane with sulfonic acid groups served as the basic material. The membrane samples with different thickness were used: MF-4SK-I (240 μm), MF-4SK-II (60 μm) and MF-4SK-III (60 μm). The MF-4SK-III membrane has rough surface due to the treating the initial membrane by the sandpaper. The bare membranes were undergone the thermal pre-testing treatment before experiments described in [25]. The process included successive boiling of the membrane in 10 % H2O2 solution, 5 % HNO3 solution and deionized water (each single step required 3 h).

An important task is the quantitative estimation of PANI content in the composites obtained by template synthesis. PANI in emeraldine-salt form is colored substance, so it permits to use the UV-vis spectrometry to determine the PANI quantity inside the modified membrane with absorbance lower than 3 abs. units (referred to hereinafter as “optically transparent”). To reveal the correlation between the quantity of PANI and absorbance of the composite membrane it is necessary to obtain the set of the samples with known modifier content. The PANI concentration inside the composite membrane prepared by template synthesis method depends on the quantity of sorbed monomer. Therefore, detailed investigation was carried out into the sorption and desorption of aniline at every stage of PANI synthesis in the membrane bulk. At the first stage of the synthesis the MF-4SK-I membrane is in contact with a mixed solution of 0.01 M aniline and 0.5 M H2SO4, and as a result the H+-ions are replaced by aniliniun cations according to the ion-exchange equation

2.1. Preparation of MF-4SK/PANI composites in static conditions PANI synthesis in static conditions was performed by immersion the membrane sample in the polymerizing solutions and included a number of stages [20]. During the first stage the initial MF-4SK-I in H+-form was immersed in a solution of 0.01 M aniline and 0.5 M H2SO4 to saturate the membrane with monomer (stage I, Fig. 1). Then the sample was placed in oxidant solution of 0.01 M FeCl3 in 0.5 M H2SO4 or (NH4)2S2O8 in water. The oxidation of adsorbed aniline under the action of oxidant in the membrane matrix took place during this stage (stage II). The membrane became green, which pointed to formation of PANI in emeraldine-salt form. Then the composite was immersed in a 0.5 M H2SO4 solution (stage III) and washed with deionized water (stage IV) to complete the removal of the reagents from the membrane phase. Due to low thickness (about 60 μm) the MF-4SK-II and MF-4SK-

2R or R

SO3 H+ + (C6 H5 NH3+) 2 SO4 SO3 H+ + C6 H5 NH3+

R

2R

SO3 C6 H5 NH3+ + H2 SO4

SO3 C6 H5 NH3+ + H+

It is known that pKa for aniline is 4.596 [27]. The simple calculation shows that 99.9 % aniline exist in cationic An+-form at pH < 1.6. The monomer solution contained 0.5 M H2SO4, so we consider that aniline is fully protonated under the current experimental conditions. The monomer sorption process is accompanied by a decrease in aniline concentration in the solution, and the saturation level (θ) of the 2

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Fig. 1. Scheme of preparation of MF-4SK/PANI composites by chemical aniline polymerization.

0.01 M FeCl3 in 0.5 M H2SO4. It appears that some anilinium ions are displaced by Fe3+ and H+ cations

3R

MF-4SK-I membrane by anilinium cations (Table 1) can be calculated according to formula

=

R

(SO3 )3Fe3 + + 3C6 H5 NH3+

At the same stage, the membrane becomes green, which points to the formation of PANI. Berezina et al. observed desorption of 25 % of absorbed aniline in FeCl3 solution using independent radioisotope data [20]. In the case of 0.005 M (NH4)S2O8 the desorption at stage II was negligible due to low content of singly charged cations in the oxidant solution. At the next stage, significant desorption of monomer in 0.5 M H2SO4 occurs, and the level of monomer saturation of the membrane decreases by 25–30 % in comparison with the previous stage. However, the concentration of aniline in the water contacted with the membrane did not exceed 4·10−6 mol/L at the stage IV (Table 1), when the sample weight was 0.8 g and the volume of water was 50 mL. Therefore, the saturation degree didn’t change at IV stage. The desorption of the monomer in the last stages of preparing the composite indicates that only a portion of aniline sorbed during the first stage participates in the polymerization reaction and becomes PANI. The data obtained from the sorption experiments makes it possible to calculate the PANI content in optically transparent composite samples prepared with 0.01 M FeCl3 + 0.5 M H2SO4, which is about 1 %. Samples obtained with 0.005 M (NH4)S2O8 contain about 2 % of PANI and are optically opaque. It was assumed that all the aniline remaining in the membrane participated in the oxidative polymerization reaction and converted to PANI. However, the validity of the assumption is debatable since the increase in sample weight in a dry state after repeated washing with deionized water was comparable to the experimental error.

Fig. 2. Cell for preparation of anisotropic composites.

i

SO3 C6 H5 NH3+ + Fe3 +

ni mQ

where Δni is the change in aniline quantity in solution, mmol; m is the sample weight, g; Q is the ion-exchange capacity of the membrane, mmol/g. The value of θI at the I stage was around 0.4 under the given experimental conditions. According to Gierke model, the perfluorinated membrane consists of clusters filled with the external solution and connected by channels. We estimated the quantity of the aniline sorbed via non-exchange mechanism taking into account the water content (20 %), ion-exchange capacity (0.6 mmol/gsw) and saturation degree (0.4) of the membrane. The portion of non-exchange monomer inside the membrane was about 0.8 % of the total sorbed monomer, and the ionexchange mechanism of aniline sorption is the main one. The aniline was detected in solutions contacted with the membrane at II and III stages (Fig. 1) that indicates its desorption from the membrane phase. As a result, the saturation level in the following stages decreases (Table 1). It can be seen from the table that the most essential reduction in saturation level (about 30 %) occurred at the stage II in the case of

Table 1 Degree of the MF-4SK-I membrane saturation θ by aniline during the composite preparation. θ

0.01 M FeCl3 + 0.5 M H2SO4 0.005 M (NH4)S2O8

Preparation stage I (monomer saturation)

II (aniline oxidation)

III (desorption in 0.5 M H2SO4)

IV (desorption in water)

0.43 ± 0.13 0.46 ± 0.04

0.28 ± 0.09 0.43 ± 0.02

0.21 ± 0.06 0.31 ± 0.07

0.21 ± 0.06 0.31 ± 0.07

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Fig. 3. Electronic absorption spectra of MF-4SK-I (a, b), MF-4SK-II (c) and MF-4SK-III (d) membranes during in situ oxidative polymerization of aniline in 0.01 M (NH4)2S2O8 (a), 0.01 M FeCl3 + 0.5 M H2SO4 (b) and monomer + 0.0025 M (NH4)2S2O8 (c, d). Legend defines the duration of the membrane contact with the oxidant solution.

3.2. Kinetics of in situ oxidative polymerization of aniline in MF-4SK membranes

with smooth surface than for the rough one. For more detailed analysis of PANI formation kinetics we determined the optical densities of absorption maxima on the electronic spectra of the composites and expressed their dependency on the contact time of the samples with the oxidant solution (Fig. 4a, b). It can be seen that PANI begins to form almost in the first minutes of contact of the monomer-saturated membrane with the oxidant solution. Based on the kinetic data it is possible to calculate the average relative rate of aniline polymerization reaction for each time interval according to the formula

We compared the kinetics of the aniline polymerization in membrane under the action of different oxidants: counter- and co-ion in relation to the basic membrane. A MF-4SK-I membrane sample previously saturated with monomer was placed in a cuvette filled with a solution of 0.01 M FeCl3 + 0.5 M H2SO4 or 0.01 M (NH4)2S2O8. The electronic absorption spectra were measured at set time intervals (Fig. 3a, b). The increase in time of the membrane contact with the oxidant solution leads to a rise in the optical densities of the samples at wavelengths around 800 and 400 nm, which indicates the formation of PANI in emeraldine form [28]. Thus, the oxidation state of PANI is independent on the redox potential of oxidants Fe3+/Fe2+ and S2O82−/ SO42−. Analysis of the obtained data shows that the rate of PANI formation is significantly higher when using the FeCl3 in 0.5 M H2SO4 as the oxidant. The membrane was colorless after 0.5 h of contact with 0.01 M (NH4)2S2O8 solution. This is due to the fact that Fe3+-ions can penetrate the membrane via both ion-exchange and diffusion mechanisms, while S2O82−-ions, being co-ions in relation to the membrane, only appear inside the membrane via the diffusion mechanism. Therefore, the concentration of S2O82−-ions should be higher than that of Fe3+ to obtain the similar reaction rate. Fig. 3c, d presents the spectra for MF-4SK-II and MF-4SK-III membranes with smooth and rough surfaces. For both membranes, the peak at 400 nm is more intensive then at 800 nm, which indicates the formation of aniline oligomers especially in the case of smooth surface. In addition, polymerization rate is significantly lower for the membrane

v=

A t

where ΔA is the change in absorbance of the membrane sample for the time period Δt. Fig. 5 shows the relative polymerization rate using the Fe3+ and S2O82−-ions as the oxidant. Both dependencies have a clear power character. The polymer formation rate at the initial stage is significantly lower than at the final one. The increasing region of the kinetic curves indicates that the limiting stage of the polymerization reaction is the diffusion. At the initial time the basic matrix contains a large quantity of monomer (about 1.2 mol/L) and no oxidant, which enters the membrane by diffusion mechanism. As the oxidant diffuses into the membrane phase, the rate of aniline polymerization reaction increases. The diffusion region for MF-4SK-II sample with smooth surface is not observed (Fig. 5b). Apparently, the nucleation is promoted by surface roughness, and aniline polymerization takes place in the solution near the membrane surface similarly to PANI synthesis on an inert substrate [29]. 4

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Fig. 4. Kinetic dependencies of absorbance of MF-4SK-I (a), MF-4SK-II (b) and MF-4SK-III (b) during in situ oxidative polymerization on the period of time the samples were in contact with solutions of 0.01 M FeCl3 + 0.5 M H2SO4 (1), 0.005 M (NH4)2S2O8 (2) and 0.01 M aniline + 0.005 M H2SO4 + 0.05 M (NH4)2S2O8 (b).

We investigated the influence of (NH4)2S2O8 concentration on the rate of aniline polymerization reaction (Fig. 6). The oxidant concentration varied from 0.005 to 0.05 M, while the saturation degree of membrane by aniline was similar in every experiment. Fig. 6 demonstrates that there is a significant increase in polymerization reaction rate and decrease in the duration of initial “induction” period, when the PANI absorption bands are practically not detected. For the highest investigated concentration, the “induction” period was not fixed. Placing the monomer saturated membrane in oxidant solution starts the number of processes: aniline oxidative polymerization; ion exchange between H+/ An+ ions in membrane phase and NH4+ ions in solution; S2O82− diffusion into membrane phase. These processes occur simultaneously and in the interface region, so determination of the limiting stage of the reaction is complicated. The polymerization reaction primarily occurs near the membrane surface, and the diffusion of An+-ions from the membrane volume to the surface can also influence the polymerization rate. Considering that the fixed ions are uniformly distributed in the membrane, and the portion of SO3--groups located on the membrane surface is 0.05, the concentration of An+ on the surface is 0.024 mmol/g (θ = 0.4, Q =0.6 mmol/g). We assumed that the thickness of the layer of solution in direct contact with the membrane is 1 mm. Diffusion of oxidant and monomer is neglected. It is known that

0.2

0.15

MF-4SK-I

0.05 M

0.025 M

0.1

0.01 M 0.05 0.005 M 0 Fig. 6. Relative rate of aniline polymerization in MF-4SK-I under the action of (NH4)2S2O8 with different concentrations.

Fig. 5. Dependencies of relative rate of aniline polymerization in MF-4SK-I (a), MF-4SK-II and MF-4SK-III (b) membrane matrix on time while using FeCl3 in H2SO4 and (NH4)2S2O8 as the oxidants.

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the predicted stoichiometric mole ratio is [peroxydisulfate]/[aniline] = 1.25 [30]. The reagents ration in the reaction region could be estimated by formulae

k=

peroxydisulfate from the membrane. At the same time, when Fe3+-cation is used as the oxidant, the absorbance of the peak maxima at 400 and 800 nm increases by 2 and 1.4 times during the first days after interruption of contact by the sample with oxidant solution and by 15–20 % during the next two weeks. It appears the desorption of Fe3+-ions proceeds sufficiently slowly because of the strong electrostatic interaction of these ions with the fixed sulfonic groups of the membrane. As a result polymerization of aniline continues inside the ion-exchange material despite the fact that the oxidant solution is replaced by 0.5 M H2SO4 solution. A series of composite membranes was prepared using the method of diffusing oxidant solution through a membrane saturated with monomer in order to assess the influence of oxidant nature on PANI distribution in the structure of MF-4SK-I. It is known that synthesis in asymmetric conditions using the co-ion as the oxidant results in anisotropic or gradient distribution of PANI in the membrane, and the membrane surface will contain a different quantity of the modifier. Composite membranes were prepared during 0.5, 1 and 2 h of membrane contact with 0.025 M solution of (NH4)2S2O8 or 0.5 h with 0.05 M FeCl3 in 0.5 M H2SO4. The frustrated total internal reflection of IRwaves was used as the surface sensitive method. Spectrum of the initial membrane (Fig. 8a) contains peaks, typical for perfluorinated membranes (1210 and 1150 are CF2 vibrations; 1055 is vibration of SO3−) [31]. The FTIR-spectra of the composite membranes obtained using ammonium peroxydisulfate are shown on Fig. 8a. The spectra of the surfaces contacted with the oxidant solution during the synthesis contain peaks at 1500 and 1575 cm-1 corresponding to the vibration of structural fragments of PANI macromolecules: benzene and quinoid rings. The spectra of the opposite sides of the membranes is identical to that of the initial membrane. Increase in the period of the membrane contact with the oxidant solution from 0.5 to 2 h causes the proportional growth in the absorption peaks corresponding to the PANI and indicates the accumulation of the modifier in the surface layer of the membrane. It is confirmed by the displacement of -CF2- peak at 1150 cm-1 to 1120 cm-1 due to the overlay of PANI peaks located in this range [5]. The meaningful content of the PANI on the side facing the water during modification was not fixed even after 2 h of synthesis, and the PANI did not penetrate through the membrane. Fig. 8b shows the FTIR-spectra for the modified surface of MF-4SKII/PANI membrane obtained using S2O82− during 1.5 h of synthesis. As can be seen, a significant decrease in absorbance of the peaks corresponding to the perfluorinated matrix and a simultaneous increase in the absorbance of peaks for PANI takes place. This confirms the assumption that PANI formation occurs on the membrane surface similar to PANI synthesis on inert substrate. When using Fe3+-ions during 2 h as oxidant the penetration of the PANI chains through the membrane takes place, and the spectra of both sides of the composite are identical. Thus, PANI synthesis takes place differently depending on the nature of the used oxidant. If the oxidant is a co-ion in relation to the initial membrane the reaction proceeds in the solution near the membrane surface similarly to PANI synthesis on an inert substrate [29]. When the oxidant is counter-ion the polymerization takes place within the membrane. This opens up the possibility of controlling the distribution of PANI in the optically transparent composites by choosing an appropriate oxidant. The discovered regularities of the monomer sorption and the kinetics of polymerization under the action of the counter- and co-ions as oxidants are of a general nature and will be similar for all cation-exchange membranes and oxidizing agents. MF-4SK/PANI composites with low PANI content could be used in different devices. The analysis of literature shows a promising performance of membrane-electrode assembly of air-hydrogen fuel cell with MF-4SK/PANI membranes at 80 °C due to superior conductivity in comparison with initial membrane [32]. Moreover, MF-4SK/PANI composites could be applied as optical pH sensors due to high

n (S2 O82 ) 1.25 n (An+)

where n is the molar quantity of ions in the reaction region. k equals 0.5 and 1 for 0.005 and 0.01 M (NH4)2S2O8 solutions, so there is an excess of monomer and deficiency of oxidant. In this case, the limiting stage is the diffusion of oxidant from solution to the membrane surface, and the “induction” period corresponding to nucleation process is observed. Increase in oxidant concentration causes the deficiency of monomer concentration in the interface region (k equals to 2.8 and 4.8 for 0.025 and 0.05 M (NH4)2S2O8 solutions), and both chemical reaction and monomer diffusion could limit the polymerization process. The anilinium diffusion is provided by the ion-exchange reaction in the membrane and depends on the concentration of NH4+-ions in solution. In addition, there is a decrease in the nucleation time to very insignificant values, not exceeding 3 min. So, the mechanism of monomer diffusion is the main one at high oxidant concentrations. Despite the assumptions made it is shown that the oxidant concentration influences on the limiting stage of the polymerization reaction and both the diffusion of oxidant from solution and diffusion of monomer from membrane phase to the interface “membrane/solution” are possible. 3.3. Influence of oxidant nature on composites stability and PANI distribution in the membrane During the preparation of bulk modified MF-4SK/PANI membranes it was discovered that their color intensity was increasing even after discontinuation of samples contact with the oxidant solution. It shows indirectly that the process of polymerization of the monomer remaining in the membrane is continuing. We investigated the possibility of controlled suspension of the aniline polymerization reaction in the matrix of the perfluorinated membrane. For this purpose, UV-vis electronic spectra for membranes obtained with counter- and co-ions as the oxidants were measured at different time intervals following the interruption of contact of the sample with the oxidant solution and immersion into in 0.5 M solution of sulfuric acid (Fig. 7). When synthesis was carried out using ammonium peroxydisulfate, the absorbance values in the absorption maxima did not change within the experimental error. It clearly indicates that the aniline polymerization reaction ceases almost immediately after discontinuation of the membrane contact with the oxidant solution and the membrane immersion in the acid solution. The concentration of co-ions in ion-exchange membrane is determined by the non-exchange electrolyte content, and immersion of the sample in acid solution causes sufficiently rapid displacement of ammonium

Fig. 7. The absorbance of MF-4SK-I/PANI samples obtained with Fe3+ and S2O82- as the oxidant during their storage in 0.5 M H2SO4. 6

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Fig. 8. FTIR-spectra of MF-4SK membrane, composites MF-4SK-I/PANI obtained with: a – (NH4)2S2O8 solution during 0.5, 1 and 2 h (modified surfaces); b – with 0.05 M FeCl3 in 0.5 M H2SO4 solution during 2 h (modified and non-modified surfaces) and MF-4SK-II/PANI prepared with (NH4)2S2O8 during 1.5 h (modified surface).

sensibility of PANI to pH and high chemical and thermal stability of perfluorinated membranes coupled with transparency in UV-vis range [33].

CRediT authorship contribution statement Natalia V. Loza: Methodology, Writing - original draft. Irina V. Falina: Methodology, Writing - review & editing, Project administration. Natalia A. Kononenko: Conceptualization, Supervision. Daria S. Kudashova: Investigation, Validation.

4. Conclusion The detailed investigation into sorption and desorption of the monomer at every stage of PANI synthesis in the perfluorinated membrane during the preparation of optically transparent composites has been performed. It permits to calculate the saturation degree of MF-4SK membrane by aniline during the synthesis, which changed from 0.43 to 0.21. It is shown that the ion exchange mechanism of aniline sorption by the membrane is the main one. The PANI content in composites obtained under the current conditions does not exceed 2 % by weight. It is demonstrated that a portion of the sorbed monomer remains nonpolymerized inside the membrane for an extended period of time after the action of the oxidant, which makes it difficult to carry out a quantitative calculation of modifier content inside the membrane by UV-vis spectroscopy of the membranes. The kinetics of aniline polymerization reaction under the action of counter- and co-ions as the oxidant has been investigated. Polymerization reaction rate is significantly higher in the case when oxidant is counter-ion towards the basic matrix than for the co-ion. Analysis of the kinetic curves shows that the limiting stage of the PANI synthesis depends on the oxidant nature and concentration. It has been shown that PANI synthesis takes place differently depending on the nature of the oxidant used, and for co-ion the reaction proceeds in the solution near the membrane surface, but for counter-ion the polymerization takes place within the membrane. The polymerization reaction can be ceased when using the co-ion as the oxidant since the diffusion is the only mechanism for anions transport in cation exchange membrane. This opens up the possibility of controlling the distribution of modifier in the optically transparent composites by choosing an appropriate oxidant.

Acknowledgements This work was supported by the Russian Foundation for Basic Research (project No 18-38-20069). The authors are grateful to F.A. Kolokolov (the collective center for Diagnostics of Structure and Properties of Nanomaterials at Kuban State University) for the FTIRspectra measurements and S.V. Timofeev (JSC Plastpolymer, St. Petersburg, Russia) for supplying MF-4SK membranes. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.synthmet.2020. 116292. References [1] G. Círić-Marjanovic, Recent advances in polyaniline research: polymerization mechanisms, structural aspects, properties and applications, Synth. Metals 177 (2013) 1–47, https://doi.org/10.1016/j.synthmet.2013.06.004. [2] H. Wang, J. Lin, Z.X. Shen, Polyaniline (PANi) based electrode materials for energy storage and conversion, J. Sci.: Adv. Materials Devices 1 (2016) 225–255, https:// doi.org/10.1016/j.jsamd.2016.08.001. [3] J. Stejskal, Polymers of phenylenediamines, Prog. Polym. Sci. 41 (2015) 1–31, https://doi.org/10.1016/j.progpolymsci.2014.10.007. [4] J. Stejskal, R.G. Gilbert, Polyaniline. Preparation of a conducting polymer (IUPAC technical report), Pure Appl. Chem. 74 (2002) 857–869, https://doi.org/10.1351/ pac200274050857. [5] M. Trchová, J. Stejskal, Polyaniline: the infrared spectroscopy of conducting polymer nanotubes (IUPAC Technical Report), Pure Appl. Chem. 83 (2011)

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