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NOVEL PVA PROTON CONDUCTING MEMBRANES DOPED WITH POLYANILINE GENERATED BY IN-SITU POLYMERIZATION
Electrochimica Acta 211 (2016) 911–917
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
NOVEL PVA PROTON CONDUCTING MEMBRANES DOPED WITH POLYANILINE GENERATED BY IN-SITU POLYMERIZATION neala a Ana-Maria Albua , Ioana Maiora,* ,1, Cristian Andi Nicolaeb , Florentina Lavinia Boca a b
Politehnica University of Bucharest, Faculty of Applied Chemistry and Materials Science, 1-7 Gh. Polizu Str., 011061 Bucharest, Romania The National Institute for Research & Development in Chemistry and Petrochemistry ICECHIM, 202 Splaiul IndependenÛei, 060021 Bucharest, Romania
A R T I C L E I N F O
Article history: Received 30 January 2016 Received in revised form 18 June 2016 Accepted 19 June 2016 Available online 20 June 2016 Keywords: polymer conducting membranes PVA PANI
A B S T R A C T
The main objective of this study consists in developing a new solid electrolyte gel-based type composite with acrylic acid (AA) and polyvinyl alcohol (PVA). These novel PVA conducting membranes containing polyaniline (PANI) were synthesized via in-situ polymerization of aniline (AN). The morphology and electrical properties of PVA/AA/AN/APS/MBA membranes were investigated by Fourier Transform Infrared Spectroscopy (FT-IR), Ultraviolet–Visible Spectroscopy (UV-Vis), Scanning Electron Microscopy (SEM), Thermogravimetric Analysis (TGA) and by Electrochemical Impedance Spectroscopy (EIS) respectively. These characterization studies reveal structural interactions between PANI and PVA due to channeling microstructure organization which thereby lead to an increase in proton conductivity up to 80%. Thus obtained PANI–PVA novel membranes could be used in semiconductor type applications. ã 2016 Elsevier Ltd. All rights reserved.
1. INTRODUCTION Polymer blending is one of the important ways for development of new polymeric materials with dedicated applications at the same time being a technique for designing materials with a wide variety of properties. The use of polymers in order to achieve materials with dedicated applications is based on their advantages related to compositional and structural flexibility which favour the accurate adjustment of the functional properties of the final material. The material systems including polymers credited with transport properties, similar to the common ionic liquid solutions, are defined as polymer electrolyte membranes (PEM) and are characterized, at the same time, by processing facility and high reliability, being one of the key-elements in fuel cell technology. The requirements for polymers used in PEM composition relate to: (i) high ionic conductivity both ambient as sub-ambient environment, (ii) good mechanical strength, (iii) thermal and electrochemical stability and (iv) good compatibility with electrodes [1–4]. Although PEM were discovered in 1973 by Fenton [5], this technology has been launched starting from 1980, when the attention of researchers in domain has been attracted and the field of applications was extended from lithium batteries to other
* Corresponding author. E-mail address: [email protected] (I. Maior). ISE member.
1
http://dx.doi.org/10.1016/j.electacta.2016.06.098 0013-4686/ã 2016 Elsevier Ltd. All rights reserved.
electrochemical devices [6]. There are three stages in the development of PEM: (i) dry solid polymer; (b) gel polymer electrolyte and (iii) polymer electrolyte composite. The first category is credited with high thermal resistance but only with 105 mS/cm conductivity. These poor performances of the electrochemical cell (only 200–300 cycles) were explained by the lower constitutive conductivity. The gel electrolytes and the composite systems show similar solid-state cohesive properties (which confers durability and superiority at mechanical level) keeping the diffusive characteristics of the liquid state [6,7]. By consequence, these material assemblies present superior thermal and mechanical properties, what recommend them for fuel cell applications. These two polymer electrolytes categories enable the improvement of the intrinsic conductivity by doping with electrochemically inert filler, such as particles with large specific surface: ZrO2, TiO2, Al2O3, silica [8–15]. The fillers increase the ionic conductivity at low temperatures and, at the same time, enhance the stability at the electrodes interface [16–24]. A large series of polymers can be used as host in preparation of PEM. The polyvinyl alcohol (PVA) films are known for their properties such as: high tensile and impact strength, resistance to alkalis, oils and solvents action, bio-inertia, biocompatibility, hydrophilic nature and for their ionic conduction abilities, what recommended them for various applications [25–34]. Their high density of hydrogen bonds favours the cross-linked structure of hydrogels, characterized by selective permeability retrieved in native or composite forms which make them applicable in the
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biomedical, biochemical, agricultural or energy production field and also in wastewater treatment [35–41]. Moreover, the recent studies reveal that the optical and thermal properties of the PVA can be controlled by doping for different applications [42–44]. An explosive diversification of the range of materials dedicated to solar cells and to high reliability capacitors was recorded after 70’s, when the researchers’ interest has focused in order to obtain conductive polymers. Polyaniline (PANI), by its specific electrical, electrochemical and optical properties, as its good stability, is recommended for manufacturing of materials with applications in energy processing and storage (alternative energy sources, storage equipments, nonlinear optics, shielding of electromagnetic interference) and also those for catalysis, sensors, antistatic coatings or controlled morphology membranes [44–49]. The improvement and stabilization of conductive properties of PANI were the consequences of the development in material studies in the presence of dopants that through physico-chemical interactions with PANI sequences allow the tuning and control of conductivity. The studies that are using, either by doping or through structuring phase separation in blending, dodecyl-sulphonic acid, camphoric acid, poly(ethylene sulphonic) acid, poly(styrene sulphonate), poly (metha)acrylic acid, poly(2-acrylamido-2-methyl-1-propane) acid, polyamic acid, and so on [50–54] are well-known. Using these three classes of materials, our study proposes the synthesis and characterization of the new solid electrolyte architectures where the matrix is PVA blended in-situ with poly (acrylic acid) and PANI.
88% hydrolyzed have been used as host. The acrylic acid (AA) from Aldrich was used as monomer after distillation at reduced pressure; the aniline (AN) as precursor of dopant polyaniline (PANI) and the N, N-methylene-bis-acrylamide (MBSA) as crosslinked agent. Other used materials were KOH (Merck) for reaction medium preparation and ammonium persulphate (APS) (Aldrich) as initiator. The PANI dopant was generated in-situ simultaneously with the AA polymerization. The solid membranes were obtained through the display of solution on glass support. All membranes were characterized by the specific absorptions using: the FT-IR spectroscopy a Brucker VERTEX 70 apparatus fitted with an ATR diamond (Harrick MVP2 diamond) device; the UV–Vis spectroscopy a Jasco 670 Spectrometer provided with a diffuse reflectance integrating sphere, between 220–1000 nm wavelengths. The surface morphology was evaluated by Scanning Electron Microscopy (SEM) with a Hitachi S–2600 N device. Thermal stability was assessed by thermogravimetric analysis using a TA Instruments TGA Q5000IR operated in dynamic conditions with a heating rate of 10 C/min to 700 C, in platinum pans and in nitrogen atmosphere (50 mL/min). Electrochemical Impedance Spectroscopy measurements were performed using a VoltaLab 40 system, provided with VoltaMaster 4 software, in the frequency range of 100 kHz–50 mHz, with the amplitude of the perturbation potential of 10 mV. The used electrolysis cell consists of two plan-parallel platinum disk electrodes (1.606 cm2 active area), housed inside the jaws of a digital precision micrometer which enables an accurate evaluation of film thickness during conductivity measurements [55].
2. EXPERIMENTAL PART 3. RESULTS AND DISCUSSIONS 2.1. Materials and apparatus 3.1. Membranes preparation In order to prepare the membranes, three types of PVA (Aldrich): PVA1: Mw = 89,000 98,000; 99% hydrolyzed; PVA2: Mw = 31,000 50,000; 99% hydrolyzed and PVA: Mw 25,000,
The experimental procedure for the membranes preparation is shown in Fig. 1.
Fig. 1. The experimental procedure for membranes preparation.
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3.2. Characterization of synthesized membranes 3.2.1. FT-IR Spectroscopy FT-IR spectroscopy is one of the most important methods for materials characterization because it provides semi-quantitative information on the specific group found in the material composition as well as on the measure of polymer–polymer or polymer– dopant interaction. In general, the polymer blend shows a characteristic spectrum where the vibrational bands characterizing each polymer are predominant. The FT-IR spectra of composites are presented in Fig. 2. As is well known, the pure PANI is characterized by an intense absorption at 3430 cm1 [54] and PVA at 3385 cm1 (Fig. 2). Thus, all analyzed materials are characterized by specific peaks of PVA, PANI, PAA, as well as additional peaks specific to PVA–PAA– PANI interactions: at 3744 cm1 and 3748 cm1 respectively are visible absorption of the NH stretching vibration; the characteristic peaks for stretching vibration of OH involved in hydrogen bonds can be found at 3675 cm1 and 3683 cm1; the absorptions assigned to the vibration of the aromatic ring appear at 3024 and 3053 cm1 respectively. The region 2933–2967 cm1 characterizes the specific vibrations of alkyl groups (CH and CH2), while at 1742 and 1754 cm1 respectively are found the vibrations for the carbonyl group, derived from the acrylic structure. The signals at 1598 and 1585 cm1 were assigned to the chinoid ring, at 1509– 1510 cm1 to the benzenoid ring, whereas at the 1459 and 1460 cm1 are revealed the specific signals of CN vibrations in chinoid ring of aniline. The signals from 1368 cm1 and 1367 cm1 are specific for plan bending stretching of the sequences CO and O H; at 1216–1217 cm1 out of plan bending vibration, specific to C N group, occurs. Only the PVA1 samples show absorption at 3288 cm1 for the stretching vibration of the free HO group. 3.2.2. UV–Vis Spectroscopy UV-Vis Spectroscopy characterizes the electronic excitations between the energy levels related to the molecular orbital of the
Fig. 3. UV–vis absorption spectra: a) original absorption; b) subtracted spectra: — PVA2; --- PVA1; — PVA.
Fig. 2. FT-IR spectra for the following membranes: a) PVA2; b) PVA1; c) PVA (88%).
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system. The UV–Vis spectra of all films were recorded at room temperature in the range of 200–1000 nm as shown in Fig. 3. In pure PVA spectrum, the peak shoulder around 293 nm is assigned to the existence of associated carbonyl groups and the large peak centered on 330 nm may be due to p ! p* (K–band) and n ! p* (R–band) electronic transitions respectively. The UV–Vis spectra of PANI–PVA–PAA membranes show absorption peaks in the 250–300 nm domain which is due to the superposition of the electronic transitions of PVA with the p ! p* transition of the benzenoid rings, much better highlighted in the subtracted spectra (Fig. 3b). The peak between 300 and 400 nm (362; 359 nm) is attributed to localized polarons, which are essentially the characteristics of protonated PANI. The two absorption peaks at 595.5 nm (PVA2) and 588.5 nm (PVA2), respectively, are attributed to the characteristic peaks of undoped PANI [56–58]. The peaks at 595.5 and 588.5 nm, respectively, are specific to the polarons of the protonated form of PANI. The bipolaronic states of PANI are revealed in the absorption at 741 nm and 860 nm respectively, with a tail up to 1000 nm. Subtracting the PVA pure spectrum from the membranes spectra (Fig. 3b) it becomes much more obvious the distribution of mentioned absorptions. Moreover, some differences in the polaron band at wavelengths longer than 700 nm are observed: in the PVA1 membrane two broad peaks found at 741 and 888 nm, while for PVA2 is one peak found at 858 nm. These peaks were assigned to the bio-polaronic state caused by doping of PANI. The presence of the polaron/bipolaron band in the UV–Vis spectra of membranes reflects a dopant–polymer interaction and outlines a potential increase in conductivity level.
3.2.3. Thermo-gravimetric analysis The thermal degradation behaviour of PVA and their blend samples PVA1, PVA2 were examined by TGA as shown in Fig. 4 and Table 1. From derivative TG curve shown for each sample, the differences in thermal decomposition are observed. The initial weight loss observed (40–180 C) is due to the moisture evaporation. In the next decomposition stages (180–280 C) a major mass loss for all three materials (35% PVA; 24% PVA2 and 26% PVA1) is attributed to structural decomposition by chain stripping reaction. For pure PVA this mass loss has the maximum value at 222 C (35%) and it is assimilated to the physical transition and the degradation temperatures of polymers. Therefore, the higher values of weight indicate the chemical degradation process resulting from bond scission (C C bonds) in the polymeric backbone. The degradation behaviour of the synthesized blend films is superior to the pure components one (with maximum around 229 C for PVA2 and 235 C for PVA1). At Table 1 Thermic ranges and mass losses. T, ( C)
PVA
PVA1
PVA2
start
end
peak
% mass loss
peak
% mass loss
peak
% mass loss
40 180 280 380 520 residue
180 280 360 520 700 700
89 222.4 – 426.7 –
7.9 35.2 – 21.9 2.4 32.5
– 228.5 318.8 442.2 –
14.9 24.1 10.5 16.1 2.3 32.1
68.3 234.5 318.8 442.2 –
11.9 26.5 10.7 16.3 2.3 32.2
Fig. 4. TGA curves recorded for: a) PVA; b) PVA2; c) PVA1.
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Fig. 5. SEM micrographs recorded for: a) PVA1; b) PVA2.
stripping elimination of H2O. The third decomposition stage at 360 C, specific only for composite blends, is attributed to the loss of PANI sequences, which has practically the same value (10%) in both cases. It is remarkable the fact that the loss weight decreases with increasing the molar mass of the PVA used in preparation
the same time, it may corresponds to the breaking of the intermolecular ester linkages: if PVA is heated above 120 C, water is eliminated to result conjugated double bonds, and it may lead to the formation of ether cross-links; i.e. on heating PVA above the decomposition temperature, the polymer starts a rapid chain-
110
A1 A2 Nafion 112
100 90
-Zi (kohm·cm²)
80 70 60 50 40 30 20
a)
10 0 0
10
20
30
40
50
Zr (kohm·cm²)
200
160 PVA1 edge * PVA1 edge * PVA1 middle
140
PVA2 middle * PVA2 middle * PVA2 middle 24 h PVA2 edge * PVA2 edge 24 h
180 160
120
-Zi (kohm·cm²)
-Zi (kohm·cm²)
140
100 80 60 40
120 100 80 60 40
b)
20
c)
20
0
0
0
10
20
30
40
Zr (kohm·cm²)
50
60
70
0
10
20
30
40
50
60
Zr (kohm·cm²)
Fig. 6. Nyquist impedance plots for the following membranes: a) A1and A2 compared to Nafion 112; b) PVA1; c) PVA2.
70
80
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Table 2 The electrochemical parameters for the basic material A1 and A2 and for the PVA1, PVA2 composites. Membrane type
A1
A2
Nafion 112
PVA1 edge
PVA1a edge
PVA1a middle
PVA2 middle
PVA2a middle
PVA2 edge
PVA2a edge
Thickness d, (cm) Ohmic resistance R1, (ohmcm2) Ionic conductivity s, (mScm1)
0.092 116.4 0.786
0.114 157.6 0.720
0.156 53.43 2.92
0.019 44.87 0.423
0.0038 16.57 0.229
0.038 42.60 0.880
0.063 49.70 1.276
0.072 65.00 1.1061
0.048 44.57 1.066
0.032 10.90 2.92
a
Hydrated.
blends. The last stage at 500 C is characterized by the low mass loss which denotes the completion of dehydrogenation stage with formation of the char. At the same time it defines the residues contribution, which keeps the same value. As a conclusion, the thermal stability of PVA is not major affected (only a little shift at the third stage level) and the formation of char is due only to the PVA degradation in all analyzed samples. Moreover, the increasing of the maximum decomposition temperatures with increasing in molar mass of PVA sort used in the composites synthesis (MPVA1 MPVA2) has suggested that the strong interaction between chains by physical bond or cross-linking mediated by in-situ reaction generates sequences of poly(acrylic acid) and PANI. 3.2.4. Scanning Electron Microscopy SEM The obtained composite membranes show a random distribution of PANI aggregates (see Fig. 5). It appears that PANI is deposited both on the surface and in bulk of the PVA hydrogel. The phase separation is conspicuous in the PVA1 sample which used the PVA with high molar mass (MW = 89,000 98,000) in the composition of continuous phase. This fact is evident if it is considered that the PVA interact with acrylic and aniline sequences by hydrogen bonding: their density is higher if the length of the macromolecular chains is greater. 3.2.5. Electrical properties In Fig. 6 and Table 2 the electrical characteristics of synthesized composites are highlighted. The measurements accomplished for the basic materials A1, A2 (without PANI) reveals a good conduction for such materials, only an order of magnitude lower than Nafion 112, which is considered as standard in terms of a much smaller film thicknesses. The composites PVA1 and PVA2 are characterized by differentiated conduction values according to the region where the measurement was accomplished. Thus, from Table 2 can be noticed the following: - the conductivity values at the edge area of dry and/or hydrated membranes are lower than the ones from central areas, in the context of smaller thickness - in all areas of the membranes, the hydration process induces increases in conduction values nearest to the value of Nafion 112 (Table 2), correlated with optical properties of materials: the highest conductivity value corresponds to PVA2, where UV–Vis spectroscopy also indicates the presence of bipolaronic sequences in higher concentration - the hydration of membranes leads to the systematic increase of conductivity, virtually doubling the values in dry state. In structural terms it can be explained by the opening of channels induced by cross-linking and by increasing the degree of freedom of PANI sequences with the induction of short range order based on Coulomb type interactions. This characteristic is shared by many materials for which the conductive particle size is reduced, that results in an increase of local order [59,60]. - the use of PVA as a support with lower hydrolysis degree (88%) does not diminish the conduction values as would be expected, furthermore it improves them; in the case of PVA2 the conductivity value is significantly increasing in the edge zone.
In conclusion, the increasing of the conductivity in hydration state is the consequence of a significantly higher charge transfer in the PANI areas that through local interaction with the polymer sequence determines the occurrence of areas of network structural distortions; these distortions lead to the emergence of negative charges at the chains level, whose consequence is the appearance of polaron and bipolaron forms in the material structure. The increase in local density of these forms results in an increase of conductivity. On the other hand, the value of the molecular weight of the PVA used as the dispersion matrix for PANI is important in terms of PANI particle size and distribution. At low molecular weight, the emergence of segregation areas of PANI with smaller sized particles is favoured, which promotes the increasing of local order degree and therefore a better charge transfer, resulting in a higher conductivity. 4. CONCLUSIONS The experimental measurements presented in this paper demonstrate the possibility of obtaining composite membranes with in-situ generation of charge carriers. The synthesized membranes were characterized through FT-IR and UV–Vis spectroscopy, by SEM, TGA and electrical measurements. The results of characterization have emphasized the peculiarities of material structuring and organizing. Thus, the FT-IR spectra indicate complex interactions at the level of molecular constituent sequences: PVA, AA and PANI, which is found in material structure both in the chinoid form as well as in the benzenoid one. The UV–Vis investigations provide preliminary results for ionic structure, supporting the values obtained from the measurement of electrical conductivity. By evaluating the thermal resistance, the interactions noticed in FT-IR spectra are highlighted, reflecting the degradation of composites according to a mechanism similar to the one of the support polymer, without major changes in the specific thermal resistances. The conductive measurements reveal high levels of ionic conductivity for such materials, which recommend their use in semiconductor type applications. Completing the obtained results requires in the first place morphological and textural investigations, and on the other hand, enhancing a method for uniform thickness deposition. References [1] W. Schmittinger, A. Vahidi, A Review of the Main Parameters Influencing LongTerm Performance and Durability of PEM Fuel Cells, J. Power Sources 180 (1) (2008) 1–14. [2] G. Merle, M. Wessling, K. Nijmeijer, Anion Exchange Membranes For Alkaline Fuel Cells: A Review, J. Membr. Sci. 377 (1–2) (2011) 1–35. [3] Y. Wang, K.S. Chen, J. Mishler, S.C. Cho, X.C. Adroher, A Review of Polymer Electrolyte Membrane Fuel Cells: Technology, Applications and Needs on Fundamental Research, Appl. Energy 88 (2011) 981–1007. [4] E. Antolini, E.R. Gonzalez, Polymer Supports for Low-Temperature Fuel Cell Catalysts, Appl. Catal. A: General 365 (2009) 1–19. [5] D.E. Fenton, J.M. Parker, P.V. Wright, Complexes of Alkali Metal Ions with Poly (ethyleneoxide), Polym. 14 (1973) 589.
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NOVEL PVA PROTON CONDUCTING MEMBRANES DOPED WITH POLYANILINE GENERATED BY IN-SITU POLYMERIZATION neala a Ana-Maria Albua , Ioana Maiora,* ,1, Cristian Andi Nicolaeb , Florentina Lavinia Boca a b
Politehnica University of Bucharest, Faculty of Applied Chemistry and Materials Science, 1-7 Gh. Polizu Str., 011061 Bucharest, Romania The National Institute for Research & Development in Chemistry and Petrochemistry ICECHIM, 202 Splaiul IndependenÛei, 060021 Bucharest, Romania
A R T I C L E I N F O
Article history: Received 30 January 2016 Received in revised form 18 June 2016 Accepted 19 June 2016 Available online 20 June 2016 Keywords: polymer conducting membranes PVA PANI
A B S T R A C T
The main objective of this study consists in developing a new solid electrolyte gel-based type composite with acrylic acid (AA) and polyvinyl alcohol (PVA). These novel PVA conducting membranes containing polyaniline (PANI) were synthesized via in-situ polymerization of aniline (AN). The morphology and electrical properties of PVA/AA/AN/APS/MBA membranes were investigated by Fourier Transform Infrared Spectroscopy (FT-IR), Ultraviolet–Visible Spectroscopy (UV-Vis), Scanning Electron Microscopy (SEM), Thermogravimetric Analysis (TGA) and by Electrochemical Impedance Spectroscopy (EIS) respectively. These characterization studies reveal structural interactions between PANI and PVA due to channeling microstructure organization which thereby lead to an increase in proton conductivity up to 80%. Thus obtained PANI–PVA novel membranes could be used in semiconductor type applications. ã 2016 Elsevier Ltd. All rights reserved.
1. INTRODUCTION Polymer blending is one of the important ways for development of new polymeric materials with dedicated applications at the same time being a technique for designing materials with a wide variety of properties. The use of polymers in order to achieve materials with dedicated applications is based on their advantages related to compositional and structural flexibility which favour the accurate adjustment of the functional properties of the final material. The material systems including polymers credited with transport properties, similar to the common ionic liquid solutions, are defined as polymer electrolyte membranes (PEM) and are characterized, at the same time, by processing facility and high reliability, being one of the key-elements in fuel cell technology. The requirements for polymers used in PEM composition relate to: (i) high ionic conductivity both ambient as sub-ambient environment, (ii) good mechanical strength, (iii) thermal and electrochemical stability and (iv) good compatibility with electrodes [1–4]. Although PEM were discovered in 1973 by Fenton [5], this technology has been launched starting from 1980, when the attention of researchers in domain has been attracted and the field of applications was extended from lithium batteries to other
* Corresponding author. E-mail address: [email protected] (I. Maior). ISE member.
1
http://dx.doi.org/10.1016/j.electacta.2016.06.098 0013-4686/ã 2016 Elsevier Ltd. All rights reserved.
electrochemical devices [6]. There are three stages in the development of PEM: (i) dry solid polymer; (b) gel polymer electrolyte and (iii) polymer electrolyte composite. The first category is credited with high thermal resistance but only with 105 mS/cm conductivity. These poor performances of the electrochemical cell (only 200–300 cycles) were explained by the lower constitutive conductivity. The gel electrolytes and the composite systems show similar solid-state cohesive properties (which confers durability and superiority at mechanical level) keeping the diffusive characteristics of the liquid state [6,7]. By consequence, these material assemblies present superior thermal and mechanical properties, what recommend them for fuel cell applications. These two polymer electrolytes categories enable the improvement of the intrinsic conductivity by doping with electrochemically inert filler, such as particles with large specific surface: ZrO2, TiO2, Al2O3, silica [8–15]. The fillers increase the ionic conductivity at low temperatures and, at the same time, enhance the stability at the electrodes interface [16–24]. A large series of polymers can be used as host in preparation of PEM. The polyvinyl alcohol (PVA) films are known for their properties such as: high tensile and impact strength, resistance to alkalis, oils and solvents action, bio-inertia, biocompatibility, hydrophilic nature and for their ionic conduction abilities, what recommended them for various applications [25–34]. Their high density of hydrogen bonds favours the cross-linked structure of hydrogels, characterized by selective permeability retrieved in native or composite forms which make them applicable in the
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biomedical, biochemical, agricultural or energy production field and also in wastewater treatment [35–41]. Moreover, the recent studies reveal that the optical and thermal properties of the PVA can be controlled by doping for different applications [42–44]. An explosive diversification of the range of materials dedicated to solar cells and to high reliability capacitors was recorded after 70’s, when the researchers’ interest has focused in order to obtain conductive polymers. Polyaniline (PANI), by its specific electrical, electrochemical and optical properties, as its good stability, is recommended for manufacturing of materials with applications in energy processing and storage (alternative energy sources, storage equipments, nonlinear optics, shielding of electromagnetic interference) and also those for catalysis, sensors, antistatic coatings or controlled morphology membranes [44–49]. The improvement and stabilization of conductive properties of PANI were the consequences of the development in material studies in the presence of dopants that through physico-chemical interactions with PANI sequences allow the tuning and control of conductivity. The studies that are using, either by doping or through structuring phase separation in blending, dodecyl-sulphonic acid, camphoric acid, poly(ethylene sulphonic) acid, poly(styrene sulphonate), poly (metha)acrylic acid, poly(2-acrylamido-2-methyl-1-propane) acid, polyamic acid, and so on [50–54] are well-known. Using these three classes of materials, our study proposes the synthesis and characterization of the new solid electrolyte architectures where the matrix is PVA blended in-situ with poly (acrylic acid) and PANI.
88% hydrolyzed have been used as host. The acrylic acid (AA) from Aldrich was used as monomer after distillation at reduced pressure; the aniline (AN) as precursor of dopant polyaniline (PANI) and the N, N-methylene-bis-acrylamide (MBSA) as crosslinked agent. Other used materials were KOH (Merck) for reaction medium preparation and ammonium persulphate (APS) (Aldrich) as initiator. The PANI dopant was generated in-situ simultaneously with the AA polymerization. The solid membranes were obtained through the display of solution on glass support. All membranes were characterized by the specific absorptions using: the FT-IR spectroscopy a Brucker VERTEX 70 apparatus fitted with an ATR diamond (Harrick MVP2 diamond) device; the UV–Vis spectroscopy a Jasco 670 Spectrometer provided with a diffuse reflectance integrating sphere, between 220–1000 nm wavelengths. The surface morphology was evaluated by Scanning Electron Microscopy (SEM) with a Hitachi S–2600 N device. Thermal stability was assessed by thermogravimetric analysis using a TA Instruments TGA Q5000IR operated in dynamic conditions with a heating rate of 10 C/min to 700 C, in platinum pans and in nitrogen atmosphere (50 mL/min). Electrochemical Impedance Spectroscopy measurements were performed using a VoltaLab 40 system, provided with VoltaMaster 4 software, in the frequency range of 100 kHz–50 mHz, with the amplitude of the perturbation potential of 10 mV. The used electrolysis cell consists of two plan-parallel platinum disk electrodes (1.606 cm2 active area), housed inside the jaws of a digital precision micrometer which enables an accurate evaluation of film thickness during conductivity measurements [55].
2. EXPERIMENTAL PART 3. RESULTS AND DISCUSSIONS 2.1. Materials and apparatus 3.1. Membranes preparation In order to prepare the membranes, three types of PVA (Aldrich): PVA1: Mw = 89,000 98,000; 99% hydrolyzed; PVA2: Mw = 31,000 50,000; 99% hydrolyzed and PVA: Mw 25,000,
The experimental procedure for the membranes preparation is shown in Fig. 1.
Fig. 1. The experimental procedure for membranes preparation.
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3.2. Characterization of synthesized membranes 3.2.1. FT-IR Spectroscopy FT-IR spectroscopy is one of the most important methods for materials characterization because it provides semi-quantitative information on the specific group found in the material composition as well as on the measure of polymer–polymer or polymer– dopant interaction. In general, the polymer blend shows a characteristic spectrum where the vibrational bands characterizing each polymer are predominant. The FT-IR spectra of composites are presented in Fig. 2. As is well known, the pure PANI is characterized by an intense absorption at 3430 cm1 [54] and PVA at 3385 cm1 (Fig. 2). Thus, all analyzed materials are characterized by specific peaks of PVA, PANI, PAA, as well as additional peaks specific to PVA–PAA– PANI interactions: at 3744 cm1 and 3748 cm1 respectively are visible absorption of the NH stretching vibration; the characteristic peaks for stretching vibration of OH involved in hydrogen bonds can be found at 3675 cm1 and 3683 cm1; the absorptions assigned to the vibration of the aromatic ring appear at 3024 and 3053 cm1 respectively. The region 2933–2967 cm1 characterizes the specific vibrations of alkyl groups (CH and CH2), while at 1742 and 1754 cm1 respectively are found the vibrations for the carbonyl group, derived from the acrylic structure. The signals at 1598 and 1585 cm1 were assigned to the chinoid ring, at 1509– 1510 cm1 to the benzenoid ring, whereas at the 1459 and 1460 cm1 are revealed the specific signals of CN vibrations in chinoid ring of aniline. The signals from 1368 cm1 and 1367 cm1 are specific for plan bending stretching of the sequences CO and O H; at 1216–1217 cm1 out of plan bending vibration, specific to C N group, occurs. Only the PVA1 samples show absorption at 3288 cm1 for the stretching vibration of the free HO group. 3.2.2. UV–Vis Spectroscopy UV-Vis Spectroscopy characterizes the electronic excitations between the energy levels related to the molecular orbital of the
Fig. 3. UV–vis absorption spectra: a) original absorption; b) subtracted spectra: — PVA2; --- PVA1; — PVA.
Fig. 2. FT-IR spectra for the following membranes: a) PVA2; b) PVA1; c) PVA (88%).
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system. The UV–Vis spectra of all films were recorded at room temperature in the range of 200–1000 nm as shown in Fig. 3. In pure PVA spectrum, the peak shoulder around 293 nm is assigned to the existence of associated carbonyl groups and the large peak centered on 330 nm may be due to p ! p* (K–band) and n ! p* (R–band) electronic transitions respectively. The UV–Vis spectra of PANI–PVA–PAA membranes show absorption peaks in the 250–300 nm domain which is due to the superposition of the electronic transitions of PVA with the p ! p* transition of the benzenoid rings, much better highlighted in the subtracted spectra (Fig. 3b). The peak between 300 and 400 nm (362; 359 nm) is attributed to localized polarons, which are essentially the characteristics of protonated PANI. The two absorption peaks at 595.5 nm (PVA2) and 588.5 nm (PVA2), respectively, are attributed to the characteristic peaks of undoped PANI [56–58]. The peaks at 595.5 and 588.5 nm, respectively, are specific to the polarons of the protonated form of PANI. The bipolaronic states of PANI are revealed in the absorption at 741 nm and 860 nm respectively, with a tail up to 1000 nm. Subtracting the PVA pure spectrum from the membranes spectra (Fig. 3b) it becomes much more obvious the distribution of mentioned absorptions. Moreover, some differences in the polaron band at wavelengths longer than 700 nm are observed: in the PVA1 membrane two broad peaks found at 741 and 888 nm, while for PVA2 is one peak found at 858 nm. These peaks were assigned to the bio-polaronic state caused by doping of PANI. The presence of the polaron/bipolaron band in the UV–Vis spectra of membranes reflects a dopant–polymer interaction and outlines a potential increase in conductivity level.
3.2.3. Thermo-gravimetric analysis The thermal degradation behaviour of PVA and their blend samples PVA1, PVA2 were examined by TGA as shown in Fig. 4 and Table 1. From derivative TG curve shown for each sample, the differences in thermal decomposition are observed. The initial weight loss observed (40–180 C) is due to the moisture evaporation. In the next decomposition stages (180–280 C) a major mass loss for all three materials (35% PVA; 24% PVA2 and 26% PVA1) is attributed to structural decomposition by chain stripping reaction. For pure PVA this mass loss has the maximum value at 222 C (35%) and it is assimilated to the physical transition and the degradation temperatures of polymers. Therefore, the higher values of weight indicate the chemical degradation process resulting from bond scission (C C bonds) in the polymeric backbone. The degradation behaviour of the synthesized blend films is superior to the pure components one (with maximum around 229 C for PVA2 and 235 C for PVA1). At Table 1 Thermic ranges and mass losses. T, ( C)
PVA
PVA1
PVA2
start
end
peak
% mass loss
peak
% mass loss
peak
% mass loss
40 180 280 380 520 residue
180 280 360 520 700 700
89 222.4 – 426.7 –
7.9 35.2 – 21.9 2.4 32.5
– 228.5 318.8 442.2 –
14.9 24.1 10.5 16.1 2.3 32.1
68.3 234.5 318.8 442.2 –
11.9 26.5 10.7 16.3 2.3 32.2
Fig. 4. TGA curves recorded for: a) PVA; b) PVA2; c) PVA1.
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Fig. 5. SEM micrographs recorded for: a) PVA1; b) PVA2.
stripping elimination of H2O. The third decomposition stage at 360 C, specific only for composite blends, is attributed to the loss of PANI sequences, which has practically the same value (10%) in both cases. It is remarkable the fact that the loss weight decreases with increasing the molar mass of the PVA used in preparation
the same time, it may corresponds to the breaking of the intermolecular ester linkages: if PVA is heated above 120 C, water is eliminated to result conjugated double bonds, and it may lead to the formation of ether cross-links; i.e. on heating PVA above the decomposition temperature, the polymer starts a rapid chain-
110
A1 A2 Nafion 112
100 90
-Zi (kohm·cm²)
80 70 60 50 40 30 20
a)
10 0 0
10
20
30
40
50
Zr (kohm·cm²)
200
160 PVA1 edge * PVA1 edge * PVA1 middle
140
PVA2 middle * PVA2 middle * PVA2 middle 24 h PVA2 edge * PVA2 edge 24 h
180 160
120
-Zi (kohm·cm²)
-Zi (kohm·cm²)
140
100 80 60 40
120 100 80 60 40
b)
20
c)
20
0
0
0
10
20
30
40
Zr (kohm·cm²)
50
60
70
0
10
20
30
40
50
60
Zr (kohm·cm²)
Fig. 6. Nyquist impedance plots for the following membranes: a) A1and A2 compared to Nafion 112; b) PVA1; c) PVA2.
70
80
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Table 2 The electrochemical parameters for the basic material A1 and A2 and for the PVA1, PVA2 composites. Membrane type
A1
A2
Nafion 112
PVA1 edge
PVA1a edge
PVA1a middle
PVA2 middle
PVA2a middle
PVA2 edge
PVA2a edge
Thickness d, (cm) Ohmic resistance R1, (ohmcm2) Ionic conductivity s, (mScm1)
0.092 116.4 0.786
0.114 157.6 0.720
0.156 53.43 2.92
0.019 44.87 0.423
0.0038 16.57 0.229
0.038 42.60 0.880
0.063 49.70 1.276
0.072 65.00 1.1061
0.048 44.57 1.066
0.032 10.90 2.92
a
Hydrated.
blends. The last stage at 500 C is characterized by the low mass loss which denotes the completion of dehydrogenation stage with formation of the char. At the same time it defines the residues contribution, which keeps the same value. As a conclusion, the thermal stability of PVA is not major affected (only a little shift at the third stage level) and the formation of char is due only to the PVA degradation in all analyzed samples. Moreover, the increasing of the maximum decomposition temperatures with increasing in molar mass of PVA sort used in the composites synthesis (MPVA1 MPVA2) has suggested that the strong interaction between chains by physical bond or cross-linking mediated by in-situ reaction generates sequences of poly(acrylic acid) and PANI. 3.2.4. Scanning Electron Microscopy SEM The obtained composite membranes show a random distribution of PANI aggregates (see Fig. 5). It appears that PANI is deposited both on the surface and in bulk of the PVA hydrogel. The phase separation is conspicuous in the PVA1 sample which used the PVA with high molar mass (MW = 89,000 98,000) in the composition of continuous phase. This fact is evident if it is considered that the PVA interact with acrylic and aniline sequences by hydrogen bonding: their density is higher if the length of the macromolecular chains is greater. 3.2.5. Electrical properties In Fig. 6 and Table 2 the electrical characteristics of synthesized composites are highlighted. The measurements accomplished for the basic materials A1, A2 (without PANI) reveals a good conduction for such materials, only an order of magnitude lower than Nafion 112, which is considered as standard in terms of a much smaller film thicknesses. The composites PVA1 and PVA2 are characterized by differentiated conduction values according to the region where the measurement was accomplished. Thus, from Table 2 can be noticed the following: - the conductivity values at the edge area of dry and/or hydrated membranes are lower than the ones from central areas, in the context of smaller thickness - in all areas of the membranes, the hydration process induces increases in conduction values nearest to the value of Nafion 112 (Table 2), correlated with optical properties of materials: the highest conductivity value corresponds to PVA2, where UV–Vis spectroscopy also indicates the presence of bipolaronic sequences in higher concentration - the hydration of membranes leads to the systematic increase of conductivity, virtually doubling the values in dry state. In structural terms it can be explained by the opening of channels induced by cross-linking and by increasing the degree of freedom of PANI sequences with the induction of short range order based on Coulomb type interactions. This characteristic is shared by many materials for which the conductive particle size is reduced, that results in an increase of local order [59,60]. - the use of PVA as a support with lower hydrolysis degree (88%) does not diminish the conduction values as would be expected, furthermore it improves them; in the case of PVA2 the conductivity value is significantly increasing in the edge zone.
In conclusion, the increasing of the conductivity in hydration state is the consequence of a significantly higher charge transfer in the PANI areas that through local interaction with the polymer sequence determines the occurrence of areas of network structural distortions; these distortions lead to the emergence of negative charges at the chains level, whose consequence is the appearance of polaron and bipolaron forms in the material structure. The increase in local density of these forms results in an increase of conductivity. On the other hand, the value of the molecular weight of the PVA used as the dispersion matrix for PANI is important in terms of PANI particle size and distribution. At low molecular weight, the emergence of segregation areas of PANI with smaller sized particles is favoured, which promotes the increasing of local order degree and therefore a better charge transfer, resulting in a higher conductivity. 4. CONCLUSIONS The experimental measurements presented in this paper demonstrate the possibility of obtaining composite membranes with in-situ generation of charge carriers. The synthesized membranes were characterized through FT-IR and UV–Vis spectroscopy, by SEM, TGA and electrical measurements. The results of characterization have emphasized the peculiarities of material structuring and organizing. Thus, the FT-IR spectra indicate complex interactions at the level of molecular constituent sequences: PVA, AA and PANI, which is found in material structure both in the chinoid form as well as in the benzenoid one. The UV–Vis investigations provide preliminary results for ionic structure, supporting the values obtained from the measurement of electrical conductivity. By evaluating the thermal resistance, the interactions noticed in FT-IR spectra are highlighted, reflecting the degradation of composites according to a mechanism similar to the one of the support polymer, without major changes in the specific thermal resistances. The conductive measurements reveal high levels of ionic conductivity for such materials, which recommend their use in semiconductor type applications. Completing the obtained results requires in the first place morphological and textural investigations, and on the other hand, enhancing a method for uniform thickness deposition. References [1] W. Schmittinger, A. Vahidi, A Review of the Main Parameters Influencing LongTerm Performance and Durability of PEM Fuel Cells, J. Power Sources 180 (1) (2008) 1–14. [2] G. Merle, M. Wessling, K. Nijmeijer, Anion Exchange Membranes For Alkaline Fuel Cells: A Review, J. Membr. Sci. 377 (1–2) (2011) 1–35. [3] Y. Wang, K.S. Chen, J. Mishler, S.C. Cho, X.C. Adroher, A Review of Polymer Electrolyte Membrane Fuel Cells: Technology, Applications and Needs on Fundamental Research, Appl. Energy 88 (2011) 981–1007. [4] E. Antolini, E.R. Gonzalez, Polymer Supports for Low-Temperature Fuel Cell Catalysts, Appl. Catal. A: General 365 (2009) 1–19. [5] D.E. Fenton, J.M. Parker, P.V. Wright, Complexes of Alkali Metal Ions with Poly (ethyleneoxide), Polym. 14 (1973) 589.
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