New low-band gap polyurethanes containing azomethine bonding: Photophysical, electrochemical, thermal and morphological properties

Journal of the Taiwan Institute of Chemical Engineers 59 (2016) 536–546

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Journal of the Taiwan Institute of Chemical Engineers journal homepage: www.elsevier.com/locate/jtice

New low-band gap polyurethanes containing azomethine bonding: Photophysical, electrochemical, thermal and morphological properties ˙ Musa Kamacı a,b, Ismet Kaya a,∗ a b

Polymer Synthesis and Analysis Laboratory, Department of Chemistry, Çanakkale Onsekiz Mart University, 17020 Çanakkale, Turkey Department of Chemistry, Faculty of Sciences and Letters, Piri Reis University, 34940 Tuzla, Istanbul, Turkey

a r t i c l e

i n f o

Article history: Received 5 June 2015 Revised 13 August 2015 Accepted 27 August 2015 Available online 28 September 2015 Keywords: Polyurethanes Thermal properties Low-band gap polymers Vanillin Azomethine

a b s t r a c t In this paper, a series of new Schiff bases with different substituents were prepared via condensation reaction of vanillin and different diamines. These monomers were converted to polyurethane derivatives using aliphatic and aromatic diisocyanates as co-monomer via step-polymerization reactions. Photophysical, electrochemical, thermal and morphological properties of these compounds were also clarified. Photophysical properties of the compounds were determined by using UV–vis and photoluminescence (PL) spectroscopy. Thermal stability and degradation behavior of Schiff bases and their poly(azomethine-urethane) derivatives were investigated by using TG-DTA technique. Also, thermal transitions properties of polyurethanes containing azomethine linkage were clarified using DSC technique. Mechanical and morphological properties of poly(azomethine-urethane)s were also determined by using DMA and SEM, respectively. Morphological photographs showed that poly(azomethine-urethane)s have different particle sizes and consist of semicrystalline particles. Electrochemical properties of the materials were investigated by using cyclic voltammetry (CV). Electrochemical results showed that PAMUs have electrochemical band gap below 2.0 eV. © 2015 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1. Introduction In the past few decades, conjugated organic and polymeric materials have attracted much attention in science and technology [1, 2]. These materials have a widespread application in fields such as biosensors [3] and super capacitors [4]. Also, they have been used as a kind of active medium in several optoelectronic devices, such as organic field effect transistors (OFET) [5], organic photodiodes [6], organic light emitting diodes (OLEDs) [7] and polymer light-emitting electrochemical cells (LECs) [8]. Polyurethanes are another important conjugated materials and they have also found applications in fields such as wound-dressing [9], eco-friendly coating [10] and electrooptic application [11], etc. Polymeric Schiff bases, which are also known as polyazomethines or polyimines, are a class of conjugated materials which contain nitrogen atoms in polymer chain [12, 13]. They contain one or more imine (–N=CH) linkages in their structure and exhibit good thermal stability [14], excellent mechanical strength [15], paramagnetism [16], electrical conductivity [17], metal-chelating ability [18], resistance to high energy due to the resonance stabilization of Schiff base unit [19] and, in some cases, liquid crystalline morphology [20]. Also, poly(azomethine-urethane) derivatives of Schiff bases have been syn-

∗

Corresponding author. Tel.: +90 286 218 00 18; fax: +90 286 218 05 33. E-mail address: [email protected], [email protected] (I.˙ Kaya).

thesized in the literature [21-23]. Their thermal stability [24], tunable multi-color emission behavior [25], non-linear optical [26] and electrochemical properties [27], solar cell [28] and ion sensor application [29-31] have also been clarified. In this paper, we firstly synthesized Schiff bases containing different substitute such as methyl (–CH3 ) and nitro (–NO2 ) as electrondonating and electron-withdrawing units. Then, these Schiff bases were converted to poly(azomethine-urethane) (PAMU) derivatives using aliphatic and aromatic diisocyanates as co-monomer. The obtained PAMUs were investigated in terms of photophysical, electrochemical, thermal and morphological characteristics. The effects of electron-donating and electron-withdrawing units were also examined. Xie et al. were reported that polymers containing electrondonating and electron-withdrawing units or groups in the polymer chain have attracted increasing attentions. In addition, optical properties and energy levels of these kinds of polymers could be effectively tuned and they could be used for probing the structureproperty relationships [32]. 2. Experimental 2.1. Materials 3-Metoxy-4-hydroxybenzaldehyde, vanillin, (VAN), o-phenylenediamine (o-PDA), 4-methyl-o-phenylenediamine (o-PDAM), 4-nitro-o-phenylenediamine (o-PDAN), 2,4-toluene diisocyanate

http://dx.doi.org/10.1016/j.jtice.2015.08.018 1876-1070/© 2015 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

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Scheme 1. Synthesis scheme of Schiff bases and poly(azomethine-urethane)s.

(TDI), hexamethylenediisocyanate (HDI), dimethylformamide (DMF), dimethylsulfoxide (DMSO), tetrahydrofurane (THF), methanol (MeOH), ethanol (EtOH), acetone, acetonitrile (MeCN), toluene, ethyl acetate, CHCl3 , CCl4 and n-hexane were supplied by Merck Chemical Co. (Germany). 2.2. Preparation of Schiff bases VAN-o-PDA, VAN-o-PDAM and VAN-o-PDAN were prepared by the condensation reaction of vanillin (VAN, 1.522 g, 20 mmol) with o-PDA (1.081 g, 10 mmol), o-PDAM (1.222 g, 10 mmol) or o-PDAN (1.531 g, 10 mmol), respectively, in MeOH (30 mL) by stirring the mixture under reflux for 3 h, and then cooling at room temperature (Scheme 1). The prepared Schiff bases were washed with MeCN (2 × 50 mL) to remove the unreacted components and dried in a vacuum desiccator at 75 °C [33]. The yields of VAN-o-PDA, VAN-o-PDAM and VAN-o-PDAN were 72, 68 and 66%, respectively. 2.3. Preparation of polyurethanes containing azomethine VAN-o-PDA-TDI, VAN-o-PDAM-TDI, VAN-o-PDAN-TDI, VAN-oPDA-HDI, VAN-o-PDAM-HDI and VAN-o-PDAN-HDI were prepared by

the step polymerization reaction of VAN-o-PDA (1.035 g, 2.75 mmol), VAN-o-PDAM (1.074 g, 2.75 mmol) and VAN-o-PDAN (1.159 g, 2.75 mmol) in DMF/THF (1/3) mixture (60 mL) with 2,4-toluene diisocyanate (0.479 g, 2.75 mmol) or hexamethylene diisocyanate (0.463 g, 2.75 mmol) in THF (30 mL), respectively, by stirring the mixture under argon atmosphere for 6 h, cooling at room temperature, and kept for 24 h (Scheme 1). PAMUs were washed with MeOH (2 × 50 mL) and MeCN (2 × 50 mL) to remove the unreacted components and dried in a vacuum oven at 75 °C for 24 h [15]. The yields of VAN-o-PDATDI, VAN-o-PDAM-TDI, VAN-o-PDAN-TDI, VAN-o-PDA-HDI, VAN-oPDAM-HDI and VAN-o-PDAN-HDI were 65, 64, 68, 78, 71 and 68%, respectively. 2.4. Characterization techniques The solubility tests were done in different solvents by using 1 mg sample and 1 mL solvent at 25 °C. The obtained materials were characterized by using FT-IR, 1 H, 13 C NMR spectra and size exclusion chromatography (SEC). The infrared spectra were obtained by Perkin Elmer FT-IR Spectrum one using universal ATR sampling accessory (4000–550 cm−1 ). 1 H and 13 C NMR spectra (Bruker AC FT NMR spectrometer operating at 400 and 100.6 MHz, respectively)

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were recorded in deuterated DMSO-d6 at 25 °C. Tetramethylsilane (TMS) was used as internal standard. 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 A˚ and 7 nm diameter loading material) 3.3 mm i.d. × 300 mm columns was used; eluent: DMF (0.4 mL/min), polystyrene standards were used. A refractive index detector (RID) was used to analyze the products at 25 °C. Thermal characterization and decomposition were determined by using TG-DTA and DSC. Thermal data were obtained by using Perkin Elmer Diamond Thermal Analysis. The TG-DTA measurements were made between 20–1000 °C (in N2 , 10 °C/min). DSC analyses were carried out between 25 and 420 °C (in N2 , 10 °C/min) using Perkin Elmer Pyris Sapphire DSC. DMA tests were carried out with Perkin Elmer Pyris Diamond DMA 115 V. Rectangular samples of 10 mm (length) × 5.5 mm (width) × 0.6 mm (thickness) were used to conduct DMA tests in single cantilever bending mode at a frequency of 1 Hz, a heating rate of 3 °C/min and in the range 20–350 °C. The sample was prepared as follows: 0.5 g of PAMU was placed into the titanium clamp (supplied from Triton Technology Ltd., United Kingdom) and extended followed by closing of the clamp from both sides. Scanning electron microscopy (SEM) photographs of PAMUs were recorded using a JEOL JSM 5600LV SEM instrument. For SEM, polymer samples were sprinkled on double-sided adhesive tape mounted on an aluminum stub. Then, they were coated with a thin gold/palladium film using a sputter coater. 2.5. Photophysical properties Ultraviolet–visible (UV–vis) spectra were recorded by Analytikjena Specord 210 Plus at 25 °C. The measurements were carried out in DMSO. The optical band gaps (Eg ) were calculated from the absorption edges as in the literature [34]. A Shimadzu RF-5301PC spectrofluorophotometer was used for fluorescence measurements. Photoluminescence (PL) spectra were obtained in DMF solutions. Solution concentrations of the compounds and slit width of the spectrofluorophotometer were adjusted to between 0.2 and 0.0625 mg/L and 3 nm for all measurements, respectively. 2.6. Electrochemical properties Cyclic voltammetry (CV) measurements were carried out with a CHI 660C Electrochemical Analyzer (CH Instruments, Texas, USA) at a potential scan rate of 20 mV/s. All the experiments were performed in a dry box filled with argon at room temperature. The system consisted of a CV cell containing glassy carbon (GCE) as the working

electrode, platinum wire as the counter electrode, and Ag wire as the reference electrode. The electrochemical potential of Ag was calibrated with respect to the ferrocene/ferrocenium (Fc/Fc+ ) couple. The half-wave potential (E1/2 ) of (Fc/Fc+ ) measured in MeCN solution of 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6 ) MeCN solution is 0.39 V with respect to Ag wire. The voltammetric measurements were carried out in MeCN/DMSO mixtures (v/v: 3/2). The Eox,onset, Ered,onset, ionization potential (IP), electron affinity (EA) and electrochemical energy band gaps (E g ) of Schiff bases and their poly(azomethine-urethane) derivatives were calculated from their CV measurements [34]. 3. Results and discussion 3.1. Characterization and solubility The synthesized VAN-o-PDA, VAN-o-PDAM and VAN-o-PDA-HDI are dark-brown colored compounds; VAN-o-PDAN, VAN-o-PDATDI, VAN-o-PDAM-TDI, VAN-o-PDAN-TDI are dark-red colored compounds; VAN-o-PDAM-HDI and VAN-o-PDAN-HDI are yellow colored compounds. The polyurethanes containing azomethine are completely soluble only in strong polar solvents like DMSO and DMF. They are partly soluble in THF while they are insoluble in ethyl acetate, MeCN, n-hexane and acetone. FT-IR spectral data all of the starting materials and synthesized compounds are summarized in Table 1. According to characteristic stretch vibrations of Schiff base samples in the literature [35], they show two stretch vibrations assigned to the characteristic functional groups such as imine (–N=CH) and hydroxyl (–OH) stretch vibrations. As shown in Table 1, imine (–N=CH) stretch vibration of Schiff base was observed in the range 1595–1600 cm−1 , and hydroxyl (–OH) stretch vibration observed between 3343 and 3402 cm−1 . In the FT-IR spectra of TDI and HDI characteristic isocyanate (–C=O) and (–C=N) stretch vibrations of diisocyanates are observed at 2234 and 1615 for TDI and 2250 and 1584 cm−1 for HDI, respectively, which agrees with the literature values [29, 36]. In the FT-IR spectra of poly(azomethineurethane)s, these characteristic hydroxyl stretch vibration (–OH) of Schiff bases and isocyanate stretch vibrations (–C=O) and (–C=N) of TDI and HDI disappear due to urethane (–NH–COO) formation. In the FT-IR spectra of PAMUs, there are three stretch vibrations assigned to the characteristic functional groups of urethane (–NH) in the range 3274–3338 cm−1 , urethane (–C=O) 1685–1719 cm−1 and imine (–N=CH) 1601–1663 cm−1 [34]. According to these FT-IR spectral data, they clearly confirm the formation of Schiff bases and polyurethanes containing azomethine. Fig. 1 shows 1 H NMR spectra of VAN-o-PDAM, VAN-o-PDAM-TDI and VAN-o-PDAM-HDI. 1 H NMR spectral data all of the materials are

Table 1 FT-IR spectra data of the starting materials, Schiff bases and polyurethanes. Compounds

VAN o-PDA o-PDAM o-PDAN TDI HDI VAN-o-PDA VAN-o-PDAM VAN-o-PDAN VAN-o-PDA-TDI VAN-o-PDAM-TDI VAN-o-PDAN-TDI VAN-o-PDA-HDI VAN-o-PDAM-HDI VAN-o-PDAN-HDI

Urethane –NH

Urethane –C=O

Imine –N=CH

Isocyanate –C=O

Isocyanate –C=N

–OH

–NH2

Aldehyde –CHO

Aromatic –CH

Aliphatic –CH

Aromatic –C=C

– – – – – – – – – 3338 3331 3274 3307 3327 3295

– – – – – – – – – 1699 1696 1714 1685 1709 1719

– – – – – – 1600 1595 1596 1602 1663 1650 1601 1658 1655

– – – – 2234 2250 – – – – – – – – –

– – – – 1615 1584 – – – – – – – – –

3166 – – – – – 3402 3346 3343 – – – – – –

– 3384 3434 3334 – – – – – – – – – – –

1662 – – – – – – – – – – – – – –

3021 3029 3029 3072 3072 – 3060 3073 3098 3069 3064 3093 3059 3083 3074

– – 2912 – 2986 2944 2934 2938 2942 2984 2982 2938 2931 2930 2934

1586,1510,1466 1590,1526,1497 1594,1506,1456 1594,1522,1468 1615,1574,1525 – 1527,1487,1462 1599,1514,1483 1510,1495,1468 1535,1495,1438 1599,1528,1501 1597,1514,1494 1533,1503,1478 1604,1513,1440 1597,1535,1514

M. Kamacı, I.˙ Kaya / Journal of the Taiwan Institute of Chemical Engineers 59 (2016) 536–546

Fig. 1.

1

539

H NMR spectra of VAN-o-PDAM (a), VAN-o-PDAM-TDI (b) and VAN-o-PDAM-HDI (c).

also summarized in Table 2. According to Fig. 1 and Table 2, characteristic hydroxyl (–OH) and imine (–N=CH) protons of Schiff bases are observed in the range 9.02–9.75 and 8.21–8.66 ppm, respectively. Also, aromatic protons in the structures of Schiff bases are observed between 6.34 and 8.09 ppm. Additionally, aliphatic methoxy (–OCH3 ) proton is observed in the range 3.83–3.89 ppm. According to Fig. 1 and Table 2, characteristic urethane (–NH) and imine (–N=CH) pro-

tons of polyurethanes are observed in the range 9.93–9.51 and 9.00– 7.93 ppm, respectively. Also, aromatic and methoxy (–OCH3 ) protons in the structures of polyurethanes are observed in the range 6.34– 8.14 ppm and 3.38–3.89 ppm, respectively. Moreover, aliphatic protons of VAN-o-PDA-HDI, VAN-o-PDAM-HDI and VAN-o-PDAN-HDI resulting from aliphatic diisocyanates (HDI) are observed between 1.04 and 3.40 ppm.

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Table 2 NMR spectra data of the prepared compounds. Compounds

Spectral data (δ ppm )

VAN-o-PDA

1

H NMR (DMSO): 9.02 (–OH), 8.66 (Imine (–N=CH)) between 6.67 and 7.66 ppm Ar–CH and 3.89 (–OCH3 ). C NMR (DMSO): 148.65 (–C–OH), 154.40 (Imine (–N=CH)), between 113.41 and 148.05 ppm Ar–CH and 55.99 (–OCH3 ). H NMR (DMSO): 9.75 (–OH, Hc, s), 8.21(Imine (–N=CH), Hf, s) 7.52 (Ha, s), 7.31 (He, d), 7.22 (Hh, d), 7.15 (Hg, d), 7.06 (Hj, s), 6.90 (Hd, d), 3.86 (–OCH3 , Hb, s) and 2.39 (–CH3 , Hi, s). 13 C NMR (DMSO): 150.74 (–C–OH, C4 -ipso), 153.92 (Imine (–N=CH), C8 –H), 148.01 (–C–O–C,C2 -ipso), 143.34 (C15 ), 136.72 (C12 -ipso), 132.02 (C10 –H), 129.12 (C11 –H), 128.37 (C7 ), 123.93 (C14 –H), 121.64 (C6 –H), 119.97 (C1 –H), 116.02 (C9 ), 113.34 (C5 –H), 55.90 (–OCH3 , C3 -ipso) and 21.87 (–CH3 , C13 ). 1 H NMR (DMSO): 9.72 (–OH), 8.66 (Imine (–N=CH)), between 6.34 and 8.09 ppm Ar–CH and 3.83 (–OCH3 ). 13 C NMR (DMSO): 148.37 (–C-OH), 149.95 (Imine (–N=CH)), between 111.01 and 146.85 ppm Ar–CH and 56.11 (–OCH3 ). 1 H NMR (DMSO): 9.51 (Urethane –NH), 8.76 (Imine (–N=CH)), between 7.12 and 7.93 ppm Ar–CH, 3.86 (–OCH3 ) and 2.09 (–CH3 ). 13 C NMR (DMSO): 154.71 (Urethane, –C=O), 153.93 (Imine (–N=CH)), between 113.22 and 148.47 ppm Ar–CH, 55.87 (–OCH3 ) and 17.60 (–CH3 ). 1 H NMR (DMSO): 9.51 (Urethane –NH, He, s), 8.77 (Imine (–N=CH), Hj, s), 7.93 (Ha, s), 7.73 (Hi, s), 7.48 (Hh, d), 7.14 (Hd, d), 7.04 (Hg, d), 6.91 (Hl, d), 6.68 (Hk, d), 6.48 (Hn, s), 6.36 (Hc, d), 3.38 (–OCH3 , Hf, s), 2.48 (–CH3 , Hm, s) and 2.09 (–CH3 , Hb, s). 13 C NMR (DMSO): 162.76 (Urethane, –C=O, C8 -ipso), 154.72 (Imine (–N=CH), C16 –H), 153.93 (–C–O–C, C14 -ipso), 148.93 (C23 ),142.89 (C9 ), 137.64 (C20 ), 136.90 (C2 ), 136.41 (C12 ), 130.63 (C7 ), 128.26 (C18 –H), 125.95 (C5 –H), 122.68 (C19 –H), 122.18 (C3 -ipso), 121.42 (C22 –H), 120.09 (C10 –H), 118.92 (C11 –H), 115.94 (C17 ), 115.02 (C6 –H), 113.35 (C1 –H), 110.67 (C13 –H), 55.84 (–OCH3 , C15 -ipso), 31.20 (–CH3 , C21 ) and 17.59 (CH3 , C4 ). 1 H NMR (DMSO): 9.75 (Urethane –NH), 8.53 (Imine (–N=CH)), between 6.75 and 8.07 ppm Ar–CH, 3.89 (–OCH3 ) and 2.19 (–CH3 ). 13 C NMR (DMSO): 162.76 (Urethane, –C=O), 157.89 (Imine (–N=CH)), between 111.01 and 152.85 ppm Ar–CH, 56.11 (–OCH3 ) and 17.91 (–CH3 ). 1 H NMR (DMSO): 9.59 (Urethane –NH), 7.93 (Imine (–N=CH)), between 6.91 and 7.93 ppm Ar–CH, 3.87 (–OCH3 ), between 1.12 and 3.40 ppm Al–CH. 13 C NMR (DMSO): 162.76 (Urethane, –C=O), 156.67 (Imine, –N=CH), between 111.05 and 152.20 ppm Ar–CH, 55.84 (–OCH3 ), between 26.39 and 36.22 ppm Al–CH. 1 H NMR (DMSO): 9.93 (Urethane –NH, Hd, s), 9.00 (Imine (–N=CH), Hi, s), 7.78 (Hg, s), 7.30 (Hf, d), 7.21 (He, d), 7.04 (Hk, d), 6.93 (Hj, d), 6.85 (Hm, s), 3.82 (–OCH3 , Hh, s), 2.94 (Hc, t), 2.40 (–CH3 , Hl, s), 1.73 (Hb, s) and 1.34 (Ha, d). 13 C NMR (DMSO): 162.78 (Urethane, –C=O, C4 -ipso), 158.54 (Imine, –N=CH, C12 –H), 152.09 (–C–O–C, C10 -ipso), 146.28 (C19 ), 141.50 (C5 ), 139.51 (C16 ), 135.60 (C8 ), 132.50 (C14 –H), 128.65 (C15 –H), 125.37 (C18 –H), 124.17(C6 –H), 121.49 (C7 –H), 118.93 (C13 ), 111.00 (C9 –H), 55.99 (–OCH3 , C11 -ipso), 47.57 (C3 –H), 30.50 (C2 –H), 26.56 (C1 –H) and 21.66 (–CH3 , C17 ). 1 H NMR (DMSO): 9.75 (Urethane –NH), 8.14 (Imine (–N=CH)), between 6.64 and 8.14 ppm Ar–CH, 3.82 (–OCH3 ) between 1.04 and 3.01 ppm Al–CH. 13 C NMR (DMSO): 163.57 (Urethane, –C=O), 159.54 (Imine, –N=CH), between 113.18 and 152.20 ppm Ar–CH, 56.46 (–OCH3 ), between 26.59 and 44.84 ppm Al–CH. 13

VAN-o-PDAM

VAN-o-PDAN VAN-o-PDA-TDI VAN-o-PDAM-TDI

VAN-o-PDAN-TDI VAN-o-PDA-HDI

VAN-o-PDAM-HDI

VAN-o-PDAN-HDI

1

13 C NMR spectra of VAN-o-PDAM, VAN-o-PDAM-TDI and VAN-oPDAM-HDI compounds are given in Fig. 2. 13 C NMR spectral data all of the compounds are also summarized in Table 2. As can be seen in Fig. 2 and Table 2, characteristic hydroxyl (– C–OH) and imine (–N= CH) carbons of Schiff bases are observed in the range 154.40–149.95 and 150.74–148.37 ppm, respectively. Also, aromatic and methoxy (–OCH3 ) carbons in the structures of Schiff bases are also observed in the range 111.01–148.05 ppm and 55.90–56.11 ppm, respectively. According to Fig. 1 and Table 2, characteristic urethane (– C=O) and imine (–N= CH) carbons of PAMUs are observed in the range 154.71–163.57 and 153.93–159.54 ppm, respectively. Moreover, aromatic and methoxy (–OCH3 ) carbons of PAMUs are observed in the range 110.67–153.93 ppm and 55.84–56.46 ppm, respectively. Additionally, aliphatic carbons of VAN-o-PDA-HDI, VAN-o-PDAM-HDI and VAN-o-PDAN-HDI are observed between 26.39 and 47.57 ppm. Consequently, 1 H and 13 C NMR spectral data confirm the structure of Schiff bases and poly(azomethine-urethane)s.

3.2. Size exclusion chromatography The number-average molecular weight (Mn ) and polydispersity index (PDI) values of polyurethanes were measured by using RI detector. According to SEC results, Mn values of VAN-o-PDA-TDI, VANo-PDAM-TDI, VAN-o-PDAN-TDI, VAN-o-PDA-HDI, VAN-o-PDAM-HDI and VAN-o-PDAN-HDI were calculated as 6400, 7900, 7350, 9060, 9580 and 10640 g/mol, respectively. Also, PDI values of polyurethanes were determined between 1.175 and 1.254. According to total average molecular weight, VAN-o-PDA-TDI, VAN-o-PDAM-TDI, VAN-o-PDANTDI, VAN-o-PDA-HDI, VAN-o-PDAM-HDI and VAN-o-PDAN-HDI have nearly 17–21, 20–25, 17–23, 24–30, 25–27 and 25–30 repeated units. The results showed that PAMUs obtained from HDI have higher average molecular weights than TDI series. 3.3. Photophysical properties Photophysical properties of Schiff bases and their PAMU derivatives were investigated using ultraviolet–visible (UV–vis) and photo-

luminescence (PL) spectroscopy. The obtained UV–vis and PL spectra of the compounds are given in Figs. 3 and 4, respectively. Also, the obtained results are summarized in Table 3. According to the structure of VAN-o-PDA, VAN-o-PDAM and VANo-PDAN (in Scheme 1), they contain lone pairs of electron (–O) and multiple bonds (–CH=N–, –C=C– and –N=O). These lone pairs of electron and multiple bonds in the structures of Schiff bases allow an extent conjugation [37]. As seen in Fig. 3, VAN-o-PDA, VAN-o-PDAM and VAN-o-PDAN exhibited maximum UV–vis absorption at 311, 310 and 292 nm because of the π –π ∗ transitions of the azomethine linkage (–N=CH) in the structure of Schiff bases. Moreover, VAN-o-PDAN exhibited maximum UV–vis absorption at 371 nm ascribed to the n→π ∗ transitions of nitro group (–NO2 ) in the structure. According to Fig. 3, polyurethanes exhibited maximum UV–vis absorption in the range 269–312 nm ascribed to the π →π ∗ transition of urethane linkage. Also, VAN-o-PDAN-TDI and VAN-o-PDAN-HDI exhibited maximum UV–vis absorption at 302 and 381 nm due to nitro group (–NO2 ) in the structure [34]. Optical band gap values of the materials were calculated by using Eq. (1) as in the literature [38].

Eg =

1242

λonset

(1)

VAN-o-PDA, VAN-o-PDAM and VAN-o-PDAN have 3.53, 3.57 and 2.77 eV optical band gap (Eg ). Also, polyurethanes have in the range 2.90–3.94 eV optical band gap value. According to this Eg result, VANo-PDAN has the lowest Eg . As seen in Scheme 1, VAN-o-PDAN has nitro (–NO2 ) group m- and o-positions of imine (–N=CH) linkage. On the other hand, VAN-o-PDA and VAN-o-PDAM also have hydrogen atom and methyl (–CH3 ) groups in the same positions of imine linkage, respectively. As known, nitro group is an electron-withdrawing group whereas methyl group is an electron-donating group. Therefore, –CH3 group partly increases the electron density of ortho and para positions of aromatic ring [36] while –NO2 group decreases the electron density [39]. Similar trend is observed in poly(azomethineurethane) derivatives of VAN-o-PDAN.

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Fig. 2.

13

541

C NMR spectra of VAN-o-PDAM (a), VAN-o-PDAM-TDI (b) and VAN-o-PDAM-HDI (c).

Photoluminescence (PL) spectroscopy was used to investigate fluorescence characteristics of the compounds. As seen in PL spectra of the compounds, Schiff bases and their polyurethane derivatives have two emission maxima except for VAN-o-PDAN, VAN-o-PDAN-TDI and VAN-o-PDAM-HDI. As shown first emission maxima of VAN-o-PDAN series, VAN-o-PDAN-TDI (395 nm) and VAN-o-PDAM-HDI (356 nm) have a 32 and 71 nm blue shift compared to VAN-o-PDAN (427 nm), respectively. On the other hand, as can be seen PL spectral data of VAN-o-PDA series, VAN-o-PDAM-TDI (376 nm) shown 3 nm blue shift while VAN-o-PDA-HDI (395 nm) shown 16 nm red shift compared to VAN-o-PDA (379 nm) at the first emission maxima.

3.4. Electrochemical properties Cyclic voltammetric (CV) measurements were performed to evaluate the electrochemical properties of Schiff bases and their poly(azomethine-urethane) derivatives as well as to estimate the ionization potential (IP), electron affinity (EA) which are important for determining the electrochemical energy band gaps. Fig. 5 shows cyclic voltammograms of the materials. As shown in Fig. 5, Schiff bases and their polyurethane derivatives have one oxidation and reduction peak potential. Reduction peak in CV of Schiff bases and their polyurethane derivatives could be azomethine nitrogen and urethane

M. Kamacı, I.˙ Kaya / Journal of the Taiwan Institute of Chemical Engineers 59 (2016) 536–546

1 0.8

Absorbance

1 = VAN-o-PDA 2 = VAN-o-PDA-TDI 3 = VAN-o-PDA-HDI

1

(a)

2

0.6

0.4

3

0.2 0 460 Wavelength (nm)

400

3

2

200

1 325

2

0.6

(b)

3

0.4

1

0

415

505 595 Wavelength (nm)

685

775

1 = VAN-o-PDAM 2 = VAN-o-PDAM-TDI 3 = VAN-o-PDAM-HDI

1000 800

600

2 400 200

1

3

0 260

360

460 560 Wavelength (nm)

660

325

1 = VAN-o-FDAN 2 = VAN-o-FDAN-TDI 3 = VAN-o-FDAN-HDI

1

0.8 Absorbance

600

660

1 = VAN-o-FDAM 2 = VAN-o-FDAM-TDI 3 = VAN-o-FDAM-HDI

1 0.8

560

Emission Intensity (a.u.)

Absorbance

360

0.2

(c)

800

0 260

(b)

1 = VAN-o-PDA 2 = VAN-o-PDA-TDI 3 = VAN-o-PDA-HDI

1000

0.6

(c)

1

0.4

3 2

0.2 0 260

360

460 560 Wavelength (nm)

660

Fig. 3. UV–vis spectra of Schiff bases and polyurethanes containing azomethine.

group. As known, azomethine nitrogen (– N=CH) and urethane oxygen (–NH–COO) are reduced to amine and alcohol taking a proton, respectively. Moreover, oxidation peak in CV of polyurethanes could be urethane group due to oxidation of this group. The ionization potential (IP), electron affinity (EA) and electrochemical energy band gaps (E g ) of the compounds were calculated by using Eqs. (2), (3) and (4), respectively, as in the literature [34] and the obtained results are also summarized in Table 4.

EIP = −(Eox.,onset − 0.39) − 4.80

(2)

EEA = −(Ered.,onset − 0.39) − 4.80

(3)

Eg = EIP − EEA

(4)

According to Table 4, the calculated EIP and EEA values of Schiff bases were calculated in the range −5.64 to −5.27 eV and −3.68 to −2.97 eV, respectively. These values of polyurethanes were calculated in the range −5.43 to −5.31 eV and −3.71 to −3.43 eV, respectively. Electrochemical energy band gaps (Eg ) of VAN-o-PDA, VAN-o-PDAM and VAN-o-PDAN are calculated as 1.64, 1.61 and 2.67 eV, respectively. Eg values of polyurethanes are calculated in the range 1.64– 1.98 eV. As can be seen these results, poly(azomethine-urethane)s

415

505 595 Wavelength (nm)

685

775

1 = VAN-o-PDAN 2 = VAN-o-PDAN-TDI 3 = VAN-o-PDAN-HDI

250 Emission Intensity (a.u.)

(a)

Emission Intensity (a.u.)

542

3 200 150 100

2

50

1

0 325

415

505 595 Wavelength (nm)

685

775

Fig. 4. Fluorescence spectra of 4-DHB-o-PDA (a), 4-DHB-o-PDA (b), 4-DHB-o-PDA (c) and polyurethanes containing azomethine.

have below 2.0 eV Eg (except for VAN-o-PDAN). Due to this property, polyurethanes can be successfully used in heterojunction solar cells [34, 40]. As can be seen the electrochemical energy band gap of both Schiff bases and polyurethanes, Schiff bases and polyurethanes containing nitro (NO2 ) group as electron-donating group have the highest electrochemical band gap due to nitro (−NO2 ) group in the structure. In our previous paper, we synthesized 4-hydroxybenzaldehyde (4-HBA) based Schiff bases and poly(azomethine-urethane)s [27]. When we compared to the electrochemical band gap of these two series of Schiff bases, VAN based Schiff bases have lower electrochemical band gap values than 4-HBA series due to VAN series have methoxy substituent in the structures. 3.5. Thermal analyses The thermal degradation behavior of Schiff bases and their polyurethane derivatives were studied by using TG-DTA technique. Fig. 6a shows TG curves of VAN-o-PDA, VAN-o-PDA-TDI and VAN-oPDA-HDI. Also, thermal degradation data all of materials were summarized in Table 5. As seen in Table 5, VAN-o-PDAM-TDI, VAN-oPDAN-TDI and VAN-o-PDAM-HDI exhibited three-stage degradation

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543

Table 3 Optical and photoluminescence spectral data of the compounds. Compounds

UV–vis

VAN-o-PDA VAN-o-PDAM VAN-o-PDAN VAN-o-PDA-TDI VAN-o-PDAM-TDI VAN-o-PDAN-TDI VAN-o-PDA-HDI VAN-o-PDAM-HDI VAN-o-PDAN-HDI a b c d e

PL

λ (nm)

λonset (nm)

a

311 310 292,371 303 312 269,302 312 277,348 271,381

351 348 449 360 351 417 363 421 428

3.53 3.57 2.77 3.45 3.54 2.98 3.42 2.95 2.90

Eg (eV)

λEx

Conc. (mg/L)

b

4.00 × 10−1 4.00 × 10−1 2.50 × 10−2 4.00 × 10−1 2.50×10−2 1.00×10−1 1.00×10−1 6.25×10−3 1.25×10−2

347 365 319 348 329 332 340 316 313

c

λEm

369 388 358 358 408 412 393 382 377

d

λmax

379, 701 396, 734 427 376, 701 412, 704 395 395, 700 373 356, 707

e

IEm

977, 125 82, 11 17 972, 82 975, 999 83 968, 358 75 241, 19

Optical band gap. Excitation wavelength for emission. Emission wavelength for excitation. Maximum emission wavelength. Maximum emission intensity.

Fig. 5. Cyclic voltammograms of Schiff bases and polyurethanes containing azomethine. Table 4 Electrochemical onset potentials and electronic energy levels of the compounds. Compounds

Eox.,onset (V)

a

VAN-o-PDA VAN-o-PDAM VAN-o-PDAN VAN-o-PDA-TDI VAN-o-PDAM-TDI VAN-o-PDAN-TDI VAN-o-PDA-HDI VAN-o-PDAM-HDI VAN-o-PDAN-HDI

0.86 0.88 1.23 1.01 0.96 0.99 0.90 1.02 1.00

−5.27 −5.29 −5.64 −5.42 −5.37 −5.40 −5.31 −5.43 −5.41

a b c

IP (eV)

Ered.,onset (V)

b

−0.78 −0.73 −1.44 −0.70 −0.71 −0.87 −0.74 −0.75 −0.98

−3.63 −3.68 −2.97 −3.71 −3.70 −3.54 −3.67 −3.66 −3.43

EA (eV)

c

Eg (eV)

1.64 1.61 2.67 1.71 1.67 1.86 1.64 1.77 1.98

Ionization potential. Electron affinity. Electrochemical energy band gaps.

while the other compounds exhibited two-stage degradation. According to Table 5, the initial degradation temperatures (Ton ) of Schiff bases were determined in the range 147–309 °C and polyurethanes determined in the range 136–244 °C. Furthermore, char at 1000 °C

of Schiff bases was between 25.25 and 53.30% and poly(azomethineurethane)s was between 10.00 and 41.50%. According to the obtained thermal degradation data, VAN-o-PDA-TDI series have higher char values than VAN-o-PDA-HDI series due to structure of VAN-o-PDATDI series. As seen in Scheme 1, VAN-o-PDA-TDI series have fully aromatic structure and they were synthesized from aromatic diisocyanates whereas VAN-o-PDA-HDI series consist of both aliphatic and aromatic groups and they were synthesized from aliphatic diisocyanates. In our previous paper, we synthesized novel polyurethane using 3-etoxy-4-hydroxybenzaldehyde instead of vanillin [41]. When we compared to the obtained thermal stability results, vanillin series have higher char at 1000 °C than 3-etoxy-4-hydroxybenzaldehyde series. Moreover, thermal stability of the polyurethane was improved using vanillin instead of 3-etoxy-4-hydroxybenzaldehyde. Thermal transitions in polyurethanes containing azomethine were studied using DSC technique. Fig. 6b shows the DSC thermogram of VAN-o-PDA-HDI, which shows glass transition temperature (Tg ), crystallization temperature (Tx ), peak crystallization temperature (Tp ) and melting point (Tm ). Table 6 shows DSC analysis results all of polyurethanes containing azomethine. As seen in Table 6,

27.00 25.25 53.30 17.30 41.20 41.50 10.90 14.00 10.00 377 372 – 317 631 574 308 347 363 333 312 411 246 249 246 228 253 311 – – – – 26.70 26.80 – 24.60 – – – – – 1000 1000 – 1000 – – – – – 469 457 – 434 – – – – – 374 377 – 396 –

Fig. 6. TG-DTG curves of VAN-o-PDA, VAN-o-PDA-TDI and VAN-o-PDA-HDI (a), TG curves of VAN-o-PDAM, VAN-o-PDAM-TDI and VAN-o-PDAM-HDI (b), and DSC curves of the VAN-o-PDA-HDI (c).

– 357 503 343 297 294 389 337 356

– 1000 1000 1000 374 377 1000 396 1000

– 65.80 26.70 31.70 26.10 12.00 61.60 41.20 79.70

the glass transition temperature (Tg ) of poly(azomethine-urethane)s is between 88 and 173 °C. The crystallization temperature (Tx ) and peak crystallization temperature of PAMUs are determined to be in the range 209–276 °C and 317 to 331 °C, respectively. Also, melting points of poly(azomethine-urethane)s are calculated between 342 and 390 °C.

3.6. Dynamical mechanical analysis

f

e

d

c

b

a

The onset temperature. Maximum weight temperature. Final thermal degradation temperature. Starting temperature of thermal degradation. 20% weight loss. 50% weight loss.

– 235 411 320 228 236 357 269 255 71.10 7.10 18.80 48.60 4.20 16.00 21.00 16.40 6.80 1000 235 411 320 228 236 357 269 255 345 187 338 263 160 178 275 241 221 309 147 277 225 136 134 244 193 183 VAN-o-PDA VAN-o-PDAM VAN-o-PDAN VAN-o-PDA-TDI VAN-o-PDAM-TDI VAN-o-PDAN-TDI VAN-o-PDA-HDI VAN-o-PDAM-HDI VAN-o-PDAN-HDI

Lost of absorbed water (%) Char at 1000 °C (%) T50 f

T20 e

Percentage of weight loss Tend c

Wmax T b

Tstart

Third degradation temperature (°C)

d

Percentage of weight loss Tend c

Wmax T b

Tstart

Second degradation temperature (°C)

d

Percentage of weight loss Tend c

Wmax T b

Ton a

First degradation temperature (°C) Compounds

Table 5 Thermal degradation values of the compounds.

1.90 1.85 1.20 2.40 1.80 3.70 1.90 3.80 3.50

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DMA analyses of polyurethanes containing azomethine were carried out to investigate the mechanical properties of the polymers. The tan delta (tan δ ) signals, storage modulus (E  ) and loss modulus (E ) of PAMUs were measured as functions of the sample temperature. Fig. 7a–c shows Tan delta (Tan δ ) signals of polyurethanes, storage modulus and loss modulus of VAN-o-PDA-HDI, VAN-o-PDAMHDI and VAN-o-PDAN-HDI, respectively. The obtained results are also summarized in Table 6. According to Fig. 7a and Table 6, glass transitions (Tg ) of polyurethane containing azomethine are determined in the range 65–250 °C. As can be seen from glass transitions of polyurethanes containing azomethine, the obtained results from DMA and DSC are a bit different. This could be the different nature of used two methods. As known, DMA measures the change in the mechanical response of the polymer chains. On the other hand, DSC measures the change in heat capacity from frozen to unfrozen chains [42]. According to Fig. 7b, the behavior of E  has a linear portion in the region 115–255, 110–253 and 20–170 °C for VAN-o-PDA-HDI, VAN-o-PDAM-HDI and VAN-o-PDAN-HDI. This is probably where the variation of the modulus is less intense, this region is denominated the glass region, and it can be characterized by low mobility of the polymeric chains. This linearity is observed at 50–155, 50–115 and

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545

Table 6 DMA and DSC data of polyurethanes containing azomethine.

Compounds VAN-o-PDA-TDI VAN-o-PDAM-TDI VAN-o-PDAN-TDI VAN-o-PDA-HDI VAN-o-PDAM-HDI VAN-o-PDAN-HDI a b c d e

Tan Delta (Tan

8

4 3

1

Tx (°C)b

Tp (°C)c

Tm (°C)d

Cp (J/g K)e

97 65 87 174 250 94

88 125 117 141 173 120

209 260 276 275 270 218

317 324 324 331 326 318

342 373 390 357 382 355

0.643 0.018 0.022 0.023 0.020 0.132

1 = VAN-o-PDA-TDI 2 = VAN-o-PDAM-TDI 3 = VAN-o-PDAN-TDI 4 = VAN-o-PDA-HDI 5 = VAN-o-PDAM-HDI 6 = VAN-o-PDAN-HDI

2

5

4

DSC Tg (°C)a

The glass transition temperature. The crystallization temperature. The peak crystallization temperature. The melting point. Change of specific heat during glass transition.

(a) 10

6

DMA Tg (°C)a

6

2 0 30

Storage Modulus (x109 Pa)

(b)

130 230 Temperature (oC)

330

30 25

3 = VAN-o-PDAN-HDI

20 1 = VAN-o-PDA-HDI

15 10 2 = VAN-o-PDAM-HDI

5 0 25

(c) Loss Modulus (x109 Pa)

3

75

125 175 225 Temperature (oC)

275

325

1 = VAN-o-PDA-HDI 2 = VAN-o-PDAM-HDI 3 = VAN-o-PDAN-HDI

2.5

Fig. 8. SEM images of VAN-o-PDA-TDI (a), VAN-o-PDAM-TDI (b), VAN-o-PDAN-TDI (c), VAN-o-PDA-HDI (d), VAN-o-PDAM-HDI (e) and VAN-o-PDAN-HDI (f).

2

2

1.5

This increase is observed in the range 61–170 °C for VAN-o-PDA-TDI, 86–204 °C for VAN-o-PDAM-TDI, 151–218 °C for VAN-o-PDAN-TDI, 141–266 °C for VAN-o-PDAM-HDI and 82–161 °C for VAN-o-PDANHDI.

3 1

1

0.5

3.7. Morphological properties

0

25

75

125 175 225 Temperature (oC)

275

325

Fig. 7. Tan δ signal of polyurethanes containing azomethine (a), storage modulus (b) and loss modulus (c) of polyurethane containing azomethine.

30–225 °C for VAN-o-PDA-TDI, VAN-o-PDAM-TDI and VAN-o-PDANTDI [43]. As seen in Fig. 7c, E curve of VAN-o-PDA-HDI increases between 123 and 189 °C. This increase may be the increase of the polymeric chains mobility causing an increase in the energy dissipation [39].

Morphological properties of polyurethanes containing azomethine were determined by using scanning electron microscopy (SEM). Fig. 8 shows SEM photograph of poly(azomethine-urethane)s in powder form. This technique was carried out to investigate surface morphology of polymers. According to the obtained photographs, poly(azomethine-urethane)s have different particle sizes and consist of semi-crystalline particles. As seen in Fig. 8a, VAN-o-PDA-TDI consists of fiber-like semi-crystalline particles. Also, VAN-o-PDAMTDI, VAN-o-PDAN-HDI and VAN-o-PDAM-HDI consist of huge semicrystalline particles.

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4. Conclusion The novel low-band gap Schiff bases and polyurethanes containing azomethine were successfully synthesized. The structures of the compounds were clarified by using FT-IR, NMR, and SEC techniques. Photophysical properties of Schiff bases and poly(azomethineurethane)s were investigated using UV–vis and PL spectroscopy. According to photophysical results, Schiff base containing nitro group has the lowest optical band gap due to the electron-withdrawing effect of nitro group in the structure. Electrochemical properties of the compounds were illuminated by using cyclic voltammetry. According to CV results, the obtained poly(azomethine-urethane)s have electrochemical band gap below 2.0 eV. Due to this property, they can be successfully used in heterojunction solar cells. As per DSC curves of polyurethanes, they contain crystallization and melting peak. Because of these peaks, they are semi-crystalline materials. Morphological properties of poly(azomethine-urethane)s were investigated by scanning electron microscopy. SEM photographs show that polyurethanes containing azomethine consist of semi-crystalline particles. References [1] Liu X, He R, Shen W, Li M. Molecular design of donor–acceptor conjugated copolymers based on C-, Si- and N-bridged dithiophene and thienopyrroledione derivatives units for organic solar cells. J Power Sources 2014;245:217–23. [2] Mondragon M, Moggio I, de-León A, Arias E. Enhanced electroluminescence of organic light-emitting diodes by using halloysite nanotubes. J Lumin 2013;144:176– 9. [3] Wang C, Tang Y, Guo Y. Adenosine deaminase biosensor combining cationic conjugated polymer-based FRET with deoxyguanosine-based photo induced electron transfer. ACS Appl Mater Interfaces 2014;6:21686–91. [4] Zhang H, Zhang Y, Gu C, Ma Y. Electropolymerized conjugated microporous poly(zinc-porphyrin) films as potential electrode materials in super capacitors. Adv Eng Mater 2015;5:1402175. [5] Kola S, Kim JH, Ireland R, Yeh ML, Smith K, Guo W, et al. Pyromellitic diimide– ethynylene-based homopolymer film as an N-channel organic field-effect transistor semiconductor. ACS Macro Lett 2013;2:664–9. [6] Colbert AE, Janke EM, Hsieh ST, Subramaniyan S, Schlenker CW, Jenekhe SA, et al. Hole transfer from low band gap quantum dots to conjugated polymers in organic/inorganic hybrid photovoltaics. J Phys Chem Lett 2013;4:280–4. [7] Ye H, Zhao B, Li D, Chen D, Xie G, Su SJ, et al. Highly efficient non-doped singlelayer blue organic light-emitting diodes based on light-emitting conjugated polymers containing trifluoren-2-ylamine and dibenzothiophene-S, S-dioxide. Synth Met 2015;205:228–35. [8] Chen HF, Liao CT, Su HC, Yeh YS, Wong KT. Highly efficient exciplex emission in solid-state light-emitting electrochemical cells based on mixed ionic holetransport triarylamine and ionic electron-transport 1,3,5-triazine derivatives. J Mater Chem C 2013;1:4647–54. [9] Xu H, Chang J, Chen Y, Fan H, Shi B. Asymmetric polyurethane membrane with inflammation-responsive antibacterial activity for potential wound dressing application. J Mater Sci 2013;48:6625–39. [10] Gaikwad MS, Gite VV, Mahulikar PP, Hundiwale DG, Yemul OS. Eco-friendly polyurethane coatings from cottonseed and karanja oil. Prog Org Coat 2015;86:164–72. [11] Choe KY, Lee JH, Ju-Yeon L. Synthesis and properties of novel X-type polyurethane containing dicyanovinylnitrocatechol of enhanced SHG thermal stability for electro-optic applications. Polym Bull 2014;71:2369–82. [12] Senol D, Kaya I.˙ Synthesis and characterization of azomethine polymers containing ether and ester groups. J Saudi Chem Soc 2015. doi:10.1016/j.jscs.2015.05.006. [13] Xiang T, Liu X, Yi P, Guo M, Chen Y, Wesdemiotis C, et al. Schiff base polymers derived from 2,5-diformylfuran. Polym Int 2013;62:1517–23. [14] Kaya I,˙ Avcı A. Synthesis, characterization, and thermal stability of novel poly(azomethine-urethane)s and polyphenol derivatives derived from 2,4dihydroxy benzaldehyde and toluene-2,4-diisocyanate. Mater Chem Phys 2012;133:269–77. [15] Kaya I,˙ Kamacı M. Novel poly(azomethine-urethane)s and their polyphenol derivatives derived from aliphatic diisocyanate compound: synthesis and thermal characterization. J Appl Polym Sci 2012;125:876–87. [16] Kaya I,˙ Uysal S. Synthesis and characterization of imine polymers containing aliphatic and aromatic groups and some of Schiff base-metal complexes. J Appl Polym Sci 2011;120:3325–36. [17] Tripathi SM, Tiwari D, Ray A. Electrical conductivity of polyazomethine nanocompozites. Indian J Chem 2014;53A:1505–12.

[18] Zaltariov MF, Cazacu M, Racles C, Musteata V, Vlad A, Airinei A. Metallopolymers based on a polyazomethine ligand containing rigid oxadiazole and flexible tetramethyldisiloxane units. J Appl Polym Sci 2015;132. doi:10.1002/app.41631. [19] Kaya I,˙ Avcı A, Gültekin Ö. Syntheses and characterization of poly (iminophenol)s derived from 4-bromobenzaldehyde: thermal, optical, electrochemical, and fluorescent properties. Chin J Polym Sci 2012;30:796–807. [20] Thaker BT, Patel P, Vansadia AD, Patel HG. Synthesis, characterization, and mesomorphic properties of new liquid-crystalline compounds involving ester– azomethine central linkages, lateral substitution, and a thiazole ring. Mol Cryst Liq Cryst 2007;466:13–22. [21] Liu CP, Wang MK, Xiao Q. Preparation, property characterization and UVconverting application of poly(conjugated azomethine-urethane)/hydroxyl polyacrylate resin. J Appl Polym Sci 2013;129:3629–39. [22] Kaya I,˙ Yıldırım M, Kamacı M, Avcı A. New poly(azomethine-urethane)s including melamine derivatives in the main chain: synthesis and thermal characterization. J Appl Polym Sci 2011;120:3027–35. [23] Kausar A, Zulfiqar S, Ishaq M, Sarwar MI. An investigation on new high performance Schiff base polyurethanes. High Perform Polym 2012;24:125–34. [24] Kaya I,˙ Avcı A, Temizkan K. Synthesis of thermally stable and low band gap poly(azomethine-urethane)s containing fluorene unit in the backbone. Korean J Chem Eng 2015;32:777–86. [25] Avcı A, Kamacı M, Kaya I,˙ Yıldırım M. Synthesis of novel crosslinked poly(azomethine-urethane)s: photophysical and thermal properties. Mater Chem Phys 2015;163:301–10. [26] Ali I, Al-Zahrani SM, Dolui SK. Thermotropic poly(azomethine-urethane)s with non linear optical properties: synthesis and characterization. Polym Sci Ser B 2012;54:342–8. [27] Kamacı M, Kaya I.˙ Synthesis, thermal and morphological properties of polyurethanes containing azomethine linkage. J Inorg Organomet Polym 2014;24:803–8180. [28] Shanavas A, Vanjinathan M, Nasar AS, Amudha S, Suthanthiraraj SA. Synthesis, thermal and solar cell application of novel hyperbranched polyurethanes containing azomethine and aryl-ether connectivities. High Perform Polym 2012;24:561– 70. [29] Kaya I,˙ Kamacı M. Highly selective and stable florescent sensor for Cd(II) based on poly(azomethine-urethane). J Fluoresc 2013;23:115–21. [30] Kamacı M, Kaya I.˙ 2,4-Diamino-6-hydroxypyrimidine based poly(azomethineurethane): Synthesis and application as a fluorescent probe for detection of Cu2+ in aqueous solution. J Fluoresc 2015. doi:10.1007/s10895-015-1624-z. [31] Kamacı M, Kaya I.˙ The novel poly(azomethine-urethane): synthesis, morphological properties and application as a fluorescent probe for detection of Zn2+ ions. J Inorg Organomet Polym 2015;25:1250–9. [32] Xie H, Zhang K, Duan C, Liu S, Huang F, Cao Y. New acceptor-pended conjugated polymers based on 3,6- and 2,7-carbazole for polymer solar cells. Polymer 2012;53:5675–83. [33] Yıldırım M, Kaya I,˙ Aydın A. Azomethine coupled fluorine-thiophenepyrrole based copolymers: electrochromic applications. React Funct Polym 2013;73:1167–74. [34] Farcas A, Resmerita AM, Aubert PH, Ghosh I, Cantin S, Nau WN. Synthesis, photophysical, and morphological properties of azomethine-persylilated α cyclodextrin main-chain polyrotaxane. Macromol Chem Phys 2015;216:662–70. [35] Kaya I,˙ Yıldırım M, Avcı A. Synthesis and characterization of fluorescent polyphenol species derived from methyl substituted aminopyridine based Schiff bases: the effect of substituent position on optical, electrical, electrochemical, and fluorescence properties. Synth Met 2010;160:911–20. [36] Kaya I,˙ Kamacı M, Arıcan F. Synthesis and characterization of polyphenols derived from 4-fluorobenzaldeyde: the effect of electron-donating group on some physical properties. J Appl Polym Sci 2012;125:608–19. [37] Marin L, Zabulica A, Sava M. Symmetric liquid crystal dimers containing a luminescent mesogen: synthesis, mesomorphic behavior and optical properties. Soft Mater 2013;11:32–9. [38] Kamacı M, Kaya I.˙ Synthesis of metal-coordinated poly(azomethine-urethane)s: thermal stability, optical and electrochemical properties. J Inorg Organomet Polym 2013;23:159–1171. [39] Albayrak Ç, Kasta ¸ s¸ G, Odabaso˘ ¸ glu M, Frank R. The prototropic tautomerism and substituent effect through strong electron-withdrawing group in (E)5-(diethylamino)-2-[(3-nitrophenylimino)methyl]phenol. Spectrochim Acta A 2013;114:205–13. [40] Kaya I,˙ Kamacı M. Synthesis, optical, electrochemical, and thermal stability properties of poly(azomethine-urethane)s. Prog Org Coat 2012;74:204–14. [41] Kamacı M, Kaya I.˙ Photophysical, electrochemical, thermal and morphological properties of polyurethanes containing azomethine bonding. J Macromol Sci A 2014;51:805–19. [42] Gurunathan T, Mohanty S, Nayak SK. Effect of reactive organoclay on physicochemical properties of vegetable oil-based waterborne polyurethane nanocomposites. RSC Adv 2015;5:11524–33. [43] Rocco JAFF, Lima JES, Lourenco VL, Batista NL, Botelho EC, Iha K. Dynamic mechanical properties for polyurethane elastomers applied in elastomeric mortar. J Appl Polym Sci 2012;126:1461–7.