ISSN 15600904, Polymer Science, Ser. B, 2013, Vol. 55, Nos. 7–8, pp. 467–471. © Pleiades Publishing, Ltd., 2013.

POLYMERIZATION

Electrical Field Influence on Molecular Mass and Electrical Conductivity of Polyaniline1 Seyed Hossein Hosseinia and S. Jamal Goharib a

Department of Chemistry, Faculty of Science, Islamic Azad University, Islamshahr branch, Tehran, Iran Department of Chemistry, Faculty of Science, Imam Hossein University, Babaee Express Way, Tehran, Iran email: [email protected], [email protected]

b

Received February 21, 2012; Revised Manuscript Received July 2, 2012

Abstract—We have described the primary studies on the conductivity and molecular weight of polyaniline in an electric field as it is used in a field effect experimental configuration. We report further studies on doped insitu deposited polyaniline. First we have chemically synthesized polyaniline by ammonium peroxodisul fate in an acidic solution, with aqueous, organic and emulsion conditions at different times. Next, we mea sured mass and conductivity and obtained the best time of polymerizations. Then, we repeated these reac tions under different electrical fields in constant time and measured mass and conductivity. The polyaniline is characterized by gel permeation chromatography (GPC), UV–visible spectroscopy and electrical conduc tivity. Polyanilines with high molecular weight are synthesized under electric field Mw = (5.2–6.8) × 105, with Mw/Mn = 2.0–2.5. The UV–visible spectra of polyanilines oxidized by ammonium peroxodisulfate and pro tonated with dodecylbenzenesulfonic acid (PANi–DBSA), in Nmethylpyrolidone (NMP) show a smeared polaron peak shifted into the visible. Electrical conductivity of polyaniline has been studied by fourprobe method. The conductivity of the films of emeraldine protonated by DBSA cast from NMP is higher than 500 S/cm under (10 kV/cm2 of potential) electric field and shows an enhanced resistance to ageing. Next, we carried chemical polymerization at the best electric field at different times. Finally, the best time and amount of electric field were determined. Polymers synthesized under an electric field probably have better physical properties regarding the existence of less branching and high electric conductivity. DOI: 10.1134/S1560090413070087 1

INTRODUCTION Since it has been discovered that polyaniline (PANi) is the generic polymer of a novel class of con ducting polymers [1], it has been the subject of numer ous studies. Most studies performed on PANi have chemically synthesized it at 1–5°C, which is, accord ing to MacDiarmid et al. [2], a synthesis derived from Green’s synthesis that became a standard procedure. This standard PANi, that is in the form of emeraldine base (EB), is only partly soluble in 1methyl2pyrro lidinone (NMP) [3, 4] and the soluble part has low molecular weight [5] (MW): Mn = (2.0–2.6) × 104, Mw = (3–6) × 104, large polydispersity Mw/Mn = 2.5– 3.0 and low inherent viscosity, ηinh = 0.8–1.2 dL/g. Since it is protonated by the camphorsulfonic acid (CSA), it is only partly soluble in mcresol [6]. Polymers that can move charge carriers can be clas sified in two groups. The first one has ionic carriers while the other one has electronic carriers. Conju gated polymers such as polypyrrole, polyaniline and their derivatives can be classified in one of these two groups according to the temperature and humidity of environmental conditions. 1

The article is published in the original.

A number of mechanisms for the electric field effects in chemical reactions have been established, well documented, and received a proper theoretical analysis [7]. Therefore, electric field effects on con ducting polymers were investigated [8, 9]. However, applications of electric field effects to polymerization, conductivity and molecular weight are rather limited. We have described preliminary studies on the depen dency of the conductivity of PANi on an electric field as it is used in a field effect experimental configura tion. Now we report further the studies on different conditions and also the studies on doped insitu deposited polyaniline. EXPERIMENTAL Reagents and Instrumentation Chemicals used in this study were American Chemical Society (ACS) grade. Aniline (Merck) was dried with NaOH, fractionally distillated under reduced pressure from sodium or CaH2. Other chem icals and solvents were purified as prestandard proce dure before use. Conductivity of all samples was mea sured at room temperature by a fourprobe device (home made, ASTM Standards, F 4393) on pellets

467

468

SEYED HOSSEIN HOSSEINI, S. JAMAL GOHARI Absorbance 2

1 1 2 0 350

550

750

950 λ, nm

UV–visible spectra of PANi prepared in NMP in (1) the absence and (2) presence electrical field.

compressed at 700 MPa, 13 mm in diameter and 1– 1.5 mm thick [10, 11]. Electric field device was equipped by Hipotronics S.O. No. 00439000, HV power supply, model 830.50 made in USA. A Fourier transform infrared spectrom eter (FTIR) (Bruker) was used in spectral measure ments of the polymer and graft copolymer. UV–VIS spectra were recorded using a Perkin Elmer Lambda 15 spectrophotometer on 0.05 wt/vol. EB solutions in spectrophotometric grade NMP. Molecular weight was measured at 30°C with a gel permeation chroma tography (GPC), (Waters Associates, model 150C). Three styragel packed columns with different pore sizes (104–106 Å) were used. The mobile phase was NMP with flow rate of 1.5 mL/min. Mw determina tions by GPC were carried out on 0.05 wt./vol.% EB solutions in HPLC grade NMP. Visually, all the sam ples were totally soluble. EB powders were dissolved by stirring for 15 h, then the solutions were filtered. We determined the Mw of each some of synthesized poly Table 1. Electrical conductivity of polyaniline produced by (NH4)2S2O8 (HCl 0.1M) as initiator in different times and absence electric field. Weight of initial monomer (aniline)⎯0.1 g Time of Color Weight of Electrical Sample polymeriza change produced conductiv tion, min time, min polymer, g ity, S/cm

mers with the high conductivities by GPC on EB solu tions in NMP. Preparation of Polyaniline in the Absence of Electric Field Project participants followed the same instructions to oxidize 0.001 mol aniline hydrochloride with 0.001 mol ammonium peroxydisulfate, (NH4)2S2O8, in aqueous medium. Aniline hydrochloride (purum; 0.1 g) was dissolved in 10 mL of 1 M HCl in a 50 mL volumetric flask. Ammonium peroxydisulfate (purum; 0.228 g) was dissolved in 10 mL of 1 M HCl, then was added. Both solutions were kept for different times (5, 10, 15, 20, 30, 40, 50, 60, 120 min) at room tempera ture (~18–24°C). The PANi precipitate was collected on a filter, washed with three 10mL portions of 0.1 M HCl, and acetone. Polyaniline hydrochloride powders were dried in air and then in vacuum at 60°C. Then its weights were measured and its electrical conductivities were measured by four probe method [4, 12]. PANi powder with (NH4)2Ce(NO3)6, FeCl3 (dry), FeCl3 · 6H2O, Fe(ClO4)3 and H2O2 as initiators were synthesized in similar manner. Therefore, PANi pow der were prepared in similar manner with H2O, etha nol, tetrahydrofuran (THF), mcrosol, dimethylsul foxide (DMSO), acetone, acetonitrile, cyclohexane, dichloromethane, benzene and nitrobenzene as sol vents. These solvents have different dielectric con stants.

1

10

3

0.07

0.0005

2

15

3

0.07

0.01

Emulsion Polymerization of Aniline

3

20

3

0.09

0.8

4

30

3

0.15

7

5

40

3

0.17

8

6

60

3

0.20

8

7

120

3

0.22

10

In a typical experiment, 1 g of sodium lauryl sul phate was dissolved in 15 mL of water and added to a three necked round bottomed flask which were con taining 10 mL of tolueneisooctane mixture. Double distilled aniline (1 mL) was then added with stirring. To this mixture, 5 g of benzoyl peroxide in 30 mL of toluene–isooctane mixture and an aqueous solution (10 mL) of 0.1 M sulphuric acid were simultaneously POLYMER SCIENCE

Series B

Vol. 55

Nos. 7–8

2013

ELECTRICAL FIELD INFLUENCE ON MOLECULAR MASS

added drop by drop. The reaction mixture turned green via blue color within 15–20 min, indicating the onset of polymerization. The reaction was allowed to proceed at room temperature for different times (5, 10, 15, 20, 30, 40, 50, 60, 120 min) without stirring. The reaction mixture was added to 100 ml of methanol for 2 h to precipitate the PANi–H2SO4 salt. It was fil tered, washed with methanol three times and then was filtered and dried for 48 h under dynamic vacuum. PANi salts with ptoluene sulphonic acid (PANi– PTSA), dodecylbenzenesulfonic acid (PANi–DBSA) and camphor sulphonic acid (PANi–CSA) as dopants were synthesized in similar manner. Preparation of Polyaniline under Electric Field Polyaniline was prepared under the same chemical circumstance but with different electric fields in 45 min at room temperature. Therefore, polyanilines prepared by the same reaction but various time spans at the best electric field. Additional polymerizations were carried out in an emulsion and organic phase. Thus, polymer will be in the organic phase was formed. It will be separated form aqueous phase, treated several times with solid sodium sulphate to remove traces of moisture and precipitated in a suit able nonsolvent. RESULTS AND DISCUSSION The obtained polymer is of high purity because the excess initiator was readily washed with acetone and the purity was confirmed by elemental analysis. The figure shows the UV–visible spectroscopy for PANi oxidized by ammonium peroxydisulfate and protonated by DBSA that was prepared in the absence (1) and the presence (2) of an electrical field in NMP. UV–visible spectrum of PANi prepared in the pres ence of the electrical field shows a peak in higher than 650–950 nm. It is related to π–π* transitions of the longer polymer chains with the higher molecular weights/masses. We know, conducting polymers have conjugated double bonds systems, so its show π–π* transitions a lot. Tables 1 and 2 show electrical conductivity and weight polyaniline produced in the absence of electric fields at different times and initiators, respectively. Electrical conductivity increases by increasing the duration of polymerization. Ammonium peroxydisul fate and ammonium ceric nitrate as oxidants are better than the other oxidants. Therefore, weight of PANi was increased in the presence power oxidants by increasing time of reaction. Tables 3 showed electrical conductivity of polya niline produced by ammonium peroxydisulfate, [(NH4)2S2O8], in different electric fields after 45 min. According to this Table, by increasing the intensity of electric field, the electrical conductivity has been increased. But beyond about 10 kV/cm2, electrical POLYMER SCIENCE

Series B

Vol. 55

Nos. 7–8

469

Table 2. Electrical conductivity of polyaniline produced by different initiators in water and absence electric field after 45 min. Weight of initial monomer (aniline)⎯0.1 g Color change time, min

Initiator

Weight of Electrical produced conductivity, polymer, g S/cm

(NH4)2S2O8

3

0.19

0.08

(NH4)2Ce(NO3)6

3

0.18

0.08

FeCl3 · 6H2O

3

0.16

0.007

FeCl3 (dry)

4

0.15

0.008

Fe(ClO4)3

4

0.16

0.01

H2O2

5

0.16

0.03

Table 3. Electrical conductivity of polyaniline produced by (NH4)2S2O8, (HCl 0.1M) under different electric fields after 45 min. Weight of initial monomer (aniline)⎯0.1 g Sample

Electric fields, kV/cm2

1

5

4

0.10

22

2

8

3.5

0.18

73

3

9

4

0.20

78

4

10

4

0.20

88

5

11

4.5

0.19

86

6

12

4.5

0.18

80

7

15

4

0.20

76

8

20

5

0.19

75

9

25

5

0.18

64

10

30

6

0.18

49

Color Weight of change produced time, min polymer, g

Electrical conductiv ity, S/cm

conductivity has been decreased steadily. This may be due to the fact that total dipole moment process decreases during a polymer chain. Considering the fact that each of two monomers has joined to each other with a special angle, this reduction is natural. Results show that although electrical conductivities initially increased, it decreased approximately at 10 kV/cm2. Table 4 shows electrical conductivity of polyaniline produced by (NH4)2S2O8 initiator in different solvents and in 10 kV/cm2 electric field after 45 min. Based on the dielectric constant, the solvent effect has been ver ified. It has been concluded that water would be the best solvent. It has been characterized according to its 2013

470

SEYED HOSSEIN HOSSEINI, S. JAMAL GOHARI

Table 4. Electrical conductivity of polyaniline produced by (NH4)2S2O8 initiator and different solvents under 10 kV/cm2 electric field after 45 min. Weight of initial monomer (aniline)⎯0.1 g. Solvent/Dopant, 0.1 M

Color Weight Electrical change of produced conducti time, min polymer, g vity, S/cm

H2O/HCl

4

0.20

89

Ethanol/HCl

4

0.07

22

CH3CN/HCl

6

0.09

14

THF/HCl

5

0.15

14

DMSO/HCl

6

0.17

15

mCrosol/HCl

4

0.20

11

Acetone/HCl

10

0.18

11

CH2Cl2/PTSA

45

0.11

0.001

Benzene/PTSA

45

0.11

0.003

Nitrobenzene/PTSA

15

0.05

0.042

Cyclohexane/PTSA

60

0.12

0.0004

In Table 6, electrical conductivities and molecular weights (from GPC method) in the absence and pres ence (10 kV/cm2) of an electric field are reported in different conditions of polymerization of aniline. For example, we showed, the molecular weight of PANi prepared in HCl solution, using (NH4)2Ce(NO3)6 as an oxidant, was about Mw = 5.1 × 105 and 6.5 × 105 in the absence and presence of electric field, respectively, while the molecular weight obtained using (NH4)2S2O8, was about Mw = 5.3 × 105 and 6.8 × 105 in the absence and presence of electric field, respectively. However, the molecular weight prepared in presence of (NH4)2S2O8, using DBSA as a dopant, was about Mw = 4.9 × 105 and 6.55 × 105 in the absence and pres ence of electric field, respectively. Increasing the electric field to (10 kV/cm2 of potential) lead to the increase of the Mw of the synthe sized polymers as it was observed by the increase of their masses. Polyaniline of higher molar mass and electrical properties are produced under electric fields. CONCLUSIONS

Table 5. Electrical conductivity of polyaniline produced by (NH4)2S2O8 initiator and different dopants under 10 kV/cm2 electric field after 45 min. Weight of initial monomer (aniline)⎯1.02 g Color change time, min

Weight of produced polymer, g

Electrical conductivity, S/cm

HCl

4

0.20

120

PTSA

4

0.22

210

DBSA

5

0.24

500

CSA

5

0.23

320

Dopant, 1M

highest dielectric constant. Respectively, we improved electrical conductivities by different dopants which have been shown in Table 5.

Application of a moderate external electric field increases the efficiency of polymerization and decreases the duration of polymerization process. Conductivity and molecular weight increase. A new inverted emulsion process has been adopted for the synthesis of polyaniline in better yields, high purity and conductivity using electric field. Electrical conductivities of the PANi powders under electric fields are higher than 500 S/cm and show an enhanced resistance to aging. The best time for polymerization is 20–40 min and ammonium per oxydisulfate and ammonium cerium nitrate are the best initiators. Therefore, the best solvent and dopant are water and DBSA respectively. Thus, the obtained polyaniline is probably less branched than standard one as indicated by better solubility, enhanced con ductivities, enhanced molecular weight (i.e. Mw) at the absence of electric field (using standard polyaniline).

Table 6. The electrical conductivities and molecular weight (from GPC method) in the absence and presence (10 kV/cm2) of electric field for different conditions. Weight of initial monomer (aniline)⎯0.1 g Condition of polymerization Solvent/Dopant

Initiator

Molecular weight (GPC) in absence electric field

Conductivity in presence electric field, S/cm

10

5.3 × 105

125

6.8 × 105

2.3

Conductivity absence electric field, S/cm

Molecular weight (GPC) Mw /Mn in presence electric field

H2O/HCl (0.1 M)

(NH4)2S2O8

H2O/HCl (0.1 M)

(NH4)2Ce(NO3)6

8

5.1 × 105

110

6.5 × 105

2.5

H2O/HCl (0.1 M)

Fe(ClO4)3

0.01

4.5 × 105

85

5.2 × 105

2.1

H2O/DBSA (0.1 M) (NH4)2S2O8

50

4.9 ×

105

POLYMER SCIENCE

6.55 ×

510 Series B

Vol. 55

105

Nos. 7–8

2.5 2013

ELECTRICAL FIELD INFLUENCE ON MOLECULAR MASS

REFERENCES 1. A. G. MacDiarmid, J. C. Chiang, M. Halpern, W. S. Huang, J. R Krawczyk, R. J. Mammone, S. L. Mu, N. L. D. Somarisi, and W. Wu, Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 24, 248 (1984). 2. A. G. MacDiarmid, J. C. Chiang, A. F. Richter, and N. L. D. Somarisi, in Conducting Polymers, Special Applications, Ed. by L. Alcacer (Reidel, Dordrecht, 1987), p. 105. 3. Y. H. Liao, T. K. Kwei, and K. Levon, Makromol. Chem. Phys. 196, 3107 (1995). 4. S. H. Hosseini, S. H. Abdi Oskooe, and A. A. Entezami, Iran. Polym. J. 14, 333 (2005). 5. M. Angelopoulos, Y. H. Liao, B. Furman, and T. Gra ham, Macromolecules 29, 3046 (1996).

POLYMER SCIENCE

Series B

Vol. 55

Nos. 7–8

471

6. C. L. Gettinger, A. J. Heeger, D. J. Pine, and Y. Cao, Synth. Met. 74, 81 (1995). 7. A. L. Buchachenko, Pure Appl. Chem. 72, 2243 (2000). 8. A. G. MacDiarmid, Angew. Chem., Int. Ed. Engl. 40, 2581 (2001). 9. S. K. Manohar, A. G. MacDiarmid, A. J. Epstein, Polym. Mater.: Sci. Eng. 86, 1 (2002). 10. S. H. Hosseini, M. Dabiri, and M. Ashrafi, Polym. Int. 55, 1081 (2006). 11. S. H. Hosseini, J. Appl. Polym. Sci. 101, 3920 (2006). 12. S. H. Hosseini and A. A. Entezami, J. Appl. Polym. Sci. 90, 63 (2003).

2013