Influence of Dispersed Nanoparticles on the Kinetics of Formation and

Article pubs.acs.org/JPCB

Influence of Dispersed Nanoparticles on the Kinetics of Formation and Molecular Mass of Polyaniline Nikolay A. Ogurtsov,*,† Sergei D. Mikhaylov,†,‡ Patrice Coddeville,‡ Jean-Luc Wojkiewicz,‡ Galina V. Dudarenko,§ and Alexander A. Pud*,† †

Institute of Bioorganic Chemistry and Petrochemistry, NAS of Ukraine, 50 Kharkivske shose, Kyiv 02160, Ukraine Atmospheric Science and Environment Engineering Department (SAGE), Mines Douai, 941 rue Charles Bourseul, F-59508 Douai, France § Institute of Macromolecular Chemistry, NAS of Ukraine, 48 Kharkivske shose, Kyiv 02160, Ukraine ‡

S Supporting Information *

ABSTRACT: Using a combination of the open circuit potential and pH profiles of aniline (An) polymerization and their mathematical treatment, we develop a new convenient semiquantitative approach to determine the influence of the dispersed nanoparticles (e.g., TiO2 nanoparticles) on the kinetic features of this process and molecular mass of the formed polyaniline (PANI). It is revealed that the reciprocal values of the polymerization stages, namely, the duration of the induction period, of homogeneous and heterogeneous pernigraniline (PN) accumulation, and of PN reduction with An, are linear functions of the weight fraction of the nanoparticles. We found that when nanoparticles are added the weight-averaged molecular weight of PANI initially increases from 56 000 to 79 000 and the polydispersity index drops from 3.9 to 1.7. However, at high TiO2 concentrations, the former dramatically decreases, whereas the latter increases. We use the relative proton concentration as a function of time and the different extents of acceleration of the consecutive stages of An polymerization to explain the changes in the molecular weight distribution of PANI with different contents of TiO2 nanoparticles in the polymerization medium. Cu3Ti4O12),20−22 metals (Ag, Ni, Pd),23−25 clay,26 carbonaceous materials (CNT, graphene),27−31 and so on have been synthesized for application in ultrasensitive chemical and biological sensing, broadband microwave absorption, electromagnetic interference shielding, electrostatic charge dissipation, energy storage, thermoelectrics, magneto- and piezo-resistive materials, and capacitive deionization and as efficient heterogeneous catalysts, electrorheological fluids, and supercapacitors. The high quality of these materials stems probably from the fact that polymerization of An in the presence of the dispersed phase in the reaction medium can have a profound effect on the properties of the PANI layer/shell formed at the surface of the dispersed particles due to the formation of intimate contacts between all participants of the polymerization process. For example, the crystallinity, molecular weight (MW), conductivity, and oxidation level of PANI doped with ptoluenesulfonic acid (TSA) in such shells on the surface of polycarbonate particles considerably changed as compared to those of pure PANI−TSA.32 A similar effect was found for

1. INTRODUCTION Currently, there exists a significant interest in the development, preparation, and investigation of multifunctional hybrid nanocomposites of intrinsically conducting polymers with different insulating, semiconducting, or conducting substances that combine the properties of all components and can be used for different high-tech applications. Among these materials, polyaniline (PANI)-based nanocomposites hold a special place due to an effective combination of conductivity and reactivity with high stability. However, the simple synthesis, easy treatment, and low price of PANI, which facilitate scalable preparation of PANI nanocomposites, are also very important factors. These nanocomposites can be prepared by mixing previously prepared emeraldine salt with another component or by polymerization of aniline (An) in the presence of this component. In particular, through the mixing approach, we can obtain synergetic properties in only some cases, whereas through the polymerization approach, these properties typically prevail. On the basis of the polymerization approach, several PANI nanocomposites with metal dioxides (TiO2, SnO2, In2O3, SiO2, Fe3O4, CeO2, ZrO2),1−13 ferrites (CoFe2O4, MnFe2O4, Ni0.5Zn0.5Fe2O4, BaFe12O19, SrSm0.3Fe11.7O19, MnaZn(1−a)Fe2O4),13−19 titanates (BaTiO3, Ca© XXXX American Chemical Society

Received: June 13, 2016 Revised: September 2, 2016

A

DOI: 10.1021/acs.jpcb.6b05944 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B

nanoparticles (MTI Corporation), with an average size of 5−10 nm and specific surface area of 210 m2 g−1, were used as received. The polymerization protocol was the same in all cases except for different contents of TiO2 nanoparticles in the reaction medium. In short, it involved mixing of 0.05 g (0.537 mmol) of An with 0.2629 g of DBSA (0.805 mmol) in 32 mL of distilled water, followed by continuous magnetic stirring for 1 h. Then, the required quantity of TiO2 nanoparticles was added to the solution. The mixture was stirred for additional 1 h at a temperature of 10 °C, maintained by a thermostat (CC1-K6, Huber). Then, a precooled (10 °C) solution of 0.1532 g of APS (0.671 mmol) in 8 mL of distilled water was added to this reaction mixture, followed by stirring for 24 h. The formed PANI nanocomposites and neat PANI were purified by dialysis against distilled water for 72 h and dried under vacuum at 60 °C to a constant weight. The process of An polymerization was continuously monitored by simultaneous OCP and pH measurements with the redox electrode Hamilton Polyplast Oxidation Reduction Potential and the pH sensor Hamilton Polyplast BNC, respectively, connected to the pH/redox/temperature-measuring instrument, GMH 3530 (Greisinger Electronics), interfaced with a computer. The MW of PANI was estimated by size-exclusion chromatography (SEC) on the Du Pont LC System 8800, with an ultraviolet detector and Azorbax bimodal exclusion columns, using a procedure described elsewhere.32 For MW analysis, the pristine neat PANI and TiO2/PANI nanocomposites were converted to the base form by treatment with a 0.5 wt % aqueous ammonia solution for 24 h. The powders were filtered, washed with distilled water, and dried under vacuum at 60 °C to a constant weight. Then, 0.1 g of the dedoped samples was dissolved in 15 mL of a 3 wt % solution of ascorbic acid in N-methylpyrrolidone to reduce the dissolved PANI in the emeraldine base state to the leucoemeraldine base. After 24 h, the obtained leucoemeraldine solutions were centrifuged for 30 min at 8000 rpm to remove the TiO2 nanoparticles. The prepared solutions were passed through a 0.5 μm filter and used for the further MW measurements. To prevent the aggregation of PANI in NMP, we used not only the leucoemeraldine state but also 0.05 wt % LiCl.

multiwalled carbon nanotubes (MWCNTs)/PANI−dodecylbenzenesulfonic acid (DBSA) nanocomposites.28 This specificity of the PANI precipitated at the surface of the nanoparticles of other composite components implies that the successful development of new PANI-based advanced nanocomposite materials requires both a deeper understanding of what is happening during the formation of PANI to tune the PANI phase properties and the development of a method to monitor the polymerization process, with an option to control it. On the other hand, from fundamental point of view, careful monitoring of the An polymerization process under various conditions can shed more light on some kinetic features and details of the polymerization mechanism, which are still not well understood and discussed.33 One of the important methods for monitoring the An polymerization process is through open circuit potential (OCP) measurements, which provide information on the redox state of the formed PANI and the rate and mechanism of the polymerization.34−37 On the basis of the OCP measurements, a new method of kinetic analysis of the separate stages of the An dispersion polymerization has been developed recently.38 This method considers characteristic points and shapes of the OCP profiles of the reaction medium and ties their lengths with the quantities of nanoparticles dispersed in this medium. However, the analysis becomes difficult if the OCP profiles have broad and flat maxima and the end of the induction period is quite blurred. Here, we resolve this problem by using the first derivatives of the OCP profiles and comparing the kinetic analysis data of the poorly shaped OCP profiles to additional kinetic information obtained from the pH profiles of the chemical oxidative An polymerization both in the absence and presence of different quantities of dispersed nanoparticles (e.g., titania nanoparticles). This approach is quite simple and allows the determination of the main kinetic parameters of the polymerization process without complementary experiments and much faster and more easily than that by the known time-consuming methods, such as linear sweep cyclic voltammetry,39 gas chromatography,40 calorimetry,41 surface pressure,42 proton NMR spectroscopy,43,44 visible absorption spectroscopy,45−47 and the quartz crystal microbalance technique.48 We also introduce relative proton concentration as a function of the polymerization time and discover that it allows us to link the kinetic data with the MW distribution of PANI. It should be emphasized here that due to consideration of the peculiarities of the physical−chemical changes in the polymerization medium the newly developed OCP/pH approach allows the estimation of the kinetic parameters of the main stages of the chemical oxidative polymerization of An in the presence or absence of different nanoparticles and therefore can be applied to, for example, earlier studied systems.1−31 In this study, our interest in TiO2 nanoparticles originates just from their promising applications in many fields, including materials for chemical sensing,1−3,6,7 microactuators, dynamic random-access memory, metal-oxide semiconductor devices,4 and microwave absorption.5 In addition, they possess a high specific surface area (210 m2 g−1).6 The characterization of the nanocomposites of TiO2 with PANI has been published elsewhere.6

3. RESULTS AND DISCUSSION The typical OCPs and pH kinetic profiles of the An polymerization in the absence and presence of TiO2 nanoparticles are shown in Figure 1. As one can see, the addition of TiO2 nanoparticles produces some change in the pH value of the reaction medium. For example, at the 20th min, the pH of the reaction mixture without TiO2 is about 1 (curve 1), but it increases to 1.36 when the value of the TiO2/An weight ratio is 80:20 (curve 3). The redox potential shifts from 571 mV (curve 1, 20 min) to 547 mV (curve 3), in agreement with the Nernst equation written for a reduction reaction of persulfate in an acidic medium.49 Obviously, such changes do not affect the mechanism of An polymerization because all main stages of the reactions occur at pH < 2.5 when the reactants are protonated.33 The growth of the conjugated chain in this case, as is known,33 proceeds via coupling of the cation-radical centers. At the same time, the addition of TiO2 nanoparticles has a significant effect on the reaction kinetics. As can be seen, the maximum magnitude and position in the OCP profiles change from 680 mV and 147 min, respectively, to 610 mV and

2. EXPERIMENTAL SECTION An (Merck) was distilled under vacuum and stored under an argon atmosphere at 3−5 °C. The reagent-grade ammonium persulfate (APS, Ukraine) and DBSA (Acros), the TiO2 anatase B


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here that the reactions of proton release and persulfate consumption are interrelated and take place in the first step of the polymerization reaction. In the second step, protons are not formed. For clarity, in addition to reaction 3, which is shown in the form of chain growth, we represent a side reaction of the formation of new PANI chains (reaction 3a),50 in which PN is depicted arbitrarily in the unprotonated form

Figure 1. Effect of the TiO2/An weight ratio on the development of the OCPs and pH profiles of the An polymerization: (1) 0:100, (2) 50:50, (3) 80:20, (4) 90:10, (5) 95:5.

45 min when the TiO2/An weight ratio is increased from 0:100 to 95:5. The time to reach the steady state of the pH profiles also decreases, which indicates the acceleration of the polymerization process of An. The observed impact of the TiO2 nanophase can be quantified. We have shown previously38 that the characteristic points and shape of the OCP profile can be used to find the effect of the dispersions of MWCNTs on the kinetics of the main stages of the oxidative chemical polymerization of An. These characteristic points correspond with the times taken to reach the end of the induction period (tip) and the OCP maximum (tmax). In turn, attaining the OCP maximum matches well with a maximum content of pernigraniline (PN) in the reaction medium.38 The falling part (reduction of PN with residual An) of the OCP profiles gives the decay constants (t1 and t2) of the double exponential decay function E(t ) = A 0 + A1 e−t / t1 + A 2 e−t / t2

In some cases, there are experimental difficulties in the assessment of the points of the OCP profile corresponding to tip and tmax. For example, the OCP profile can exhibit a very broad vertex.36 To accurately determine the duration of the induction period, one has to use continuous visual observation or simultaneous UV−vis and OCP monitoring for detection of the dimer (p-aminodiphenylamine) appearance.37 We find that these difficulties can be eliminated if we use the first derivatives of the OCP curves (curve 2, Figures 2 and S1−S4). Our study

(1)

It has been revealed that the reciprocal values of tip, (tmax − tip), t1, and t2 are linear functions of the weight fraction of MWCNT (f wtCNT), which allows the determination of both the values of the effective reaction rate constants of the polymerization stages and the degree of impact of the concentration of the inorganic phase. The induction period, PN accumulation, and reduction of PN with residual An correspond to the indicated times. The last two steps constitute essential reaction features, called the two-step mechanism of chemical polymerization of An34,38,50,51

Figure 2. OCP (1), its first derivative (2), and pH (3) profiles of the An polymerization at a TiO2/An weight ratio of 80:20. The characteristic points of the curves are shown also.

shows that the induction period (tip(E)) correlates well with a break point in the graph of the first derivative. As shown below, the inflection point in the falling part of the OCP profile (tinf_p) can be used instead of the tmax point and is easily found from the position of the minimum of the first derivative. Apparently, the first point more accurately defines the termination of PN formation as well as APS consumption because the pH value continues to fall slightly until the tinf_p point is reached. The difference between the values of tmax and tinf_p is usually noticeable when the maximum/plateau of the OCP profile is wide. In contrast, both the points almost coincide when the peak is sharp (e.g., see Figure 3 in ref 38). To obtain more insight into the kinetic features of An polymerization in dispersions of TiO2 nanoparticles, we have

As the degree of protonation of PN is not known, it is represented in this scheme arbitrarily. It should be emphasized C


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Figure 3. (a) Dependence of the reciprocal values of tip(E) (1), tmax − tip(E) (2), and tinfl p − tip(E) (3) on the weight fraction of TiO2 in the reaction medium. (b) Dependence of the reciprocal values of tip(pH) (1), tbp2 − tip(pH) (2), tbp1 − tip(pH) (3), and tbp2 − tbp1 (4) on the weight fraction of TiO2 in the reaction medium. The lines are the best linear fits for each case.

applied, for the first time, the method of characteristic points to the pH profiles. As can be seen from Figures 2 and S1−S4, there are at least three characteristic points in all of the profiles. These break points (tip(pH), tbp1, and tbp2) allow us to estimate the times of the induction period and the two separate stages of the PN accumulation stage. As the tbp1 point locates close to the appearance of insoluble PN phase (tipp)37,52 and divides regions with strongly different slopes of pH curve, we can identify them as the stages of homogeneous and heterogeneous PN accumulation, respectively. It should be noted here that the tipp point could not be used to determine the duration of the last two stages because it was not observed in some curves (Figure 1, curves 4 and 5). The experimental data indicate that the reciprocal value of the characteristic time (tch) is a linear function of the weight fraction of the TiO2 nanoparticles (Figures 3 and 4) 1/tch = a + bfwt

TiO2

tip(pH)), or both stages of the homogeneous (tbp1 − tip(pH)) and heterogeneous (tbp2 − tbp1) PN accumulation, or the decay constants (t1 and t2) of the function 1, characterizing the reduction of PN with residual An. The results of the linear fittings are summarized in Table 1. Almost all reciprocal values of the indicated parameters show Table 1. Values of the Constants for Linear Fitting (Equation 4) Presented in Figures 3 and 4, with the Corresponding Correlation Coefficients (r) tch

a

Induction Period tip(E) 0.046 tip(pH) 0.055 PN Accumulation tmax − tip(E) 0.0078 tinf_p − tip(E) 0.0069 tbp2 − tip(pH) 0.0062 Homogeneous PN Accumulation tbp1 − tip(pH) 0.014 PN Heterogeneous Accumulation tbp2 − tbp1 0.011 Reduction of PN with Residual An t1 0.021 t2 0.0029

(4)

where tch can be the duration of the induction period (tip(E), tip(pH)), the PN accumulation (tmax − tip(E), tinf_p − tip(E), tbp2 −

b

r

1.02 3.66

0.976 0.998

1.06 0.88 0.73

0.997 0.992 0.993

1.42

0.996

1.5

0.989

5.1 0.17

0.941 0.998

good linear relationships (see the correlation coefficients). Only the value of 1/t1 shows a moderate linear trends (r = 0.94). Perhaps it stems from the increase in the error of the t1 determination due to a diminishing contribution of the first exponential term of eq 1 to the decay parts of the OCP profiles (Figures S5 and S6, fit parameters in Table S1) when the weight fraction of TiO2 is increased. A comparison of the OCP and pH approaches reveals that they give close results for the semiquantitative estimation of kinetic parameters. The obtained dataset clearly shows that the reaction rates increase in the presence of TiO2 nanoparticles compared to those in the polymerization solution without the dispersion phase. However, the most important aspect of this result is the fact that each consecutive stage of the An polymerization is accelerated to a different extent. As shown

Figure 4. Dependence of the reciprocal values of decay constants t1 (1) and t2 (2) on the weight fraction of TiO2 in the reaction medium. The lines are the best linear fits for each case. D


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Figure 5. (a) Effect of the TiO2/An weight ratio on the relative proton concentration, H(t), produced by An polymerization: (1) 0:100, (2) 50:50, (3) 80:20, (4) 90:10, (5) 95:5. (b) The H(t) profile changes for the TiO2/An weight ratio of 80:20. The double-headed arrow indicates the proton molar fraction produced during the stages of homogeneous (H1) and heterogeneous (H2) PN accumulation and the reduction of PN with residual An (H3).

the kinetics of the An polymerization in the presence of MWCNT and SiO2 nanoparticles.38 The effect of the different accelerations of the main stages of the An polymerization can be observed in the OCP profiles. This effect is visualized by decreasing both the magnitude of the PN maximum in the OCP profiles (Figure 1) and the input of the term A1 e−t/t1 of eq 1 (Figure S6) when increasing the weight fraction of TiO2. Moreover, one can see some decrease in the pH of the polymerization medium after reaching tbp2 (Figures 2 and S4). This behavior is predetermined by the consecutive reactions 2 and 3, in which PN is the intermediate product. As f wtTiO2 is increased, the ratio of these reaction rates (υ3(t1)/υ2 = 0.021/0.0078 = 2.7 and (0.021 + 5.1 × 0.023)/ (0.0078 + 1.06 × 0.023) = 5.7 for TiO2/An weight ratios of 0:100 and 95:5, respectively) increases (see Table 1). [For the discussed conditions, we consider that this ratio (υ3(t1)/υ2) is equivalent to the ratio of the reciprocal values of the lengths of the stages of PN accumulation (1/(tmax − tip(E)) and PN consumption in the reaction of reduction by An (1/t1).] This inevitably leads to a decrease in both the PN maximum magnitude and the time at which this maximum appears. This PN maximum behavior is similar to that of an intermediate of two consecutive first-order reactions55 but with a more complex kinetic description. To study this phenomenon in more detail, we have examined the variation of proton concentration with time. For this purpose, we consider the relative proton concentration

below, this phenomenon leads to a MW dependence of PANI on the weight fraction of TiO2 in the reaction medium. As follows from Table 1 and eq 4, the reaction rates of consecutive stages (the induction period (a); the PN accumulation (b) in general, including the homogeneous (b1) and heterogeneous (b2) accumulations; and the reduction of PN (c) with residual An) are increased by factors of (a) 1.5− 2.5, (b) 3.7−4.1, (b1) 3.3, (b2) 4.1, and (c) 6.6 (t1) to 2.3 (t2), respectively, for the case of TiO2 nanoparticle contents corresponding to f wtTiO2 = 0.023 (TiO2/An weight ratio 95:5). If we compare these data with those obtained in dispersions of MWCNT,38 we see that the respective rates are increased by factors of (a) 2.8, (b) 3.3, and (c) 9.1 (t1) to 7.7 (t2) at the same f wtCNT. Assessment of the impact of the specific surface area (σ) on the observable rate constant (k) value (k = k′σ, where k′ is the rate constant)40 allows us to perform a correct comparison of the rates of these two sets. In particular, assuming that TiO2 and MWCNT particles have spherical and cylindrical shapes, the ratio (R) of their specific surface areas (S) can be easily calculated as R = STiO2 /SCNT = 3ρCNT DCNT /2ρTiO DTiO2 2

where ρTiO2, ρCNT, DTiO2, and DCNT are the densities and diameters of TiO2 and MWCNT, accordingly. Using ρTiO2 = 3.83 g/cm3,53 ρCNT = 2.6 g/cm3,54 DTiO2 = 7.5 nm, and DCNT = 15 nm,38 we found that the specific surface area of MWCNT is 2.0 times higher than that of the TiO2 nanoparticles. Therefore, the rates of the An polymerization stages in dispersions of MWCNT should be halved for comparison with those in TiO2 dispersions. As a consequence, this estimation shows that the corrected reaction rates of PN accumulation and reduction of PN with An (t1) are about 2 times lower in dispersions of MWCNT than those in dispersions of TiO2. It should be noted here that the reaction rate would be even higher if pH growth did not occur in the presence of TiO2 nanoparticles, which slows the rate of An polymerization.40 It follows from here that the degree of acceleration of each stage depends not only on the specific surface area but also on the nature of the substrate surface that is added to the reaction system. A similar finding for the PN accumulation stage was observed when comparing

H(t ) = ([H+]t − [H+]ip )/([H+]max − [H+]ip )

where [H+]t represents the instantaneous proton concentration, [H+]max is the maximum proton concentration, and [H+]ip is the proton concentration at time t = tip(pH). The concentrations of protons in the reaction media are calculated from the pH values of the polymerization media. Neglecting the activity coefficient of the H+ ions should not lead to large errors because the used reagent concentrations were low enough. Besides, the presentation of the proton concentration in the form of the relative characteristic, H(t), should minimize the inaccuracy of this approximation. The moderate rates of the An polymerization in a solution of DBSA and thermostatic control of the reaction media also reduce the E


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Table 2. Effect of the Weight Content of the TiO2 Nanoparticles on the Molar Fraction of Protons (H1, H2, H3) Produced at the Main Stages of the An Polymerization (see Figure 5b) and the Weight-Average Molecular Weight (Mw), Number-Average Molecular Weight (Mn), and Polydispersity Index (PDI = Mw/Mn) of PANI f wtTiO2

TiO2/An weight ratio

H1

H2

H3

Mw

Mn

PDI

t(E = 0.55 V) − tmax, min

0 0.0049 0.011 0.023

0/100 80/20 90/10 95/5

0.153 0.15 0.128 0.054

0.728 0.765 0.74 0.544

0.119 0.085 0.132 0.402

56 000 75 800 79 000 38 000

14 300 44 300 38 000 11 700

3.9 1.7 2.1 3.3

59.4 24.3 20.1 26.2

Figure 6. General block scheme of the oxidative An polymerization.

the addition of TiO2 nanoparticles, the Mw of PANI increases by more than 40% and exhibits a maximum at f wtTiO2 = 0.11 (Table 2). After that, the Mw of PANI drops sharply to 38 000 at f wtTiO2 = 0.23, whereas, in contrast, the PDI decreases from 3.9 to 1.7 at f wtTiO2 = 0.004 and then rises to 3.3 at f wtTiO2 = 0.23. By comparing the Mw value with the molar fractions of protons (H1, H2, H3) produced at the main stages of the An polymerization, we may conclude that the MW of PANI is enhanced when the H1 fraction of the protons decreases and the H3 value is less than about 0.15. However, when the H3 value becomes high, the MW decreases and PDI increases. This can also indicate that the stages of homogeneous and heterogeneous PN accumulation and the reduction of PN with An to emeraldine salt can be viewed as equivalent to the initiation, propagation, and termination steps, respectively. From this point of view, when the propagation rate rises faster than the initiation rate (the ratio of coefficients b of eq 4 for the stages of homogeneous and heterogeneous PN accumulation is less than 1), the amount of growth centers of the polymer chains decreases (i.e., H1 decreases). Consequently, the MW of PANI increases. Accordingly, the strong growth of the termination rate (the value of a + bf wtTiO2 for the stage of reduction of PN with An) lowers the propagation rate (and its H2 yield) and the MW due to a sharp drop in the instantaneous PN concentration (compared to the OCP maximums for curves 1 and 5 in Figure 1). In this case, the propagation reaction (eq 2) is observed for the time period t > tbp2 (with a sharp increase in the H3 value) when there is usually almost complete domination of the termination reaction (eq 3, with a low or zero H3 value). It can be assumed that the induction period and stage of homogeneous PN accumulation are two consecutive subprocesses of the initiation step, which perform the different functions. A sufficient amount of dimer (and may be some amount of oligomer) is produced during the first substep to accelerate the polymerization reaction. During the next substep,

error in the determination of the proton concentration. It should be emphasized here that the value of [H+]ip is taken instead of that of [H+]0 (at time t = 0 min) because An is not practically consumed during the induction period37 and therefore protons are not generated at this stage. The effect of the TiO2/An weight ratio on the relative proton concentration during the An polymerization is shown in Figure 5a. As seen, using the H(t) parameter facilitates comparison of the quantity of protons, which are produced during the main stages of the polymerization under different conditions. We also present the example of determination of the mole fractions of protons produced during the stages of homogeneous (H1) and heterogeneous (H2) PN accumulation and during the reduction of PN with residual An (Figure 5b). The possibility of their determination is based on a sharp difference in the rate of reaction 2 between the stages of homogeneous and heterogeneous PN accumulation (because of the autoaccelerated character of the reaction) and its almost complete attenuation in the stage of PN reduction. It is clearly seen from Figure 5 that the proton yield during each of the three mentioned stages depends strongly on the weight fraction of the TiO2 nanoparticles. As expected, the main yield of the protons is observed in the first two stages (H1 and H2), when reaction 2 is dominant in the reaction media. However, this reaction can play a significant role during the time of the dominance of the consecutive reaction, reaction 3, especially at high TiO2 concentrations (Figure 5a, curve 5). The effect of the weight content of the TiO2 nanoparticles on the molar fraction of the protons of the stages is presented in Table 2. There is no doubt that the change in the main kinetic relations will affect the properties of PANI, for example, an important polymer feature such as its MW. Indeed, we have found that the Mw of PANI synthesized in absence of TiO2 is 56 000 (polydispersity Mw/Mn = 3.9) (Table 2). This PANI feature is comparable to that of PANI typically synthesized in HCl solution (Mw ≈ 53 000 and PDI ≈ 2.08).56 However, with F


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by the project “The formation, properties, and interactions of nanocomposites of conducting polymers and bioactive compounds in heterophase systems” of the program of fundamental research and by the complex scientific-technical program “Sensing devices for medical−ecological and industrial−technological problems: metrology support and trial operation” of the National Academy of Sciences of Ukraine.

the process of nucleation occurs, and the main part of the growth centers of the polymer is formed (Figure 6). It should be noted here that the early report on controlled synthesis of high-MW PANI attempted to determine a correlation between the MW distribution and features of the OCP profile.56 It was found that an increase in the value of the time period between the maximum potential and the potential of 0.55 V (vs SCE) in the falling OCP profile was paralleled with an increase in MW.56 Using this time period for our case as value t(E = 0.55 V) − tmax, we have tried to apply this correlation. However, as one can see from Table 2, our results do not show any correlation of this parameter with the MW.

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4. CONCLUSIONS The enhanced OCP approach has been developed to determine the kinetic parameters of the main stages of the chemical oxidative polymerization of An in the absence and presence of dispersed (nano)particles. It has been shown that using the characteristic features underlying this approach can be also expanded to the pH profiles of the polymerization. This allows division of the stage of PN accumulation into two steps: homogeneous and heterogeneous steps. On the basis of the OCP and pH approaches, we have shown that the reciprocal values of the durations of the stages of the induction period; PN accumulation as a whole, including the homogeneous and heterogeneous steps; and the reduction of PN by An are linear functions of a weight fraction of TiO2 nanoparticles in the reaction medium. On the basis of the relative proton concentration as the function of time, we determine an interrelation between the found kinetic features and MW of PANI. We show, for the first time, that the average MW and MW distribution of PANI depend not only on the content of the dispersed (nano)particles in the polymerization medium but also on the ratio of the rates of the main consecutive stages of the An polymerization (the homogeneous and heterogeneous accumulation of PN and the reduction of PN by An). The obtained data enable us to identify these stages as the steps of initiation, propagation, and termination of the An polymerization.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.6b05944. OCP, its first derivative, and pH profiles of An polymerization under different TiO2/An weight ratios; decay portions of the OCP profiles; double-exponential fitting parameters for Figure S5 (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +38 044 503 3110 (N.A.O.). *E-mail: [email protected]. Phone: +38 044 559 70 03 (A.A.P.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Sergei Mikhaylov acknowledges the Mines Douai and University of Lille 1 for PhD scholarship and Armines for the partial financial support. This work is also partially supported G


Article

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