The crosslinked poly(azomethine-urethane)s containing o-hydroxyazomethine: Tunable multicolor emission, photophysical and thermal properties

Progress in Organic Coatings 88 (2015) 325–336

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The crosslinked poly(azomethine-urethane)s containing o-hydroxyazomethine: Tunable multicolor emission, photophysical and thermal properties Musa Kamacı a,b , Ali Avcı a,c , I˙ smet Kaya a,∗ a

Polymer Synthesis and Analysis Lab., Department of Chemistry, C¸anakkale Onsekiz Mart University, 17020 C¸anakkale, Turkey Faculty of Sciences and Letters, Department of Chemistry, Piri Reis University, Tuzla, 34940 Istanbul, Turkey c Faculty of Sciences and Arts, Department of Chemistry, Celal Bayar University, 45140 Manisa, Turkey b

a r t i c l e

i n f o

Article history: Received 28 March 2015 Received in revised form 13 July 2015 Accepted 16 July 2015 Available online 11 August 2015 Keywords: Tunable multi color emission Photophysical Electrochemical Morphological and thermal properties Crosslinked polyurethane

a b s t r a c t This article presents synthesis, photophysical, electrochemical, thermal, and morphological properties of the crosslinked polyurethanes (CLPUs). CLPUs were synthesized in two main steps. In the first step, aldehyde functionalized polyurethane prepolymer was synthesized using 2,4-dihydroxy benzaldehyde and hexamethylene diisocyanates. In the second step, the prepared prepolymer was converted to the crosslinked polyurethane derivatives using different diamines via condensation reaction. Diamines with various chain length and side-group substitutions were used as crosslinker. Photophysical properties of the crosslinked polyurethanes were investigated using UV–vis and photoluminescence (PL) spectra techniques. Fluorescence measurements showed that CLPUs exhibited multicolor emission behavior. Additionally, a linear relationship is determined between the excitation energies and the obtained emission maxima and, this property allows tuning the PL color by changing the source light energy on the desired scale. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Polyazomethines (PAZs) are an interesting class of polymers which can be easily prepared by a simple reaction of diamine and dialdehyde [1]. By now, this class of polymers attracted much attention because of their useful properties such as high thermal stability, excellent mechanical strength, metal chelating ability, semi-conducting, and good optoelectronic properties [2,3]. Because of these superior properties they were used to prepare composites, thermostabilisators, fotoresists, graphite materials, epoxy oligomer, and block copolymers [4,5]. However, their low solubility is a serious problem and so PAZs should be prepared as soluble forms to make them more useful for applications [6,7]. An effective method for improving the solubility of PAZs is the incorporation of several flexible units into the macromolecule backbone and the preparation of some copolymers such as poly (azomethine-ether), poly(azomethine-sulfone), poly(azomethineester), and poly(azomethine-urethane) [8–12].

∗ Corresponding author. Tel.: +90 286 218 00 18; fax: +90 286 218 05 33. E-mail address: [email protected] (I˙ . Kaya). http://dx.doi.org/10.1016/j.porgcoat.2015.07.017 0300-9440/© 2015 Elsevier B.V. All rights reserved.

Polyurethanes (PUs) are versatile polymeric materials. They can be synthesized on a large scale and processed in a variety of different ways. Because of these properties, they are widely used in the industry, for example as adhesives [13], coatings [14], synthetic leather [15], as well as for construction materials [16], and even as flame retardants [17]. In addition, over the past 40 years, polyurethanes have also been used in biomedical devices due to their biocompatibility and mechanical flexibility [18,19]. On the other hand, PUs are often used as implants for tissue repair and drug delivery systems [20]. Essentially, photoluminescence (PL) color tuning is known the alteration of the transition energy levels of the polymers and it can be acquired by different physical and chemical strategies [21]. Bilici et al. stated that these physical and chemical strategies are: (i) different substituents in the polymer backbone, (ii) controlling polymer molecular weight, (iii) inter/intrachain interactions, (iv) altering polymer concentration, (v) particle size, and (vi) applied excitation wavelength [22]. To the best of our knowledge, there is no paper in the literature on the crosslinked polyurethanes with tunable multi color emission. For this reason, we firstly synthesized the aldehyde functionalized polyurethane prepolymer with 2,4-dihydroxy benzaldehyde and hexamethylene diisocyanate, and then converted to

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Scheme 1. Synthesis scheme of the aldehyde functionalized prepolymer.

Scheme 2. Synthesis scheme of the crosslinked poly(azomethine-urethane)s.

the crosslinked polyurethane derivatives via condensation reaction with different diamines. Secondly, we investigated photophysical, electrochemical, thermal and morphological properties of the crosslinked polyurethanes. We surprisingly found that CLPUs exhibited multicolor emission behavior, and this property allows tuning the PL color by changing the source light energy on the desired scale.

2. Experimental

n-hexane were supplied from Merck Chemical Co. (Germany), and all the reagents were used without further purification. 2.2. Synthesis of aldehyde functionalized prepolymer The aldehyde functionalized prepolymer has been synthesized in our previous paper by copolymerization reaction of DHBA with HDI under Argon atmosphere as in Ref. (Scheme 1) [23]. Yields: 64%; GPC: Mn: 3300, Mw /Mn : 1.424.

2.1. Materials

2.3. Synthesis of the crosslinked poly(azomethine-urethane)s

1,6-Diaminohexane (DAH), 1,12-diaminododecane (DAD), 1,3-bis(aminopropyl)tetramethyl disiloxane (APMS), 2,4,8,10tetraoxaspiro [5,5]undecane-3,9-dipropan amine (DPA), 2,4dihydroxy benzaldehyde (DHBA), hexamethylene diisocyanate (HDI), dimethylformamide (DMF), dimethylsulfoxide (DMSO), tetrahydrofurane (THF), methanol (MeOH), ethanol (EtOH), acetone, acetonitrile (MeCN), toluene, ethyl acetate, CHCl3 , CCl4 , and

The crosslinked polyurethanes were synthesized by condensation reaction of the preformed prepolymer and different diamines and, they abbreviated as CLPU-1, CLPU-2, CLPU-3, and CLPU-4. Synthesis procedure of CLPUs was as follows: the aldehyde functionalized prepolymer (1.56 g, 5.00 × 10−3 mol) was dissolved in 20 mL DMF/MeOH mixture (1/3) and added into a 100 mL threenecked round-bottom flask which was fitted with condenser and

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327

Fig. 1. FT-IR spectra of the prepolymer, DAH, and the crosslinked polyurethanes.

magnetic stirrer. An excess of DAH (1.162 g, 10 × 10−3 mol), DAD (2.000 g, 10 × 10−3 mol), APMS (2.485 g, 10 × 10−3 mol) or DPA (2.743 g, 10 × 10−3 mol) were dissolved in 20 mL MeOH and added to the solution drop by drop. Reaction mixture was heated up to 60 ◦ C and maintained for 3 h, cooled at the room temperature. The obtained CLPUs were washed by MeOH (2 × 50 mL) and MeCN (2 × 50 mL) to remove the unreacted components (Scheme 2) and powder polymers were obtained as yellow, orange, dark yellow and dark orange color in 78%, 75%, 72%, 76% yield for CLPU-1, CLPU-2, CLPU-3, and CLPU-4, respectively [24]. Mn = 14,200, Mw /Mn = 1.211 for CLPU-1, Mn = 14,800, Mw /Mn = 1.195 for CLPU-2, Mn = 15,700,

Mw /Mn = 1.172 for CLPU-3 and Mn = 19,000, Mw /Mn = 1.142 for CLPU-4. 2.4. Characterization techniques The infrared spectra were obtained by Perkin Elmer FTIR Spectrum one using the 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) were recorded in deuterated DMSO-d6 at 25 ◦ C. Tetramethylsilane (TMS) was used as internal standard. The number average

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Fig. 2.

1

H-NMR spectra of prepolymer (a), CLPU-1 (b), CLPU-3 (c), and 13 C-NMR spectrum of CLPU-1 (d).

M. Kamacı et al. / Progress in Organic Coatings 88 (2015) 325–336

Spectral data (ıppm )

CLPU-1

1

CLPU-2

CLPU-3

CLPU-4

H-NMR (DMSO-d6 ): 9.91(Urethane –NH, Hd, 2H), 8.28 (Imine (–N CH), Hh, 1H), 7.50 (Hg, 1H), 7.13 (He, 1H), 6.37 (Hf, 1H), 2.92 (Hc, 4H), 2.73 (Hi, 2H), 2.53 (Hb, 5H), 1.56 (Hj, 2H), 1.32 (Ha, 4H) and 1.20 ppm (Hk, 2H). 13 C-NMR (DMSO-d6 ): 168.76 (Urethane –C O, C11 ), 164.55 (Imine (–N CH), C4 ), 161.18 (C10 ), 157.58 (C8 ), 136.02 (C6 ), 121.93 (C5 ), 111.24 (C7 ), 104.76 (C9 ), 58.75 (C3 ), 39.90 (C12 ), 32.09 (C2 ), 28.41 (C13 ), 25.16 (C1 ) and 22.12 ppm (C14 ). 1 H-NMR (DMSO-d6 ): 9.93 (Urethane –NH, 2H), 8.29 (Imine (-N CH), 1H), between 6.10 and 7.93 ppm aromatic protons (3H), between 1.21 and 2.94 ppm aliphatic protons (36 H). 1 H-NMR (DMSO-d6 ): 9.90 (Urethane –NH, Hd, 2H), 8.28 (Imine (–N CH), Hh, 1H), 7.50 (Hg, 1H), 7.12 (He, 1H), 6.38 (Hf, 1H), 2.91 (Hc, 5H), 2.48 (Hi, 2H), 1.56 (Hb, 4H), 1.31 (Ha, 4H), 1.19 (Hj, 2H), 0.41 (Hk, 4H), 0.10 ppm (Si-CH3 , Hl, 11 H). 1 H-NMR (DMSO-d6 ): 9.92 (Urethane –NH, 2H), 8.48 (Imine (–N CH), 1H), between 6.11 and 8.29 ppm aromatic region (3H), between 1.33 and 4.48 ppm aliphatic region (33 H).

molecular weight (Mn ), the weight-average molecular weight (Mw ) and Mw /Mn were determined by gel permeation chromatographylight scattering (GPC-LS) device of Malvern Viscotek GPC Dual 270 max. The GPC measurement was investigated by dual columns with 300 mm length and 8.00 mm in diameter. Addition 1 g/L of lithium bromide in DMF (1 mL/min) was used as solvent. Light scattering (LS) detector and a refractive index detector (RID) were used to analyze the polymer at 55 ◦ C. Thermal data were obtained by using Perkin Elmer Diamond Thermal Analysis system. The TG-DTA measurements were made between 20 and 1000 ◦ C (in N2 , rate 10 ◦ C/min). DSC analyses were carried out between 25–420 ◦ C (in N2 , rate 10 ◦ C/min) using a Perkin Elmer Pyris Sapphire DSC. DMA tests of the polymers were carried out by Perkin Elmer Pyris Diamond DMA 115 V using 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 samples were prepared as follows: 0.5 g of the crosslinked polyurethanes was placed into the titanium clamp (supplied from Triton Technology Ltd., United Kingdom) and extended followed by closing of the clamp from both sides. 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 Ref. [25]. Photoluminescence (PL) measurements were performed in solution form (in DMF) using a Shimadzu RF-5301PC spectrofluorometer. Concentration of polymer solutions and slit width of the spectrofluorophotometer were adjusted to between 0.8 × 10−3 and 1.65 × 10−3 mg/L and 5 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

1

1 = CLPU-1 2 = CLPU-2 3 = CLPU-3 4 = CLPU-4

0.8 Absorbance

Compounds

a

0.6 0.4

2 3

0.2 4 0

b P.L. Intensity (a.u.)

Table 1 1 H and 13 C-NMR spectra data of the crosslinked polyurethanes.

329

250

1

350

450 550 Wavelength (cm-1)

750

1 = CLPU-1 2 = CLPU-2 3 = CLPU-3 4 = CLPU-4

600 500

650

3

400 300

1

200

4

100 0

2

300

400

500 600 Wavelength (nm)

700

800

Fig. 3. UV–vis (a) and PL spectra (b) of the crosslinked polyurethanes.

(Fc/Fc+ ) couple. The half-wave potential (E1/2 ) of (Fc/Fc+ ) measured in MeCN solution of 0.1 M tetrabutylammoniumhexafluorophosphate (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 HOMO–LUMO energy levels and electrochemical band gaps (E g ) were calculated from the oxidation and reduction onset values as in Ref. [26]. 2.7. Morphological properties Atomic force microscopy (AFM) topography and 3D images of CLPU-3 and CLPU-4 were recorded using WITec Alpha 300A AC mode (cantilever 42 N/m 285 kHz). For AFM measurements, the crosslinked polyurethane films were prepared by spin coating technique onto glass in THF solution of CLPU-3 and CLPU-4 at room temperature. The surface morphologies of CLPU-2 and CLPU-3 were monitored by using a Jeol JSM-7100F Schottky field emission scanning electron microscope. For SEM measurements, the surface of CLPU2 and CLPU-3 powder was coated with a thin layer of gold using a sputter coater prior to microscopy. 3. Results and discussion 3.1. Solubility and characterization The solubility tests of the crosslinked polyurethanes were done in different solvents by using 1 mg sample and 1 mL solvent at 25 ◦ C. Solubility results showed that CLPUs are completely soluble only in strong polar solvents like DMSO and DMF. They are partly soluble in THF while they are insoluble in EtOH, MeOH, ethyl acetate, MeCN, n-hexane, and acetone. Typical FT-IR spectra of the prepolymer, DAH and CLPUs were displayed in Fig. 1. According to FT-IR spectrum data of the prepolymer, the characteristic aldehyde (–CHO) and urethane (–NH), (–C O), and (–C–O) stretch vibrations are observed at 1705, 3321, 1666 and 1159 cm−1 , respectively [23]. Some additional peaks including aliphatic (C–H stretch) and aromatic (C C

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Table 2 Photophysical properties of the crosslinked polyurethanes. Compounds

UV–vis

CLPU-1 CLPU-2 CLPU-3 CLPU-4

e

Eg (eV)a

Conc. (mg/L)

Ex b

Em c

max d

Ie

283, 313, 384 280, 308, 381 282, 307, 386 281, 306, 388

455 477 444 453

2.73 2.60 2.80 2.74

1.6 × 10−3 8.0 × 10−4 1.6 × 10−3 8.0 × 10−4

446 295 447 447

501 350 496 505

506, 550 346, 430 494 469, 554

235, 282 72, 79 580 227, 152

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

700

PL Intensity (a.u.)

a

3

400

2

300

1

200 100 440

490

400

540 590 640 Wavelength (nm)

CLPU-3

350 PL Intensity (a.u.)

Ex. WL - Em. WL 1 = 440 nm - 553 nm 2 = 460 nm - 556 nm 3 = 480 nm - 558 nm 4 = 500 nm - 560 nm

4

500

0

c

CLPU-1

600

300

3

1

250

690

Ex. WL - Em. WL 1 = 460 nm - 539 nm 2 = 480 nm - 556 nm 3 = 500 nm - 575 nm

2

100 50 0

120

d

CLPU-2

550 650 Wavelength (nm)

750

3

4

Ex. WL - Em. WL 1 = 360 nm - 445 nm 2 = 400 nm - 477 nm 3 = 440 nm - 500 nm 4 = 480 nm - 554 nm

60 30

350

400

300

450

500 550 600 Wavelength (nm)

4

CLPU-4

250

1

3

200

2

650

700

750

Ex. WL - Em. WL 1 = 460 nm - 551 nm 2 = 480 nm - 556 nm 3 = 500 nm - 565 nm 4 = 520 nm - 566 nm

150 100 50 0

450

2 1

90

0

740

200 150

b PL Intensity (a.u.)

c d

onset (nm)

PL Intensity (a.u.)

a b

PL

 (nm)

450

550

650

750

Wavelength (nm)

Fig. 4. PL spectra of CLPU-1 (a), CLPU-2 (b), CLPU-3 (c), and CLPU-4 (d) in DMF excited by different wavelengths.

Fig. 5. Photographs of the crosslinking polyurethanes solution excited by different wavelengths and corresponding emission spectra CLPUs in DMF. Photographs were recorded in PL analysis cell (a: CLPU-1, b: CLPU-2, c: CLPU-3, and d: CLPU-4).

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331

Fig. 6. Linear relationship between emission maxima observed versus applied excitation wavelength (a: CLPU-1, b: CLPU-2, c: CLPU-3, and d: CLPU-4).

stretch) vibrations are observed in the range 2861–2936 cm−1 and 1477–152 cm−1 , respectively. By crosslinked prepolymer with different diamine monomers (DAH, DAD, APMS or DPA), the crosslinked polyurethanes were obtained. In addition, stretch vibration of aldehyde (–CHO) at 1705 cm−1 is not appears in the spectra of CLPUs, which indicated that –CHO groups in prepolymer is completely reacted with amine groups of diamine monomers as used crosslinker. According to FT-IR spectra of CLPUS, the new stretch vibration is formed instead of aldehyde (–CHO) stretch vibration. This newly formed stretch vibration (–N CH) is observed at 1609, 1613, 1618, and 1619 cm−1 for CLPU-1, CLPU2, CLPU-3, and CLPU-4, respectively. Urethane (–NH), (–C O) and (–C–O) stretch vibrations of CLPU-1, CLPU-2, CLPU-3, and CLPU-4 are observed in the range 3315–3320, 1714–1722, and 1163–1183 cm−1 , respectively [27]. Chemical structures of CLPUs were also evaluated by 1 H and 13 C-NMR spectra and Fig. 2 shows 1 H-NMR spectra of the prepolymer, CLPU-1, CLPU-3 and 13 C-NMR spectrum of CLPU-1. Also, 1 H and 13 C-NMR data of CLPUs are given in Table 1. As can be seen in

Fig. 7. Cyclic voltammograms of the crosslinked polyurethanes.

Table 3 Fluorescence-related data of the crosslinked polyurethanes solutions in DMF. Ex (nm)

360 380 400 420 440 460 480 500 520

EEx. (eV)

3.45 3.27 3.11 2.96 2.82 2.70 2.59 2.45 2.39

CLPU-1

CLPU-2

CLPU-3

CLPU-4

Em (nm)

Iem.

Em (nm)

Iem.

Em (nm)

Iem.

Em (nm)

Iem.

– – – – 553 556 558 560 –

– – – – 231 379 534 659 –

445 – 477 – 500 – 554 – –

95 – 103 – 105 – 110 – –

– – – – – 539 556 575 –

– – – – – 329 355 371 –

– – – – – 551 556 565 566

– – – – – 149 188 228 283

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Table 4 Electrochemical onset potentials and electronic energy levels of the crosslinked polyurethanes. Compounds

Eox. (V)

HOMO (eV)a

Ered. (V)

LUMO (eV)b

E g (eV)c

CLPU-1 CLPU-2 CLPU-3 CLPU-4

1.1419 0.9552 0.9013 1.0094

−5.53 −5.35 −5.29 −5.40

−0.9600 −0.9696 −0.9109 −0.9301

−3.43 −3.42 −3.48 −3.46

2.10 1.93 1.81 1.94

a b c

Highest occupied molecular orbital. Lowest unoccupied molecular orbital. Electrochemical band gap.

Fig. 2a, characteristic aldehyde (–CHO) proton of the prepolymer is observed at 10.93 ppm and urethane (–NH) proton observed at 6.76 ppm. Also, some additional protons of the prepolymer such as aromatic and aliphatic –CH are observed in the range 7.65–7.70 and 1.32–3.06 ppm, respectively. According to 1 H-NMR spectra of CLPUs (Figs. 2b, c and Table 1) the characteristic aldehyde proton of the prepolymer was disappeared and the new azomethine protons appeared instead of this proton due to form crosslinked azomethine structure. This newly formed imine (–N CH) proton of CLPUs is observed in the range 8.28–8.48 ppm. Also, urethane (–NH) proton of the crosslinked poly(azomethine-urethane)s is observed between 9.90 and 9.93 ppm. Some additional protons of CLPUs such as aliphatic and aromatic –CH are also observed in the range 0.41–4.48 and 6.10–8.29 ppm, respectively. When we compared to urethane protons of the prepolymer and crosslinked polymers, proton of the polymer is shifted low field (high frequency). This can be probably due to the newly formed imine (–N CH) bonding in polymer chain of the crosslinked poly(azomethine-urethane)s. As known, urethane (–NH) proton was observed in the range 6.50–10.0 ppm depending upon its structure. Also, similar tendency was observed in previous papers [23,24,28].

According to 13 C-NMR spectrum of CLPU-1, urethane (–C O) and imine (–N CH) carbons are observed at 168.76 and 164.55 ppm, respectively. Also, aromatic and aliphatic carbons are observed in the range of 104.76–161.18 and 22.12–58.75 ppm, respectively. The number-average molecular weight (Mn ), the weightaverage molecular weight (Mw ) and the molecular weight distribution (Mw /Mn ) of CLPUs were estimated with gel permeation chromatography (GPC) in DMF solution with polystyrene standards. The values of Mn , Mw and Mw /Mn of CLPUs were determined as 14,200, 17,200 and 1.211 (CLPU-1), 14,800, 17,700 and 1.203 (CLPU-2), 15,700, 18,400 and 1.172 (CLPU-3), and 19,000, 21,700 and 1.142 (CLPU-4), respectively. 3.2. Photophysical properties To examine the electronic states of the crosslinked polyurethanes, UV–vis absorption and photoluminescence measurements were performed. Fig. 3a shows UV–vis spectra of CLPUs, and the obtained results are also summarized in Table 2. According to UV–vis spectra of crosslinked polyurethanes, they are shown

Fig. 8. TG-DTA curves of the crosslinked polyurethanes.

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333

Fig. 9. DSC curves of the crosslinked polyurethanes.

three absorption band. The first absorption band is observed between 280 and 283 nm due to ␲ → ␲* transition of urethane linkage. The second and third absorption bands are observed in the range 306–313 and 381–388 nm due to ␲ → ␲* and n → ␲* transitions of azomethine linkage, respectively [28]. Optical band gap of CLPUs were calculated by using Eq. (1) as in Ref. [29]. Eg =

1242 onset

(1)

According to Table 2, onset values of CLPU-1, CLPU-2, CLPU3, and CLPU-4 are 455, 477, 444, and 453 nm, respectively. Also, optical band gap (Eg ) of these compounds changes from 2.60 to 2.80 eV. The photoluminescence (PL) properties of crosslinked polyurethanes were studied in DMF solution. Polymer concentration was adjusted as 1.6 × 10−3 mg/L for CLPU-1 and CLPU-3, and 8.0 × 10−4 for CLPU-2 and CLPU-4 in these measurements. The obtained spectra and data are given in Fig. 3b and Table 2, respectively. According to Fig. 3b, CLPU-3 is shown single emission maxima while the other crosslinked polyurethanes are shown two emission maxima. As can be seen in Fig. 3b, CLPU-3 has the highest fluorescence intensity while CLPU-2 has the lowest fluorescence intensity. To determine the effect of excitation wavelength on emission intensity, PL spectra of the crosslinked polymers were measured by excited with various wavelengths. We surprisingly found that when CLPUs excited by different wavelength, they shown different emission colors (Fig. 4). According to PL spectra of CLPU-1, when it excited by 440, 460, 480 and 500 nm it has max (Em) of 553, 556, 558, and 560 nm, respectively. According to these results, CLPU-1 shown 3, 5, and 7 nm bathochromic shift compared to 440 nm, respectively. Similarly, CLPU-2, CLPU-3, and CLPU-4 are excited by various wavelengths in the range 360–480, 460–560, and 460–520 nm, respectively. Emission wavelength of these compounds are determined in the range 445–554, 539–575, and 551–566 nm, respectively. According to these results, CLPU-2, CLPU-3, and CLPU-4 shown bathochromic shift in the range 32 and 109, 17 and 36, and 5 and 15 nm, respectively. This multicolor emission characteristic of the crosslinked poly(azomethine-urethane)s are attributed to o-hydroxyazomethine structure in polymer chain. As known, polymer chains having different PL signals due to the conjugation length varieties or excited state intramolecular proton transfer (ESIPT) process in o-hydroxyazomethine polymers [22]. Fig. 5 shows photographs of the crosslinked polyurethanes solution excited by different wavelengths and corresponding emission spectra CLPUs in DMF. These obtained photographs were

Fig. 10. Tan ı (a), storage (E ) modulus (b), and loss (E ) modulus (c) curves of the crosslinked polyurethanes.

recording in PL analysis cell. As can be seen in Fig. 5, when CLPU-1, CLPU-2, CLPU-3, and CLPU-4 excited by 440, 360, 440, and 460 nm, respectively, they are no emit light. On the other hand, when CLPU-1 excited by 460 and 500 nm, it emits indigo and turquoise light, respectively. When CLPU-2 excited by 400, 440, and 480 nm it emits indigo, light-blue, and green light, respectively. CLPU3 excited by 460, 480, and 500 nm it emits blue, turquoise, and green light, respectively. Similarly, CLPU-4 excited by 480, 500, and 520 nm it emits indigo, turquoise and green light, respectively. These observed colors in PL analysis cell are clearly visible by the naked eye. We were also found that a linear relationship is observed between the excitation energies and the obtained emission maxima expect CLPU-3. Fig. 6 shows linear relationship between emission maxima observed versus applied excitation wavelength and the obtained data are also summarized in Table 3. The obtained equations showed that CLPUs are shown linear relationship between the excitation energies and the obtained emission maxima. According to literature, there are only a few reports on an adjustable emission color by changing the source light energy on the desired scale. 3.3. Electrochemical properties The electrochemical behaviors of CLPUs are investigated by cyclic voltammetry (CV) with a three-electrode electrochemical

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Table 5 Thermal degradation, DMA and DSC data of the crosslinked polyurethanes. Compounds

CLPU-1 CLPU-2 CLPU-3 CLPU-4 a b c d e f

TG-DTA

DMA

DSC

Ton a

Wmax Tb

T20c

T50d

Char at 1000 ◦ C (%)

Tg (◦ C)e

Tg (◦ C)e

Cp (J/g K)f

175 99 314 126

181, 324, 441 190, 356, 455 348, 445 163, 348

284 334 324 318

354 420 362 374

12 11 18 16

77 83 207 122

114 81 163 118

0.459 0.716 0.053 0.538

The onset temperature. Maximum weight temperature. 20% weight loss. 50% weight loss. The glass transition temperature. Change of specific heat during glass transition.

cell. The highest occupied molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO) and electrochemical band gap (E g ) values of CLPUs were calculated due to understand electronic structures of the synthesized materials. Fig. 7 shows cyclic voltammograms of the crosslinked polyurethanes. HOMO, LUMO and E g values of polyurethanes were calculated by using Eqs. (2), (3) and (4), respectively, as in Ref. [30] and the obtained results summarized in Table 4. EHOMO = −(4.39 + Eox )

(2)

ELUMO = −(4.39 + Ered )

(3)

Eg = ELUMO − EHOMO

(4)

The onset oxidation potential (Eox. ) of the crosslinked polyurethanes is in the range 0.9013–1.1419 V and HOMO energy level of these compounds were determined between −5.53 and −5.29 eV. The oxidation peaks in the CVs of the crosslinked poly(azomethine-urethane)s could be probably urethane group in

polymer chain. Similarly, the onset reduction potential (Ered. ) of the polyurethanes is between −0.9696 and −0.9109 V, and LUMO energy level of these materials were calculated in the range −3.48 to −3.42 eV. The reduction peaks in the CVs of the polymers could be probably azomethine and urethane groups in polymer chain. As known, azomethine and urethane groups are reduced to amine and alcohol taking a proton, respectively. The electrochemical band gap (E g ) of CLPU-1, CLPU-2, CLPU-3, and CLPU-4 was also calculated as 2.10, 1.93, 1.81, and 1.94 eV, respectively. As can be seen the E g values, the crosslinked polyurethanes have quite low-electrochemical band gap.

3.4. Thermal analyses The thermal behavior of CLPUs was investigated by using TGDTA and DSC techniques to determine the thermal degradation pattern and glass transition temperature. TG-DTA curves of the

Fig. 11. AFM images of CLPU-3 and CLPU-4.

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Fig. 12. SEM images of CLPU-2 (a, b) and CLPU-3 (c, d).

crosslinked polyurethanes are shown in Fig. 8 and results summarized in Table 5. As can be seen in Fig. 8, CLPU-1 and CLPU-2 decompose in three steps whereas CLPU-3 and CLPU-4 decompose in two steps. According to TG-DTA thermogram of the crosslinked polyurethanes, the onset temperature of CLPU-1, CLPU-2, CLPU-3, and CLPU-4 was determined as 175, 99, 314, and 126 ◦ C, respectively. As can be seen the these onset temperature of the polymers, the crosslinked poly(azomethine-urethane)s decomposed at low temperature. These can be probably structures and molecular weights of the polymers. According to the synthesis scheme of the crosslinked poly(azomethine-urethane)s, these compounds were synthesized using different diamines with various chain length and side-group substitutions. As known, aliphatic compounds exhibit poor thermal stability. Additionally, the crosslinked polymers have low molecular weights distribution. T20 and T50 weight loss temperatures of CLPUs were determined in the range 284–334 ◦ C and 354–420 ◦ C, respectively. Char at 1000 ◦ C was also found as 12, 11, 18, and 16 for CLPU-1, CLPU-2, CLPU-3, and CLPU-4, respectively. Fig. 9 shows DSC curves of CLPUs. The obtained results from these curves are also summarized in Table 5. According to DSC curves, the glass transition temperature (Tg ) of CLPU-1, CLPU-2, CLPU-3, and CLPU-4 was found as 114, 81, 163, and 118 ◦ C, respectively. DMA analyses of CLPUs were carried out as a functional of temperature in the range 20–350 ◦ C. Tan ı curves of the crosslinked polyurethanes are shown in Fig. 10a. The temperature associated with the first peak magnitude of tan ı is determined as the glass transition temperature (Tg ) and the obtained Tg values of the crosslinked polyurethanes are also summarized in Table 5. The glass transition temperature (Tg ) of CLPU-1, CLPU-2, CLPU-3, and CLPU4 was determined as 77, 83, 207, and 122 ◦ C, respectively. When compared to glass-transition temperature obtained from DMA and DSC, Tg values of CLPU-2 and CLPU-4 are much the same while Tg values of CLPU-1 and CLPU-4 are a slight difference due to the different nature of these 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 [31]. Fig. 10b shows storage modulus (E ) curves of CLPUs. Storage modulus (E ) is a measure of material stiffness and it can be used to provide information regarding polymer molecular weight and degree of crosslinked density [32,33]. As known, the difference between the storage modulus (E ) in the plateau regions before and after the glass transition is associated with the degree of crosslinked density of polymer. A smaller E of polymer is related to greater crosslinked density [32,34]. As can be seen in Fig. 10b, CLPU-2 and CLPU-4 have smaller E than CLPU-1 and CLPU-3. Similar tendency is seen at GPC results. According to the molecular weight of the crosslinked polyurethanes, CLPU-2 and CLPU-4 have higher molecular weight than CLPU-1 and CLPU-3. Loss modulus (E ) curves of the crosslinked polyurethanes are given in Fig. 10c. According to E curves of the crosslinked polyurethanes, CLPU-2 and CLPU-4 exhibited border E than CLPU1 and CLPU-3. Broad E suggest non-uniformity of crosslinked [35]. 3.5. Morphological properties Morphological properties of CLPU-3 and CLPU-4 were investigeted using atomic force microscopy (AFM) and scanning electron microscopy (SEM). AFM technique was used in order to evaluate the evolution of topography and the phase roughness of CLPU-3 and CLPU-4 (Fig. 11). Also, provided variable heights (Sy) and root mean square roughness (Sq) of CLPU-3 and CLPU-4 were also determined due to investigate of roughness parameters of these compounds [36]. Sy and Sq values of CLPU-3 and CLPU-4 were determined as 44–5.17 and 46–5.43 nm, respectively. According to AFM images of CLPU-3 and CLPU-4, the surface of these compounds seems to be smooth and dense with uniform dispersion. SEM technique was used to investigate surface morphology of the crosslinked polyurethanes. Fig. 12 shows SEM photographs of CLPU-2 and CLPU-3 at different particle sizes. According to SEM images of CLPU-2 and CLPU-3, CLPU-2 has a relatively smooth surface while CLPU-3 has tightly packed a globular

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surface. This difference is attributable to the variations of diamines used as crosslinked in molecular structures of the crosslinked polyurethanes. 4. Conclusion The crosslinked poly(azomethine-urethane)s were synthesized using the aldehyde functionalized prepolymer and different diamines. Diamines were used as crosslinker. The obtained crosslinked polyurethanes were characterized using FT-IR, 1 HNMR, 13 C-NMR, and GPC techniques. Photophysical properties of the compounds were investigated using UV–vis and PL spectroscopy. PL results showed that CLPUs exhibited multicolor emission behavior. In addition, a linear relationship was determined between the excitation energies and the obtained emission maxima. Electrochemical behaviors of the crosslinked polyurethanes were investigated by CV technique. CV data showed that the crosslinked polyurethanes have quite low-electrochemical band gap (in the range 1.81–2.10 eV). Also, morphological properties of the crosslinked polyurethanes were investigeted using AFM and SEM. According to AFM images, the surface of CLPU-3 and CLPU4 seems to be smooth and dense with uniform dispersion. Also, SEM images shown that CLPU-2 has a relatively smooth surface while CLPU-3 has tightly packed a globular surface. References [1] A. Iwan, B. Boharewicz, I. Tazbir, M. Malinowski, M. Filapek, T. Kła, B. Luszczynska, I. Glowacki, K.P. Korona, M. Kaminska, J. Wojtkiewicz, M. Lewandowska, A. Hreniak, New environmentally friendly polyazomethines with thiophene rings for polymer solar cells, Sol. Energy 117 (2015) 246–259. [2] I˙ . Kaya, A. Avcı, Synthesis, characterization, and thermal stability of novel poly(azomethine-urethane)s and polyphenol derivatives derived from 2,4-dihydroxy benzaldehyde and toluene-2,4-diisocyanate, Mater. Chem. Phys. 133 (2012) 269–277. [3] A. Iwan, B. Boharewicz, I. Tazbir, M. Filapek, Enhanced power conversion efficiency in bulk heterojunction solar cell based on new polyazomethine with vinylene moieties and [6,6]-phenyl C61 butyric acid methyl ester by adding 10-camphorsulfonic acid, Electrochim. Acta 159 (2015) 81–92. [4] I˙ . Kaya, F. Baycan, F. Do˘gan, Synthesis, characterization, conductivity, band gap, and kinetic of thermal degradation of poly-4-[(2-mercaptophenyl) imino methyl] phenol, J. Appl. Polym. Sci. 112 (2009) 1234–1243. [5] D. S¸enol, I˙ . Kaya, Synthesis and characterization of aromatic compounds containing imine and amine groups via oxidative polycondensation, Des. Monomers Polym. 17 (2014) 557–575. [6] I˙ . Kaya, M. Yıldırım, Synthesis and characterization of graft copolymers of melamine: Thermal stability, electrical conductivity, and optical properties, Synth. Met. 159 (2009) 1572–1582. [7] A. Iwan, B. Boharewicz, K. Parafiniuk, I. Tazbir, L. Gorecki, A. Sikora, M. Filapek, E. Schab-Balcerzak, New air-stable aromatic polyazomethines with triphenylamine or phenylenevinylene moieties towards photovoltaic application, Synth. Met. 195 (2014) 341–349. [8] H.J. Yen, G.S. Liou, Novel blue and red electrochromic poly(azomethine ether)s based on electroactive triphenylamine moieties, Org. Electron. 11 (2010) 299–310. [9] E.C. Buruiana, M. Olaru, B.C. Simionescu, Synthesis and properties of some new polyazomethine-urethanes, Eur. Polym. J. 38 (2002) 1079–1086. [10] L. Marin, V. Cozan, M. Bruma, V.C. Grigoras, Synthesis and thermal behaviour of new poly(azomethine-ether), Eur. Polym. J. 42 (2006) 1173–1182. [11] I˙ . Kaya, S. C¸ulhao˘glu, Syntheses and characterizations of oligo(azomethine ether)s derived from 2,2 -[1,4-enylenebis (methyleneoxy)]dibenzaldehyde and 2,2 -[1,2-phenylenebis(methyleneoxy)]dibenzaldehyde, Chin. J. Polym. Sci. 30 (2012) 682–693. [12] K.I. Aly, M.A. Abbady, S.A. Mahgoub, M.A. Hussein, Liquid crystalline polymers IX Main chain thermotropic poly(azomethine-ether)s containing thiazole

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