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Synthesis and characterization of graft copolymers of melamine: Thermal stability, electrical conductivity, and optical properties
Synthetic Metals 159 (2009) 1572–1582
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Synthesis and characterization of graft copolymers of melamine: Thermal stability, electrical conductivity, and optical properties I˙ smet Kaya ∗ , Mehmet Yıldırım C¸anakkale Onsekiz Mart University, Faculty of Sciences and Arts, Department of Chemistry, 17020, C¸anakkale, Turkey
a r t i c l e
i n f o
Article history: Received 17 April 2008 Received in revised form 17 March 2009 Accepted 20 April 2009 Available online 20 May 2009 Keywords: Melamine Polyazomethine Oligophenol Graft copolymer Schiff base oligomer or polymers Conjugated polymers Thermal analysis Differential scanning calorimetry Conductivity and band gap
a b s t r a c t In this study, novel three Schiff bases of melamine were synthesized via condensation reaction of melamine with salicylaldehyde, 3-hydroxybenzaldehyde, and 4-hydroxybenzaldehyde namely N,N ,N ,tris[(2-hydroxyphenyl)methylene]-1,3,5-triazine-2,4,6-triamine (2-HPMTT), N,N ,N -tris[(3-hydroxyphenyl)methylene]-1,3,5-triazine-2,4,6-triamine (3-HPMTT), N,N ,N -tris[(4-hydroxyphenyl)methylene]-1,3,5-triazine-2,4,6-triamine (4-HPMTT), respectively. Then, oligo/polyphenol derivatives of these Schiff bases were obtained by grafting melamine onto oligosalicylaldehyde (OSA), oligo-3-hydroxybenzaldehyde, and oligo-4-hydroxybenzaldehyde that have generate names of poly-N,N ,N -tris[(2-hydroxyphenyl)methylene]-1,3,5-triazine-2,4,6-triamine (P-2-HPMTT), poly-N, N ,N -tris[(3-hydroxyphenyl)methylene]-1,3,5-triazine-2,4,6-triamine (P-3-HPMTT), and oligoN,N ,N -tris[(4-hydroxyphenyl)methylene]-1,3,5-triazine-2,4,6-triamine (O-4-HPMTT), respectively. The structures of the synthesized compounds were confirmed by FT-IR, UV–vis, 1 H NMR, and 13 C NMR techniques. The characterization was made by TG-DTA, DSC, size exclusion chromatography (SEC), and solubility tests. Electrical conductivities of the synthesized materials were measured by four-point probe technique using a Keithley 2400 electrometer showing that the synthesized oligo/polyphenols have higher electrical conductivities than the monomeric Schiff bases. Also considerable increases in the conductivities were observed when they were doped with iodine as a doping agent. The order of increase rates of the conductivities were found as follows: 2-HPMTT > P-2-HPMTT > P-3-HPMTT > 3-HPMTT > 4HPMTT > P-4-HPMTT. Additionally, the optical band gaps (Eg ) were calculated by using the absorption spectra and found to be 2.78, 2.17, 3.58, 3.30, 4.03, and 2.82 eV for 2-HPMTT, P-2-HPMTT, 3-HPMTT, P-3-HPMTT, 4-HPMTT, and O-4-HPMTT, respectively. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Polyazomethines (PAMs) have had continual increasing interest with a wide range of applications since they were firstly synthesized by the condensation of terephthaldehyde with benzidine and dianisidine [1]. PAMs have a lot of useful properties like high thermal stability, excellent mechanical strength, and good optoelectronic properties [2,3]. However, their low solubility is a serious problem and so PAMs, especially aromatic derivatives, should be prepared as soluble forms to make them more useful for applications. For this aim, a lot of PAMs have been synthesized like poly(azomethine ether)s [4], poly(acrylate-azomethine)s [5], poly(azomethine carbonate)s [6], poly(amide-azomethine-ester)s [7], and poly(azomethine sulfone)s [8]. Also, PAMs containing methoxy substituents have been presented with good solubility and high thermal stability [9].
∗ Corresponding author. Tel.: +90 286 218 00 18; fax: +90 286 218 05 33. E-mail address: [email protected] (I˙ . Kaya). 0379-6779/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2009.04.019
Oligophenols and their azomethine derivatives had been previously synthesized by oxidative polycondensation method and presented in the literature with their several useful properties [10–14]. This method is easy to apply and environmentally harmless due to the use of water as a medium. Also using cheap oxidants such as NaOCl, H2 O2 , and air O2 is another advantage of this method. Additionally, because of the synthesized products having good properties like paramagnetism, semi-conductivity, electrochemical cell, and resistance to high energy, this class of PAMs has become more attractive for researchers. During the last decade, this class of PAMs has been studied by Kaya and co-workers with thermal, electrochemical, optical, and electrical properties [15–18]. Moreover, considerable increases in conductivities have been obtained when the synthesized PAMs are exposed to iodine vapor. Antimicrobial activities of the synthesized products have been also investigated [19]. It is seen that the synthesized PAMs have semi-conductivity due to their polyconjugated structures [20–22]. Also, they have high thermal stability because of their highly aromatic structures. Oligo/polyphenol derivatives of melamine with Schiff base substituents have not been studied with their useful properties so far.
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This class of PAMs may yield a new research area due to their special characteristics such as the structures containing high rate of imine bond and phenolic groups. For these reasons, in this study we firstly synthesized novel melamine Schiff bases and than their oligo/polyphenol derivatives having polyconjugated structures via graft copolymerization. They were expected to have good electrical conductivity, low band gaps, and high resistance for thermal degradation due to their polyconjugated aromatic structures. In the second part of the study we characterized these synthesized products using FT-IR, UV–vis spectra, 1 H and 13 C NMR, and SEC analyses. TG-DTA technique was used to determine the stabilities to thermal degradation. DSC analyses of the oligo/polyphenols were also carried out to determine the glass transition temperatures (Tg ). Additionally, optical properties of these materials were determined by using UV–vis spectra and the optical band gaps were calculated from absorption edges. Electrical properties of doped and undoped oligo/polymers and Schiff bases were also determined showing that the synthesized oligo/polyphenols have higher electrical conductivities than those of their Schiff base monomers. 2. Experimental 2.1. Materials Melamine, salicylaldehyde, 3-hydroxybenzaldehyde, 4-hydroxybenzaldehyde, dimethylformamide (DMF), dimethylsulfoxide
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(DMSO), tetrahydrofurane (THF), dioxane, methanol, ethanol, 1buthanol, 2-propanol, acetonitrile, benzene, toluene, ethyl acetate, heptane, hexane, CCl4 , and CHCl3 were supplied from Merck Chemical Co. and they were used as received. Oligosalicylaldehyde [15], oligo-3-hydroxybenzaldehyde [23], and oligo4-hydroxybenzaldehyde [24] were prepared according to the literature and used as received. 2.2. Syntheses of the melamine Schiff bases The melamine Schiff bases abbreviated as 2-HPMTT, 3-HPMTT, and 4-HPMTT were synthesized by the condensation reaction in DMF. Reactions were made as follows: melamine (0.630 g, 0.005 mol) was placed into a 250 ml three-necked round-bottom flask which was fitted with condenser, thermometer, and magnetic stirrer. 80 ml DMF was added into the flask. Reaction mixture was heated up to 140 ◦ C and at this temperature melamine was solved. A solution of excessive amount of salicylaldehyde, 3hydroxybenzaldehyde, or 4-hydroxybenzaldehyde (2.440 g, 0.02 mol—for each aldehyde) in 20 ml. DMF was added into the flask. All reactions were maintained for 3 h under reflux (Scheme 1). Reaction mixture was cooled at room temperature and was slowly dropped into 200 ml toluene to precipitate the Schiff base. The product was filtered and washed with methanol/toluene (1:1) to separate unreacted aldehyde from the condensation product. Then it was dried in a vacuum desiccator (yields: 15, 38, and 37% for 2-HPMTT, 3-HPMTT, and 4-HPMTT, respectively).
Scheme 1. Syntheses of melamine Schiff bases.
Scheme 2. Syntheses of oligomer and polymers.
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2.3. Syntheses of graft copolymers Melamine Schiff base oligomer and polymers shortened as P-2-HPMTT, P-3-HPMTT, and O-4-HPMTT were synthesized by the grafting of melamine onto oligosalicylaldehyde, oligo-3hydroxybenzaldehyde, and oligo-4-hydroxybenzaldehyde, respectively. Reactions were made as follows: melamine (0.378 g, 0.003 mol) was placed into a 250 ml three-necked round-bottom flask which was fitted with condenser, thermometer, and magnetic stirrer. 50 ml DMF was added into the flask. Reaction mixture was heated up to 140 ◦ C. Excessive amounts of oligosalicylaldehyde,
oligo-3-hydroxybenzaldehyde, or oligo-4-hydroxybenzaldehyde (1.200 g, containing nearly 0.01 mol aldehyde groups) were dissolved in 20 ml DMF and were added into the flask. Reactions were maintained under reflux for 5 h. The reaction mixtures were cooled at the room temperature and were slowly dropped into toluene mediums each of which contains 200 ml toluene to precipitate the oligo/poly-Schiff bases (Scheme 2). Products were filtered and washed with methanol and toluene to separate from the unreacted oligohydroxyaldehydes. Then, they were dried in a vacuum desiccator (yields: 17, 42, and 35% for P-2-HPMTT, P-3-HPMTT, and O-4-HPMTT, respectively) (Scheme 3).
Scheme 3. (A) 2D view, (B) 3D view of O-4-HPMTT (H atoms are not shown at 3D view).
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Scheme 4. Schematic illustration of conductivity measurement and possible doping reaction of 4-HPMTT.
2.4. Electrical properties
2.6. Solubility and characterization techniques
Solid state electrical conductivities of the synthesized materials were measured by four-point probe technique using a Keithley 2400 Electrometer. The pellets of the materials were pressed on a hydraulic press developing up to 1687.2 kg/cm2 . Thickness of the pellets was about 1–3 mm and the diameter of all was 10 mm. Iodine doping was carried out by exposure of the pellets to iodine vapor at atmospheric pressure and room temperature in a desiccator [20]. Measurements were carried out in 24 h time periods. A schematic view of conductivity measurements of 4-HPMTT was shown in Scheme 4.
2-HPMTT, 3-HPMTT, and 4-HPMTT have light-colored powder forms. However, their oligo/polyphenol derivatives shown as P-2HPMTT, P-3-HPMTT, and O-4-HPMTT are dark-colored in powder forms. The solubility tests were done in different solvents by using 1 mg sample and 1 ml solvent at 25 ◦ C and the results are shown in Table 1. According to Table 1 the Schiff bases and oligo/poly-Schiff bases are insoluble in many organic solvents (except for DMF and DMSO). They are all soluble in conc. H2 SO4 . The Schiff bases are soluble in hot water while the solubilities of their oligo/polymer derivatives in water are various. Although P-3-HPMTT is completely soluble in hot water, O-4-HPMTT is partially soluble, but P-2-HPMTT is insoluble. The infrared and ultraviolet–visible spectra were measured by Perkin Elmer FT-IR Spectrum one and Perkin Elmer Lambda 25, respectively. The FT-IR spectra were recorded using universal ATR sampling accessory (4000–550 cm−1 ). 3-HPMTT, P-3-HPMTT, 4HPMTT, and O-4-HPMTT were also characterized by using 1 H and 13 C NMR spectra (Bruker AC FT-NMR spectrometer operating at 400 and 100.6 MHz, respectively) and recorded by using deuterated DMSO-d6 as a solvent at 25 ◦ C. The tetramethylsilane was used as an internal standard. UV–vis measurements were carried out by
2.5. Optical properties The optical band gaps (Eg ) of the Schiff bases and oligo/polySchiff bases were calculated by their absorption edges. Ultraviolet–visible (UV–vis) spectra were measured by Perkin Elmer Lambda 25 double beam spectrophotometer. The absorption spectra were recorded in 10 mm × 10 mm × 45 mm quartz cells by using DMSO, at 25 ◦ C, in the range of 250–700 nm (scan speed: 2880 nm/min; slit width: 1.00; interval: 1.200 nm). Table 1 Solubility tests of the synthesized melamine compounds. Solvents Methanol Ethanol Acetone Acetonitrile Cyclohexane CHCl3 Ethyl acetate 0 Dioxane Methylene chloride THF DMF (25 ◦ C) DMF (100 ◦ C) Water (25 ◦ C) Water (100 ◦ C) Conc. H2 SO4 DMSO
2-HPMTT
3-HPMTT
4-HPMTT
P-2-HPMTT
P-3-HPMTT
O-4-HPMTT
− − − − − − ± − ± − ± ± − + + +
− − ± − − − − − − − ± + − + + +
− − ± − − − − − − ± + + − + + +
− − − − − − − − − − + + − − + +
+ + + ± − + ± − ± + + + − + + +
− − − − − − − − − − + + − ± + +
(+) soluble, (−) insoluble, (±) partially soluble.
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Fig. 1. FT-IR spectra of 2-HPMTT, 3-HPMTT, 4-HPMTT, and melamine.
Fig. 2. FT-IR spectra of P-2-HPMTT, P-3-HPMTT, and O-4-HPMTT.
using DMSO. Thermal data were obtained by using Perkin Elmer Diamond Thermal Analysis. The TG-DTA measurements were made between 20 and 1000 ◦ C (in N2 , rate 10 ◦ C/min). DSC analyses were carried out by using Perkin Elmer Pyris Sapphire DSC. DSC measurements were made between 25 and 420 ◦ C (in N2 , rate 20 ◦ C/min). The number average molecular weight (Mn ), weight average molecular weight (Mw ), and polydispersity index (PDI) were determined by size exclusion chromatography (SEC) techniques of Shimadzu Co. For SEC investigations an SGX (100 Å and 7 nm diameter loading material) 3.3 mm i.d. × 300 mm columns was used; eluent: DMF (0.4 ml/min), polystyrene standards. A refractive index detector was used to analyze the products at 25 ◦ C.
structures of the synthesized compounds. Some of the other peaks observed in the FT-IR spectra are also given in Table 2. The UV–vis analyses results are also shown in Table 2. In these spectra the absorption peaks of the synthesized oligo/polymers spread on much broader range of wavelength compared to the monomers. 1 H and 13 C NMR spectra of the 3-HPMTT, P-3-HPMTT, 4-HPMTT, and O-4-HPMTT are shown in Figs. 3–6, respectively. Also, the NMR analyses results are given in Table 3. According to these results, at the 1 H NMR spectra of P-3-HPMTT a new singlet peak at 7.30 ppm appear as a result of polymerization indicated –Hb proton. Also, according to 13 C NMR spectra of P-3-HPMTT, the values of C2, C4, and C6 – where the polymerization progress – shift to higher chemical shifts. At the 13 C NMR spectra of P-3-HPMTT, the chemical shift values of C2, C4, and C6 are 123.33, 122.86, and 130.48 ppm whereas at the 13 C NMR spectra of 3-HPMTT, they are 114.50, 121.00, and 121.75 ppm, respectively. When we compare the 1 H NMR spectra of 4-HPMTT with the spectra of O-4-HPMTT, it is seen that after the polymerization a new peak appear at 7.95 ppm indicated –Ha proton, but for –Hb proton any new peak is not observed. This is because of the combination of monomer units occurring at the ortho position of the –OH group during the radicalic polymerization. However, the doublet peaks at 7.75 ppm and 6.95 ppm at the 1 H NMR spectrum of O-4-HPMTT are owing to the –Ha and –Hb protons of the terminal units. The radicalic polymerization mechanism of oligophenols had been previously studied and presented in the literature21 . Additionally, as seen in the 13 C NMR spectra of O-4-HPMTT, the peak value of C2 shifts from 117.92 to 130.60 ppm as a result of intermolecular combination of the monomer units. The observed results confirm the structures of the synthesized compounds.
3. Results and discussions 3.1. Structures of the compounds The FT-IR spectra of the synthesized monomers and the oligo/polymers are given in Figs. 1 and 2, respectively. The FT-IR spectrum of melamine is also shown in Fig. 1 as the comparison spectrum. As seen in Fig. 1, the peaks at 3417 and 3468 cm−1 on the spectrum of melamine indicate the three –NH2 groups of melamine. However, on the other spectra these peaks disappear as a result of the condensation of the –NH2 groups with the –CHO groups of the aldehydes. Additionally, the new imine bonds (–CH N) formed after the condensation reactions appear at 1625, 1633, and 1635 cm−1 for the 2-HPMTT, 3-HPMTT, and 4-HPMTT, respectively. Also, Fig. 2 indicates the peaks of new imine bonds at 1633, 1635, and 1633 cm−1 for P-2-HPMTT, P-3-HPMTT, and O4-HPMTT, respectively. And also, –NH2 group peaks of melamine disappear. Consequently, the FT-IR analyses results confirm the Table 2 Some of the spectral data and optical band gaps (Eg ) of the synthesized compounds. Compounds
2-HPMTT P-2-HPMTT 3-HPMTT P-3-HPMTT 4-HPMTT O-4-HPMTT
FT-IR (cm−1 )
UV
CH N
C C
C–OH
1625 1633 1633 1635 1635 1633
1530, 1435 1532, 1451, 1433 1521, 1435, 1442 1522, 1451 1522, 1452 1526, 1440
1253 1246 1253 1251 1250 1248
max (nm) 261, 332, 390 268 260, 316 260 286 262
Eg (eV) 2.78 2.17 3.58 3.30 4.03 2.82
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Fig. 3.
1
H NMR (a) and 13 C NMR (b) spectra of 3-HPMTT.
According to SEC chromatograms, the values of number average molecular weight (Mn ), weight average molecular weight (Mw ), and polydispersity index (PDI) of P-2-HPMTT, P-3-HPMTT, and O4-HPMTT are calculated in accordance with a polystyrene standard
calibration curve and given in Table 4. According to the SEC analyses, the number average molecular weight (Mn ), weight average molecular weight (Mw ), and polydispersity index (PDI) values of total of P-2-HPMTT, P-3-HPMTT, and O-4-HPMTT are found to be
Table 3 1 H and 13 C NMR spectral data of 3-HPMTT, P-3-HPMTT, 4-HPMTT, and O-4-HPMTT. Compound
1
3-HPMTT
H NMR (DMSO): ı ppm: 11.15 (d, –OH), 9.30 (d, –CH N–), 7.42 (t, –Hb ), 7.35 (d, –Ha ), 7.25 (s, –Hd ), 7.10 (d, –Hc ). 13 C NMR (DMSO): ppm: 167.25 (C8-ipso), 162.50 (C7-H), 159.25 (C3-ipso), 137.50 (C1-ipso), 130.00 (C5-H), 121.75 (C6-H), 121.00 (C4-H), 114.50 (C2-H).
P-3-HPMTT
1 H NMR (DMSO): ı ppm: 11.04 (d, –OH), 9.30 (d, –CH N–), 7.41 (t, –Hb -terminal), 7.36 (d, –Ha -terminal), 7.30 (s, –Hb ), 7.25 (s, –Hd -terminal), 7.10 (d, –Hc -terminal). 13 C NMR (DMSO): ppm: 167.86 (C8-ipso), 164.29 (C7-H), 163.10 (C3-ipso), 130.48 (C6-ipso), 129.29 (C5-H), 123.33 (C2-ipso), 122.86 (C4-ipso), 122.14 (C1-ipso).
4-HPMTT
1
H/13 C NMR data
1
H NMR (DMSO): ı ppm: 10.92 (d, –OH), 9.29 (d, –CH N–), 7.76 (d, –Ha ), 6.93 (d, –Hb ). C NMR (DMSO): ppm: 167.50 (C6-ipso), 164.17 (C1-ipso), 162.71 (C5-H), 134.17 (C3-H), 129.79 (C4-ipso), 117.92 (C2-H).
13
O-4-HPMTT
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1 H NMR (DMSO): ı ppm: 10.55 (s, –OH), 9.29 (s, –CH N–), 7.95 (s, –Ha ), 7.75 (d, –Ha -terminal), 6.95 (d, –Hb -terminal). 13 C NMR (DMSO): ppm: 167.80 (C6-ipso), 164.00 (C1-ipso), 162.80 (C5-H), 134.60 (C3-H), 130.60 (C2-ipso), 129.60 (C4-ipso), 118.20 (C2-H-terminal).
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Fig. 4.
1
H NMR (a) and 13 C NMR (b) spectra of P-3-HPMTT.
6000, 7700 g mol−1 and 1.283; 5600, 6000 g mol−1 and 1.071; and 1900, 2050 g mol−1 and 1.079, respectively.
indicate easy electron transfer between HOMO and LUMO energy levels. The synthesized oligo/polymers thus have higher electrical conductivities related to lower band gaps.
3.2. Optical properties 3.3. Electrical conductivities The absorption spectra recorded by using DMSO are shown in Fig. 7a (for 2-HPMTT and P-2-HPMTT), Fig. 7b (for 3-HPMTT and P-3-HPMTT), and Fig. 7c (for 4-HPMTT and O-4-HPMTT). max and the optical band gaps are calculated as in the literature [25] and shown in Table 2. As seen in Table 2, max (nm) and the optical band gaps (Eg /eV) are found as 390 and 2.78 for 2-HPMTT, 268 and 2.17 for P-2-HPMTT, 316 and 3.58 for 3-HPMTT, 260 and 3.30 for P-3-HPMTT, 286 and 4.03 for 4-HPMTT, and 262 and 2.82 for O4-HPMTT, respectively. Because of their polyconjugated structures P-2-HPMTT, P-3-HPMTT, and O-4-HPMTT have lower band gaps than 2-HPMTT, 3-HPMTT, and 4-HPMTT. These results support the electrical conductivity measurements because the lower band gaps
Electrical conductivities of the synthesized monomers and the oligo/polymers and the changes of these values related to doping time with iodine are determined and shown in Table 5. The changes of the electrical conductivities are also shown schematically in Fig. 8. As seen in Table 5 and Fig. 8, although the initial conductivity of P-2-HPMTT is higher than 2-HPMTT, when doped with iodine for 168 h, its conductivity becomes a little lower than the monomer between 10−6 and 10−5 S/cm. The increases in the conductivities of similar kinds of PAMs which derived from salicylaldehyde were previously studied under doping conditions. The initial conductivities of P-2-MPIMP [17], OFPIMP [20], O-4-HBAB [18], and O-2M-4-
Table 4 The number average molecular weight (Mn ), weight average molecular weight (Mw ), polydispersity index (PDI) and % values of the synthesized oligo/polymers. Compounds
Total Mn
P-2-HPMTT P-3-HPMTT O-4-HPMTT
6000 5600 1900
Molecular weight distribution parameters Mw
7700 6000 2050
PDI
1.283 1.071 1.079
Fraction I
Fraction II
Fraction III
Mn
Mw
PDI
%
Mn
Mw
PDI
%
Mn
Mw
PDI
%
700 700 700
900 900 900
1.286 1.286 1.286
65 80 75
7700 7500 7500
8800 8600 8600
1.143 1.150 1.147
10 15 15
67,900 64,000 41,500
86,900 75,000 42,200
1.280 1.172 1.017
25 05 10
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Fig. 5.
Fig. 6.
1
1
H NMR (a) and 13 C NMR (b) spectra of 4-HPMTT.
H NMR (a) and 13 C NMR (b) spectra of O-4-HPMTT.
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Fig. 7. Absorption spectra of 2-HPMTT and P-2-HPMTT (a), 3-HPMTT and P-3-HPMTT (b), 4-HPMTT and O-4-HPMTT (c).
MPIMP [22] were ∼10−12 , 10−10 –10−9 , 10−10 –10−9 , and 10−8 S/cm, respectively. However, their conductivities could be increased by ∼104 , 102 , 103 , and 103 orders of their magnitudes and reached up to 3 × 10−8 , 6 × 10−8 , 7.82 × 10−7 , and 1 × 10−5 S/cm when doped with iodine for 168 h at the same conditions. P-2-HPMTT, which we synthesized by grafting melamine onto OSA, has the conductivity increase by ∼3 × 106 order of its magnitude when doped for 168 h and this increase is quite greater than the previously presented. This is probably because of higher imine bond rate of P-2-HPMTT than those of refereed above. This causes higher iodine coordination with the polymer and consequently a higher increase in conductivity is obtained. The initial conductivities of 3-HPMTT and P-3-HPMTT are nearly the same. But, the conductivity of P-3-HPMTT increased up to 10−7 S/cm whereas the resultant conductivity of 3-HPMTT is about eight times lower than P-3-HPMTT. Additionally, the initial conductivities of 4-HPMTT and O-4-HPMTT are nearly 10−10 and 10−11 S/cm, respectively. When doped with iodine, the conductiv-
ity of 4-HPMTT can be increased by about four and half orders of magnitude (up to 10−6 S/cm) whereas the conductivity of O4-HPMTT can be increased by about four orders of magnitude (up to 10−8 S/cm). As a result, the order of the increase rates in conductivities by doping for 168 h is as follows: 2-HPMTT > P2-HPMTT > P-3-HPMTT > 3-HPMTT > 4-HPMTT > P-4-HPMTT. In the doping of the monomers and the oligo/polymers with iodine, it is found that the conductivities firstly increase greatly with doping time, but then tend to level-off. The increasing conductivity can indicate that a charge-transfer complex between the polymers (or monomers) and dopant iodine is continuously formed. Consequently, Fig. 8 not only shows the conductivity/doping time relationship but also indicates doping kinetics of the synthesized materials. The experiments show that a longer doping time is needed to obtain the maximal conductivity. As a result, the conductivity/doping time curves vary with doping conditions. In order to exclude the influence of doping conditions, the conductivities of the doped monomers and oligo/polymers have
Table 5 Electrical conductivity results of I2 -doped melamine compounds vs. doping time at 25 ◦ C. Time (h)
0 24 48 72 96 120 144 168
Conductivity (×10−13 S/cm−1 ) 2-HPMTT
P-2-HPMTT
66.7 12,038,257 49,280,016 73,639,910 102,033,984 155,102,748 186,203,995 193,040,438
234 2,660,394 15,183,022 31,230,298 54,730,926 65,299,202 76,592,732 77,500,330
3-HPMTT 174 420,063 1,178,220 1,869,286 2,202,739 3,065,243 3,451,982 3,599,460
P-3-HPMTT
4-HPMTT
O-4-HPMTT
138 6,57,839 3,241,192 6,549,553 12,804,033 24,102,751 27,204,066 28,673,049
83.8 6540 35538 116047 291003 655077 947220 1145287
777 802039 3234722 3880153 4180372 4295471 4340110 4358490
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Fig. 10. DTG curves of 2-HPMTT, P-2-HPMTT, 3-HPMTT, P-3-HPMTT, 4-HPMTT, and O-4-HPMTT.
Fig. 8. Changes of the electrical conductivities of the I2 -doped synthesized compounds vs. doping time at 25 ◦ C.
Fig. 11. DTA curves of 2-HPMTT, P-2-HPMTT, 3-HPMTT, P-3-HPMTT, 4-HPMTT, and O-4-HPMTT.
Fig. 9. TGA curves of 2-HPMTT, P-2-HPMTT, 3-HPMTT, P-3-HPMTT, 4-HPMTT, and O-4-HPMTT.
been related with doping extent (shown in Fig. 8). Diaz et al. suggested the conductivity mechanisms of Schiff base polymers for doping with iodine [26]. Nitrogen is a very electronegative element and it is capable of coordinating with an iodine molecule. Coordination of iodine with Schiff base polymers was presented in the literature [17,18]. 3.4. Thermal analyses TGA, DTG, and DTA curves for 2-HPMTT, P-2-HPMTT, 3-HPMTT, P-3-HPMTT, 4-HPMTT, and O-4-HPMTT are shown in Figs. 9–11, respectively. Using these curves, the calculated thermal degradation data are summarized in Table 6. As seen in Table 6, the initial degradation temperature (◦ C) and the carbine residue (%) at 1000 ◦ C are 118, 281 and 0.83 for 2-HPMTT; 106, 240, 407 and 37.21 for
P-2-HPMTT; 93, 257 and 2.76 for 3-HPMTT; 98, 257 and 2.20 for P-3HPMTT; 101, 258 and 2.43 for 4-HPMTT; and 106, 255, 757 and 1.59 for O-4-HPMTT, respectively. According to these values P-2-HPMTT is more stable than 2-HPMTT to temperature and thermal decomposition because of its polyconjugated bond system. However, it is seen in Table 6 that P-3-HPMTT and O-4-HPMTT lose nearly the same percentage of their weights compared to their monomers at 1000 ◦ C. Consequently, P-2-HPMTT is found to be the most thermally stable polymer among the synthesized oligo/polymers. DSC curves of P-2-HPMTT, P-3-HPMTT, and O-4-HPMTT are shown in Fig. 12 and using these curves the calculated glass transition temperatures (Tg ) and changes of specific heat values during glass transitions (Cp ) are also given in Table 6. According to these results, P-3-HPMTT has the most Tg value among the synthesized oligo/polymers. Also, as seen in the DSC curves, absence of any endothermic or exothermic peak of P-3-HPMTT agree with its DTA analysis results. However, DSC analyses results of P-3-HPMTT and O-4-HPMTT differ from their DTA results with new endothermic peaks observed at the temperatures of 187.04 and 353.82 ◦ C for P-
Table 6 Thermal degradation values of the synthesized monomers, oligomer, and polymers. Compounds
2-HPMTT P-2-HPMTT 3-HPMTT P-3-HPMTT 4-HPMTT O-4-HPMTT a b c d
Ton a
118, 281 106, 240, 407 93, 257 98, 257 101, 258 106, 255, 757
Wmax. Tb
138, 320 132, 276, 469 116, 285 125, 282 121, 288 123, 284, 803
The onset temperature. Maximum weight temperature. Glass transition temperature. Change of specific heat during glass transition.
DTA
20% weight losses
50% weight losses
% Carbine residue at 1000 ◦ C
Exo
254 249 230 127 224 164
305 572 275 267 279 279
0.83 37.21 2.76 2.20 2.43 1.59
– – – – – –
DSC Endo 141, 325 – 294 290 295 130, 293, 816
Tg (◦ C)c
Cp (j/g. ◦ C)d
– 71.73 – 170.34 – 66.05
– 0.356 – 1.042 – 0.168
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undergo any exothermic event during the heating up until 1000 ◦ C. The Tg values of the synthesized oligo/polymers were also determined using the DSC curves and found as 71.73, 170.34, and 66.05 ◦ C for P-2-HPMTT, P-3-HPMTT, and O-4-HPMTT, respectively. The electrical conductivity measurements showed that the synthesized melamine compounds were semiconductors and their conductivities could be increased considerable via doping with iodine. What is more, 2-HPMTT had the greatest conductivity increase among the synthesized. The properties of the synthesized compounds with low optical band gap characteristics were determined. The optical band gap values of the Schiff bases were found to be higher than the oligo/polyphenol derivatives. The observed band gaps were sufficiently low to make these oligo/polymers highly promising for electronic, optoelectronic, electroactive, and photovoltaic applications. References Fig. 12. DSC thermograms of P-2-HPMTT, P-3-HPMTT, and O-4-HPMTT.
3-HPMTT; and 176.86, 301.80, 357.17 and 366.40 ◦ C for O-4-HPMTT. On the other hand, it is also supported by DSC analyses that there is no exothermic event during the synthesized oligo/polymers until when heated up to 420 ◦ C as suggested from DTA analyses. 4. Conclusions The Schiff bases and the oligo/polyphenol derivatives of melamine abbreviated as 2-HPMTT, 3-HPMTT, 4-HPMTT, P-2HPMTT, P-3-HPMTT, and O-4-HPMTT were synthesized with the yields of 15, 38, 37, 17, 42, and 35%, respectively. The solubility tests showed that the synthesized melamine Schiff bases and the copolymers had little solubilities in many organic solvents. But they completely dissolved in DMSO and H2 SO4 . Also, with the exception of 2-HPMTT and 3-HPMTT they are all dissolved in DMF. What is more, although they are insoluble in cold water, many of them are soluble in hot water (except for P-2-HPMTT and O-4-HPMTT). Thermal analysis results demonstrated that there was sufficient resistance to thermal degradation of P-2-HPMTT and O-4-HPMTT while the others had lower stabilities to thermal degradation. According to TG analysis results, 2-HPMTT, 3-HPMTT, 4-HPMTT, P2-HPMTT, P-3-HPMTT, and O-4-HPMTT lost half of their weights at 305, 275, 279, 572, 267, and 279 ◦ C, respectively. DTA and DSC analyses results showed that the synthesized compounds did not
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Synthesis and characterization of graft copolymers of melamine: Thermal stability, electrical conductivity, and optical properties I˙ smet Kaya ∗ , Mehmet Yıldırım C¸anakkale Onsekiz Mart University, Faculty of Sciences and Arts, Department of Chemistry, 17020, C¸anakkale, Turkey
a r t i c l e
i n f o
Article history: Received 17 April 2008 Received in revised form 17 March 2009 Accepted 20 April 2009 Available online 20 May 2009 Keywords: Melamine Polyazomethine Oligophenol Graft copolymer Schiff base oligomer or polymers Conjugated polymers Thermal analysis Differential scanning calorimetry Conductivity and band gap
a b s t r a c t In this study, novel three Schiff bases of melamine were synthesized via condensation reaction of melamine with salicylaldehyde, 3-hydroxybenzaldehyde, and 4-hydroxybenzaldehyde namely N,N ,N ,tris[(2-hydroxyphenyl)methylene]-1,3,5-triazine-2,4,6-triamine (2-HPMTT), N,N ,N -tris[(3-hydroxyphenyl)methylene]-1,3,5-triazine-2,4,6-triamine (3-HPMTT), N,N ,N -tris[(4-hydroxyphenyl)methylene]-1,3,5-triazine-2,4,6-triamine (4-HPMTT), respectively. Then, oligo/polyphenol derivatives of these Schiff bases were obtained by grafting melamine onto oligosalicylaldehyde (OSA), oligo-3-hydroxybenzaldehyde, and oligo-4-hydroxybenzaldehyde that have generate names of poly-N,N ,N -tris[(2-hydroxyphenyl)methylene]-1,3,5-triazine-2,4,6-triamine (P-2-HPMTT), poly-N, N ,N -tris[(3-hydroxyphenyl)methylene]-1,3,5-triazine-2,4,6-triamine (P-3-HPMTT), and oligoN,N ,N -tris[(4-hydroxyphenyl)methylene]-1,3,5-triazine-2,4,6-triamine (O-4-HPMTT), respectively. The structures of the synthesized compounds were confirmed by FT-IR, UV–vis, 1 H NMR, and 13 C NMR techniques. The characterization was made by TG-DTA, DSC, size exclusion chromatography (SEC), and solubility tests. Electrical conductivities of the synthesized materials were measured by four-point probe technique using a Keithley 2400 electrometer showing that the synthesized oligo/polyphenols have higher electrical conductivities than the monomeric Schiff bases. Also considerable increases in the conductivities were observed when they were doped with iodine as a doping agent. The order of increase rates of the conductivities were found as follows: 2-HPMTT > P-2-HPMTT > P-3-HPMTT > 3-HPMTT > 4HPMTT > P-4-HPMTT. Additionally, the optical band gaps (Eg ) were calculated by using the absorption spectra and found to be 2.78, 2.17, 3.58, 3.30, 4.03, and 2.82 eV for 2-HPMTT, P-2-HPMTT, 3-HPMTT, P-3-HPMTT, 4-HPMTT, and O-4-HPMTT, respectively. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Polyazomethines (PAMs) have had continual increasing interest with a wide range of applications since they were firstly synthesized by the condensation of terephthaldehyde with benzidine and dianisidine [1]. PAMs have a lot of useful properties like high thermal stability, excellent mechanical strength, and good optoelectronic properties [2,3]. However, their low solubility is a serious problem and so PAMs, especially aromatic derivatives, should be prepared as soluble forms to make them more useful for applications. For this aim, a lot of PAMs have been synthesized like poly(azomethine ether)s [4], poly(acrylate-azomethine)s [5], poly(azomethine carbonate)s [6], poly(amide-azomethine-ester)s [7], and poly(azomethine sulfone)s [8]. Also, PAMs containing methoxy substituents have been presented with good solubility and high thermal stability [9].
∗ Corresponding author. Tel.: +90 286 218 00 18; fax: +90 286 218 05 33. E-mail address: [email protected] (I˙ . Kaya). 0379-6779/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2009.04.019
Oligophenols and their azomethine derivatives had been previously synthesized by oxidative polycondensation method and presented in the literature with their several useful properties [10–14]. This method is easy to apply and environmentally harmless due to the use of water as a medium. Also using cheap oxidants such as NaOCl, H2 O2 , and air O2 is another advantage of this method. Additionally, because of the synthesized products having good properties like paramagnetism, semi-conductivity, electrochemical cell, and resistance to high energy, this class of PAMs has become more attractive for researchers. During the last decade, this class of PAMs has been studied by Kaya and co-workers with thermal, electrochemical, optical, and electrical properties [15–18]. Moreover, considerable increases in conductivities have been obtained when the synthesized PAMs are exposed to iodine vapor. Antimicrobial activities of the synthesized products have been also investigated [19]. It is seen that the synthesized PAMs have semi-conductivity due to their polyconjugated structures [20–22]. Also, they have high thermal stability because of their highly aromatic structures. Oligo/polyphenol derivatives of melamine with Schiff base substituents have not been studied with their useful properties so far.
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This class of PAMs may yield a new research area due to their special characteristics such as the structures containing high rate of imine bond and phenolic groups. For these reasons, in this study we firstly synthesized novel melamine Schiff bases and than their oligo/polyphenol derivatives having polyconjugated structures via graft copolymerization. They were expected to have good electrical conductivity, low band gaps, and high resistance for thermal degradation due to their polyconjugated aromatic structures. In the second part of the study we characterized these synthesized products using FT-IR, UV–vis spectra, 1 H and 13 C NMR, and SEC analyses. TG-DTA technique was used to determine the stabilities to thermal degradation. DSC analyses of the oligo/polyphenols were also carried out to determine the glass transition temperatures (Tg ). Additionally, optical properties of these materials were determined by using UV–vis spectra and the optical band gaps were calculated from absorption edges. Electrical properties of doped and undoped oligo/polymers and Schiff bases were also determined showing that the synthesized oligo/polyphenols have higher electrical conductivities than those of their Schiff base monomers. 2. Experimental 2.1. Materials Melamine, salicylaldehyde, 3-hydroxybenzaldehyde, 4-hydroxybenzaldehyde, dimethylformamide (DMF), dimethylsulfoxide
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(DMSO), tetrahydrofurane (THF), dioxane, methanol, ethanol, 1buthanol, 2-propanol, acetonitrile, benzene, toluene, ethyl acetate, heptane, hexane, CCl4 , and CHCl3 were supplied from Merck Chemical Co. and they were used as received. Oligosalicylaldehyde [15], oligo-3-hydroxybenzaldehyde [23], and oligo4-hydroxybenzaldehyde [24] were prepared according to the literature and used as received. 2.2. Syntheses of the melamine Schiff bases The melamine Schiff bases abbreviated as 2-HPMTT, 3-HPMTT, and 4-HPMTT were synthesized by the condensation reaction in DMF. Reactions were made as follows: melamine (0.630 g, 0.005 mol) was placed into a 250 ml three-necked round-bottom flask which was fitted with condenser, thermometer, and magnetic stirrer. 80 ml DMF was added into the flask. Reaction mixture was heated up to 140 ◦ C and at this temperature melamine was solved. A solution of excessive amount of salicylaldehyde, 3hydroxybenzaldehyde, or 4-hydroxybenzaldehyde (2.440 g, 0.02 mol—for each aldehyde) in 20 ml. DMF was added into the flask. All reactions were maintained for 3 h under reflux (Scheme 1). Reaction mixture was cooled at room temperature and was slowly dropped into 200 ml toluene to precipitate the Schiff base. The product was filtered and washed with methanol/toluene (1:1) to separate unreacted aldehyde from the condensation product. Then it was dried in a vacuum desiccator (yields: 15, 38, and 37% for 2-HPMTT, 3-HPMTT, and 4-HPMTT, respectively).
Scheme 1. Syntheses of melamine Schiff bases.
Scheme 2. Syntheses of oligomer and polymers.
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2.3. Syntheses of graft copolymers Melamine Schiff base oligomer and polymers shortened as P-2-HPMTT, P-3-HPMTT, and O-4-HPMTT were synthesized by the grafting of melamine onto oligosalicylaldehyde, oligo-3hydroxybenzaldehyde, and oligo-4-hydroxybenzaldehyde, respectively. Reactions were made as follows: melamine (0.378 g, 0.003 mol) was placed into a 250 ml three-necked round-bottom flask which was fitted with condenser, thermometer, and magnetic stirrer. 50 ml DMF was added into the flask. Reaction mixture was heated up to 140 ◦ C. Excessive amounts of oligosalicylaldehyde,
oligo-3-hydroxybenzaldehyde, or oligo-4-hydroxybenzaldehyde (1.200 g, containing nearly 0.01 mol aldehyde groups) were dissolved in 20 ml DMF and were added into the flask. Reactions were maintained under reflux for 5 h. The reaction mixtures were cooled at the room temperature and were slowly dropped into toluene mediums each of which contains 200 ml toluene to precipitate the oligo/poly-Schiff bases (Scheme 2). Products were filtered and washed with methanol and toluene to separate from the unreacted oligohydroxyaldehydes. Then, they were dried in a vacuum desiccator (yields: 17, 42, and 35% for P-2-HPMTT, P-3-HPMTT, and O-4-HPMTT, respectively) (Scheme 3).
Scheme 3. (A) 2D view, (B) 3D view of O-4-HPMTT (H atoms are not shown at 3D view).
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Scheme 4. Schematic illustration of conductivity measurement and possible doping reaction of 4-HPMTT.
2.4. Electrical properties
2.6. Solubility and characterization techniques
Solid state electrical conductivities of the synthesized materials were measured by four-point probe technique using a Keithley 2400 Electrometer. The pellets of the materials were pressed on a hydraulic press developing up to 1687.2 kg/cm2 . Thickness of the pellets was about 1–3 mm and the diameter of all was 10 mm. Iodine doping was carried out by exposure of the pellets to iodine vapor at atmospheric pressure and room temperature in a desiccator [20]. Measurements were carried out in 24 h time periods. A schematic view of conductivity measurements of 4-HPMTT was shown in Scheme 4.
2-HPMTT, 3-HPMTT, and 4-HPMTT have light-colored powder forms. However, their oligo/polyphenol derivatives shown as P-2HPMTT, P-3-HPMTT, and O-4-HPMTT are dark-colored in powder forms. The solubility tests were done in different solvents by using 1 mg sample and 1 ml solvent at 25 ◦ C and the results are shown in Table 1. According to Table 1 the Schiff bases and oligo/poly-Schiff bases are insoluble in many organic solvents (except for DMF and DMSO). They are all soluble in conc. H2 SO4 . The Schiff bases are soluble in hot water while the solubilities of their oligo/polymer derivatives in water are various. Although P-3-HPMTT is completely soluble in hot water, O-4-HPMTT is partially soluble, but P-2-HPMTT is insoluble. The infrared and ultraviolet–visible spectra were measured by Perkin Elmer FT-IR Spectrum one and Perkin Elmer Lambda 25, respectively. The FT-IR spectra were recorded using universal ATR sampling accessory (4000–550 cm−1 ). 3-HPMTT, P-3-HPMTT, 4HPMTT, and O-4-HPMTT were also characterized by using 1 H and 13 C NMR spectra (Bruker AC FT-NMR spectrometer operating at 400 and 100.6 MHz, respectively) and recorded by using deuterated DMSO-d6 as a solvent at 25 ◦ C. The tetramethylsilane was used as an internal standard. UV–vis measurements were carried out by
2.5. Optical properties The optical band gaps (Eg ) of the Schiff bases and oligo/polySchiff bases were calculated by their absorption edges. Ultraviolet–visible (UV–vis) spectra were measured by Perkin Elmer Lambda 25 double beam spectrophotometer. The absorption spectra were recorded in 10 mm × 10 mm × 45 mm quartz cells by using DMSO, at 25 ◦ C, in the range of 250–700 nm (scan speed: 2880 nm/min; slit width: 1.00; interval: 1.200 nm). Table 1 Solubility tests of the synthesized melamine compounds. Solvents Methanol Ethanol Acetone Acetonitrile Cyclohexane CHCl3 Ethyl acetate 0 Dioxane Methylene chloride THF DMF (25 ◦ C) DMF (100 ◦ C) Water (25 ◦ C) Water (100 ◦ C) Conc. H2 SO4 DMSO
2-HPMTT
3-HPMTT
4-HPMTT
P-2-HPMTT
P-3-HPMTT
O-4-HPMTT
− − − − − − ± − ± − ± ± − + + +
− − ± − − − − − − − ± + − + + +
− − ± − − − − − − ± + + − + + +
− − − − − − − − − − + + − − + +
+ + + ± − + ± − ± + + + − + + +
− − − − − − − − − − + + − ± + +
(+) soluble, (−) insoluble, (±) partially soluble.
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Fig. 1. FT-IR spectra of 2-HPMTT, 3-HPMTT, 4-HPMTT, and melamine.
Fig. 2. FT-IR spectra of P-2-HPMTT, P-3-HPMTT, and O-4-HPMTT.
using DMSO. Thermal data were obtained by using Perkin Elmer Diamond Thermal Analysis. The TG-DTA measurements were made between 20 and 1000 ◦ C (in N2 , rate 10 ◦ C/min). DSC analyses were carried out by using Perkin Elmer Pyris Sapphire DSC. DSC measurements were made between 25 and 420 ◦ C (in N2 , rate 20 ◦ C/min). The number average molecular weight (Mn ), weight average molecular weight (Mw ), and polydispersity index (PDI) were determined by size exclusion chromatography (SEC) techniques of Shimadzu Co. For SEC investigations an SGX (100 Å and 7 nm diameter loading material) 3.3 mm i.d. × 300 mm columns was used; eluent: DMF (0.4 ml/min), polystyrene standards. A refractive index detector was used to analyze the products at 25 ◦ C.
structures of the synthesized compounds. Some of the other peaks observed in the FT-IR spectra are also given in Table 2. The UV–vis analyses results are also shown in Table 2. In these spectra the absorption peaks of the synthesized oligo/polymers spread on much broader range of wavelength compared to the monomers. 1 H and 13 C NMR spectra of the 3-HPMTT, P-3-HPMTT, 4-HPMTT, and O-4-HPMTT are shown in Figs. 3–6, respectively. Also, the NMR analyses results are given in Table 3. According to these results, at the 1 H NMR spectra of P-3-HPMTT a new singlet peak at 7.30 ppm appear as a result of polymerization indicated –Hb proton. Also, according to 13 C NMR spectra of P-3-HPMTT, the values of C2, C4, and C6 – where the polymerization progress – shift to higher chemical shifts. At the 13 C NMR spectra of P-3-HPMTT, the chemical shift values of C2, C4, and C6 are 123.33, 122.86, and 130.48 ppm whereas at the 13 C NMR spectra of 3-HPMTT, they are 114.50, 121.00, and 121.75 ppm, respectively. When we compare the 1 H NMR spectra of 4-HPMTT with the spectra of O-4-HPMTT, it is seen that after the polymerization a new peak appear at 7.95 ppm indicated –Ha proton, but for –Hb proton any new peak is not observed. This is because of the combination of monomer units occurring at the ortho position of the –OH group during the radicalic polymerization. However, the doublet peaks at 7.75 ppm and 6.95 ppm at the 1 H NMR spectrum of O-4-HPMTT are owing to the –Ha and –Hb protons of the terminal units. The radicalic polymerization mechanism of oligophenols had been previously studied and presented in the literature21 . Additionally, as seen in the 13 C NMR spectra of O-4-HPMTT, the peak value of C2 shifts from 117.92 to 130.60 ppm as a result of intermolecular combination of the monomer units. The observed results confirm the structures of the synthesized compounds.
3. Results and discussions 3.1. Structures of the compounds The FT-IR spectra of the synthesized monomers and the oligo/polymers are given in Figs. 1 and 2, respectively. The FT-IR spectrum of melamine is also shown in Fig. 1 as the comparison spectrum. As seen in Fig. 1, the peaks at 3417 and 3468 cm−1 on the spectrum of melamine indicate the three –NH2 groups of melamine. However, on the other spectra these peaks disappear as a result of the condensation of the –NH2 groups with the –CHO groups of the aldehydes. Additionally, the new imine bonds (–CH N) formed after the condensation reactions appear at 1625, 1633, and 1635 cm−1 for the 2-HPMTT, 3-HPMTT, and 4-HPMTT, respectively. Also, Fig. 2 indicates the peaks of new imine bonds at 1633, 1635, and 1633 cm−1 for P-2-HPMTT, P-3-HPMTT, and O4-HPMTT, respectively. And also, –NH2 group peaks of melamine disappear. Consequently, the FT-IR analyses results confirm the Table 2 Some of the spectral data and optical band gaps (Eg ) of the synthesized compounds. Compounds
2-HPMTT P-2-HPMTT 3-HPMTT P-3-HPMTT 4-HPMTT O-4-HPMTT
FT-IR (cm−1 )
UV
CH N
C C
C–OH
1625 1633 1633 1635 1635 1633
1530, 1435 1532, 1451, 1433 1521, 1435, 1442 1522, 1451 1522, 1452 1526, 1440
1253 1246 1253 1251 1250 1248
max (nm) 261, 332, 390 268 260, 316 260 286 262
Eg (eV) 2.78 2.17 3.58 3.30 4.03 2.82
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Fig. 3.
1
H NMR (a) and 13 C NMR (b) spectra of 3-HPMTT.
According to SEC chromatograms, the values of number average molecular weight (Mn ), weight average molecular weight (Mw ), and polydispersity index (PDI) of P-2-HPMTT, P-3-HPMTT, and O4-HPMTT are calculated in accordance with a polystyrene standard
calibration curve and given in Table 4. According to the SEC analyses, the number average molecular weight (Mn ), weight average molecular weight (Mw ), and polydispersity index (PDI) values of total of P-2-HPMTT, P-3-HPMTT, and O-4-HPMTT are found to be
Table 3 1 H and 13 C NMR spectral data of 3-HPMTT, P-3-HPMTT, 4-HPMTT, and O-4-HPMTT. Compound
1
3-HPMTT
H NMR (DMSO): ı ppm: 11.15 (d, –OH), 9.30 (d, –CH N–), 7.42 (t, –Hb ), 7.35 (d, –Ha ), 7.25 (s, –Hd ), 7.10 (d, –Hc ). 13 C NMR (DMSO): ppm: 167.25 (C8-ipso), 162.50 (C7-H), 159.25 (C3-ipso), 137.50 (C1-ipso), 130.00 (C5-H), 121.75 (C6-H), 121.00 (C4-H), 114.50 (C2-H).
P-3-HPMTT
1 H NMR (DMSO): ı ppm: 11.04 (d, –OH), 9.30 (d, –CH N–), 7.41 (t, –Hb -terminal), 7.36 (d, –Ha -terminal), 7.30 (s, –Hb ), 7.25 (s, –Hd -terminal), 7.10 (d, –Hc -terminal). 13 C NMR (DMSO): ppm: 167.86 (C8-ipso), 164.29 (C7-H), 163.10 (C3-ipso), 130.48 (C6-ipso), 129.29 (C5-H), 123.33 (C2-ipso), 122.86 (C4-ipso), 122.14 (C1-ipso).
4-HPMTT
1
H/13 C NMR data
1
H NMR (DMSO): ı ppm: 10.92 (d, –OH), 9.29 (d, –CH N–), 7.76 (d, –Ha ), 6.93 (d, –Hb ). C NMR (DMSO): ppm: 167.50 (C6-ipso), 164.17 (C1-ipso), 162.71 (C5-H), 134.17 (C3-H), 129.79 (C4-ipso), 117.92 (C2-H).
13
O-4-HPMTT
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1 H NMR (DMSO): ı ppm: 10.55 (s, –OH), 9.29 (s, –CH N–), 7.95 (s, –Ha ), 7.75 (d, –Ha -terminal), 6.95 (d, –Hb -terminal). 13 C NMR (DMSO): ppm: 167.80 (C6-ipso), 164.00 (C1-ipso), 162.80 (C5-H), 134.60 (C3-H), 130.60 (C2-ipso), 129.60 (C4-ipso), 118.20 (C2-H-terminal).
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Fig. 4.
1
H NMR (a) and 13 C NMR (b) spectra of P-3-HPMTT.
6000, 7700 g mol−1 and 1.283; 5600, 6000 g mol−1 and 1.071; and 1900, 2050 g mol−1 and 1.079, respectively.
indicate easy electron transfer between HOMO and LUMO energy levels. The synthesized oligo/polymers thus have higher electrical conductivities related to lower band gaps.
3.2. Optical properties 3.3. Electrical conductivities The absorption spectra recorded by using DMSO are shown in Fig. 7a (for 2-HPMTT and P-2-HPMTT), Fig. 7b (for 3-HPMTT and P-3-HPMTT), and Fig. 7c (for 4-HPMTT and O-4-HPMTT). max and the optical band gaps are calculated as in the literature [25] and shown in Table 2. As seen in Table 2, max (nm) and the optical band gaps (Eg /eV) are found as 390 and 2.78 for 2-HPMTT, 268 and 2.17 for P-2-HPMTT, 316 and 3.58 for 3-HPMTT, 260 and 3.30 for P-3-HPMTT, 286 and 4.03 for 4-HPMTT, and 262 and 2.82 for O4-HPMTT, respectively. Because of their polyconjugated structures P-2-HPMTT, P-3-HPMTT, and O-4-HPMTT have lower band gaps than 2-HPMTT, 3-HPMTT, and 4-HPMTT. These results support the electrical conductivity measurements because the lower band gaps
Electrical conductivities of the synthesized monomers and the oligo/polymers and the changes of these values related to doping time with iodine are determined and shown in Table 5. The changes of the electrical conductivities are also shown schematically in Fig. 8. As seen in Table 5 and Fig. 8, although the initial conductivity of P-2-HPMTT is higher than 2-HPMTT, when doped with iodine for 168 h, its conductivity becomes a little lower than the monomer between 10−6 and 10−5 S/cm. The increases in the conductivities of similar kinds of PAMs which derived from salicylaldehyde were previously studied under doping conditions. The initial conductivities of P-2-MPIMP [17], OFPIMP [20], O-4-HBAB [18], and O-2M-4-
Table 4 The number average molecular weight (Mn ), weight average molecular weight (Mw ), polydispersity index (PDI) and % values of the synthesized oligo/polymers. Compounds
Total Mn
P-2-HPMTT P-3-HPMTT O-4-HPMTT
6000 5600 1900
Molecular weight distribution parameters Mw
7700 6000 2050
PDI
1.283 1.071 1.079
Fraction I
Fraction II
Fraction III
Mn
Mw
PDI
%
Mn
Mw
PDI
%
Mn
Mw
PDI
%
700 700 700
900 900 900
1.286 1.286 1.286
65 80 75
7700 7500 7500
8800 8600 8600
1.143 1.150 1.147
10 15 15
67,900 64,000 41,500
86,900 75,000 42,200
1.280 1.172 1.017
25 05 10
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Fig. 5.
Fig. 6.
1
1
H NMR (a) and 13 C NMR (b) spectra of 4-HPMTT.
H NMR (a) and 13 C NMR (b) spectra of O-4-HPMTT.
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Fig. 7. Absorption spectra of 2-HPMTT and P-2-HPMTT (a), 3-HPMTT and P-3-HPMTT (b), 4-HPMTT and O-4-HPMTT (c).
MPIMP [22] were ∼10−12 , 10−10 –10−9 , 10−10 –10−9 , and 10−8 S/cm, respectively. However, their conductivities could be increased by ∼104 , 102 , 103 , and 103 orders of their magnitudes and reached up to 3 × 10−8 , 6 × 10−8 , 7.82 × 10−7 , and 1 × 10−5 S/cm when doped with iodine for 168 h at the same conditions. P-2-HPMTT, which we synthesized by grafting melamine onto OSA, has the conductivity increase by ∼3 × 106 order of its magnitude when doped for 168 h and this increase is quite greater than the previously presented. This is probably because of higher imine bond rate of P-2-HPMTT than those of refereed above. This causes higher iodine coordination with the polymer and consequently a higher increase in conductivity is obtained. The initial conductivities of 3-HPMTT and P-3-HPMTT are nearly the same. But, the conductivity of P-3-HPMTT increased up to 10−7 S/cm whereas the resultant conductivity of 3-HPMTT is about eight times lower than P-3-HPMTT. Additionally, the initial conductivities of 4-HPMTT and O-4-HPMTT are nearly 10−10 and 10−11 S/cm, respectively. When doped with iodine, the conductiv-
ity of 4-HPMTT can be increased by about four and half orders of magnitude (up to 10−6 S/cm) whereas the conductivity of O4-HPMTT can be increased by about four orders of magnitude (up to 10−8 S/cm). As a result, the order of the increase rates in conductivities by doping for 168 h is as follows: 2-HPMTT > P2-HPMTT > P-3-HPMTT > 3-HPMTT > 4-HPMTT > P-4-HPMTT. In the doping of the monomers and the oligo/polymers with iodine, it is found that the conductivities firstly increase greatly with doping time, but then tend to level-off. The increasing conductivity can indicate that a charge-transfer complex between the polymers (or monomers) and dopant iodine is continuously formed. Consequently, Fig. 8 not only shows the conductivity/doping time relationship but also indicates doping kinetics of the synthesized materials. The experiments show that a longer doping time is needed to obtain the maximal conductivity. As a result, the conductivity/doping time curves vary with doping conditions. In order to exclude the influence of doping conditions, the conductivities of the doped monomers and oligo/polymers have
Table 5 Electrical conductivity results of I2 -doped melamine compounds vs. doping time at 25 ◦ C. Time (h)
0 24 48 72 96 120 144 168
Conductivity (×10−13 S/cm−1 ) 2-HPMTT
P-2-HPMTT
66.7 12,038,257 49,280,016 73,639,910 102,033,984 155,102,748 186,203,995 193,040,438
234 2,660,394 15,183,022 31,230,298 54,730,926 65,299,202 76,592,732 77,500,330
3-HPMTT 174 420,063 1,178,220 1,869,286 2,202,739 3,065,243 3,451,982 3,599,460
P-3-HPMTT
4-HPMTT
O-4-HPMTT
138 6,57,839 3,241,192 6,549,553 12,804,033 24,102,751 27,204,066 28,673,049
83.8 6540 35538 116047 291003 655077 947220 1145287
777 802039 3234722 3880153 4180372 4295471 4340110 4358490
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Fig. 10. DTG curves of 2-HPMTT, P-2-HPMTT, 3-HPMTT, P-3-HPMTT, 4-HPMTT, and O-4-HPMTT.
Fig. 8. Changes of the electrical conductivities of the I2 -doped synthesized compounds vs. doping time at 25 ◦ C.
Fig. 11. DTA curves of 2-HPMTT, P-2-HPMTT, 3-HPMTT, P-3-HPMTT, 4-HPMTT, and O-4-HPMTT.
Fig. 9. TGA curves of 2-HPMTT, P-2-HPMTT, 3-HPMTT, P-3-HPMTT, 4-HPMTT, and O-4-HPMTT.
been related with doping extent (shown in Fig. 8). Diaz et al. suggested the conductivity mechanisms of Schiff base polymers for doping with iodine [26]. Nitrogen is a very electronegative element and it is capable of coordinating with an iodine molecule. Coordination of iodine with Schiff base polymers was presented in the literature [17,18]. 3.4. Thermal analyses TGA, DTG, and DTA curves for 2-HPMTT, P-2-HPMTT, 3-HPMTT, P-3-HPMTT, 4-HPMTT, and O-4-HPMTT are shown in Figs. 9–11, respectively. Using these curves, the calculated thermal degradation data are summarized in Table 6. As seen in Table 6, the initial degradation temperature (◦ C) and the carbine residue (%) at 1000 ◦ C are 118, 281 and 0.83 for 2-HPMTT; 106, 240, 407 and 37.21 for
P-2-HPMTT; 93, 257 and 2.76 for 3-HPMTT; 98, 257 and 2.20 for P-3HPMTT; 101, 258 and 2.43 for 4-HPMTT; and 106, 255, 757 and 1.59 for O-4-HPMTT, respectively. According to these values P-2-HPMTT is more stable than 2-HPMTT to temperature and thermal decomposition because of its polyconjugated bond system. However, it is seen in Table 6 that P-3-HPMTT and O-4-HPMTT lose nearly the same percentage of their weights compared to their monomers at 1000 ◦ C. Consequently, P-2-HPMTT is found to be the most thermally stable polymer among the synthesized oligo/polymers. DSC curves of P-2-HPMTT, P-3-HPMTT, and O-4-HPMTT are shown in Fig. 12 and using these curves the calculated glass transition temperatures (Tg ) and changes of specific heat values during glass transitions (Cp ) are also given in Table 6. According to these results, P-3-HPMTT has the most Tg value among the synthesized oligo/polymers. Also, as seen in the DSC curves, absence of any endothermic or exothermic peak of P-3-HPMTT agree with its DTA analysis results. However, DSC analyses results of P-3-HPMTT and O-4-HPMTT differ from their DTA results with new endothermic peaks observed at the temperatures of 187.04 and 353.82 ◦ C for P-
Table 6 Thermal degradation values of the synthesized monomers, oligomer, and polymers. Compounds
2-HPMTT P-2-HPMTT 3-HPMTT P-3-HPMTT 4-HPMTT O-4-HPMTT a b c d
Ton a
118, 281 106, 240, 407 93, 257 98, 257 101, 258 106, 255, 757
Wmax. Tb
138, 320 132, 276, 469 116, 285 125, 282 121, 288 123, 284, 803
The onset temperature. Maximum weight temperature. Glass transition temperature. Change of specific heat during glass transition.
DTA
20% weight losses
50% weight losses
% Carbine residue at 1000 ◦ C
Exo
254 249 230 127 224 164
305 572 275 267 279 279
0.83 37.21 2.76 2.20 2.43 1.59
– – – – – –
DSC Endo 141, 325 – 294 290 295 130, 293, 816
Tg (◦ C)c
Cp (j/g. ◦ C)d
– 71.73 – 170.34 – 66.05
– 0.356 – 1.042 – 0.168
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undergo any exothermic event during the heating up until 1000 ◦ C. The Tg values of the synthesized oligo/polymers were also determined using the DSC curves and found as 71.73, 170.34, and 66.05 ◦ C for P-2-HPMTT, P-3-HPMTT, and O-4-HPMTT, respectively. The electrical conductivity measurements showed that the synthesized melamine compounds were semiconductors and their conductivities could be increased considerable via doping with iodine. What is more, 2-HPMTT had the greatest conductivity increase among the synthesized. The properties of the synthesized compounds with low optical band gap characteristics were determined. The optical band gap values of the Schiff bases were found to be higher than the oligo/polyphenol derivatives. The observed band gaps were sufficiently low to make these oligo/polymers highly promising for electronic, optoelectronic, electroactive, and photovoltaic applications. References Fig. 12. DSC thermograms of P-2-HPMTT, P-3-HPMTT, and O-4-HPMTT.
3-HPMTT; and 176.86, 301.80, 357.17 and 366.40 ◦ C for O-4-HPMTT. On the other hand, it is also supported by DSC analyses that there is no exothermic event during the synthesized oligo/polymers until when heated up to 420 ◦ C as suggested from DTA analyses. 4. Conclusions The Schiff bases and the oligo/polyphenol derivatives of melamine abbreviated as 2-HPMTT, 3-HPMTT, 4-HPMTT, P-2HPMTT, P-3-HPMTT, and O-4-HPMTT were synthesized with the yields of 15, 38, 37, 17, 42, and 35%, respectively. The solubility tests showed that the synthesized melamine Schiff bases and the copolymers had little solubilities in many organic solvents. But they completely dissolved in DMSO and H2 SO4 . Also, with the exception of 2-HPMTT and 3-HPMTT they are all dissolved in DMF. What is more, although they are insoluble in cold water, many of them are soluble in hot water (except for P-2-HPMTT and O-4-HPMTT). Thermal analysis results demonstrated that there was sufficient resistance to thermal degradation of P-2-HPMTT and O-4-HPMTT while the others had lower stabilities to thermal degradation. According to TG analysis results, 2-HPMTT, 3-HPMTT, 4-HPMTT, P2-HPMTT, P-3-HPMTT, and O-4-HPMTT lost half of their weights at 305, 275, 279, 572, 267, and 279 ◦ C, respectively. DTA and DSC analyses results showed that the synthesized compounds did not
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