The Effect of Azo Moiety on the Electrical Conductivity of Oxadiazole and Triazole Containing Polymers

: Phenolic mercapto oxadiazole and triazole monomers were prepared and then azotized with aniline and Para-phenylene diamine. The prepared monomers have been characterized with FTIR and1HNMR. The six prepared monomers were polymerized by condensation with formaldehyde in a basic medium. The polymer was recognized by FTIR, thermal analysis, and fine elemental analysis (CHN). The results have shown that the presence of azo moiety decreases the thermal stability and the glass transition temperature of the polymers. Also, the oxadiazole polymers are more thermally stable and have higher Tg than the triazole polymers. Doping with different ratios of iodine vapor was carried out on the polymer. The electrical conductivity of the pure and doped polymers was measured by using three probe cells. The results also revealed that the azo moiety increases the conductivity of the pure polymers to some extent (from 1.45534E-11 to 4.69038E-10 ohm-1cm-1 for oxadiazole polymers and from 1.15263E-10 to 3.2680E-10 ohm-1cm-1 for triazole polymers). It was also shown that the electrical conductivity was increased by many orders of magnitude (2-3 orders).by increasing the ratio of the doping.


1-Introduction:
The polyconjugation structure in the main chain of the polymer is the major characteristic of the electrically conductive polymers [1]. The essential structure of the conjugated polymers is the presence a of quasi-infinite Π system which extended over the monomer units. The conjugation of Π electrons gives characteristics for the polymer to be highly susceptible to oxidized or reduced chemically or electrically [2]. Oxadiazole and triazole groups can be considered as starting materials used as active heterocyclic compounds in many fields of medicinal, agricultural, and different industrial fields [3]. Oxadiazole and triazole moieties containing polymers represent important types of highly conjugated polymers that have many applications in various fields. The most important applications are electrically conductive polymer synthesis. The electrical conductivity of oxadiazole and triazole containing polymers was studied [4]. It was shown that the electrical conductivity was affected by the chemical structure of the main chain of the polymer. Polymers containing triazole moiety as a side group or in the main chain were used as proton conductivity polymers [5,6]. In the application of solar cell, it was found that the triazolebased polymers are suitable for bulk hetero junction solar cell, whereby the prepared polymers behaves as a donor [7]. Ayoob et al., proved that the conjugated polymers with diazole ring can behave as n or p type via doping with different dopants [8]. These polymers are used successfully for manufacturing solar cells with acceptable efficiency [9]. On the other hand, the azo group can also enhance the conjugation of the polymer chains. Studies on polyamide [10,11] and polyester [12] containing azo moiety within the main chain show increase in the electrical conductivity with increasing the number of the azo moieties. Organic polymers with an azo active group were used in lithium ion batteries, whereby the azo moiety can enhance the electrical conductivity by connecting the conjugated structure in one molecular unit and maintaining the electrochemical activity [13]. Polystyrene sulfonate was grafted with azo moiety in order to get photo-illuminated film through the isomerization of the azo group [14]. In the present work, oxadiazole and triazole monomers containing azo moiety as a side group were prepared. Condensation polymers of the prepared monomers were prepared. The electrical conductivity of the pure and doped polymers was investigated.

2-Experimental:
Materials: Table (1) shows all the used chemicals which were used as received from the sources without any purification except phenylene diamine which was recrystallized from ethanol before used, and the aniline was distilled off twice under vacuum . Equipment: FTIR spectra were accomplished by using (BRUKER F.T.IR Infrared. The spectra are obtained using the powder of the sample. 1 HNM was performed by using Bruker Bio GmbH Spectrophotometer (400 MHz) by using DMSO as a solvent. Thermal analyses were measured by Metler, the heating speed of measuring TGA and DTA of the prepared samples was 20 o C\min. CHN elemental analysis was measured by using Veri OEL V5. The volume electrical conductivity measurements for the pure and doped polymers were performed by using a lab -mad 3probe Dc cell [15].

Methodology:
3-hydroxyl ethyl benzoate: was prepared from 3-hydroxyl benzoic acid by using conc. H2SO4 as a catalyst. was added and reflux the mixture for a period of 8hrs.. After that, the excess ethanol was distilled off. 30ml of crushed ice was added to the mixture and neutralized by 10% Na2 CO3. The precipitated product was filtered and dried under vacuum, m.p.= 117 0 C, yield= 81%.
3-hydroxyl benzohydrazide: 15 ml of hydrazine hydrate was added to 4.2g (0.02mol) of the above prepared ester. The mixture was refluxed for 3hrs, 50ml of ethanol was added to the mixture, and continued reflux for another 3hrs. After completion of the period of reaction, the excess ethanol was distilled off and crushed ice was added to the residue. The precipitated product was filtered and washed with cold ethanol, m.p.= 265-267 0 C, yield= 85%.

1,3,4-Oxadiazole (M1):
0.05 mole of 3-hydroxyl benzohydrazide was dissolved in 75ml of abs. ethanol, 3g of KOH was added and 3ml of CS2 was added drop wise with stirring in an ice path. The reaction mixture was refluxed for a period of 10 hrs., the appearance of yellow vapor as H2S refers to the completion of the reaction. Excess ethanol was distilled of and the product was precipitated by adding crushed ice and acidifying the medium to pH=2 by 1NHCl. The precipitate was filtered and recrystallized from ethanol-water mixed solvent. m.p= 224-226 0 C, yield = 76% . Polymers preparation: Six condensation polymers were prepared from the oxadiazole and triazole monomer. The general procedure for the preparation of polymers (I,II,III,IV,V and VI) is as follows: 0.01 mole of oxadiazole or triazole monomer was mixed with 25ml of formaldehyde solution (37%) in a round bottom flask. 5% NaOH was used to control the pH of the reaction medium at 9.5-10. The mixture was heated to 95 0 C and stirred for 8hrs. After completing the period, the mixture was neutralized with 5%H3PO4 to pH= 7.5. the produced polymer was collected, washed with deionized water and dried under vacuum Doping of polymers: The iodine was used as a dopant. Vapor-phase doping process was followed, whereby the polymer disc has been exposed to the vapor of the iodine in an evacuated tube for different periods of time. Electrical conductivity measurements : Disc was performed from the pure or doped polymers with 2cm diameter and about 0.5mm thickness under 3-4 ton/cm 2 pressure. Electrical volume conductivity measurements are performed using the standard 3-probe D.C technique according to the ASTM method [15] by using the equation: σ = Id/AE; where: σ =volume conductivity (S.cm -1 ) , I= the flow current (ampere), d=thickness of the specimen (cm), A=active surface area of the specimen (cm 2 ) and E=applied voltage (volt).

3-Results and discussion: Synthetic routes: Monomers preparation:
The oxadiazole and triazole monomers are prepared with routine work. The characteristic groups of the monomers show a distinctive absorption bands in the FTIR spectra (figures 1-6). The absorption bands of the monomers are shown in Table (2).  Tables (3) and Figures (7-12) indicate the chemical shifts appeared in 1 H NMR of the prepared monomers.

Polymers synthesis:
Six phenoplast polymers were synthesized via condensation polymerization of the oxadiazole and triazole monomers with formaldehyde in a basic medium (Table 4). Whereby the phenol moiety within the monomer structure has two functional groups (two ortho position) that can be condensed with formaldehyde molecule in a basic medium to form the phenol formaldehyde polymers. The chemical structure of the prepared polymers was recognized by FTIR spectroscopy (Figures 13-14).
In comparison with the spectra of the monomers, the spectra of all polymers show new bands appeared at about 2924-2942 cm -1 which belong to the CH2 group formed through the polymerization process with formaldehyde to form phenoplast polymers. The other absorption bands of the major monomers also appeared in the spectra of the polymers. The chemical formula of the polymers has been confirmed by fine elemental analysis CHN (Table 5) Thermal analysis : DSC analysis for the pure prepared polymers was performed over 600 0 C under a nitrogen atmosphere (Figures 14and15). Table ( 6) shows some thermal characteristics of the polymers. The values of Tg can be observed in the range between 366 0 C to 433.5 0 C. The results show the effect of the azo moiety on decreasing the Tg as going from P1 to P2 to P3 or from P4 to P5 to P6. The TG analysis was carried out to investigate the thermal stability of the prepared polymers. TGA and DTA curves show two distinct stages for weight loss at 250 0 C and 600-700 0 C. The results reveal that the degradation of the polymers is affected by the presence of the azo group which is accelerating the degradation. On the other hand, the oxadiazole polymers (P1,P2, and P3) seem to be more thermally stable than the triazole polymers (P4,P5, and P6) at the two temperatures, whereby the presence of amine (NH2) as a substituent on the ring of the azo moiety may decrease the stability.

Electrical conductivity measurements:
The electrical volume conductivity of the pure prepared polymers was accomplished at room temperature. The numerical values (σ) are listed in Table (7). It is noticed clearly that the chemical structures of the investigated polymers are the dominant factors in affecting the electrical conductivity. On the other hand, it was noticed that the presence of azo moiety will enhance the electrical conductivity, as it on going from P1 to P2 or from P4 to P5. This can be explained by that the azo moieties led to an increase in the polarization of the chain and enhanced the conductivity [11]. In comparison between oxadiazole and triazole polymers (P1 and P4),it was noticed clearly that the presence of NH2 in triazole will enhance the electrical conductivity with one order, whereby the lone pair of electron of NH2 group increases the power of conjugation and enhances the conductivity. This idea was confirmed in the conductivity of P3 and P6, whereby there is another NH2 was added to the phenyl ring of the azo moiety and increased the conductivity.

Effect of Doping on Conductivity:
The delocalization of electrons along the conjugated chains and their jumping from one chain to another through the dopant molecules will represent the process by which charges are transported through the polymeric material [12]. Iodine was carried out as a dopant for the polymers. The iodine was known as an electron donor and enhanced the electrically electronic conductivity [8]. The kinetics of doping exhibit a strong dependence on the doping time. The limitation of the dopants ratio depends on getting the higher values of electrical conductivity or the conductivity become decreasing at higher concentrations of dopant. On the other hand, the polymers may lose their mechanical properties at higher concentrations of the dopants [16]. The values of the volume electrical conductivity of the investigated doped polymers with iodine are shown in tables (8). In comparison with Table 7, the electrical conductivity of the doped polymers was increased by many orders of magnitude by doping (2-3 order). The results can be explained in terms of the active moieties (oxadiazole, triazole and azo) can be oxidized with the iodine and forming a charge carriers and enhancing the conductivity. It is well known that the efficiency of the dopant is related to its ability to form some type of complexes that act like charge carriers with the unsaturation units in the polymer chain [17].

4-Conclusions:
1-The oxadiazole and triazole moieties can provide a conjugation for the polymer chains and enhance the electrical conductivity.
2-The azo group as a graft substituent on the chain of oxadiazole and triazole polymers decreases the thermal resistance of the polymers. 3-The azo group can participate in the conjugation of the polymer chains and increase the conductivity to some extent. 4-Doping with iodine vapor increases the electrical conductivity of the polymers by many orders of magnitude.