Chemically oxidative polymerization of aromatic

Polymer 54 (2013) 505e512

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Polymer journal homepage: www.elsevier.com/locate/polymer

Chemically oxidative polymerization of aromatic diamines: The first use of aluminium-triflate as a co-catalyst Ismael Amer*, Desmond Austin Young Focus Area, Chemical Resource Beneficiation, Catalysis and Synthesis Research Group, North-West University, Private Bag X6001, Potchefstroom 2520, South Africa

a r t i c l e i n f o

a b s t r a c t

Article history: Received 21 August 2012 Received in revised form 6 November 2012 Accepted 28 November 2012 Available online 4 December 2012

Aromatic diamine monomers, including o-phenylenediamine (oPD), p-phenylenediamine (pPD), 4,40 diaminodiphenylenemethane (DADPM) and benzidine (BZN), were polymerized by chemical oxidation using sodium persulfate, potassium persulfate, and ammonium persulfate as oxidant catalysts. Aluminium-triflate (Al(OTf)3) was also used for the first time as a co-catalyst under various polymerization conditions. The homopolymers obtained are characterized by FT-IR, 1H and 13C NMR, GPC, WAXD, DSC and TGA. The yield, solubility, structure and molecular weight of the polymers are significantly dependent on the oxidative catalyst and polymerization conditions. The polymers show different molecular structures, good thermal stability and decompose above 400 C in nitrogen. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Aluminium-triflate Diamines Oxidative polymerization

1. Introduction Chemically oxidative polymerization has produced various functional polymers. Typical polymers include polyaniline, polytoluidine, polypyrrole, polyaminopyridine, polyaminonaphthalene, polyaminoquinoline, polymethylquinoline and polyphenylenediamine, which are used in the production of rechargeable batteries, electrocatalysts, smart windows, microelectronic and electrochromic devices, sensors, and actuators. Novel multifunctionality, which includes good redox reversibility, variable conductivity, strong electroactivity, colorful electrochromism, adsorption of heavy metal ions and good environmental stability has enabled the use of diamine polymers in various technological applications [1e 11]. Polyphenylenediamines are considered to be conductive polymers which have attracted attention lately because they display high gas separation ability [12,13] and lyotropic liquid crystallinity [14,15]. Furthermore, it is reported that polyphenylenediamines produced by chemically oxidative polymerization with sodium, potassium, or ammonium persulfate as oxidants show ladder and ladder-like structures having highly aromatic nitrogenous heterocycles and show unusually high thermostability [16e22]. The nitrogenous heterocyclic ladder structure is considered to be of great benefit to the preparation of advanced air separation

* Corresponding author. Tel.: þ27 18 299 2336; fax: þ27 18 299 1667. E-mail address: [email protected] (I. Amer). 0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.polymer.2012.11.078

membranes [23]. Moreover, the solubility of the polyphenylenediamines in most of the common organic solvents is low and depends on solvent composition and on the oxidant that was used for polymerization [24,25]. Only a few studies on oxidative homopolymerization of three phenylenediamine isomers have been described [16e22,26e29]. Chemically oxidative polymerization of p-phenylenediamine (pPD) was first reported as an additive of aniline (AN) polymerization in order to increase the rate and yield of polymerization [30]. Chan, Rawat, and coworkers reported the chemically oxidative polymerization of different aromatic diamines with persulfate as the oxidant in acidic aqueous solution from 0 C to room temperature [18,31]. Improvements in synthetic techniques, characterization of structure and properties lead to the design of functional materials of the polymers [2,32e34]. Metal triflates have received wide attention for their role as Lewis acids in a number of reactions [35e38]. Water-tolerant metal triflates are especially attractive from both an economical and environmental perspective as they can be recycled easily and repeatedly [39]. Thus, aluminiumtriflate (Al(OTf)3) is a reusable catalyst, which tolerates water effectively and is stable in aqueous medium [40]. Al(OTf)3 has not been explored as extensively as other metal triflates, for example the rare earth metal triflates in particular, despite it being comparatively more affordable [40,41]. Al(OTf)3 was used in our study as co-catalyst to polymerize different phenylenediamines instead of protic acids which are normally used for polymerizations. Al(OTf)3 showed an improvement on the yield and had different effects on the molecular weight

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I. Amer, D.A. Young / Polymer 54 (2013) 505e512

of phenylenediamine polymers, compared to polymerization reactions in which protic acids were used. This article reports the chemically oxidative polymerization and characterization of a series of aromatic diamines, including oPD, pPD, 4,40 -diaminodiphenylenemethane (DADPM) and benzidine (BZN), using different oxidants and acidic co-catalysts. 2. Experimental section 2.1. Materials The oPD, pPD, DADPM, BZN and all other solvents and reagents were obtained from Aldrich or Fluka and were used as received. 2.2. General polymerization procedure The oxidative polymerization was generally accomplished according to Ref. [19]. A typical procedure for the preparation of polymers was as follows: Phenelynediamine monomer (1.7 g) (oPD, pPD, DADPM or BZN) was added to an acidic solution (150 ml of HCl, glacial acetic acid or water) in a 500-ml glass flask in an oil bath at certain temperatures (Table 1) and was magnetically stirred. In a separate flask, the oxidative catalyst or co-catalyst (4.0 g) (sodium persulfate, potassium persulfate, ammonium persulfate or Al(OTf)3) was dissolved in 20 ml of water. The polymerization reaction was started by steadily adding oxidant solution dropwise into the monomer solution at a rate of one drop every 3 s over a period of 30 min. The dropwise addition of oxidant solution produced polymers with a relatively high molecular weight and narrow molecular weight distribution, as the highly exothermic nature of the polymerization reaction could then be effectively controlled. Immediately after the addition of the first few drops of oxidant, the color of the reaction solution changed as shown in Table 1. The reaction mixture was vigorously stirred for at least 8 h. Thereafter, the reaction mixture was filtered and washed several times with an excess of distilled water to remove the oxidant and

oligomers. Finally, the resulting powder products were left to dry in ambient air for one week. 2.3. Equipments Fourier transform infrared spectroscopy (FT-IR) was used to characterize the molecular structure of aromatic diamine polymers. Infrared spectra of the samples were recorded on a Bruker VERTEX 80 FT-IR spectrometer at room temperature. The analysis was performed between 500 and 4000 cm1, the resolution used was 2 cm1 and the results were based on an average of 22 scans. 1 H and 13C nuclear magnetic resonance spectroscopy (1H and 13 C NMR) spectra were recorded at room temperature on a 600 SB Ultra ShieldÔ Plus NMR spectrometer equipped with an Oxford magnet (14.09 T), operating at 600 MHz. Samples (20e30 mg) for NMR analyses were dissolved in deuterated dimethyl sulfoxide (DMSO-d6). Gel permeation chromatography (GPC) was used to determine weight average molecular weight (Mw ) and molecular weight distribution (MWD). Samples were analyzed with a PL-GPC 220. A flow rate of 1.0 ml/min was used. The analyses were carried out in dimethylacetamide (DMAC). Wide-angle X-ray diffraction (WAXD) analysis was performed on a Bruker AXS D8 Advance diffractometer at room temperature with filtered CuKa radiation. All samples were scanned at 2q angles, ranging from 0 to 80 , with a sampling width of 0.02 , where 2q is the diffraction angle. Differential Scanning Calorimetry (DSC) analysis was performed on a TA Instruments Q100 DSC. All measurements were conducted under a nitrogen atmosphere, at a purge gas flow rate of 50 ml/min. Three cycles were performed for each sample. First the samples were heated in crimped aluminium pans from 25 C to 450 C at a rate of 10 C/min. Samples were then cooled from 450 C to 25 C. Finally, the samples were heated for a second time at rate of 10 C/ min to 450 C. Thermogravimetric analysis (TGA) measures the change in the weight of polymer sample as a function of temperature which was

Table 1 Shows preparation and properties of the polymer powders from aromatic diamines by chemically oxidative polymerization under various conditions using different catalysts. Run 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 a b c d e f

Polymer c

PoPD PoPD PoPD PoPD PoPD PoPD PoPD PoPD PpPDd PpPD PpPD PpPD PpPD PpPD PpPD PpPD PDADPMe PDADPM PDADPM PBZNf PBZN PBZN

Powder color

Acid/solvent

Ta [ C]

Time [h]

Oxidant/catalyst

Yield [wt%]

Mw

PDb

e e Black Orange Dark Orange Orange e Black e Black Black Dark Black Black Purple Brown Brown e Brown Brown Gray

Glacial CH3COOH CH3COOH/H2O Glacial CH3COOH HCl/H2O H2O HCl/H2O HCl/H2O Al-triflate/H2O Glacial CH3COOH CH3COOH/H2O Glacial CH3COOH HCl/H2O H2O HCl/H2O HCl/H2O Al-triflate/H2O H2O HCl/H2O Al-triflate/H2O H2O HCl/H2O Al-triflate/H2O

118 100 118 0e2 100 30 30 100 118 100 118 0e2 100 30 30 100 100 30 100 100 30 100

72 8 72 72 8 24 24 24 72 8 72 72 8 24 24 24 8 24 24 8 24 24

None None (NH4)2S2O8 (NH4)2S2O8 (NH4)2S2O8 K2S2O8 Na2S2O8 None None None (NH4)2S2O8 (NH4)2S2O8 (NH4)2S2O8 K2S2O8 Na2S2O8 None (NH4)2S2O8 (NH4)2S2O8 None (NH4)2S2O8 (NH4)2S2O8 None

0.0 0.0 34.0 8.0 85.0 6.0 12.0 0.0 16.0 0.0 44.0 56.0 93.0 53.0 54.0 15.0 91.0 38.0 0.0 98.0 95.0 82.0

e e 16,365 4816 5573 4252 4576 e 1010 e 9882 e e 16,992 26,445 e e e e e 14,258 850

e e 2.28 1.10 1.91 1.11 1.10 e 1.04 e 2.32 e e 2.86 1.17 e e e e e 2.73 1.03

Reaction temperature. Polydispersity. Poly(o-phenylenediamine). Poly(p-phenylenediamine). Poly(diaminodiphenylenemethane). Poly(benzidine).


determined on a TGA Q100 instrument (TA Instruments) with a resolution of 0.1 mg, in a nitrogen atmosphere. Approximately 5e 10 mg of each sample was analyzed, by heating from 20 to 800 C at a rate of 10 C/min. 3. Results and discussion Few reports in the literature have described the synthesis and characterization of aromatic diamine polymers produced by chemical oxidation. Table 1 lists the synthesis conditions, polymerization yield, and molecular weight of the oPD, pPD, DADPM and BZN polymers produced in our experiments. 3.1. Polymerization yield Table 1 shows that the polymerization yield was dependent on the oxidative catalyst and experimental conditions. It was found that the presence of the oxidant was not necessary for the polymerization of pPD and BZN. For example, pPD in oxidant-free glacial acetic acid and Al(OTf)3 solutions achieved polymerization with yield of 16% and 15%, respectively, as shown in runs 9 and 16 (Table 1). However, the oxidant was required for the polymerization of oPD as no oxidative polymer of oPD was obtained even if the reaction was run at high temperature (118 C) in aqueous acetic acid solution over an extended time (72 h) as shown in runs 1 and 2 (Table 1). Furthermore, the oxidative polymerization yield of oPD was usually lower than that of pPD. The highest polymerization yields of oPD, pPD, DADPM and BZN were obtained when water was used as a solvent and ammonium persulfate as an oxidative catalyst, as can be seen for runs 5, 13, 17 and 20 respectively (Table 1). It was also found that the chemical oxidative polymerizations gave different colored powdered precipitates, depending on the oxidant used. In this study and for the first time, the chemical oxidative polymerizations of pPD and BZN with Al(OTf)3 as co-catalyst in aqueous solution were achieved. Purple and gray polymer powders were obtained with polymerization yields of 15% and 82%, respectively, as shown in Table 1. In contrast, no polymers of oPD and DADPM were obtained using Al(OTf)3 as co-catalyst. On other hand, it was possible to prepare polymers of oPD, pPD, DADPM and BZN with high yield, using a mixture of Al(OTf)3 and ammonium persulfate as oxidant in water as can be seen in Table 2. Black and brown powders were obtained with a polymerization yield of 80e94%. This high yield, obtained using Al(OTf)3 as a cocatalyst, is probably due to the fact that Al(OTf)3 does not form monomer salts which lower the polymerization yield, as is the case with the use of protic acids. 3.2. Solubility and molecular weight The solubility of aromatic diamine polymers depended greatly on their macromolecular structures. The oPD and pPD polymers were relatively soluble in DMSO, DMF and partly soluble in THF. The

Table 2 Shows preparation of the polymer powders from aromatic diamines by chemically oxidative polymerization under various conditions using a mixture of Al(OTf)3 and ammonium persulfate as oxidative catalyst. Run

Polymer

Powder color

Acid/solvent

T [ C]

Time [h]

Oxidant/ catalyst

Yield [wt%]

23 24 25 26

PoPD PpPD PDADPM PBZN

Black Black Brown Brown

Al-triflate/H2O Al-triflate/H2O Al-triflate/H2O Al-triflate/H2O

100 100 100 100

24 24 24 24

(NH4)2S2O8 (NH4)2S2O8 (NH4)2S2O8 (NH4)2S2O8

80.0 93.0 92.0 94.0

507

solubility and molecular weight of diamine polymers depended on the monomer and composition of the solution that was used for the polymerization. Table 1 shows that polymerization conditions led to different molecular weights with no obvious consistency. Unfortunately, the molecular weights of all polymers could not be determined due to their limited solubility in most common solvents. Generally, the molecular weights of pPD polymers prepared under the same conditions were higher than those of oPD polymers. For instance, the molecular weights of pPD polymers prepared using potassium and sodium persulfates as oxidant in HCl aqueous solution were 16,992 and 26,455 respectively (runs 14 and 15 in Table 1), which were higher than the oPD polymers prepared under same conditions, 4252 and 4576 (runs 6 and 7). Moreover, a small amount of oxidant was more effective for obtaining polymers with relatively higher molecular weights, as indicated by comparing molecular weights corresponding to runs 9 and 11with those of runs 21 and 22. On the other hand, the degree of polymerization was not very high, and the polymers were composed of 40e150 units of monomeric oPD and 90e245 units of monomeric pPD (excluding run 9, which was polymerized without oxidant catalyst). One of the main challenges for the industrial application and commercialization of the oxidative polymers of aromatic diamines is the increase in their molecular weight. There are several methods to achieve this efficiently. For oPD with ammonium persulfate as oxidant, elevating the polymerization temperature to 118 C together with the use of glacial acetic acid of high boiling point is one of the better methods to prepare the polymer of relatively high molecular weight, as indicated for run 3 in Table 1. This is in agreement with the results reported by Premasiri and Euler [17]. In contrast with oPD, conducting the reaction at room temperature for the oxidative polymerization of pPD with sodium persulfate in HCl solution produced pPD polymer of relatively high molecular weight (run 15, Table 1). Therefore, one can reasonably expect that the molecular weight of aromatic diamine polymers will increase under the optimum polymerization conditions including monomer/oxidant ratio, temperature, and reaction medium. The molecular weight of BZN polymer produced by ammonium persulfate was higher than the corresponding BZN polymer prepared using Al(OTf)3 (compare runs 21 and 22, Table 1). The powder material produced using Al(OTf)3 in run 22, which had a molecular weight of 850, corresponding to a degree of polymerization of 5, which can be considered as an oligomer and not a polymer. 3.3. Molecular structure and spectroscopy The molecular structure of aromatic diamine polymers was characterized by means of FT-IR and NMR. It is believed that the polymerization procedure has some effect on the structure of these polymers. 3.3.1. FT-IR spectroscopy FT-IR spectroscopy provided valuable information about the linkages of aromatic diamines for oxidative polymerization. In the FT-IR spectra of oPD and pPD polymers, shown in Figs. 1 and 2, almost all polymers exhibited two or three broad bands at 3400e 3200 and 3150e3100 cm1, suggesting NeH stretching vibrations of eNH and eNH2 groups, respectively [16,17,21]. The differences were observed between the FT-IR spectra of the oPD and pPD polymers. Note that the pPD polymers (except spectrum (C) in Fig. 2) did not exhibit the absorption bands at 3400e3200 cm1 as is the case with oPD polymers. This indicates that there were almost

508


(A) (A)

Transmittance

(B)

Transmittance

(B)

(C)

(C)

4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber (cm ) 4000

3500

3000

2500

2000

1500

1000

500

Wavenumber ( cm -1 ) Fig. 1. FT-IR spectra of oPD polymers prepared by chemically oxidative polymerization using: (A) ammonium persulfate in water, (B) potassium persulfate in hydrochloric acid, and (C) sodium persulfate in hydrochloric acid.

no free secondary amino groups in the pPD polymer chains, which is in agreement with the results reported by Li X-G [19]. In the region between 1580 and 1650 cm1, as well as between 1400 and 1550 cm1, the polymers showed strong peaks associated with aromatic ring stretching. Generally, the peak at 1610e 1620 cm1 was assigned to the following quinoid ring

N

N

N

NH2

(A)

Transmittance

(B)

(C)

4000

3500

3000

2500

2000

1500

1000

500

Wavenumber ( cm -1) Fig. 2. FT-IR spectra of pPD polymers prepared by chemically oxidative polymerization using: (A) potassium persulfate in hydrochloric acid, (B) sodium persulfate in hydrochloric acid, and (C) ammonium persulfate in water.

Fig. 3. FT-IR spectra of BZN polymers prepared by chemically oxidative polymerization using: (A) ammonium persulfate in water, (B) ammonium persulfate in hydrochloric solution and (C) ammonium persulfate in Al(OTf)3 solution.

whereas the peak at 1470e1480 cm1 was assigned to the following benzenoid ring [22].

N

NH

N

NH2

The peaks at 940e820, 750e760 and 570e590 cm1 were attributed to the out-of-plane bending motions of CeH of 1,2,4,5tetrasubstituted benzene nuclei of phenazine units, implying that the polymers have the basic phenazine skeleton. Furthermore, the peaks at 1350 and 1230e1240 cm1 were attributable to the CeN stretching vibration in quinoid imine units and to the CeN stretching in the benzenoid unit respectively. We conclude that the oxidative polymerization of oPD and pPD polymers led to relatively small differences in the FT-IR spectra of the final polymers, as was also found in a study by Li X-G [2]. The FT-IR spectra of oPD polymers suggest that oPD polymers have a 1,4-substituted benzenoid-quinoid backbone structure, containing one eNH2 group on each 1,4-substituted benzenoid or quinoid unit [42]. It is reported that the use of different levels of oxidizing catalysts do not cause a large difference in the IR spectra of the pPD polymers obtained, but lead to differences in the intensity of some bands [16]. Nevertheless, the FT-IR spectrum of pPD polymer in Fig. 2(C) showed a relatively different trend, compared to the FT-IR spectra of pPD polymers in Fig. 2(A) and (B). The presence of the peaks at 1620 and 1350 cm1 in spectrum (C) is probably due to the higher quinoid imine unit content in the polymer chains of sample (C), compared to samples (A) and (B), which mainly contain benzenoid units in their polymer chains. Similar results were reported by Cataldo [16]. Fig. 3 shows the FT-IR of BZN polymers produced by different polymerization procedures. All polymers showed two or three broad bands at 3400e3200 and 3150e3100 cm1 due to NeH stretching vibrations of eNH and eNH2 groups, respectively. The peaks at 1170, 1420, and 1495 cm1 in the spectra shown in Fig. 3(A), (B) and (C) were attributed to the CeN, N]N and CeC stretching vibrations respectively. The appearance of these peaks in the spectra of the polymer indicates that para coupling reactions are taking place during the polymerization. Similar results were reported by Do Nascimento


509

GM et al. [43]. Furthermore, the peaks at 1260, 1300, and 1360 cm1 can be attributed to the inter-rings CeC stretching of the azo, radical cation, and dication segments, respectively, in the polymeric chains [44]. These differences in the frequency values are due to the degree of the double bond character of the inter-rings CeC of the diphenyl moiety. The FT-IR spectra of BZN polymers in Fig. 3(B) and (C) showed other weak peaks at 1220 and 1400 cm1 which may be attributed to CeN and CeC stretching vibrations in the phenazine-like segments that are probably formed by ortho reaction coupling, as reported by D’Eramo et al. [45]. The formation of phenazine rings in the polymeric chains of BZN polymers in Fig. 3(B) and (C) are likely to have caused to differences observed in the BZN polymer spectrum in Fig. 3(A). 3.3.2. NMR spectroscopy 3.3.2.1. 1H NMR of the polymers. 1H NMR spectra of the oPD and pPD polymers are shown in Figs. 4 and 5. We found the spectra to be quite different from each other. The 1H NMR spectra of the oPD polymers in deuterated DMSO obtained in glacial acetic acid and water using ammonium persulfate as a catalyst, are characterized by three sharp peaks at 7.0, 7.1, and 7.2 ppm, which possibly correspond to eNH2 and eNH protons on phenazine unit, as shown in Fig. 4(B) and (C). The second strongest broad peaks

Fig. 5. 1H NMR spectra of pPD polymers prepared by chemically oxidative polymerization with: (A) ammonium persulfate in hydrochloric solution, (B) sodium persulfate in hydrochloric solution, and (C) ammonium persulfate with acetic acid in deuterated DMSO at 600.17 MHz.

Fig. 4. 1H NMR spectra of oPD polymers prepared by chemically oxidative polymerization with: (A) sodium persulfate in hydrochloric solution, (B) ammonium persulfate in acetic acid, and (C) ammonium persulfate with water in deuterated DMSO at 600.17 MHz.

centered at 8.0, 8.2 and 9.4 ppm in Fig. 4(C) and the peak centered at 8.15 ppm in Fig. 4(B) are attributed to the aromatic protons on phenazine unit. These results correlate well with results obtained in previous studies [2,17,21,22,24,42]. However, the oPD polymer formed in hydrochloric acid solution, using sodium persulfate, exhibited two strong peaks at 6.3 and 6.9 ppm for eNH2 and eNH protons and two other strong peaks at 7.6 and 7.9 ppm for the aromatic protons, as can be seen in Fig. 4(A) [2]. Additionally, a sharp strong peak appears at 1.9 ppm in Fig. 4(B) due to the methyl group of acetic acid on the oPD polymer [2,22,24]. Fig. 4(A), (B) and (C), thus indicates that there is a significant influence of oxidant and solvent on the 1H NMR characteristics of the oPD polymers. This is in agreement with the findings of previous studies [2,24]. As seen in Fig. 5(A) and (B), the 1H NMR spectra of the pPD polymers obtained in hydrochloric acid solution, using ammonium persulfate and sodium persulfate, respectively, as oxidant exhibited a very strong sharp doublet peak at 5.6 ppm, due to eNH2 proton; and a very strong sharp quadruplet peaks at 6.6e7.2 ppm due to aromatic protons. Similar results were obtained by Li, Huang and Cataldo [2,16,19]. Moreover, two broad weak peaks centered at 7.3 and 7.4 ppm; and the peaks at 8.3, 9.2 and 9.6 ppm due to the protons on 1,2,4,5tetrasubstituted benzene rings, which is similar to the results reported by Li et al. [46]. In contrast, the 1H NMR spectrum of pPD polymer in Fig. 5(C) obtained using acetic acid, with ammonium persulfate as the oxidant, showed a strong broad peak centered at 7.0e8.9 ppm due to eNH2 protons and a weak peak at 6.9 ppm due to aromatic protons. This is similar to the results reported by Ichinohe and Muranaka, in which hydrogen peroxide was used as an oxidant [47].

510


The significant differences in the 1H NMR spectra of pPD polymers could result from different molecular structure and molecular weight. A possible explanation of the differences in the spectra is that the molecular weights of the pPD polymers in the spectra (A) and (B) in Fig. 5 is higher than that in spectra (C), thus causing much smaller peaks from the eNH2 and eNH protons. 3.3.2.2. 13C NMR of the polymers. So far, solution 13C NMR spectra of phenylenediamine polymers have not been reported, probably due to low solubility of phenylenediamine polymers [17,42,47]. Fortunately, in this study it was possible to get useful 13C NMR spectra for some of pPD and oPD polymers as can be seen in Fig. 6(A) and (B). This is could be due to the higher resolution of the spectra in our study which was ascribed to the higher magnetic frequency (600.17 MHz) and the larger number of scans (NS ¼ 4096) than that used in the previous studies. As shown in Fig. 6(A), the 13C NMR spectra of pPD polymer obtained in acetic acid without any catalyst showed the strongest peak at 119 ppm, which can be assigned to the hydrogen-bonded carbon (CeH) in the benzenoid unit and the peak at 137 ppm was assigned to the nitrogen-bonded carbon (CeNH) in the benzenoid unit [46]. The peak at 169 ppm due to C]N in the quinoid unit. Furthermore, another peak at 22 ppm should be due to the methyl group of acetic acid on the polymer. These results are comparable with results obtained by Sestrem and Do Nascimento, for oligomers of oPD [21]. On the other hand, the 13C NMR spectrum of oPD polymer prepared in hydrochloric acid solution using sodium persulfate as the oxidant shows down field shift of the chemical shifts as can be seen in Fig. 6(B). The strongest two peaks at 127 and 128 which should be due to the carbon in CeH bond, while the peak at 102 ppm was attributed to the carbon in CeN bond. The peaks at 140e142 ppm due to C]N bond in the quinoid unit in the polymer.

3.3.3. Wide-angle X-Ray diffractograms of polymers Wide-angle X-ray diffractograms of oPD, pPD and BZN polymers are shown in Fig. 7(A), (B) and (C), respectively. oPD and pPD polymers appeared to show the strongest diffraction intensity at 2q 6 and two medium broad peaks placed at 2q ¼ 18 and 26 , which are attributed to the diffraction by an amorphous polymer. Note that the peak at 2q ¼ 26 , in the case of oPD polymer, (Fig. 7(A)) is stronger than in the case of pPD polymer (Fig. 7(B)). This result indicates that the supermolecular structure of the oPD polymer is different to that of the pPD polymer, because the two polymers, oPD and pPD exhibited different macromolecular structures, as proved by the FT-IR and NMR spectra [2,19,22,24]. In contrast, the XRD pattern of BZN polymer (Fig. 7(C)) shows a series of sharp peaks in the region of 5 < 2q > 45 , and there is small broad reflection due to amorphous components, which indicates that BZN polymer is polycrystalline in nature and has good crystallinity, regular chain structure and long range ordering [2,43,48]. 3.4. Electrical conductivity The bulk electrical conductivity of the polymers was measured by a two-disk method using a UT 70A multimeter at ambient temperature. Most of the polymers showed conductivity lower than 1 108 S/cm. Unfortunately, it was not possible to see the effect of the oxidant and polymerization procedure on the electrical conductivity of these polymers. The low electrical conductivity of phenylenediamine polymers showed can be attributed to the bulky eNH2 substituents which cause main-chain twisting, leading to a reduction in its coplanarity, and resulting in a barrier to the intrachain transfer and interchain jumping of the electrons, thereby shortening the conjugation length [2,4,10,33,34]. It was reported by Premasiri at al. [17] that oPD polymers synthesized by means of oxidative polymerization, in the absence of a catalyst, are not good conductors, since their conductivities are lower than 106 S/cm at room temperature. The poor conductivities were attributed to the lack of charge carriers (i.e., protonation at amine sites) on the main

Diffraction Intensity

(A)

(B)

(C) 70

60

50

40

30

20

10

Diffraction Angle 2 θ (degree) Fig. 6. 13C NMR spectra of (A) pPD polymer prepared with acetic acid without any catalyst and (B) oPD polymer prepared with sodium persulfate in hydrochloric solution in deuterated DMSO at 600.17 MHz.

Fig. 7. WAXD diffractograms of (A) oPD polymer prepared with ammonium persulfate in water, (B) pPD polymer prepared with potassium persulfate in hydrochloric solution and (C) BZN polymer prepared with potassium persulfate in hydrochloric solution.

I. Amer, D.A. Young / Polymer 54 (2013) 505e512 -0.4

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3.5. Thermal analysis of polymers

(A)

-0.8

Heat Flow (W/g)

-1.2 -1.6 -2.0

(B) (C) (D)

-2.4 -2.8 -3.2 -3.6 -4.0 150

200

250

300

350

400

Temperature (°C) Fig. 8. DSC thermograms of (A) pPD polymer prepared with ammonium persulfate in hydrochloric solution, (B) pPD polymer prepared with ammonium persulfate in acetic acid, (C) oPD polymer prepared with ammonium persulfate in hydrochloric solution and (D) oPD polymer prepared with sodium persulfate in hydrochloric solution.

structures of the oPD polymers. Copolymerization of phenylenediamines with other comonomers, which have different functional groups, may possibly improve the electrical conductivity of these polymers [2,4,10,33,34].

(A)

Acetic acid + ammonium persulfate Water + ammonium persulfate Al(OTf) 3 + ammonium persulfate

110

3.5.2. TGA of the polymers Fig. 9(A), (B), (C) and (D) shows the TGA curves of polymer powders of oPD, pPD, DADPM and BZN respectively, in flowing nitrogen. The polymers exhibited two weight loss processes (except

(B) 100

90

90

80

80

70 60 50 40

Acetic acid no catalyst HCl acid + ammonium persulfate Al(OTf) 3 + ammonium persulfate

110

Weight (%)

Weight (%)

100

3.5.1. DSC of the polymers There are not many studies in which DSC was used to analyze phenylenediamine oxidative polymers. Fig. 8(A), (B), (C) and (D) presents a waterfall plot of the DSC endotherms of pPD and oPD polymers. As can be seen from Fig. 8(A), (B), (C) and (D), all polymers showed glass transition temperatures (Tg) in the range of 258e260 C. This was taken as the midpoint of the change in slope of the baseline in DSC curve. The high glass transition temperatures of these polymers are probably due to the high rigidity of the polymer chains [49]. However, DSC detected no melting transitions of phenylenediamine polymers. These DSC results indicate that the pPD and oPD polymers have amorphous polymer structures as proved by XRD results. Thus, the DSC results reveal that pPD and oPD polymers not only have excellent thermal stability in high temperatures, which would make them useful for utilization by the engineering industry [50].

70 60 50 40 30

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0

0

-10 100

200

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400

500

600

700

800

900 1000

100

200

300

Temperature ( C)

500

600

700

800

900 1000

Temperature ( C)

(C)

(D) Water + ammonium persulfate HCl acid + ammonium persulfate Al(OTf) 3 + ammonium persulfate

110 100 90

100

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90

70

80

60 50 40 30 20 10

Water + ammonium persulfate HCl acid + ammonium persulfate Al(OTf) 3 + ammonium persulfate

110

Weight (%)

Weight (%)

400

o

o

70 60 50 40 30

0

20

-10

10

-20 100

200

300

400

500

600

700

800

0 100

200

300

400

500

600

700

o

Temperature ( C)

o

Temperature ( C)

Fig. 9. TGA curves of the polymer powders of (A) PoPD, (B) PpPD, (C) PDADPM, and (D) PBZN, in nitrogen at a heating rate of 10 C/min.

800

512


References

Table 3 TGA thermal data of the polymer powders. Polymer

T0 [ C]

T10 [ C]

Tmaxa [ C]

Char yield at 600 C [%]

PoPD PpPD PDADPM PBZN

250e260 200e295 200e230 200e215

263e280 240e315 210e235 220e250

550e650 355e590 360e420 300e380

44e72 0e68 0e63 21e48

a

Maximum decomposition temperature.

pPD polymer prepared in acetic acid without a catalyst as can be seen in Fig. 9(B)). The first weight loss of 6e8% at 60e70 C should be due to the evaporating of water molecules trapped by polymer chains. The second weight loss starting at about 200e350 C was attributed to the thermal degradation of polymer chains. The oPD, pPD, DADPM and BZN polymers showed different thermal behavior due to their difference of molecular structure. Also oPD polymers exhibited higher thermal stability than other polymers as can be seen in Fig. 9(A), (B), (C) and (D). The polymers showed the initial decomposition temperature (T0) and the temperature for 10% weight loss (T10) in the range of 200e350 C and 210e400 C, respectively. Moreover, the char yields of the polymers at 600 C were about 0e72%. The obtained results of oPD, pPD, DADPM and BZN polymers, from TGA were summarized in Table 3. According to the obtained data, these polymers showed good thermally stability and heat resistance due to their rigid chain structure, and is less dependent on their molecular weight. 4. Conclusions We polymerized oPD, pPD, DADPM and BZN using sodium persulfate, potassium persulfate and ammonium persulfate as oxidant catalysts in different solvents and temperatures. Moreover, we found that the presence of the oxidant was not necessary for the polymerization of pPD and BZN. Al(OTf)3 was successfully used as a co-catalyst to polymerize pPD and also BZN, but we were unable to polymerize oPD and DADPM using Al(OTf)3 as a catalyst on its own. The polymerization yield was dependent on the oxidative catalyst, solvent and temperature of the reactions. In general, the molecular weights of pPD polymers were higher than those of oPD polymers. FT-IR spectra of oPD and pPD polymers revealed relatively small differences among them but showed that these polymers consist of aminophenazine units containing both quinoid and benzenoid structures. In addition, NMR spectra and WAXD diffractograms of the oPD and pPD polymers provided additional detail on the molecular structures, including that the two kinds of polymers differed structurally from each other, depending on the catalyst and solvent used to prepare them. The polymers showed good thermal stability, but they showed a different thermal behavior due to their different of molecular structure. Acknowledgments The authors wish to thank (1) North West University, Potchefstroom, for awarding a research fellowship to I. Amer, (2) SASOL (South Africa) for the financial support and (3) Dr Dawie Joubert and Dr Jonathan Molefi of SASOL for TGA measurements.

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