Effects of solvents on post-irradiation grafting of …Effects of solvents on post-irradiation grafting of styrene onto fluoropolymer films Adriana Napoleão Geraldes,* Heloísa Augusto

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e-Polymers 2007, no. 063 http://www.e-polymers.org

ISSN 1618-7229

Effects of solvents on post-irradiation grafting of styrene onto fluoropolymer films Adriana Napoleão Geraldes,* Heloísa Augusto Zen, Duclerc Fernandes Parra, Henrique Perez Ferreira, Ademar Benévolo Lugão

*Instituto de Pesquisas Energéticas e Nucleares (IPEN), Centro de Química e Meio Ambiente (CQMA), Av Professor Lineu Prestes, 2242, 05508-900, São Paulo, Brazil; fax: +55 11 3031-9249; e-mail: [email protected] (Received: 27 August, 2007; published: 11 May, 2008)

Abstract: Due to the interest in the development of low cost membranes with similar capabilities to Nafion®, grafting of styrene on fluorinated polymers by simultaneous irradiation and pre-irradiation methods at high temperature (higher than 50 °C) followed by sulfonation has been a process studied broadly. However, there is very little information about grafting yield at room temperature after simultaneous irradiation. Previous works reported that the surface morphology of the styrene grafts has showed an uneven profile under the studied conditions. This work aims to evaluate the grafting of styrene onto these films after simultaneous irradiation (in post-irradiation condition), using different solvent in periods until 28 days at room temperature. Films of poly(tetrafluoroethylene) (PTFE) and poly(vinylidene fluoride) (PVDF) were immersed in styrene/toluene 1:4 or styrene/methanol 1:4 and then submitted to gamma radiation at 40 and 80 kGy doses. The degree of grafting (DOG) was determined gravimetrically and physical and chemical changes were evaluated by infrared spectroscopic analysis (FTIR), differential scanning calorimeter analysis (DSC), thermogravimetric analysis (TGA), and also by scanning electron microscopy (SEM). For PTFE films, the highest DOG was achieved using toluene as solvent while for PVDF films, the highest DOG was achieved with methanol. Surface images of the grafted films by SEM technique have presented a strong effect of the solvents on the films morphology. Finally, PVDF-g-PS in styrene/methanol film revealed higher DOG, although the surface showed small cavities and discontinuities. On the contrary, the surface of PTFE-g-PS in styrene/toluene presented homogeneous surface.

Introduction There is considerable interest in proton exchange membrane fuel cells (PEMFCs) for their applications in electric utility, portable power and transportation. The proton exchange membrane (PEM) is a vital component in this type of fuel cell; it acts as a separator to prevent mixing of the reactant gases and, as an electrolyte, performs protons transportation from the anode to the cathode. Nafion® by Dupont is the most frequently used PEM material because of its chemical stability and commercial availability. However, in face of costs, efforts have been focused in the development of lower cost membrane materials. Radiation induced grafting of monomers into fluorinated membranes was designed as an alternative route to obtain the proton conduction membranes for PEMFCs applications. Generally, there are two methods for radiation induced grafting: i) Pre-irradiation: the base polymer films is irradiated and in the sequence monomer is added and grafted [1]; ii) Simultaneous irradiation: the base polymer films are soaked

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with monomer solution and then irradiated [2]. Nasef et al has reviewed the preparation of various types of ion exchange membranes by radiation induced grafting copolymerization [3]. A membrane to perform as polymeric electrolyte needs to have a combination of requirements in order to maintain good separation and electrochemical capabilities. These include high ionic conductivity, low swelling behaviour, high chemical resistance and mechanical integrity. Many groups all over the world have studied commercial fluoropolymer films which were irradiated, grafted with styrene and sulfonated [1, 4-12]. Novel proton-conducting membranes for hydrogen and methanol for fuel cells were developed by G. G. Scherer group in Paul Scherrer Institute [4-8]. The radiation grafting of monomers onto various trunk polymers (e.g., FEP, ETFE, etc.) with subsequent sulfonation of the grafted side chains and 4,000 hours membrane lifetimes was studied. Other authors have also investigated the grafting of styrene or mixtures of styrene and other monomers on fluorinated polymers as well as the structural modification of fluoropolymers by radiation crosslinking. Li et al. have studied PTFE-g-PS (grafting polystyrene), PTFE-g-AMS/STY (grafting α-methylstyrene/styrene), PTFE-g-PS/DVB (styrene/divinylbenzene) and radiation-crosslinked poly(tetrafluoroethylene), (RX-PTFE) films with further grafting and sulfonation [9-12]. Horsfall et al. investigated a range of hydrocarbon and fluorocarbon polymers at two different methods of irradiation [13]. Nasef et al., studying poly(tetrafluoroethylene)-graft-polystyrene sulfonic acid (PTFE-g-PSSA), found changes on polymer structure and on physical-chemical properties as a direct result of irradiation [3]. In addition, Kallio et al studied the effect of different thickness of PVDF films by radiation grafting with styrene followed by sulfonation [14]. Among the properties of membranes, thermal stability is the most important. It determines the operating temperature of the membrane electrochemical cell. Thermal stability of polystyrene grafted films prepared by radiation grafting onto PTFE [15-17], tetrafluoroethylene-co-hexafluoropropylene, FEP [18], PVDF [19, 20], and poly(tetrafluoroethylene-co-perfluorovinyl ether (PFA) was studied using TGA and DSC under nitrogen atmosphere. Scanning electron microscopy (SEM) provides information on the morphology of grafted copolymer systems [21]. Grafting conditions are playing an important role when determining the degree of grafting and the structure built up inside the grafted polymer [22]. In addition, the solvent to be used for monomer dilution is of special interest as it is one of the essential elements towards successful radiation-induced grafting processes. Solvents are usually used during grafting to bring about swelling of the base polymer, therefore increasing the monomer accessibility to the grafting sites. The use of a poor solvent is most likely to lead to surface grafting due to the reduction in monomer diffusibility and, eventually, low degrees of grafting are obtained. However, the use of good solvent results in homogeneous grafting in the bulk. Increasing the swelling, enhances the diffusion of monomer into the internal layers of the polymer substrate, and thus the interactions between the internal active sites and the monomer molecules increase, leading to higher degrees of grafting. Some works have reported the effect of solvents in the grafting styrene onto fluorinated polymer [22-25]. Cardona et al. and Nasef observed an increase of styrene graft for PFA in benzene or dichloromethane when compared with methanol. [24, 25]. The viscosity of the

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substrates controls the diffusion of the monomer to the grafting sites. The solvent, on its turn, was useful to induce swelling of the grafted layers, enhancing the monomer acessibility to grafting sites [23]. That being so, the choice of solvent is one of the key parameters to control the grafting reaction. Differences between toluene and methanol are indicated by physical parameters. The solubility parameters [22] for toluene and methanol solvents found at the literature are 18.2 MPa and 29.7 MPa. The solubility parameters for polystyrene, PTFE and PVDF are 17.5 MPa, 12.6 MPa and 23.2 MPa, respectively. Then, according to these parameters it is expected that the grafting reaction of styrene will be promoted in the toluene solution for PTFE films because they have similar solubility parameter and in the methanol solution will be favored by the grafting yield for PVFD films. The chain transfer constant of methanol is high (0.29) [22], which cause a fast termination in polystyrene growing chains. Besides, as the polystyrene homopolymer formed is insoluble in methanol, there will be a small dilution of the grafted layer. Consequently, low grafting yield will be obtained. Toluene has low chain transfer constant (0.18) and it acts as solvent for polystyrene, so the grafted layer will be diluted, leading to higher grafting yield. The process of styrene grafting on fluoropolymer is usually conducted at temperatures higher than 50 ºC for high grafting yield [1, 4, 11, 13, 24]. Nonetheless, Walsby et al presented a work on styrene grafting induced by simultaneous irradiation at room temperature, which showed that no polymerization at all takes place. They also reported that the grafts obtained at high temperature were distributed in an uneven way at the surface [1]. The goal of the present work is to study the grafting reaction after simultaneous irradiation, both at room temperature. The post-irradiation time was established at 7, 14, 21 and 28 days; the graft and ungraft samples were characterized by TGA, DSC, FTIR, SEM and gravimetric analysis was employed for degree of grafting (DOG) determination. The effects of solvents on grafting yield and surface morphology were accessed by diluting styrene in toluene or methanol during simultaneous irradiation and post-irradiation. Results and discussion Figures 1 and 2 show the relationship between time and the degree of grafting at 40 and 80 kGy dose in presence of styrene diluted in methanol and toluene. The highest DOG was achieved after 21 days of grafting. Therefore, the study of the solvents effect on the grafting of styrene in methanol and toluene was concentrated in this period of time. Figure 1 shows how the dilution of styrene with toluene increases the degree of grafting in PTFE films when compared with methanol. Figure 2 demonstrates that the dilution of styrene with methanol enhanced the degree of grafting in PVDF films when compared with toluene dilution. After irradiation, the styrene/methanol solution seemed to be more viscous, indicating a higher formation of homopolymer. On the other hand, the styrene/toluene solution showed no change in the solution viscosity. This lack of viscosity change was also observed by Nasef in graft systems with styrene/methanol and styrene/benzene [22]. The results confirmed that the degree of grafting was strongly dependent upon the type of solvent. The choice of solvent to a monomer/substrate combination proved to enhance the yield in the radiation induced

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grafting. The maximum grafting yield in PTFE has been observed with the use of dichloromethane as diluent [26].

7 14 21 28

2

4

6

8

10

12

DO

G /

%

Days

PTFE 40 kGy methanol PTFE 80 kGy methanol PTFE 40 kGy toluene PTFE 80 kGy toluene

Fig. 1. Relation between degree of grafting after irradiation and time for PTFE films at 40 and 80 kGy.

7 14 21 28

2

4

6

8

10

DO

G /

%

Days

PVDF 40 kGy methanol PVDF 80 kGy methanol PVDF 40 kGy toluene PVDF 80 kGy toluene

Fig. 2. Relation between degree of grafting after irradiation and time for PVDF films at 40 and 80 kGy. Methanol, which has a higher chain transfer constant (0.29) than toluene (0.18) [22], caused faster termination of the graft growing polystyrene chains, leading to lower degree of grafting on PTFE. Due to the homopolymer formation, it was also observed an increased viscosity of the grafting solution during irradiation, which leads to a lower diffusion of styrene in PTFE, and therefore decreases the DOG.

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Toluene, on the other hand, has smaller chain transfer constant than methanol. Thus, it can allow a slower termination reaction, yielding higher DOG on PTFE. Better dissolution of the grafted polystyrene chains in the toluene/styrene solution enhances the diffusion of monomer through the grafted into PTFE matrix and, as a result, leads to an increase in the grafting yield for the PTFE films. The results of PVDF grafting suggested the importance of the structure in the reactivity of the matrix. PVDF showed higher graft level than PTFE. The grafting yield results increased in both of solvents, except for the sample in toluene at 80 kGy. These results suggested that matrix viscosity, other than chain transfer constants, as well as the solvent, are important parameters. Such results are in accordance with the findings of Dargarville et al, which reported the grafting of styrene onto PFA in dichloromethane, toluene and methanol. They measured the grafting yield as a function of the concentration of styrene in the grafting solution and have reported that yield appeared not to be dependent on the solvent power for PS alone. They suggested that the solvent, the polymer chain, and the viscosity monomer/solvent system are equally important factors [28, 30].

4000 3500 3000 2500 2000 1500

Tran

smita

nce

/ u.a

.

W avelength / cm-1

PVDF pure PVDF graft

Fig. 3. Infrared transmission spectra of PVDF pure (-----) and grafted (----) at 80 kGy, in the region 4000-1500 cm-1. As shown in figures 1 and 2, the degree of grafting increases with the irradiation dose (40 and 80 kGy). This is a result of the higher formation of free radicals in the grafting system, which subsequently leads to a higher degree of grafting. In order to confirm the grafting of polystyrene in PVDF and PTFE films, FTIR spectra was conducted. The characteristic peaks in the PVDF base polymer (Figure 3) are those near to 3000 cm-1 representing C-H stretching. In infrared spectra of the graft PVDF films, new peaks appeared in the region 3080 - 3010 cm-1 owing to aromatic C-H stretching; 2975 - 2840 cm-1 due to aliphatic C-H stretching; and 1601-1500 cm-1

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attributed to aromatic C=C stretching [31].

4000 3500 3000 2500 2000 1500

Tran

smita

nce

/ u.a

.

Wavelength / cm-1

PTFE pure PTFE graft

Fig. 4. Infrared transmission spectra of PTFE pure (-----) and grafted (----) at 80 kGy, in the region 4000-1500 cm-1. In the FTIR spectra of the graft PTFE films (Figure 4), new peaks appeared in the region 3100 to 3000 cm-1 as a result of =C-H stretching of the styrene groups. The band at 2920 cm-1 is the asymmetric stretching and 2850 cm-1 (symmetric stretching) was attributed to the aliphatic CH2 group of the polystyrene grafts. The band at 1600 cm-1 is the skeletal C=C stretching and 1490 - 1460 cm-1 are the skeletal C=C in plane deformation of polystyrene grafts [6]. TGA results of grafting polystyrene in PVDF and PTFE films present a two-step degradation pattern. The first degradation step is attributed to the degradation of polystyrene grafts and the second can be attributed to film matrix degradation. This is in accordance with the view that the polystyrene grafts have no miscibility with the PTFE or PVDF matrix and form separation phases in micro domains in the grafted polymer [10, 32]. The initial degradation temperature (Tonset) of non grafted PVDF film was 436.9 °C and the polystyrene grafts start to degrade at around 393 °C for PVDF films. The Tonset of the second step, attributed to the matrix (Table 1), is displaced to Tonset 443.7 °C (40 kGy) and 448.1 °C (80 kGy) in methanol and 449.4 °C (40 kGy) in toluene. Those displacement indicated modification to a more stable PVDF matrix. The Tonset for the non grafted PTFE film was 555.4 °C. The polystyrene grafts start to degrade at around 400 °C. The second step is the degradation of PTFE matrix, reported in Table 2. The displacement to Tonset 561.9 °C (40 kGy) and 561.4 °C (80 kGy) in methanol solution; and to 559.7 °C(40 kGy) and 554.8 °C (80 kGy) in toluene solution are in agreement with the literature [11].

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Tab. 1. Experimental temperatures by TGA for PVDF films.

Film DOG (%) Dose (kGy) Tonset (oC) PVDF pure - - 436.9

PVDF in methanol 6.7 40 443.7 PVDF in methanol 8.8 80 448.1 PVDF in toluene 3.9 40 449.4 PVDF in toluene 4.5 80 436.8

DOG = degree of grafting; Tonset=Initial degradation temperature Tab. 2. Experimental temperatures by TGA for PTFE films.

Film DOG (%) Dose (kGy) Tonset (oC) PTFE pure - - 555.4

PTFE in methanol 1.9 40 561.9 PTFE in methanol 2.0 80 561.4 PTFE in toluene 6.1 40 559.7 PTFE in toluene 11.4 80 554.8

DOG = degree of grafting; Tonset=Initial degradation temperature The overall degree of crystallinity of samples was calculated using Equation (2).

Xc = (ΔHm/ ΔHm100) . 100 (2)

where, ΔHm is the heat of fusion of PTFE or PVDF film and ΔHm100 is the heat of fusion for a 100% crystalline polymer, 92.9 J g-1 [9] for PTFE and 104.7 J g-1 for PVDF [11]. To the calculation of crystallinity of the grafted samples, it was assumed that those values were still valid as the grafting reaction proceeded only in the amorphous region. The DSC data for the PVDF and PTFE films are presented in Table 3. The original PVDF melting temperature (Tm) was 170.1 °C and polystyrene incorporation into PVDF films caused decrease of about 3 °C in both solvents. The Tc results were 139.1 °C for the original PVDF and for the grafted PVDF decrease to Tc, 136.9 °C and 136.4 °C in methanol solution at 40 and 80 kGy respectively. The decrease of Tm and Tc is in agreement with the literature [3], which was reported the decrease of 167.9 °C to 157.4 °C in 50% graft in PVDF attributed to formation of partial crystal disorder. Tm of pure PTFE was 330 ºC and it showed a small increase in methanol, as shown by the Tm of 331.2 ºC for the 40 kGy sample and the Tm of 332.7 ºC for the 80 kGy sample. On the contrary, the Tm showed a small decrease in toluene as shown by the Tm of 326.9 ºC for the 40 kGy sample and the Tm of 328.7 ºC for the 80 kGy sample. The small increase in Tm can be explained as experimental deviation. On the other hand, the small decrease in Tm is the expected behavior for irradiated PTFE. In addition, Nasef et. al [16] reported Tm data 325.2 °C to ungraft PTFE film and 324.6 °C to graft at 36%.

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Table 3 shows the effect of grafting on the heat of fusions and degree of crystallinity of PTFE and PVDF films. The heat of melting (ΔHm) and the degree of crystallinity (Xc) of PVDF films show no significant changes with the dose and solvent used, whereas those of PTFE films increased with the dose and increased in both solvents. This indicated an increase in crystallinity induced by irradiation while grafting. There are some reports suggesting that the degree of crystallinity of PTFE increases upon irradiation and the degree of such increase varies with the irradiation condition dose [8, 33, 34]. In the present study such increase was at about 11.6% and 21.8% in methanol; 8.3% and 11.0% in toluene solvent, at 40 and 80 kGy respectively. In both films, the small variation in the Tm despite the increase in the degree of grafting confirms that the polystyrene grafts occurred only in the amorphous region. Therefore the crystalline region was not affected or affected in minor extension [11]. Tab. 3. Results of heat of fusion and degree of crystallinity of PTFE and PVDF films.

Sample DOG (%) Tc (oC) Tm(oC) ΔHm(J g-1) Xc(%)

PVDF pure - 139.1 170.1 41.5 39.6 PVDF 40 kGy in

methanol 6.7 136.9 166.8 40.0 38.2

PVDF 80 kGy in methanol

8.8 136.4 167.5 36.6 35.0

PVDF 40 kGy in toluene

3.9 138.1 168.9 43.6 37.9

PVDF 80 kGy in toluene

4.5 137.5 167.2 39.6 37.8

PTFE pure - 301.4 330.3 19.6 21.1

PTFE 40kGy in methanol

1.9 295.3 331.2 30.4 32.7

PTFE 80 kGy in methanol

2.0 290.3 332.7 39.8 42.9

PTFE 40 kGy in toluene

6.1 299.5 326.9 27.3 29.4

PTFE 80 kGy in toluene

11.4 293.1 328.7 29.7 32.1

DOG = degree of grafting; Tm and Tc = melting and crystallization temperatures; ΔHm = heat of melting; Xc = degree of crystallinity. The morphological changes on the surface of PTFE and PVDF films induced by the solvents upon grafting conditions were observed and compared at the surface level. Figures 5c to 5j had the surfaces almost totally covered with the polystyrene layer. The PVDF-g-PS in styrene/methanol films (Figures 5c and 5e) despite the higher DOG content, showed small cavities and discontinuities.

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Samples PTFE pure PVDF pure

(a) (b) Styrene/methanol Styrene/toluene

PVDF 40 kGy

(c) (d)

PVDF 80 kGy

(e) (f)

PTFE 40 kGy

(g) (h)

PTFE 80 kGy

(i) (j)

Fig. 5. SEM images of a surface in: PTFE (a) and PVDF (b) pure; PVDF-g-PS (c, e) and PTFE-g-PS (g, i) in styrene/methanol; PVDF-g-PS (d, f) and PTFE-g-PS (h, j) in styrene/toluene solutions. 10,000X magnification.

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The surface of PTFE-g-PS in styrene/toluene (Figures 5h and 5j) presents more homogeneous surfaces. Comparing the left and right columns of the figure the behaviour of polystyrene layer in presence of methanol and toluene is also evident. In methanol cavities and holes were observed, while in toluene, the surface of films was more homogeneous. The SEM figures showed that the effect of solvent was more remarkable than the substrate or even than radiation dose influence in the surface topography, which agrees with the strong effect of solvents reported by others authors [1, 21]. Conclusions We have described the post-irradiation grafting of styrene onto PVDF and PTFE films using gamma radiation at room temperature. The highest level of grafting occurred after 21 days of post irradiation conditions at the highest dose. The slow but steady increase in grafting yield with time was only possible due to the high stability of radicals from amorphous phase and slow migration of radicals from crystalline phase. The influence of irradiation dose using different solvent showed that the toluene was a better choice to the grafting onto PTFE films and methanol was better for the PVDF films. The yield of radio-induced grafting depends on the solvent (monomer plus diluent) affinity to the matrix and by the solvent affinity to the PS branch. The grafting reaction was successfully accomplished by means of IR technique. Thermogravimetric analysis has showed that grafted PVDF is thermally more stable than the pure polymer, and that grafted PTFE suffered no changes whatsoever. When toluene is used, the SEM result has demonstrated that the films' surface is homogeneous, while when methanol is used, their surface is heterogeneous and presents cavities. These observations confirm the fact that the effect of solvent is not only more significant than the substrate, but also that radiation dose influences the surface topography. Seemingly, the non-homogeneous surface results from the creation of PS homopolymer, which is temporarily deposited on the surface but remains non homogeneous as PS grafting due to the mixture of solvents with methanol. Experimental part Preparation of films PTFE was used in the form of 0.2 mm thick film and was obtained from ULTRAHI PLÁSTICOS. PVDF was supplied in the form of pellets by ARKEMA GROUP. It was pressed between two finely polished inox steel plates and through this process, films of PVDF of 0.12 mm have been made. The styrene was supplied by Maxepoxi Ind. Com; toluene and methanol from Merck. Films of PTFE and PVDF were immersed in styrene/toluene or styrene/methanol 1:4 in N2 atmosphere and irradiated. Irradiations were preformed by Co60 source (dose rate of 10 kGy h-¹) at 40 and 80 kGy doses at room temperature and N2 atmosphere. In the sequence, thermal treatment of the grafted samples was made in vacuum oven for 8 h at 70 °C. Afterwards, the films were washed with toluene in soxhlet extractor and then dried in vacuum until constant weight. The extraction to remove the remaining homopolymer was effective after 8 h.

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Instrumental analysis The films were analyzed before and after the grafting process. After irradiation at room temperature the samples were maintained at N2 atmosphere and evaluated. The grafting time was 0, 7, 14, 21 and 28 days in order to observe the degree of grafting. The mass of the samples was measured to determine the degree of grafting (DOG) according to Equation (1): DOG (%) = [(Wg - Wo )/ Wo] .100 (1) where Wg and Wo are the masses of the samples after and before grafting respectively. The infrared analyses were performed on Nexus 670 Thermo Nicolet, MID - FTIR with films cited into pieces and analyzed. The SEM images were obtained in a Phillips XL 30 Microscope and were amplified to 10,000 X. The samples were covered with gold in a Sputter Coater BAL-TEC SCD 050. TGA technique was accomplished in a Mettler - Toledo TGA / SDTA 851 thermobalance, using temperature program of 10 ºC min-1 heating rate, at N2 atmosphere from 25 to 750 oC. DSC curves were obtained in a Mettler-Toledo DSC 822, under nitrogen atmosphere, for PTFE, from 0 to 400 oC, at 10 oC min-1 heating rate. As for PVDF, the temperature ranged from -25 to 300 oC at 10 oC min-1 heating rate. Acknowledgements This work was supported by CNPq - Conselho Nacional de Pesquisa e Desenvolvimento process number: 507100/2004-2, 505205/2004-1, 472340/2006-9, 554921/2006-5, 555842/2006-1; IPEN - Instituto de Pesquisas Energéticas e Nucleares; CCTM/IPEN - Centro de Ciência e Tecnologia de Materiais; EMBRARAD - Empresa Brasileira de Radiações; Fábio Paganini, Arkema Group, for the PVDF pellets and Ultrahi Plasticos for the PTFE films. References [1] Walsby, N.; Paronen, J.; Juhanoja, M.; Sundholm F. J. Appl. Pol. Sci. 2001, 81, 1572. [2] Nasef M. M.; Saidi, H. J. Polym. Res. 2005, 12, 305. [3] Nasef, M. M.; Hegazy, E. S. A. Prog. Polym. Sci. 2004, 29, 499. [4] Buchi, F.N.; Scherer, G.G. J. Electroanal. Chem., 1996, 404, 37. [5] Gubler, L.; Beck, N.; Gürsel, S. A.; Hajbolouri, F.; Kramer, D.; Reiner, A.; Steiger, B.; Scherer, G. G.; Wokaun, A.; Rajesh, B.; Thampi, K. R. Chimia 2004, 58, 826. [6] Scherer, G. G. Chimia, 2004, 58, 824. [7] Brack, H. P.; Ruegg, D.; Bührer, H.; Slaki, M.; Alkan, S.; Scherer, G. G. J. Polym. Sci.: Part B, 2004, 42, 2612. [8] Gubler, L.; Prost, N.; Gürsel, S. A.; Scherer, G. G. Solid State Ionic, 2005, 176, 2849. [9] Li, J.; Matsuura, A.; Kakigi, T.; Miura, T.; Oshima, A.; Washio, M. J. Powder Sources, 2006, 161, 99. [10] Li, J.; Muto, F.; Miura, T.; Oshima, A.; Washio, M.; Ikeda, S.; Matsuura, C.; Katsumura, Y. Eur. Polym. J. 2006, 42, 1222.

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