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The effect of heat treatment on the optical properties of thin films of polyazomethine with flexible-side chains
Journal of Non-Crystalline Solids 379 (2013) 27–34
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The effect of heat treatment on the optical properties of thin films of polyazomethine with flexible-side chains B. Jarząbek a,⁎, B. Kaczmarczyk a, J. Jurusik a, J. Weszka a,b a b
Centre of Polymer and Carbon Materials Polish Academy of Sciences, 34M. Curie-Sklodowska Str., 41-819 Zabrze, Poland Silesian University of Technology, Faculty of Mechanical Engineering, Institute of Engineering and Biomaterials, 18a Konarski Str., 44-100 Gliwice, Poland
a r t i c l e
i n f o
Article history: Received 17 June 2013 Received in revised form 22 July 2013 Available online xxxx Keywords: Polyazomethines; Flexible side chains; Thin films; Absorption edge parameters; Thermal stability
a b s t r a c t Thermal stability of a soluble derivative of a basic aromatic polyazomethine: poly(1,4-(2,5-bisoctyloxyphenylene-methylenenitrilo)-1,4-phenylene-nitrilomethylene) BOO-PPI, in thin films, has been studied by “in situ” UV-Vis-NIR transmission measurements during the heat treatment. Optical investigations of the BOO-PPI films have been completed by the wide angle X-ray diffraction (WAXRD) and atomic force microscopy (AFM) studies. Absorption spectra of the BOO-PPI films did not change during the first annealing, like as for unsubstituted PPI films. Study of temperature dependences of the absorption edge parameters (the Urbach energy EU and energy gap EG) allowed us assessment of changes in structural order, conformation and conjugation of the BOO-PPI polymer chain during the successive heating and cooling runs. The value of EG (2.19 eV) was constant, while EU changed linearly in the range 71–140 meV, within the temperature range 25–250 °C. This reversible and recurrent dependence of EU is connected with the structural changes of flexible bis-octyloxy side chains, while the conjugation in the main chain is preserved (constant EG). Attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy studies confirmed invariable conjugation: C N stretching vibration band was unchanged during the heat treatment. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Conjugated polyazomthines (polyimines) also known as Schiff bases, belong to the aromatic polymers class, with an extended π-system and possess C N imine groups alternating with benzene rings in the main chain. These polymers have been of widespread interest from many years, as corrosion inhibitors [1], catalyst carriers [2,3], metal ion complexing agents [4], thermo-stable materials [5,6] in biological systems [7,8] and in optoelectronic devices [9,10], due to good thermal stability, photo- and electro-luminescence efficiency and nonlinear optical (NLO) properties. The basic aromatic polyazomethine is poly (1,4-phenylenemethylenenitrilo-1,4-phenylene-nitrilomethylene) (PPI), where the main chain has the benzene rings and CH N groups alternately. The groups CH N and CH C are isoelectronic, so PPI being isoelectronic with poly(phenylene-vinylene) PPV, may have also potential applications in light emitting diodes (LED), thin film transistors (TFT) and solar cells (SC). All these optoelectronic and photovoltaic devices are exposed to the effect of higher temperatures that is why the thermal stability of polymer thin films used in these devices is so important. Due to insolubility of polyazomethines to prepare thin films, usually the thermal vacuum evaporation (TVE) or chemical vapor deposition (CVD) methods are used [11–15], because of economic reasons spin coating methods are preferred. There are several methods, reported in ⁎ Corresponding author. Tel.: +48 32 2716077; fax: +48 32 27112969. E-mail address: [email protected] (B. Jarząbek). 0022-3093/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnoncrysol.2013.07.034
the literature, to improve solubility and processability of polyimines. Several approaches based on the reversible Lewis acid-base (GaCl3) or di-m-cresol phosphate (DCP) complexations [16,17], or modification of polymer structure by unsymmetrical [18] or symmetrical [19] substitution with the flexible alkyl or alkoxy side chains have been applied to polyimines. The flexible side chains, attached to the rigid main chain, not only improve the solubility of polymers, but also these polymers can form a novel, layered structure in their crystalline phase and mesophase [20–25]. Polyazomethines having flexible n-alkyloxy methyl side chains have been reported, by Kim et al., in several papers [26–29]. X-ray diffraction is the main method used to investigate the detailed crystal structure of these types of polymers, designated as Cm-PAMs (where m = 4, 6, 8, 12, …) [26–28]; the temperature dependence of the layer spacing of Cm-PAMs was presented in refs. [26,27] and discussed as a function of the length of the side chain (m value). Other soluble, para-linked aromatic polyazomethines with methoxy and 2-ethylhexyloxy side chains (MEH-PPI) have been presented in ref. [30]. The structure of the MEH-PPI resembles that of well-known MEHPPV i.e. poly(2-methoxy-5-(2′ethylhexyloxy)-1,4-phenylenevinylene); hence there is an interest in this type of soluble polyazomethine. The effect of annealing treatment and the temperature dependence of molecular dynamics in MEH-PPV films have been presented on the basis of the wide angle X-ray spectroscopy (WAXS), solid-state nuclear magnetic resonance (NMR) and fluorescence spectroscopy (PL) studies [31] and using the FTIR, UV-Vis and scanning electronic microscopy (SEM) methods [32]. Such detailed investigations have not been applied for
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thin films of polyazomethines with flexible side-chains, except “in situ” temperature WAXS studies of the Cm-PAMs films [26,27]. Recently, in our previous work [33] we have described the optical properties of thin films of BOO-PPI (i.e. PPI with the linear, symmetric, flexible 2,5–bisoctyloxy side chains, para-linked by oxygen atoms to the phenyl rings connected with carbon imine atoms in the main chain) and then we have compared these results with the properties of unsubstituted PPI films. The same BOO-PPI and PPI films have been investigated during the heat treatment and the results are shown in this work. In the present work, the temperature dependences of optical properties of BOO-PPI thin films have been examined, in relation with the properties of an unsubstituted PPI films. The BOO-PPI polymer, synthesized by us, is different than other soluble polyazomethines, described up to now in the literature, except our works [33–35]; only recently, we have reported the results of studies on the time-stability of diodes based on BOO-PPI films [34] and the effects of BOO-PPV and BOO-PPI protonation [35]. Herein, we only briefly report the BOO-PPI synthesis and thin-film preparation (more details in ref. [33]) and the main part of this work is devoted to the results of UV-Vis-NIR spectroscopy measurements of these films during their successive, gradual annealing and cooling. Also the high temperature-controlled attenuated total reflection (ATR) module was used in FTIR spectroscopy measurements to investigate the BOO-PPI films during the heat treatment. Other complementary methods, applied at room temperature, such as the wide angle X-ray diffraction (WAXRD) and atomic force microscopy (AFM) allowed us to evaluate the changes in atomic structure and surface morphology of the BOO-PPI films, after annealing. So far, such detailed studies of the heat treatment effect on optical properties of thin films of polyazomethines with flexible side chains have not been reported. 2. Experimental 2.1. Technological details The soluble derivative of polyazometine, i.e. poly(1,4-(2,5bisoctyloxy-phenylene-methylenenitrilo)-1,4-phenylene-nitrilomethylidyne) (BOO-PPI) was synthesized according to the modified procedure, reported in detail in ref. [33,35]. Para-phenylene-diamine (PPDA) and 2,5-bis(octyloxy)-terephthaldehyde (BOO-TPA) have been used as the initial monomers in synthesis of the BOO-PPI polymer and the scheme of this reaction is shown in Fig. 1. The glass transition temperature (Tg) for such synthesized BOO-PPI polymer was obtained at 32.7 °C, as the midpoint at half height of the increase of the specific heat associated with the transition, during differential scanning calorimetry (DSC) experiments. Thin films of BOO-PPI have been prepared by spin coating from the polymer solution in chloroform, onto quartz substrates. The deposition process has been carried out at room temperature, in atmospheric pressure air, and then the films were annealed for 20 min at 100 °C. Thin films of BOO-PPI obtained by spin coating were transparent, yellow and good quality. Thicknesses of the BOO-PPI films, determined with an interference microscope, were about 100 (±20) nm. Thicker films (above 1 μm), suitable for the ATR-FTIR measurements, were obtained by pouring out a more concentrated polymer solution on glass or quartz substrates.
The chemical vapor deposition (CVD) method, via polycondensation process, of the PPDA and TPA monomers with pure argon as a transport agent, has been used to obtain pure PPI thin films. Technological details of the PPI film preparation by CVD, using different arrangements, have been described earlier in refs. [11–13]. 2.2. Characterization techniques The obtained BOO-PPI films were characterized by the ATR FTIR and UV-Vis-NIR spectroscopes during successive, gradual heating and cooling processes. Other methods, such as wide angle X-ray diffraction (WAXRD) and atomic force microscopy (AFM), were applied at room temperature, before and after annealing process. Crystal structure of the films has been examined by WAXRD technique, with a conventional θ–2θ TUR M-62 diffractometer, using the Ni-filtered Cu Kα radiation, while the surface morphology and roughness have been investigated by an atomic force microscope (AFM) TopoMetrix Explorer, working in the contact mode in the air, in the constant force regime. The infrared spectra of thin-film samples coated onto the glass or quartz windows were acquired on a Bio-Rad FTS-40A Fourier transform infrared spectrometer, in the range 4000–400 cm−1, using a JASCO ATR high temperature-controlled attachment, with a resolution of 2 cm−1 and for accumulated 120 scans. Spectra recorded at elevated temperatures were obtained in the temperature range 20–80 °C with a step of 10 °C and with a 2 °C/min heating rate under the nitrogen atmosphere. Optical transmission and reflectivity measurements have been carried out within the spectral interval 200–2500 nm, using a two-beam UV-Vis-NIR spectrophotometer, JASCO V-570. During the reflectivity measurements, a special two-beam reflectance arrangement was used with an Al mirror in the reference beam as a reflectance standard. Temperature dependences of transmission spectra have been investigated during “in situ” heat treatment of thin films, using a special high temperature-controlled equipment of the JASCO spectrophotometer, enabling us to measure transmission spectra, at precisely definite temperatures, with the accuracy of ± 0.5 °C. 3. Results 3.1. First annealing and cooling effects 3.1.1. UV-Vis-NIR(T) measurements Good thermal stability of polyazomehines (polyimines) is one of the typical features of this group of polymers. Our previous studies [36] confirmed thermal stability of the pristine PPI film with thickness 1.4 μm, for which we have investigated the annealing effect on the absorption edge parameters, i.e. the Urbach energy (EU) and the energy gap (EG). Both parameters of this thick PPI film, turned out to be almost invariable, within the temperature range 25–225 °C [36], thereby thermal stability was proved. Hence, our idea was to check thermal stability of the BOO-PPI thin films in the same way, using the results of “in situ” optical measurements, during slow, gradual annealing process, within the same temperature range, with a step of 25 °C. For the purpose of comparison, we have repeated this experiment for unsubstituted PPI thin films with a comparable thickness, of about 100 nm. The absorption coefficient (α) has been
H3C(H2C)6H2CO
H3C(H2C)6H2CO
O
n H2N
NH2
+
H N
n H
PPDA
O
- (2n-1) H2O
C H
OCH2(CH2)6CH3
BOO-TPA Fig. 1. Formation of the BOO-PPI polymer, via the polycondensation reaction.
BOO-PPI
C
NH
H OCH2(CH2)6CH3
n
B. Jarząbek et al. / Journal of Non-Crystalline Solids 379 (2013) 27–34
calculated as a function of photon energy (E) on the basis of transmission and reflectivity measurements, according to the formulae given in ref. [37]. Figs. 2(a) and (b) present the absorption spectra at different temperatures for the PPI and BOO-PPI films, respectively. As it is seen in Figs. 2 (a) and (b), significant changes in the absorption spectra during annealing are not registered over the whole investigated spectral range. This result can suggest the same good thermal stability of both films. The main difference between PPI and BOO-PPI spectra, seen in Fig. 2, is the bathochromic shift of the absorption edge and lack of vibronic progression in the low energy absorption band of BOO-PPI films, compared to its parent PPI films To find precisely the positions of vibronic band peak energies the second derivative method has been used and then this vibronic progression has been deconvoluted [15]. The values of vibronic bands (E1, E2, E3) of the PPI films are shown in Table 1. Next, the PPI and BOO-PPI films were cooled slowly from 225 to 25 °C and then transmission at room temperature for both films has been measured once more, and the obtained absorption spectra are presented in Figs. 3(a) and (b). An unexpected result has been obtained for the BOO-PPI films [see Fig. 3 (b)]: namely, a distinct red shift of the absorption edge and simultaneously the appearance of vibronic peaks at the low-energy absorption band are registered, in contrast to the PPI films for which no differences were observed, as is depicted in Fig. 3 (a). Positions of the BOO-PPI vibronic bands are shown in Table 1. To discuss the heat treatment effect on the structural order and chain conjugation of the BOO-PPI films, we have determined the absorption edge parameters, EU and EG, seen in Figs. 4 (a) and (b), respectively, both before and after thermal relaxation. As it is seen in Fig. 4 (a), the slopes of the exponential edges follow the Urbach relation [38] α ∝ exp (E/EU), allowing us to determine the values of EU, of 91 meV and 71 meV for the BOO-PPI films, before and after annealing, respectively. Least squares estimation method was used to the Urbach edges and then the Urbach energies were calculated,
a)
Table 1 Optical parameters, obtained at room temperature. Film
E⁎1 (eV)
E⁎2 (eV)
E⁎3 (eV)
EU (meV)
EG (eV)
n
k
PPI, PPIa BOO-PPI BOO-PPIa
2.64 – 2.44
2.83 2.70 2.59
3.00 – 2.78
130 91 71
2.30 2.20 2.19
1.79 1.76 1.76
0.19 0.08 0.08
E⁎1 , E⁎2 , E⁎3 — the energies of vibronic bands, each of uncertainty (±0.01) eV; Uncertainties: EU (±10) meV; EG (±0.03) (eV); n, k (±0.02). a After thermal relaxation of the investigated films.
as: EU = 1/a, where a is a direction coefficient of line: y = a · x + b). To obtain the energy gap, we have used the well-known equation α ∝ (E − EG)x, with x depending on the type of optical transition [39]. The best fit to the absorption edge of BOO-PPI films turned out to be for x = 2, corresponding to a linear dependence (α E) 1/2 = f (E), well known as the Tauc relation [40] and shown in Fig. 4 (b). Linear approximation to the energy access (by the least squares estimation method) was used to the Tauc dependences, shown in Fig. 4 (b), to obtain the energy gaps, where: EG = E for (αE)1/2 = 0. This relation is typical for amorphous semiconductors and is often used to obtain the energy gaps of polymer films, including different polyazomethine thin films [15]. Amorphous character of the obtained PPI and BOO-PPI films has been demonstrated in ref. [33], on the basis of the X-ray diffraction patterns recorded for these films. As it is seen in Fig. 4 (b), the thermal relaxation process (carried out by annealing up to 225 °C and then by cooling to 25 °C) of the BOO-PPI film has not resulted in a change of the energy gap, that remains almost constant and equal to 2.20–2.19 eV. Both parameters of the absorption edge (EU and EG) obtained at room temperature, for the PPI and BOO-PPI thin films, before and after thermal relaxation, are gathered in Table 1. The Kramers–Kronig relations [41] applied to the reflectance spectra of the investigated films yield the values of refractive index (n) and extinction coefficient (k) in the low energy range. Both parameters are
a) PPI
4
25 0C
abs. coef., α (104 cm-1)
abs. coef., α (104 cm-1)
6
225 0C
2
0 1
2
3
4
5
a)
6
PPI 4
2
6
5
0
b)b)
1 6
3
BOO-PPI
2 25 0C
1 0
225 C
2
3
4
5
6
b)
4
abs. coef., α (104 cm-1)
abs. coef., α (104 cm-1)
29
5 4 BOO-PPI 3 2 1
0 1
2
3
4
5
6
energy,E(eV)
0 1
2
3
4
5
6
energy,E(eV) Fig. 2. Absorption spectra during first annealing of the PPI (a) and BOO-PPI (b) thin films; obtained at 25, 50, 75, 100, 125, 150, 175, 200 and 225 °C respectively (the spectra turned out almost unchangeable).
Fig. 3. Absorption spectra obtained at room temperature, before (dashed lines) and after (solid lines) thermal relaxation of the (a) PPI and (b) BOO-PPI thin films.
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4
a
400
EU = 71 meV
3
EU = 91 meV
Intensity (a.u.)
abs. coef., α (104 cm-1)
600
a)
5
2 1 2.2
2.4
2.6
2.8
200
600
400
b
energy, E(eV) 200
(αE)1/2 (eV/cm)1/2
b) 300
10
20
30
40
50
2 Theta 200
Fig. 5. X-ray diffractograms, obtained at room temperature, of the BOO-PPI thin films; line (A)—before and line (B)—after thermal relaxation.
100
0 1.8
EG= 2.19 eV
2.0
2.2
EG = 2.20 eV
2.4
2.6
2.8
energy, E(eV) Fig. 4. Absorption edges, at room temperature, of the BOO-PPI films, before (dashed lines) and after (solid lines) thermal relaxation: (a) the Urbach dependence (b) the Tauc relation. Linear regressions result in the following equations: (a) y = 14.057 · x − 32.52 for the solid line and y = 10.996 · x − 25.45 for the dashed line, (b) y = −4065.167 · x + 1843.79 for the solid line and y = −2548.351 · x + 1163.73 for the dashed line; R2 = 0.998 for all lines.
slightly smaller for the BOO-PPI than for pristine PPI films. All these coefficients are also included in Table 1. 3.1.2. X-ray diffraction, AFM and ATR FTIR(T) studies X-ray diffraction patterns obtained at room temperature, for the BOO-PPI films before and after the thermal relaxation, are shown in Fig. 5. The diffractogram of the BOO-PPI films before the heat treatment [line(A)] shows a predominant amorphous character, with the broad peak and a small feature, seen at about 2θ = 21°, corresponding to that reported for bulk PPI [42]. This small feature can indicate a little more ordered structure of the BOO-PPI films than the quite amorphous structure of PPI films [11,15,33,35]. After the heat treatment of the BOO-PPI films, we can observe a small, distinct feature at about 2θ = 12° on the diffractogram, as line (B) in Fig. 5. The AFM topography images (5 μm × 5 μm) of the BOO-PPI films surface, before and after annealing, are shown in Figs. 6(a) and (b), respectively. Before the heat treatment, the BOO-PPI films surface topography exhibits not very homogeneous grained structure, but relatively flat areas with the root mean square (RMS) roughness of about 3.2 nm. Particles of different size, scratches and elongated grains can be also observed. As it is seen in Fig. 6 (b), the surface of BOO-PPI films after annealing and cooling process shows a similar appearance, although the RMS roughness has increased to about 10 nm. The detailed assignments of the absorbance bands of FTIR spectra of the BOO-PPI polymer are as follows: 3060, 3027, 2951 and 2997 (ν CH), 2951, 2923 (νa CH2 and CH3), 2869, 2853 (νs CH2 and CH3), 2734 (ν = CH), 1604 (ν C N and ν Ph), 1580sh, 1503, 1488, 1427 and 1391 (ν Ph), 1468 (δa CH3 and CH2), 1370 (δs CH3 and CH), 1287 1273, 1202, 1164 (δ CH), 1029 (υ C-O), 971 (ω = CH), 879 and 839 (γ
CH), 722 (δ Ph), 671 (γ = CH). The absence of the bands at about 1700 cm− 1 (in relation to the PPI spectra), corresponding to the stretching vibrations of the aldehyde C O groups, proves formation of a polymer. In turn, the band attributed to the stretching vibrations of the C N group, occurs at 1604 cm− 1, i.e. at a lower wavenumber than in the case of the simplest polyazomethine PPI, where this band appears at 1612 cm− 1 [5,36]. During “in situ” gradual heating of the BOO-PPI films to 180 °C, no changes in the ATR-FTIR spectra were detected. Consequently, the spectrum obtained at 180 °C was the same as that obtained at room temperature. This spectrum is depicted as a dashed line (A) in Fig. 7. However, during cooling, at 130 °C, the bands at 1503 and 1393 cm−1 began to decrease their intensity. Simultaneously, intensity of the shoulder at about 1287 cm−1 started to decrease. Thus, after gradual cooling of the BOO-PPI films to 25 °C, only shoulders at 1503 and 1393 cm−1, and a broad band with two shoulders at 1276 and 1287 cm−1 were observed. All the abovementioned bands correspond to the stretching vibrations of the phenyl rings. The changes are also observed in the case of the band attributed to the stretching vibrations of the C-O groups. During cooling, a shoulder at 1045 cm−1 appeared at 130 °C which, after cooling to 25 °C, formed a doublet with the band at 1029 cm−1. It should be emphasized that the band attributed to the stretching vibrations of the C N imine groups, (at 1604 cm−1) did change during heating and cooling runs. The ATR-FTIR spectrum of the BOO-PPI films after cooling to room temperature is shown in Fig. 7 as a solid line (B). 3.2. Successive annealing and cooling runs; UV-Vis-NIR(T) spectroscopy After the thermal relaxation processes described above (by heating to about 200 °C and then cooling to the room temperature), we checked thermal stability of the BOO-PPI films during successive heating and cooling runs. Fig. 8(a) shows the temperature dependence of the absorption coefficient of the BOO-PPI relaxed films, during annealing from 25 to 250 °C with a step of 25 °C. Changes of the spectra, obtained during annealing process, are seen only in the region of the absorption edge for the first, low-energy absorption band, while positions of other bands are invariable with respect to the increase in temperature. The spectral range of the absorption edge is presented in detail in Fig. 8 (b), where the influence of temperature on the slope of the absorption edge and the shape of low-energy band of the BOO-PPI films are clearly seen. The vibronic progression of this band disappears with increasing temperature and above 100–125 °C is not seen at all. The
B. Jarząbek et al. / Journal of Non-Crystalline Solids 379 (2013) 27–34
31
Fig. 6. AFM topography images (5 μm × 5 μm) of the BOO-PPI films: (a) before and (b) after thermal relaxation; color scale: (a) 0–38 nm and (b) 0–94 nm.
edge of the absorption spectra becomes more sloped with respect to the energy axis, when the temperature increases, resulting in an increase of the Urbach energy. We have determined the EU values for each temperature, using the same procedure, as described before [see Fig. 4 (a)]. Simultaneously, the energy gaps (EG) values have been calculated for each temperature, using the Tauc method [as in Fig. 4 (b)]. It turned out that
the energy gap EG ≅ 2.19 eV is almost constant, irrespectively of temperature. During the second cooling of the BOO-PPI films, from 250 °C to room temperature, we have obtained the same dependence of the absorption spectra, as in Fig. 8. The energy gap remains almost constant, while the Urbach energy demonstrates the linear dependence on temperature, as it is depicted in Fig. 9, where one can see that EU changes
Fig. 7. FTIR-ATR spectra of the BOO-PPI films, recorded at room temperature, before [line (A)] and after [line (B)] thermal relaxation.
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B. Jarząbek et al. / Journal of Non-Crystalline Solids 379 (2013) 27–34
abs. coef., α (104 cm-1)
5
out to be almost invariable during annealing, up to 225 °C. In this work, the same investigations have been applied for the thin films of polyazomethine having bisoctyloxy elastic side chains and the obtained results allowed us to assess thermal stability of the BOO-PPI films.
a)
4 25οC
second annealing
3
25οC
2
250οC
4.1. First annealing and cooling process effects
ο
250 C
1
second cooling 25οC
250οC
0 2
3
4
5
6
energy, E (eV)
abs. coef., α (104 cm-1)
5 25οC
b)
250οC
4 1
3
10
2
250οC
25οC
1 250οC
25οC
0 2.0
2.2
2.4
2.6
2.8
3.0
energy, E (eV) Fig. 8. Absorption spectra during the second heating and cooling runs at: 25, 50, 75, 100, 125, 150, 175, 200, 225 and 250 °C respectively (a) whole spectral range; (b) absorption edge and low-energy band range; the spectrum (1) was obtained at 25 °C, then spectra in turn, up to the spectrum (10) at 250 °C.
linearly in the range 71–140 meV, within the temperature range 25– 250 °C. Linear regression, by the least squares estimation method has resulted in the following relation: EU (T [K]) = 0.29 · T [K] − 13.73 (with the coefficient of determination: R2 = 0.998). This linear dependence appears to be reproducible and recurrent for the next successive heating and cooling runs. 4. Discussion
Urbach energy, EU (meV)
The results of our investigations of the optical properties of BOO-PPI films presented herein and in ref. [33] confirm that the presence of 2,5bisoctyloxy flexible side chains attached to the PPI main chains, not only improves solubility but also leads to the changes of polyazomethine properties. Thermal stability of the pristine PPI film has been studied in ref. [36], where the absorption edge parameters (EU and EG) turned
140
BOO-PPI
120 second cooling
second heating
100
y = 0.29 x -13.73
80 300
350
400
450
500
550
temperature, T (K) Fig. 9. Temperature dependence of the Urbach energy obtained for the BOO-PPI film, during successive heating (black circles) and cooling (white circles).
4.1.1. UV-Vis-NIR(T) measurements The absorption spectra of the PPI and BOO-PPI thin films (Fig. 2) indicate good thermal stability; since no changes in both spectra were registered over the whole investigated spectral range, during annealing up to 225 °C. As it is shown in ref. [33], the red shift of the absorption edge of bisoctyloxy (BOO) substituted PPI films, in comparison with the edge of parent PPI polymer films, is caused by a greater electron density of the former π-cojugated polymer, being a result of the strongly electron-donating ability of the bis-octyloxy groups, like the methoxy groups [5]. Also a better planarization of the BOO-PPI chains (as compared to the PPI structure) has been confirmed by the geometry optimization (smaller torsion angles in the main chain) [33]. Hence, there is a better conjugation and smaller energy gap (EG) of the BOO-PPI films than for pristine PPI films. The absorption edge parameters (EU and EG), together with the energies (E1⁎, E2⁎, E3⁎) of vibronic bands and the values of refractive index (n) and extinction coefficient (k), obtained for PPI and BOO-PPI films at room temperature before and after annealing, are collected in Table 1. The first low energy band is caused by the interband transitions between delocalized states (D → D*) which are derived from the interaction of benzene and azomethine dimer π-orbitals. The vibronic progression of this band is connected with the electron–phonon interaction, due to benzene ring stretching mode, as it is shown and discussed for the PPI films in refs. [11,12,15]. The low-energy band of the BOO-PPI films is very narrow and the vibronic progression is absent. The absence of vibronic bands can be explained by the effect of the bis-octyloxy side chains attached in 2 and 5 positions to alternate phenylene rings. A clearly smaller value of the Urbach energy (EU) obtained for the BOO-PPI films than for the PPI films (see Table 1) indicates that the presence of side chains results in more ordered and planar structure of the polymer backbone. Based on the Urbach energy, we can estimate the width of the tails of localized states in the optical gap [38]. Structural defects in polymers, like break, torsion or aberrations of chains can be responsible for these localized states and the Urbach-like behavior of the polymer absorption edge. The results of first annealing (to 225 °C) of PPI and BOO-PPI films could indicate the same thermal stability of both films, because the absorption spectra of both films, over the investigated range, remain unchanged during heating (Fig. 2). Then, after cooling to 25 °C, we have compared the absorption spectra at the room temperature [Figs. 3 (a) and (b)] for the PPI and BOO-PPI films, before and after heat treatment (by heating to 225 °C and then slow cooling to 25 °C). The PPI absorption spectrum remained still invariable, while that of the BOO-PPI films turned out to be different than the spectra, initially obtained at room temperature and during the first heating run. The main differences between these BOO-PPI spectra [Fig. 3 (b)] are seen in the range of the absorption edge and concern the first low-energy band, where the vibronic bands occurred. This vibronic progression (as for the spectra of PPI films) reveals the electron–phonon interaction and is connected with the thermal relaxation of defects and more ordered planar polymer chains. The absorption edge of the BOO-PPI films, being the low-energy wing of the spectrum, became more “sharp” after such thermal relaxation [Fig. 3 (b)]. The energy gap of the films remained almost constant [Fig. 4 (b)] however the Urbach energy decreased [Fig. 4 (a)]. This means that conjugation of the BOO-PPI polymer chains did not change, but the structural order can be improved due to the thermal relaxation process. In the case of conjugated polymers, the energy gap depends on the conjugation length, while the Urbach energy is connected with the structural defects which are responsible for the localized states inside the energy gap. The thermal
B. Jarząbek et al. / Journal of Non-Crystalline Solids 379 (2013) 27–34
relaxation process of the BOO-PPI films did not influence the conjugation in the rigid main chain but improved the structural order in the side chains, resulting in relaxation of defects and a little more ordered, partially layered structure of polymer, as it is shown in ref. [26,27] for Cm-PAMs films. 4.1.2. X-ray diffraction, AFM and ATR FTIR(T) studies The results of X-ray diffraction investigations, seen in Fig. 5, also confirmed a little more ordered structure of the BOO-PPI films, after relaxation by a suitable heat treatment. Additionally, the AFM images, presented in Fig. 6, show that the surface of BOO-PPI films after heat treatment has larger grains; the RMS roughness has increased from 3.2 nm to about 10 nm, after the heat treatment. The ATR-FTIR(T) investigations of the BOO-PPI films (Fig. 7) support and complete the results, described above. During “in situ” annealing to 180 °C, no changes in the ATR-FTIR spectra of the investigated films were detected. Then, during slow cooling, all changes of the FTIR spectra of BOO-PPI films were related to the bands corresponding to the stretching vibrations of the phenyl rings and vibrations of the C-O groups. At the same time, the band attributed to the stretching vibrations of the C N imine groups did not change, which means that the conjugation in polymer was preserved and the phenyl rings had to preserve the spatial arrangement in relation to the C N bond plane. This, in turn, suggests that changes observed during cooling are associated with changes in the order of particular polymer chains relative to each other. The lack of changes in the bands, characteristic for vibrations of the aliphatic groups, indicates that changes during cooling of the BOOPPI films are only the consequence of spatial ordering of the phenyl rings forming particular polymer chains, relative to each other [42]. These results of ATR FTIR(T) studies of the BOO-PPI films are in a good agreement with the UV-Vis NIR(T) investigations. 4.2. UV-Vis-NIR(T) spectroscopy during successive heating and cooling runs Successive heating and cooling runs of the BOO-PPI films affected the absorption spectra only in the range of absorption edge and the first, low-energy absorption band, while the bands, seen in the higher energy range, were at the same positions. Additionally, these changes in the absorption edge slope and the shape and position of the first low-energy band turned out to be reversible and recurrent (see Fig. 8). During annealing, the vibronic progression disappeared and the band maximum decreased and shifted to a higher energy. These changes indicate more structural defects and weaker delocalization in the BOO-PPI chains at higher temperatures. The absorption edge of the BOO-PPI films during annealing became more sloped with respect to the energy axis, resulting in an increase of the Urbach energy. Reversible linear dependences of EU = f (T) have been obtained, both for heating and cooling successive runs. Annealing of the relaxed BOO-PPI films causes the structural changes in the flexible bisoctyloxy side chains, leading to changes in order of particular polymer chains, relative to each other and to an increase of structural defects, while during cooling these defects disappeared. The heat-inducted structural changes involve elastic side chains and these changes can be responsible for the spatial disorder and increase in the density of localized states, affecting an increase of the Urbach energy and decrease of delocalization (lack of the vibronic structure of this band). The energy gap turned out to be almost constant during heat treatment of the BOO-PPI films, which means that the conjugation is preserved and seems to be independent on temperature (up to 250 °C). 5. Conclusions In the present work we have shown the annealing and cooling effects on the optical properties of thin films of polyazomethine (PPI) with 2,5-bis octyloxy (BOO) flexible side-chains. The presence of these linear, symmetric side-chains, linked to the phenyl rings and connected
33
with the carbon atoms of the imine group in the polyazmethine backbone, significantly improves the solubility of PPI and also changes other properties of polymer. Thanks to the different analysis techniques, such as the UV-Vis-NIR(T) and ATR-FTIR(T) spectroscopy measurements, together with the WAXRD and AFM complementary methods, we could assess the optical properties and thermal stability of the BOO-PPI thin films. Summing up our results, we can conclude that: – annealing, for 20 min at 100 °C, just after films preparation by spin coating, is efficient to remove remains of solvent but insufficient to obtain the ordered polymer structure. Only the slow, gradual cooling from about 200 °C to room temperature, allows one to obtain a more ordered polymer structure of the BOO-PPI films. – all structural changes during heat treatment of the BOO-PPI films are connected with the presence of flexible bis-octyloxy side-chains, not influencing conjugation in the main, rigid chains. – successive heating and cooling runs of the BOO-PPI films cause the reversible and recurrent changes of the polymer structural order, connected with the effect of heat-induced movement of elastic side chains. To conclude, the presence of flexible side-chains improves solubility and causes the better conjugation and planarization of polymer chains, resulting in a smaller energy gap of BOO-PPI thin films, than for unsubstituted PPI films. Moreover, it appears that during the heat treatment of BOO-PPI thin films, the structural changes are connected with the flexible side chains but the conjugation in the main chains is preserved and the energy band gap is constant. It means that we can use the spin coating technique to obtain thin films of BOO-PPI with a smaller energy gap than for pristine PPI films and with the same good thermal stability, so the BOO-PPI thin films show promise for (opto)electronic device applications. Acknowledgements Polish National Science Centre, Grants no NN 507 227040 (B. J. and B. K.) and NN 508626840 (J. W.) supported these researches. The authors would like to thank Dr Mariola Siwy for the polymer synthesis, Dr Barbara Hajduk for the film preparation and M.Sc. Marian Domanski for the X-ray diffraction measurements. References [1] K.C. Emregul, E. Duzgun, O. Atakol, Corros. Sci. 48 (2006) 873. [2] R. Drozdzak, B. Allaert, N. Ledoux, I. Dragutan, V. Dragutan, R. Verpoort, Coord. Chem. Rev. 249 (2005) 3055. [3] J.L. Sesssler, P.J. Melfi, G. Dan Pantos, Coord. Chem. Rev. 250 (2006) 816. [4] I. Kaya, A.R. Vilayetoglu, H. Mart, Polymer 42 (2001) 4859. [5] C.J. Yang, S.A. Jenekhe, Macromolecules 28 (1995) 1180. [6] S. Destri, I.A. Khotina, W. Porzio, Macromolecules 31 (1998) 1079. [7] D.R. Larkin, J. Org. Chem. 55 (1990) 1563. [8] J. Vanco, O. Svajlenova, E. Racanska, J. Muselik, J. Valentova, J. Trace Elem. Med. Biol. 18 (2004) 155. [9] R. Trivedi, P. Sen, P.K. Dutta, P.K. Sen, Nonlinear Opt. 29 (2002) 51. [10] A. Iwan, D. Sęk, Prog. Polym. Sci. 33 (2008) 289. [11] B. Jarzabek, J. Weszka, M. Domański, J. Jurusik, J. Cisowski, J. Non-Cryst. Solids 352 (2006) 1660. [12] J. Weszka, M. Domański, B. Jarząbek, J. Jurusik, J. Cisowski, A. Burian, Thin Solid Films 516 (2008) 3089. [13] M.S. Weaver, D.D.C. Bradley, Synth. Met. 83 (1996) 61. [14] T. Yoshomiura, S. Tatsuura, W. Sotoyama, A. Matsuura, T. Hayano, Appl. Phys. Lett. 60 (1992) 268. [15] B. Jarzabek, J. Weszka, M. Domanski, J. Jurusik, J. Cisowski, J. Non-Cryst. Solids 354 (2008) 856. [16] Ch.-J. Yang, S.A. Jenekhe, Chem. Mater. 3 (1991) 878. [17] Ch.-J. Yang, S.A. Jenekhe, Chem. Mater. 6 (1994) 196. [18] K.S. Lee, J.C. Won, J.C. Jun, Makromol. Chem. 190 (1989) 1547. [19] S.-B. Park, H. Kim, W.C. Jung, Macromolecules 26 (1993) 285. [20] M. Ballauff, Macromolecules 19 (1986) 1366. [21] J.M. Rodriquez-Parada, R. Duran, G. Wegner, Macromolecules 22 (1989) 2507. [22] R. Stern, M. Ballauff, G. Lieser, G. Wegner, Polymer 32 (1991) 11. [23] J. Watanabe, B.R. Harkness, M. Sone, H. Ichimura, Macromolecules 27 (1994) 507. [24] J.C. Jung, S.-B. Park, Polym. Bull. 35 (1995) 423. [25] H.R. Kricheldorf, A. Domschke, Maromolecules (1996) 1337.
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Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/ locate/ jnoncrysol
The effect of heat treatment on the optical properties of thin films of polyazomethine with flexible-side chains B. Jarząbek a,⁎, B. Kaczmarczyk a, J. Jurusik a, J. Weszka a,b a b
Centre of Polymer and Carbon Materials Polish Academy of Sciences, 34M. Curie-Sklodowska Str., 41-819 Zabrze, Poland Silesian University of Technology, Faculty of Mechanical Engineering, Institute of Engineering and Biomaterials, 18a Konarski Str., 44-100 Gliwice, Poland
a r t i c l e
i n f o
Article history: Received 17 June 2013 Received in revised form 22 July 2013 Available online xxxx Keywords: Polyazomethines; Flexible side chains; Thin films; Absorption edge parameters; Thermal stability
a b s t r a c t Thermal stability of a soluble derivative of a basic aromatic polyazomethine: poly(1,4-(2,5-bisoctyloxyphenylene-methylenenitrilo)-1,4-phenylene-nitrilomethylene) BOO-PPI, in thin films, has been studied by “in situ” UV-Vis-NIR transmission measurements during the heat treatment. Optical investigations of the BOO-PPI films have been completed by the wide angle X-ray diffraction (WAXRD) and atomic force microscopy (AFM) studies. Absorption spectra of the BOO-PPI films did not change during the first annealing, like as for unsubstituted PPI films. Study of temperature dependences of the absorption edge parameters (the Urbach energy EU and energy gap EG) allowed us assessment of changes in structural order, conformation and conjugation of the BOO-PPI polymer chain during the successive heating and cooling runs. The value of EG (2.19 eV) was constant, while EU changed linearly in the range 71–140 meV, within the temperature range 25–250 °C. This reversible and recurrent dependence of EU is connected with the structural changes of flexible bis-octyloxy side chains, while the conjugation in the main chain is preserved (constant EG). Attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy studies confirmed invariable conjugation: C N stretching vibration band was unchanged during the heat treatment. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Conjugated polyazomthines (polyimines) also known as Schiff bases, belong to the aromatic polymers class, with an extended π-system and possess C N imine groups alternating with benzene rings in the main chain. These polymers have been of widespread interest from many years, as corrosion inhibitors [1], catalyst carriers [2,3], metal ion complexing agents [4], thermo-stable materials [5,6] in biological systems [7,8] and in optoelectronic devices [9,10], due to good thermal stability, photo- and electro-luminescence efficiency and nonlinear optical (NLO) properties. The basic aromatic polyazomethine is poly (1,4-phenylenemethylenenitrilo-1,4-phenylene-nitrilomethylene) (PPI), where the main chain has the benzene rings and CH N groups alternately. The groups CH N and CH C are isoelectronic, so PPI being isoelectronic with poly(phenylene-vinylene) PPV, may have also potential applications in light emitting diodes (LED), thin film transistors (TFT) and solar cells (SC). All these optoelectronic and photovoltaic devices are exposed to the effect of higher temperatures that is why the thermal stability of polymer thin films used in these devices is so important. Due to insolubility of polyazomethines to prepare thin films, usually the thermal vacuum evaporation (TVE) or chemical vapor deposition (CVD) methods are used [11–15], because of economic reasons spin coating methods are preferred. There are several methods, reported in ⁎ Corresponding author. Tel.: +48 32 2716077; fax: +48 32 27112969. E-mail address: [email protected] (B. Jarząbek). 0022-3093/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnoncrysol.2013.07.034
the literature, to improve solubility and processability of polyimines. Several approaches based on the reversible Lewis acid-base (GaCl3) or di-m-cresol phosphate (DCP) complexations [16,17], or modification of polymer structure by unsymmetrical [18] or symmetrical [19] substitution with the flexible alkyl or alkoxy side chains have been applied to polyimines. The flexible side chains, attached to the rigid main chain, not only improve the solubility of polymers, but also these polymers can form a novel, layered structure in their crystalline phase and mesophase [20–25]. Polyazomethines having flexible n-alkyloxy methyl side chains have been reported, by Kim et al., in several papers [26–29]. X-ray diffraction is the main method used to investigate the detailed crystal structure of these types of polymers, designated as Cm-PAMs (where m = 4, 6, 8, 12, …) [26–28]; the temperature dependence of the layer spacing of Cm-PAMs was presented in refs. [26,27] and discussed as a function of the length of the side chain (m value). Other soluble, para-linked aromatic polyazomethines with methoxy and 2-ethylhexyloxy side chains (MEH-PPI) have been presented in ref. [30]. The structure of the MEH-PPI resembles that of well-known MEHPPV i.e. poly(2-methoxy-5-(2′ethylhexyloxy)-1,4-phenylenevinylene); hence there is an interest in this type of soluble polyazomethine. The effect of annealing treatment and the temperature dependence of molecular dynamics in MEH-PPV films have been presented on the basis of the wide angle X-ray spectroscopy (WAXS), solid-state nuclear magnetic resonance (NMR) and fluorescence spectroscopy (PL) studies [31] and using the FTIR, UV-Vis and scanning electronic microscopy (SEM) methods [32]. Such detailed investigations have not been applied for
28
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thin films of polyazomethines with flexible side-chains, except “in situ” temperature WAXS studies of the Cm-PAMs films [26,27]. Recently, in our previous work [33] we have described the optical properties of thin films of BOO-PPI (i.e. PPI with the linear, symmetric, flexible 2,5–bisoctyloxy side chains, para-linked by oxygen atoms to the phenyl rings connected with carbon imine atoms in the main chain) and then we have compared these results with the properties of unsubstituted PPI films. The same BOO-PPI and PPI films have been investigated during the heat treatment and the results are shown in this work. In the present work, the temperature dependences of optical properties of BOO-PPI thin films have been examined, in relation with the properties of an unsubstituted PPI films. The BOO-PPI polymer, synthesized by us, is different than other soluble polyazomethines, described up to now in the literature, except our works [33–35]; only recently, we have reported the results of studies on the time-stability of diodes based on BOO-PPI films [34] and the effects of BOO-PPV and BOO-PPI protonation [35]. Herein, we only briefly report the BOO-PPI synthesis and thin-film preparation (more details in ref. [33]) and the main part of this work is devoted to the results of UV-Vis-NIR spectroscopy measurements of these films during their successive, gradual annealing and cooling. Also the high temperature-controlled attenuated total reflection (ATR) module was used in FTIR spectroscopy measurements to investigate the BOO-PPI films during the heat treatment. Other complementary methods, applied at room temperature, such as the wide angle X-ray diffraction (WAXRD) and atomic force microscopy (AFM) allowed us to evaluate the changes in atomic structure and surface morphology of the BOO-PPI films, after annealing. So far, such detailed studies of the heat treatment effect on optical properties of thin films of polyazomethines with flexible side chains have not been reported. 2. Experimental 2.1. Technological details The soluble derivative of polyazometine, i.e. poly(1,4-(2,5bisoctyloxy-phenylene-methylenenitrilo)-1,4-phenylene-nitrilomethylidyne) (BOO-PPI) was synthesized according to the modified procedure, reported in detail in ref. [33,35]. Para-phenylene-diamine (PPDA) and 2,5-bis(octyloxy)-terephthaldehyde (BOO-TPA) have been used as the initial monomers in synthesis of the BOO-PPI polymer and the scheme of this reaction is shown in Fig. 1. The glass transition temperature (Tg) for such synthesized BOO-PPI polymer was obtained at 32.7 °C, as the midpoint at half height of the increase of the specific heat associated with the transition, during differential scanning calorimetry (DSC) experiments. Thin films of BOO-PPI have been prepared by spin coating from the polymer solution in chloroform, onto quartz substrates. The deposition process has been carried out at room temperature, in atmospheric pressure air, and then the films were annealed for 20 min at 100 °C. Thin films of BOO-PPI obtained by spin coating were transparent, yellow and good quality. Thicknesses of the BOO-PPI films, determined with an interference microscope, were about 100 (±20) nm. Thicker films (above 1 μm), suitable for the ATR-FTIR measurements, were obtained by pouring out a more concentrated polymer solution on glass or quartz substrates.
The chemical vapor deposition (CVD) method, via polycondensation process, of the PPDA and TPA monomers with pure argon as a transport agent, has been used to obtain pure PPI thin films. Technological details of the PPI film preparation by CVD, using different arrangements, have been described earlier in refs. [11–13]. 2.2. Characterization techniques The obtained BOO-PPI films were characterized by the ATR FTIR and UV-Vis-NIR spectroscopes during successive, gradual heating and cooling processes. Other methods, such as wide angle X-ray diffraction (WAXRD) and atomic force microscopy (AFM), were applied at room temperature, before and after annealing process. Crystal structure of the films has been examined by WAXRD technique, with a conventional θ–2θ TUR M-62 diffractometer, using the Ni-filtered Cu Kα radiation, while the surface morphology and roughness have been investigated by an atomic force microscope (AFM) TopoMetrix Explorer, working in the contact mode in the air, in the constant force regime. The infrared spectra of thin-film samples coated onto the glass or quartz windows were acquired on a Bio-Rad FTS-40A Fourier transform infrared spectrometer, in the range 4000–400 cm−1, using a JASCO ATR high temperature-controlled attachment, with a resolution of 2 cm−1 and for accumulated 120 scans. Spectra recorded at elevated temperatures were obtained in the temperature range 20–80 °C with a step of 10 °C and with a 2 °C/min heating rate under the nitrogen atmosphere. Optical transmission and reflectivity measurements have been carried out within the spectral interval 200–2500 nm, using a two-beam UV-Vis-NIR spectrophotometer, JASCO V-570. During the reflectivity measurements, a special two-beam reflectance arrangement was used with an Al mirror in the reference beam as a reflectance standard. Temperature dependences of transmission spectra have been investigated during “in situ” heat treatment of thin films, using a special high temperature-controlled equipment of the JASCO spectrophotometer, enabling us to measure transmission spectra, at precisely definite temperatures, with the accuracy of ± 0.5 °C. 3. Results 3.1. First annealing and cooling effects 3.1.1. UV-Vis-NIR(T) measurements Good thermal stability of polyazomehines (polyimines) is one of the typical features of this group of polymers. Our previous studies [36] confirmed thermal stability of the pristine PPI film with thickness 1.4 μm, for which we have investigated the annealing effect on the absorption edge parameters, i.e. the Urbach energy (EU) and the energy gap (EG). Both parameters of this thick PPI film, turned out to be almost invariable, within the temperature range 25–225 °C [36], thereby thermal stability was proved. Hence, our idea was to check thermal stability of the BOO-PPI thin films in the same way, using the results of “in situ” optical measurements, during slow, gradual annealing process, within the same temperature range, with a step of 25 °C. For the purpose of comparison, we have repeated this experiment for unsubstituted PPI thin films with a comparable thickness, of about 100 nm. The absorption coefficient (α) has been
H3C(H2C)6H2CO
H3C(H2C)6H2CO
O
n H2N
NH2
+
H N
n H
PPDA
O
- (2n-1) H2O
C H
OCH2(CH2)6CH3
BOO-TPA Fig. 1. Formation of the BOO-PPI polymer, via the polycondensation reaction.
BOO-PPI
C
NH
H OCH2(CH2)6CH3
n
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calculated as a function of photon energy (E) on the basis of transmission and reflectivity measurements, according to the formulae given in ref. [37]. Figs. 2(a) and (b) present the absorption spectra at different temperatures for the PPI and BOO-PPI films, respectively. As it is seen in Figs. 2 (a) and (b), significant changes in the absorption spectra during annealing are not registered over the whole investigated spectral range. This result can suggest the same good thermal stability of both films. The main difference between PPI and BOO-PPI spectra, seen in Fig. 2, is the bathochromic shift of the absorption edge and lack of vibronic progression in the low energy absorption band of BOO-PPI films, compared to its parent PPI films To find precisely the positions of vibronic band peak energies the second derivative method has been used and then this vibronic progression has been deconvoluted [15]. The values of vibronic bands (E1, E2, E3) of the PPI films are shown in Table 1. Next, the PPI and BOO-PPI films were cooled slowly from 225 to 25 °C and then transmission at room temperature for both films has been measured once more, and the obtained absorption spectra are presented in Figs. 3(a) and (b). An unexpected result has been obtained for the BOO-PPI films [see Fig. 3 (b)]: namely, a distinct red shift of the absorption edge and simultaneously the appearance of vibronic peaks at the low-energy absorption band are registered, in contrast to the PPI films for which no differences were observed, as is depicted in Fig. 3 (a). Positions of the BOO-PPI vibronic bands are shown in Table 1. To discuss the heat treatment effect on the structural order and chain conjugation of the BOO-PPI films, we have determined the absorption edge parameters, EU and EG, seen in Figs. 4 (a) and (b), respectively, both before and after thermal relaxation. As it is seen in Fig. 4 (a), the slopes of the exponential edges follow the Urbach relation [38] α ∝ exp (E/EU), allowing us to determine the values of EU, of 91 meV and 71 meV for the BOO-PPI films, before and after annealing, respectively. Least squares estimation method was used to the Urbach edges and then the Urbach energies were calculated,
a)
Table 1 Optical parameters, obtained at room temperature. Film
E⁎1 (eV)
E⁎2 (eV)
E⁎3 (eV)
EU (meV)
EG (eV)
n
k
PPI, PPIa BOO-PPI BOO-PPIa
2.64 – 2.44
2.83 2.70 2.59
3.00 – 2.78
130 91 71
2.30 2.20 2.19
1.79 1.76 1.76
0.19 0.08 0.08
E⁎1 , E⁎2 , E⁎3 — the energies of vibronic bands, each of uncertainty (±0.01) eV; Uncertainties: EU (±10) meV; EG (±0.03) (eV); n, k (±0.02). a After thermal relaxation of the investigated films.
as: EU = 1/a, where a is a direction coefficient of line: y = a · x + b). To obtain the energy gap, we have used the well-known equation α ∝ (E − EG)x, with x depending on the type of optical transition [39]. The best fit to the absorption edge of BOO-PPI films turned out to be for x = 2, corresponding to a linear dependence (α E) 1/2 = f (E), well known as the Tauc relation [40] and shown in Fig. 4 (b). Linear approximation to the energy access (by the least squares estimation method) was used to the Tauc dependences, shown in Fig. 4 (b), to obtain the energy gaps, where: EG = E for (αE)1/2 = 0. This relation is typical for amorphous semiconductors and is often used to obtain the energy gaps of polymer films, including different polyazomethine thin films [15]. Amorphous character of the obtained PPI and BOO-PPI films has been demonstrated in ref. [33], on the basis of the X-ray diffraction patterns recorded for these films. As it is seen in Fig. 4 (b), the thermal relaxation process (carried out by annealing up to 225 °C and then by cooling to 25 °C) of the BOO-PPI film has not resulted in a change of the energy gap, that remains almost constant and equal to 2.20–2.19 eV. Both parameters of the absorption edge (EU and EG) obtained at room temperature, for the PPI and BOO-PPI thin films, before and after thermal relaxation, are gathered in Table 1. The Kramers–Kronig relations [41] applied to the reflectance spectra of the investigated films yield the values of refractive index (n) and extinction coefficient (k) in the low energy range. Both parameters are
a) PPI
4
25 0C
abs. coef., α (104 cm-1)
abs. coef., α (104 cm-1)
6
225 0C
2
0 1
2
3
4
5
a)
6
PPI 4
2
6
5
0
b)b)
1 6
3
BOO-PPI
2 25 0C
1 0
225 C
2
3
4
5
6
b)
4
abs. coef., α (104 cm-1)
abs. coef., α (104 cm-1)
29
5 4 BOO-PPI 3 2 1
0 1
2
3
4
5
6
energy,E(eV)
0 1
2
3
4
5
6
energy,E(eV) Fig. 2. Absorption spectra during first annealing of the PPI (a) and BOO-PPI (b) thin films; obtained at 25, 50, 75, 100, 125, 150, 175, 200 and 225 °C respectively (the spectra turned out almost unchangeable).
Fig. 3. Absorption spectra obtained at room temperature, before (dashed lines) and after (solid lines) thermal relaxation of the (a) PPI and (b) BOO-PPI thin films.
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B. Jarząbek et al. / Journal of Non-Crystalline Solids 379 (2013) 27–34
4
a
400
EU = 71 meV
3
EU = 91 meV
Intensity (a.u.)
abs. coef., α (104 cm-1)
600
a)
5
2 1 2.2
2.4
2.6
2.8
200
600
400
b
energy, E(eV) 200
(αE)1/2 (eV/cm)1/2
b) 300
10
20
30
40
50
2 Theta 200
Fig. 5. X-ray diffractograms, obtained at room temperature, of the BOO-PPI thin films; line (A)—before and line (B)—after thermal relaxation.
100
0 1.8
EG= 2.19 eV
2.0
2.2
EG = 2.20 eV
2.4
2.6
2.8
energy, E(eV) Fig. 4. Absorption edges, at room temperature, of the BOO-PPI films, before (dashed lines) and after (solid lines) thermal relaxation: (a) the Urbach dependence (b) the Tauc relation. Linear regressions result in the following equations: (a) y = 14.057 · x − 32.52 for the solid line and y = 10.996 · x − 25.45 for the dashed line, (b) y = −4065.167 · x + 1843.79 for the solid line and y = −2548.351 · x + 1163.73 for the dashed line; R2 = 0.998 for all lines.
slightly smaller for the BOO-PPI than for pristine PPI films. All these coefficients are also included in Table 1. 3.1.2. X-ray diffraction, AFM and ATR FTIR(T) studies X-ray diffraction patterns obtained at room temperature, for the BOO-PPI films before and after the thermal relaxation, are shown in Fig. 5. The diffractogram of the BOO-PPI films before the heat treatment [line(A)] shows a predominant amorphous character, with the broad peak and a small feature, seen at about 2θ = 21°, corresponding to that reported for bulk PPI [42]. This small feature can indicate a little more ordered structure of the BOO-PPI films than the quite amorphous structure of PPI films [11,15,33,35]. After the heat treatment of the BOO-PPI films, we can observe a small, distinct feature at about 2θ = 12° on the diffractogram, as line (B) in Fig. 5. The AFM topography images (5 μm × 5 μm) of the BOO-PPI films surface, before and after annealing, are shown in Figs. 6(a) and (b), respectively. Before the heat treatment, the BOO-PPI films surface topography exhibits not very homogeneous grained structure, but relatively flat areas with the root mean square (RMS) roughness of about 3.2 nm. Particles of different size, scratches and elongated grains can be also observed. As it is seen in Fig. 6 (b), the surface of BOO-PPI films after annealing and cooling process shows a similar appearance, although the RMS roughness has increased to about 10 nm. The detailed assignments of the absorbance bands of FTIR spectra of the BOO-PPI polymer are as follows: 3060, 3027, 2951 and 2997 (ν CH), 2951, 2923 (νa CH2 and CH3), 2869, 2853 (νs CH2 and CH3), 2734 (ν = CH), 1604 (ν C N and ν Ph), 1580sh, 1503, 1488, 1427 and 1391 (ν Ph), 1468 (δa CH3 and CH2), 1370 (δs CH3 and CH), 1287 1273, 1202, 1164 (δ CH), 1029 (υ C-O), 971 (ω = CH), 879 and 839 (γ
CH), 722 (δ Ph), 671 (γ = CH). The absence of the bands at about 1700 cm− 1 (in relation to the PPI spectra), corresponding to the stretching vibrations of the aldehyde C O groups, proves formation of a polymer. In turn, the band attributed to the stretching vibrations of the C N group, occurs at 1604 cm− 1, i.e. at a lower wavenumber than in the case of the simplest polyazomethine PPI, where this band appears at 1612 cm− 1 [5,36]. During “in situ” gradual heating of the BOO-PPI films to 180 °C, no changes in the ATR-FTIR spectra were detected. Consequently, the spectrum obtained at 180 °C was the same as that obtained at room temperature. This spectrum is depicted as a dashed line (A) in Fig. 7. However, during cooling, at 130 °C, the bands at 1503 and 1393 cm−1 began to decrease their intensity. Simultaneously, intensity of the shoulder at about 1287 cm−1 started to decrease. Thus, after gradual cooling of the BOO-PPI films to 25 °C, only shoulders at 1503 and 1393 cm−1, and a broad band with two shoulders at 1276 and 1287 cm−1 were observed. All the abovementioned bands correspond to the stretching vibrations of the phenyl rings. The changes are also observed in the case of the band attributed to the stretching vibrations of the C-O groups. During cooling, a shoulder at 1045 cm−1 appeared at 130 °C which, after cooling to 25 °C, formed a doublet with the band at 1029 cm−1. It should be emphasized that the band attributed to the stretching vibrations of the C N imine groups, (at 1604 cm−1) did change during heating and cooling runs. The ATR-FTIR spectrum of the BOO-PPI films after cooling to room temperature is shown in Fig. 7 as a solid line (B). 3.2. Successive annealing and cooling runs; UV-Vis-NIR(T) spectroscopy After the thermal relaxation processes described above (by heating to about 200 °C and then cooling to the room temperature), we checked thermal stability of the BOO-PPI films during successive heating and cooling runs. Fig. 8(a) shows the temperature dependence of the absorption coefficient of the BOO-PPI relaxed films, during annealing from 25 to 250 °C with a step of 25 °C. Changes of the spectra, obtained during annealing process, are seen only in the region of the absorption edge for the first, low-energy absorption band, while positions of other bands are invariable with respect to the increase in temperature. The spectral range of the absorption edge is presented in detail in Fig. 8 (b), where the influence of temperature on the slope of the absorption edge and the shape of low-energy band of the BOO-PPI films are clearly seen. The vibronic progression of this band disappears with increasing temperature and above 100–125 °C is not seen at all. The
B. Jarząbek et al. / Journal of Non-Crystalline Solids 379 (2013) 27–34
31
Fig. 6. AFM topography images (5 μm × 5 μm) of the BOO-PPI films: (a) before and (b) after thermal relaxation; color scale: (a) 0–38 nm and (b) 0–94 nm.
edge of the absorption spectra becomes more sloped with respect to the energy axis, when the temperature increases, resulting in an increase of the Urbach energy. We have determined the EU values for each temperature, using the same procedure, as described before [see Fig. 4 (a)]. Simultaneously, the energy gaps (EG) values have been calculated for each temperature, using the Tauc method [as in Fig. 4 (b)]. It turned out that
the energy gap EG ≅ 2.19 eV is almost constant, irrespectively of temperature. During the second cooling of the BOO-PPI films, from 250 °C to room temperature, we have obtained the same dependence of the absorption spectra, as in Fig. 8. The energy gap remains almost constant, while the Urbach energy demonstrates the linear dependence on temperature, as it is depicted in Fig. 9, where one can see that EU changes
Fig. 7. FTIR-ATR spectra of the BOO-PPI films, recorded at room temperature, before [line (A)] and after [line (B)] thermal relaxation.
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abs. coef., α (104 cm-1)
5
out to be almost invariable during annealing, up to 225 °C. In this work, the same investigations have been applied for the thin films of polyazomethine having bisoctyloxy elastic side chains and the obtained results allowed us to assess thermal stability of the BOO-PPI films.
a)
4 25οC
second annealing
3
25οC
2
250οC
4.1. First annealing and cooling process effects
ο
250 C
1
second cooling 25οC
250οC
0 2
3
4
5
6
energy, E (eV)
abs. coef., α (104 cm-1)
5 25οC
b)
250οC
4 1
3
10
2
250οC
25οC
1 250οC
25οC
0 2.0
2.2
2.4
2.6
2.8
3.0
energy, E (eV) Fig. 8. Absorption spectra during the second heating and cooling runs at: 25, 50, 75, 100, 125, 150, 175, 200, 225 and 250 °C respectively (a) whole spectral range; (b) absorption edge and low-energy band range; the spectrum (1) was obtained at 25 °C, then spectra in turn, up to the spectrum (10) at 250 °C.
linearly in the range 71–140 meV, within the temperature range 25– 250 °C. Linear regression, by the least squares estimation method has resulted in the following relation: EU (T [K]) = 0.29 · T [K] − 13.73 (with the coefficient of determination: R2 = 0.998). This linear dependence appears to be reproducible and recurrent for the next successive heating and cooling runs. 4. Discussion
Urbach energy, EU (meV)
The results of our investigations of the optical properties of BOO-PPI films presented herein and in ref. [33] confirm that the presence of 2,5bisoctyloxy flexible side chains attached to the PPI main chains, not only improves solubility but also leads to the changes of polyazomethine properties. Thermal stability of the pristine PPI film has been studied in ref. [36], where the absorption edge parameters (EU and EG) turned
140
BOO-PPI
120 second cooling
second heating
100
y = 0.29 x -13.73
80 300
350
400
450
500
550
temperature, T (K) Fig. 9. Temperature dependence of the Urbach energy obtained for the BOO-PPI film, during successive heating (black circles) and cooling (white circles).
4.1.1. UV-Vis-NIR(T) measurements The absorption spectra of the PPI and BOO-PPI thin films (Fig. 2) indicate good thermal stability; since no changes in both spectra were registered over the whole investigated spectral range, during annealing up to 225 °C. As it is shown in ref. [33], the red shift of the absorption edge of bisoctyloxy (BOO) substituted PPI films, in comparison with the edge of parent PPI polymer films, is caused by a greater electron density of the former π-cojugated polymer, being a result of the strongly electron-donating ability of the bis-octyloxy groups, like the methoxy groups [5]. Also a better planarization of the BOO-PPI chains (as compared to the PPI structure) has been confirmed by the geometry optimization (smaller torsion angles in the main chain) [33]. Hence, there is a better conjugation and smaller energy gap (EG) of the BOO-PPI films than for pristine PPI films. The absorption edge parameters (EU and EG), together with the energies (E1⁎, E2⁎, E3⁎) of vibronic bands and the values of refractive index (n) and extinction coefficient (k), obtained for PPI and BOO-PPI films at room temperature before and after annealing, are collected in Table 1. The first low energy band is caused by the interband transitions between delocalized states (D → D*) which are derived from the interaction of benzene and azomethine dimer π-orbitals. The vibronic progression of this band is connected with the electron–phonon interaction, due to benzene ring stretching mode, as it is shown and discussed for the PPI films in refs. [11,12,15]. The low-energy band of the BOO-PPI films is very narrow and the vibronic progression is absent. The absence of vibronic bands can be explained by the effect of the bis-octyloxy side chains attached in 2 and 5 positions to alternate phenylene rings. A clearly smaller value of the Urbach energy (EU) obtained for the BOO-PPI films than for the PPI films (see Table 1) indicates that the presence of side chains results in more ordered and planar structure of the polymer backbone. Based on the Urbach energy, we can estimate the width of the tails of localized states in the optical gap [38]. Structural defects in polymers, like break, torsion or aberrations of chains can be responsible for these localized states and the Urbach-like behavior of the polymer absorption edge. The results of first annealing (to 225 °C) of PPI and BOO-PPI films could indicate the same thermal stability of both films, because the absorption spectra of both films, over the investigated range, remain unchanged during heating (Fig. 2). Then, after cooling to 25 °C, we have compared the absorption spectra at the room temperature [Figs. 3 (a) and (b)] for the PPI and BOO-PPI films, before and after heat treatment (by heating to 225 °C and then slow cooling to 25 °C). The PPI absorption spectrum remained still invariable, while that of the BOO-PPI films turned out to be different than the spectra, initially obtained at room temperature and during the first heating run. The main differences between these BOO-PPI spectra [Fig. 3 (b)] are seen in the range of the absorption edge and concern the first low-energy band, where the vibronic bands occurred. This vibronic progression (as for the spectra of PPI films) reveals the electron–phonon interaction and is connected with the thermal relaxation of defects and more ordered planar polymer chains. The absorption edge of the BOO-PPI films, being the low-energy wing of the spectrum, became more “sharp” after such thermal relaxation [Fig. 3 (b)]. The energy gap of the films remained almost constant [Fig. 4 (b)] however the Urbach energy decreased [Fig. 4 (a)]. This means that conjugation of the BOO-PPI polymer chains did not change, but the structural order can be improved due to the thermal relaxation process. In the case of conjugated polymers, the energy gap depends on the conjugation length, while the Urbach energy is connected with the structural defects which are responsible for the localized states inside the energy gap. The thermal
B. Jarząbek et al. / Journal of Non-Crystalline Solids 379 (2013) 27–34
relaxation process of the BOO-PPI films did not influence the conjugation in the rigid main chain but improved the structural order in the side chains, resulting in relaxation of defects and a little more ordered, partially layered structure of polymer, as it is shown in ref. [26,27] for Cm-PAMs films. 4.1.2. X-ray diffraction, AFM and ATR FTIR(T) studies The results of X-ray diffraction investigations, seen in Fig. 5, also confirmed a little more ordered structure of the BOO-PPI films, after relaxation by a suitable heat treatment. Additionally, the AFM images, presented in Fig. 6, show that the surface of BOO-PPI films after heat treatment has larger grains; the RMS roughness has increased from 3.2 nm to about 10 nm, after the heat treatment. The ATR-FTIR(T) investigations of the BOO-PPI films (Fig. 7) support and complete the results, described above. During “in situ” annealing to 180 °C, no changes in the ATR-FTIR spectra of the investigated films were detected. Then, during slow cooling, all changes of the FTIR spectra of BOO-PPI films were related to the bands corresponding to the stretching vibrations of the phenyl rings and vibrations of the C-O groups. At the same time, the band attributed to the stretching vibrations of the C N imine groups did not change, which means that the conjugation in polymer was preserved and the phenyl rings had to preserve the spatial arrangement in relation to the C N bond plane. This, in turn, suggests that changes observed during cooling are associated with changes in the order of particular polymer chains relative to each other. The lack of changes in the bands, characteristic for vibrations of the aliphatic groups, indicates that changes during cooling of the BOOPPI films are only the consequence of spatial ordering of the phenyl rings forming particular polymer chains, relative to each other [42]. These results of ATR FTIR(T) studies of the BOO-PPI films are in a good agreement with the UV-Vis NIR(T) investigations. 4.2. UV-Vis-NIR(T) spectroscopy during successive heating and cooling runs Successive heating and cooling runs of the BOO-PPI films affected the absorption spectra only in the range of absorption edge and the first, low-energy absorption band, while the bands, seen in the higher energy range, were at the same positions. Additionally, these changes in the absorption edge slope and the shape and position of the first low-energy band turned out to be reversible and recurrent (see Fig. 8). During annealing, the vibronic progression disappeared and the band maximum decreased and shifted to a higher energy. These changes indicate more structural defects and weaker delocalization in the BOO-PPI chains at higher temperatures. The absorption edge of the BOO-PPI films during annealing became more sloped with respect to the energy axis, resulting in an increase of the Urbach energy. Reversible linear dependences of EU = f (T) have been obtained, both for heating and cooling successive runs. Annealing of the relaxed BOO-PPI films causes the structural changes in the flexible bisoctyloxy side chains, leading to changes in order of particular polymer chains, relative to each other and to an increase of structural defects, while during cooling these defects disappeared. The heat-inducted structural changes involve elastic side chains and these changes can be responsible for the spatial disorder and increase in the density of localized states, affecting an increase of the Urbach energy and decrease of delocalization (lack of the vibronic structure of this band). The energy gap turned out to be almost constant during heat treatment of the BOO-PPI films, which means that the conjugation is preserved and seems to be independent on temperature (up to 250 °C). 5. Conclusions In the present work we have shown the annealing and cooling effects on the optical properties of thin films of polyazomethine (PPI) with 2,5-bis octyloxy (BOO) flexible side-chains. The presence of these linear, symmetric side-chains, linked to the phenyl rings and connected
33
with the carbon atoms of the imine group in the polyazmethine backbone, significantly improves the solubility of PPI and also changes other properties of polymer. Thanks to the different analysis techniques, such as the UV-Vis-NIR(T) and ATR-FTIR(T) spectroscopy measurements, together with the WAXRD and AFM complementary methods, we could assess the optical properties and thermal stability of the BOO-PPI thin films. Summing up our results, we can conclude that: – annealing, for 20 min at 100 °C, just after films preparation by spin coating, is efficient to remove remains of solvent but insufficient to obtain the ordered polymer structure. Only the slow, gradual cooling from about 200 °C to room temperature, allows one to obtain a more ordered polymer structure of the BOO-PPI films. – all structural changes during heat treatment of the BOO-PPI films are connected with the presence of flexible bis-octyloxy side-chains, not influencing conjugation in the main, rigid chains. – successive heating and cooling runs of the BOO-PPI films cause the reversible and recurrent changes of the polymer structural order, connected with the effect of heat-induced movement of elastic side chains. To conclude, the presence of flexible side-chains improves solubility and causes the better conjugation and planarization of polymer chains, resulting in a smaller energy gap of BOO-PPI thin films, than for unsubstituted PPI films. Moreover, it appears that during the heat treatment of BOO-PPI thin films, the structural changes are connected with the flexible side chains but the conjugation in the main chains is preserved and the energy band gap is constant. It means that we can use the spin coating technique to obtain thin films of BOO-PPI with a smaller energy gap than for pristine PPI films and with the same good thermal stability, so the BOO-PPI thin films show promise for (opto)electronic device applications. Acknowledgements Polish National Science Centre, Grants no NN 507 227040 (B. J. and B. K.) and NN 508626840 (J. W.) supported these researches. The authors would like to thank Dr Mariola Siwy for the polymer synthesis, Dr Barbara Hajduk for the film preparation and M.Sc. Marian Domanski for the X-ray diffraction measurements. References [1] K.C. Emregul, E. Duzgun, O. Atakol, Corros. Sci. 48 (2006) 873. [2] R. Drozdzak, B. Allaert, N. Ledoux, I. Dragutan, V. Dragutan, R. Verpoort, Coord. Chem. Rev. 249 (2005) 3055. [3] J.L. Sesssler, P.J. Melfi, G. Dan Pantos, Coord. Chem. Rev. 250 (2006) 816. [4] I. Kaya, A.R. Vilayetoglu, H. Mart, Polymer 42 (2001) 4859. [5] C.J. Yang, S.A. Jenekhe, Macromolecules 28 (1995) 1180. [6] S. Destri, I.A. Khotina, W. Porzio, Macromolecules 31 (1998) 1079. [7] D.R. Larkin, J. Org. Chem. 55 (1990) 1563. [8] J. Vanco, O. Svajlenova, E. Racanska, J. Muselik, J. Valentova, J. Trace Elem. Med. Biol. 18 (2004) 155. [9] R. Trivedi, P. Sen, P.K. Dutta, P.K. Sen, Nonlinear Opt. 29 (2002) 51. [10] A. Iwan, D. Sęk, Prog. Polym. Sci. 33 (2008) 289. [11] B. Jarzabek, J. Weszka, M. Domański, J. Jurusik, J. Cisowski, J. Non-Cryst. Solids 352 (2006) 1660. [12] J. Weszka, M. Domański, B. Jarząbek, J. Jurusik, J. Cisowski, A. Burian, Thin Solid Films 516 (2008) 3089. [13] M.S. Weaver, D.D.C. Bradley, Synth. Met. 83 (1996) 61. [14] T. Yoshomiura, S. Tatsuura, W. Sotoyama, A. Matsuura, T. Hayano, Appl. Phys. Lett. 60 (1992) 268. [15] B. Jarzabek, J. Weszka, M. Domanski, J. Jurusik, J. Cisowski, J. Non-Cryst. Solids 354 (2008) 856. [16] Ch.-J. Yang, S.A. Jenekhe, Chem. Mater. 3 (1991) 878. [17] Ch.-J. Yang, S.A. Jenekhe, Chem. Mater. 6 (1994) 196. [18] K.S. Lee, J.C. Won, J.C. Jun, Makromol. Chem. 190 (1989) 1547. [19] S.-B. Park, H. Kim, W.C. Jung, Macromolecules 26 (1993) 285. [20] M. Ballauff, Macromolecules 19 (1986) 1366. [21] J.M. Rodriquez-Parada, R. Duran, G. Wegner, Macromolecules 22 (1989) 2507. [22] R. Stern, M. Ballauff, G. Lieser, G. Wegner, Polymer 32 (1991) 11. [23] J. Watanabe, B.R. Harkness, M. Sone, H. Ichimura, Macromolecules 27 (1994) 507. [24] J.C. Jung, S.-B. Park, Polym. Bull. 35 (1995) 423. [25] H.R. Kricheldorf, A. Domschke, Maromolecules (1996) 1337.
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