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Optical properties of new aliphatic–aromatic co-polyimides
Journal of Non-Crystalline Solids 299–302 (2002) 1057–1061 www.elsevier.com/locate/jnoncrysol
Optical properties of new aliphatic–aromatic co-polyimides B. Jarzaz bek a
a,*
, E. Schab-Balcerzak a, T. Chamenko b, D. Sez k a, J. Cisowski a, A. Volozhin b
Polish Academy of Sciences, Center of Polymer Chemistry, P.O. Box 20, 34 M. Curie–Skłodowska str., 41-819 Zabrze, Poland b Institute of Physical Organic Chemistry, National Academy of Sciences, Minsk, Byelorussia
Abstract Optical transmission of a series of aliphatic–aromatic co-polyimides have been investigated in the range 200–3000 nm. The short wavelength edge of transmission depends on the ratios of diamines (aliphatic to aromatic) and on the type of aliphatic diamine. To obtain the optical parameters the approach proposed by Tauc for amorphous semiconductors has been used, because of the similarity of the absorption edges. The values of pseudogaps were found from 1.01 to above 4 eV, while the Urbach energy changed in the range 75–800 meV. All determined parameters have been found to be related to the influence of the polymer chain structure. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 78.40.Me; 78.66Qn
1. Introduction Polyimide materials belong to the group of polymers characterized by good mechanical and dielectric properties and wide applications in the electronic industries [1]. Most extensive studies were concentrated on wholly aromatic polyimides [2], because of their high-temperature resistance. However, the processability of fully aromatic polyimides is difficult and most of them are insoluble in conventional organic solvents, significantly limiting their application [3]. Almost all fully aromatic polyimides are colored from pale yellow to dark
* Corresponding author. Tel.: +48-32 271 3424; fax: +48-32 271 0658. E-mail address: [email protected] (B. Jarzaz bek).
brown, causing strong absorption in the visible region [4,5]. These polymers cannot be used where colorlessness and transparency are required, for example, as covers for solar cells, orientation films in liquid crystal display devices and optical waveguides for communication interconnectors. To enhance transparency, to decrease the dielectric constant and simultaneously to improve solubility, aliphatic monomers can be introduced, but then the thermal stability is compromised [6]. So, in recent years many studies of aliphatic polyimides have been carried out because of their potential applications as liquid crystal orientation layers or materials with the low dielectric constant [7,8]. Using aliphatic monomers as co-monomers, for example, aliphatic diamines or cyclic dianhydrides were presented in [6,9]. These procedures allowed one to obtain a compromise between processability and thermal properties.
0022-3093/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 1 1 3 0 - 9
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B. Jarzaz bek et al. / Journal of Non-Crystalline Solids 299–302 (2002) 1057–1061
In our work we have investigated a series of new mixed amorphous aromatic–aliphatic co-polyimides, for which details of synthesis and properties were described in [10]. The aim of our paper was to study the optical properties of these polyimides foils in the ultraviolet, visible and near infrared spectral range as well as the influence of the ratio of diamine (aromatic–aliphatic) and type of aliphatic diamine ðCH2 Þy on the transmission spectra and position of the absorption edge. To obtain the energy gap and other energies describing the absorption edge, we have applied the method proposed by Tauc for amorphous materials. Then, on the basis of these values, the influence of polymer chain structure and structural disorder on the optical properties and probable electronic transitions have been considered.
2. Experimental The investigated polyimides have been prepared using high temperature polycondensation at 190 °C for 15 h from monomers: 1; 2; 3; 4-cyclopentanetetracarboxylic dianhydride and 4; 40 -methylenebis(2; 6-diethyaniline) and different aliphatic diamine (ðCH2 Þy , y ¼ 4; 6; 7; 9; 10; 12). This reaction was carried out in protic solvent: m-cresol at the presence of 1; 4-diazabicyclo[2; 2; 2]octane, as a catalyst. Polyimides with various ratios of aromatic to aliphatic diamine have been synthesized. Polymer films (foils) have been prepared by dissolving polymers in DMF and casting on glass plates and then heating up to 200 °C in vacuum during 7 h and striping from these glass substrates. The thickness of such obtained foils changed from 40 to 200 lm. The amorphous character of these polymers was confirmed by wide angle X-ray diffraction measurements, performed in reflection mode [10]. The general chemical structure of investigated aliphatic–aromatic copolyimides is shown in Fig. 1, where one can see aromatic part with 4; 40 -methylenebis(2; 6-methyaniline) rings, aliphatic part with ðCH2 Þy methylene units and characteristic for polyimides group of atoms: In our study we used four groups of aromatic– aliphatic co-polyimides foils, where the ratios of aromatic to aliphatic parts were: 75:25, 60:40, 50:50,
Fig. 1. Chemical structure of aliphatic–aromatic co-polyimide, where x ¼ 0:4–0:75 and y ¼ 4; 6; 7; 9; 10; 12.
40:60. For each ratio, aliphatic diamines with different numbers of ðCH2 Þy groups ðy ¼ 4; 6; 7; 9; 10; 12Þ were used. Depending on the co-polymer structure the foils were transparent (colorless) to brown. The optical transmission measurements were performed at room temperature on a Beckman Acta M-IV spectrophotometer within the 200– 3000 nm range. A deuterium lamp was used as a source of ultraviolet, while tungsten lamps were used for visible and near infrared region in this spectrophotometer.
3. Results and discussion Typical transmission (T) spectra of investigated polyimides in the whole measured range are shown in Fig. 2. A sharp cutoff on the short wavelength side at 280 nm is seen for the transparent foil (2a), while the edge is much less abrupt on the transmission curve for the light brown one (2b). The ratio of aliphatic to aromatic part is 50:50 while the
Fig. 2. Typical overall transmission spectra of polyimide foils (a) transparent, (b) colored. The ratio of aliphatic to aromatic is 50:50 and the numbers of aliphatic groups are: (a) y ¼ 4, d ¼ 65 lm, (b) y ¼ 10, d ¼ 70 lm.
B. Jarzaz bek et al. / Journal of Non-Crystalline Solids 299–302 (2002) 1057–1061
Fig. 3. Absorption coefficient of two polyimide foils (the same as in Fig. 2); (a) j – transparent, (b) – colored one.
numbers of aliphatic ðCH2 Þy groups are y ¼ 4 and y ¼ 10; the films thickness is 65 and 70 lm for (a) and (b) plots, respectively. The long wavelength edge at 2750 nm is the same for all investigated polyimides and do not depend on their structure. The level of transmission usually reaches 85% for all colorless polyimides, and is distinctly lower for colored foils. In order to obtain the absorption coefficient a from the transmission data, the expression a ¼ ð1=dÞ lnð1=T Þ, where d is the foil thickness, has been used. The absorption edges and the Tauc plots, as a function of photon energy (for the same two samples as in Fig. 2) are presented in Figs. 3 and 4, respectively. The shape of both edges is very similar to the behavior proposed by Tauc for a typical amorphous semiconductor [11–13], but the level of absorption ð101 –103 cm1 Þ is lower than for amorphous, inorganic thin films, as for example in [14]. This means that the strength of absorption in the polymers studied is lower than in inorganic semiconductors, which is thought to be due to a smaller degree of bonding delocalization in polymer materials [15]. Each of the absorption edges from Fig. 3 exhibits two different exponential regions with different slopes and a saturation region for higher energy. Both exponential parts are described by the relation: a expðE=AÞ, where A becomes either EU in the high-energy exponential region or ET describing the low-energy exponential part of the absorption coefficient. The absorption
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Fig. 4. The Tauc dependence of polyimide foils (the same as in – colored one. Figs. 2 and 3); (a) j – transparent, (b)
edges for all investigated polyimides have been 2 found to follow the Tauc power law: a ðE EG Þ in the range where the photon energy is greater than EG , so this dependence was used to obtain the Tauc optical energy gaps EG , as it is shown in Fig. 4. This approach, typical for amorphous semiconductors, has been also applied by us earlier for amorphous semiladder polymer foils [16]. The values of the obtained optical parameters for all types of polyimides under study are gathered in Table 1, together with the values of the film thickness and the experimental uncertainties are also indicated. Looking at the optical parameters presented in Table 1, one may observe that most polyimides are transparent, i.e., the energy gap EG values are greater than 3.26 eV. However, for each group, with different ratio of diamines (Ar–Al), there exists one or more colored foils. Unfortunately, we cannot connect this fact with the type of aliphatic monomer or with the evenness of the y value. Also the methylene chain length does not determine the color of the investigated polyimides. The polymer color is a sensitive property which may be influenced by many different factors, as for example in [9], where air oxidation of the end group (aromatic amino group) results in colored by-products. Nevertheless, as is seen in Table 1, for all transparent foils the values of EG decrease with increasing aromatic part of diamine. The same tendency of
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B. Jarzaz bek et al. / Journal of Non-Crystalline Solids 299–302 (2002) 1057–1061
Table 1 Optical parameters of investigated polyimides Ar–Al
d ðlmÞ ð5 lmÞ
ET ðmeVÞ ð5 meVÞ
EU ðmeVÞ ð2 meVÞ
EG ðeVÞ ð0:01 eVÞ
y¼4
75:25 60:40 50:50 40:60
200 50 65 75
1052 844 614 1265
149 149 153 300
3.47 4.05 4.09 1.43
y¼6
75:25 60:40 50:50 40:60
105 55 45 90
748 587 1004 935
74 200 122 162
3.86 3.88 4.11 4.02
y¼7
75:25 60:40 50:50 40:60
85 50 65 50
1820 496 1007 1101
418 131 167 747
1.01 4.03 4.05 3.11
y¼9
75:25 60:40 50:50 40:60
130 40 65 50
1359 738 1066 747
511 149 145 120
2.23 4.03 4.11 4.09
y ¼ 10
75:25 60:40 50:50 40:60
70 35 70 45
833 686 1220 774
104 163 495 133
3.79 4.02 1.42 4.11
y ¼ 12
75:25 60:40 50:50 40:60
160 35 60 60
1021 1189 687 1189
83 444 142 818
3.75 3.00 4.14 2.50
*
Colored foils.
the energy gap is not very different for colored foils. This means, that a higher amount of aromatic monomer influences the intensity of the polymer color, up to deep brown, what is also shown in [4,5,9,17]. In general, the coloration of an aromatic polymer is due to its conjugated aromatic structure and/or the intermolecular and intramolecular charge-transfer complex (CTC) formation. In our polyimides, it may be the formation between alternating electron-donor (diamine) and electronacceptor (dianhydride) moieties, like in [18]. Using aliphatic monomers as co-monomers seems to be reasonable to reduce the chain–chain interaction and disrupt interaction between aromatic moiety, because aliphatic diamines are more flexible and then the transparency increases. However the ratio of diamines seems not to influence the EU and ET values both for the transparent foils and the colored ones, but these two parameters are distinctly
higher for colored foils. The values of the Urbach energy are in the range 74–200 meV for transparent polyimides, while for colored foils they increase from 300 to about 800 meV. Also the ET energies for colorless polyimide foils are lower than for colored ones, for which ET is greater than 1100 meV. These two parameters are related to the localized states induced by the polymer atomic structure. Such possible structure defects like the break, the abbreviation or the torsion of polymer chains seem to be responsible for low-energy absorption, described by the ET parameter.
4. Conclusions New amorphous aliphatic–aromatic polyimides, with different ratio of aliphatic to aromatic parts and different types of aliphatic diamine, seem to be
B. Jarzaz bek et al. / Journal of Non-Crystalline Solids 299–302 (2002) 1057–1061
very interesting materials, because of their optical properties. Most of these flexible foils are transparent, with transmission above 85% in the whole visible spectral range. A few of them are colored from pale yellow to brown and we have not found any detailed explanation of this fact based on their chemical structure. The absorption edges of all investigated foils were found to be similar to the typical edge for amorphous semiconductors, allowing us to obtain the optical parameters. Increasing the aromatic part of polyimides caused a decrease of the energy gap, both for transparent and color foils. The formation of CTC seems to be responsible for this tendency. Larger structural disorder in color polyimides may cause an increase of the Urbach energy and of the parameter ET describing the low-energy part of the absorption edge.
References [1] Y.T. Chern, J. Polym. Sci. A 34 (1966) 125. [2] M. Goyal, M. Inoue, T. Kakimoto, M. Imai, J. Polym. Sci. A 36 (1998) 39.
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[3] L. Litauszki, F.E. Karasz, K. Gr€ ungerg, W. Berger, J. Polym. Sci.: Part A: Polym. Chem. 30 (1992) 1227. [4] Ch.P. Yang, R.S. Chen, K.S. Hunk, Polymer 42 (2001) 4569. [5] Q. Jin, T. Yamashita, K. Horie, J. Polym. Sci. A 32 (1994) 503. [6] Z. Falcigno, S. Jasne, M. King, J. Polym. Sci. A 30 (1992) 1433. [7] T. Matsumoto, in: Polycondensation, Tokyo, Japan, 70, 2000. [8] T. Chern, H.Ch. Shiue, Macromolecules 30 (1997) 4646. [9] T. Matsumoto, T. Kurosaki, Macromolecules 30 (1997) 993. [10] E. Schab-Balcerzk, D. Sez k, A. Volozhin, T. Chamenko, B. Jarzaz bek, Eur. Pol. J., in press. [11] E.A. Davis, N.F. Mott, Philos. Mag. 22 (1970) 903. [12] J. Tauc, A. Menth, D.L. Wood, Phys. Rev. Lett. 25 (1970) 749. [13] J. Tauc, A. Menth, J. Non-Cryst. Solids 8–10 (1971) 569. [14] B. Jarzaz bek, J. Weszka, A. Burian, G. Pocztowski, Thin Solid Films 279 (1996) 204. [15] J.P. Lowe, in: Quantum Chemistry, 2nd Ed., Academic Press, Boston, MA, 1993, p. 530. [16] B. Jarzaz bek, T. Orlik, J. Cisowski, E. Schab-Balcerzak, D. Sez k, Acta Phys. Pol. A 98 (2000) 655. [17] T. Matsumoto, Macromolecules 32 (1999) 4933. [18] A. Dine-Hart, W.W. Wright, Macromol. Chem. 143 (1971) 189.
Optical properties of new aliphatic–aromatic co-polyimides B. Jarzaz bek a
a,*
, E. Schab-Balcerzak a, T. Chamenko b, D. Sez k a, J. Cisowski a, A. Volozhin b
Polish Academy of Sciences, Center of Polymer Chemistry, P.O. Box 20, 34 M. Curie–Skłodowska str., 41-819 Zabrze, Poland b Institute of Physical Organic Chemistry, National Academy of Sciences, Minsk, Byelorussia
Abstract Optical transmission of a series of aliphatic–aromatic co-polyimides have been investigated in the range 200–3000 nm. The short wavelength edge of transmission depends on the ratios of diamines (aliphatic to aromatic) and on the type of aliphatic diamine. To obtain the optical parameters the approach proposed by Tauc for amorphous semiconductors has been used, because of the similarity of the absorption edges. The values of pseudogaps were found from 1.01 to above 4 eV, while the Urbach energy changed in the range 75–800 meV. All determined parameters have been found to be related to the influence of the polymer chain structure. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 78.40.Me; 78.66Qn
1. Introduction Polyimide materials belong to the group of polymers characterized by good mechanical and dielectric properties and wide applications in the electronic industries [1]. Most extensive studies were concentrated on wholly aromatic polyimides [2], because of their high-temperature resistance. However, the processability of fully aromatic polyimides is difficult and most of them are insoluble in conventional organic solvents, significantly limiting their application [3]. Almost all fully aromatic polyimides are colored from pale yellow to dark
* Corresponding author. Tel.: +48-32 271 3424; fax: +48-32 271 0658. E-mail address: [email protected] (B. Jarzaz bek).
brown, causing strong absorption in the visible region [4,5]. These polymers cannot be used where colorlessness and transparency are required, for example, as covers for solar cells, orientation films in liquid crystal display devices and optical waveguides for communication interconnectors. To enhance transparency, to decrease the dielectric constant and simultaneously to improve solubility, aliphatic monomers can be introduced, but then the thermal stability is compromised [6]. So, in recent years many studies of aliphatic polyimides have been carried out because of their potential applications as liquid crystal orientation layers or materials with the low dielectric constant [7,8]. Using aliphatic monomers as co-monomers, for example, aliphatic diamines or cyclic dianhydrides were presented in [6,9]. These procedures allowed one to obtain a compromise between processability and thermal properties.
0022-3093/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 0 1 ) 0 1 1 3 0 - 9
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B. Jarzaz bek et al. / Journal of Non-Crystalline Solids 299–302 (2002) 1057–1061
In our work we have investigated a series of new mixed amorphous aromatic–aliphatic co-polyimides, for which details of synthesis and properties were described in [10]. The aim of our paper was to study the optical properties of these polyimides foils in the ultraviolet, visible and near infrared spectral range as well as the influence of the ratio of diamine (aromatic–aliphatic) and type of aliphatic diamine ðCH2 Þy on the transmission spectra and position of the absorption edge. To obtain the energy gap and other energies describing the absorption edge, we have applied the method proposed by Tauc for amorphous materials. Then, on the basis of these values, the influence of polymer chain structure and structural disorder on the optical properties and probable electronic transitions have been considered.
2. Experimental The investigated polyimides have been prepared using high temperature polycondensation at 190 °C for 15 h from monomers: 1; 2; 3; 4-cyclopentanetetracarboxylic dianhydride and 4; 40 -methylenebis(2; 6-diethyaniline) and different aliphatic diamine (ðCH2 Þy , y ¼ 4; 6; 7; 9; 10; 12). This reaction was carried out in protic solvent: m-cresol at the presence of 1; 4-diazabicyclo[2; 2; 2]octane, as a catalyst. Polyimides with various ratios of aromatic to aliphatic diamine have been synthesized. Polymer films (foils) have been prepared by dissolving polymers in DMF and casting on glass plates and then heating up to 200 °C in vacuum during 7 h and striping from these glass substrates. The thickness of such obtained foils changed from 40 to 200 lm. The amorphous character of these polymers was confirmed by wide angle X-ray diffraction measurements, performed in reflection mode [10]. The general chemical structure of investigated aliphatic–aromatic copolyimides is shown in Fig. 1, where one can see aromatic part with 4; 40 -methylenebis(2; 6-methyaniline) rings, aliphatic part with ðCH2 Þy methylene units and characteristic for polyimides group of atoms: In our study we used four groups of aromatic– aliphatic co-polyimides foils, where the ratios of aromatic to aliphatic parts were: 75:25, 60:40, 50:50,
Fig. 1. Chemical structure of aliphatic–aromatic co-polyimide, where x ¼ 0:4–0:75 and y ¼ 4; 6; 7; 9; 10; 12.
40:60. For each ratio, aliphatic diamines with different numbers of ðCH2 Þy groups ðy ¼ 4; 6; 7; 9; 10; 12Þ were used. Depending on the co-polymer structure the foils were transparent (colorless) to brown. The optical transmission measurements were performed at room temperature on a Beckman Acta M-IV spectrophotometer within the 200– 3000 nm range. A deuterium lamp was used as a source of ultraviolet, while tungsten lamps were used for visible and near infrared region in this spectrophotometer.
3. Results and discussion Typical transmission (T) spectra of investigated polyimides in the whole measured range are shown in Fig. 2. A sharp cutoff on the short wavelength side at 280 nm is seen for the transparent foil (2a), while the edge is much less abrupt on the transmission curve for the light brown one (2b). The ratio of aliphatic to aromatic part is 50:50 while the
Fig. 2. Typical overall transmission spectra of polyimide foils (a) transparent, (b) colored. The ratio of aliphatic to aromatic is 50:50 and the numbers of aliphatic groups are: (a) y ¼ 4, d ¼ 65 lm, (b) y ¼ 10, d ¼ 70 lm.
B. Jarzaz bek et al. / Journal of Non-Crystalline Solids 299–302 (2002) 1057–1061
Fig. 3. Absorption coefficient of two polyimide foils (the same as in Fig. 2); (a) j – transparent, (b) – colored one.
numbers of aliphatic ðCH2 Þy groups are y ¼ 4 and y ¼ 10; the films thickness is 65 and 70 lm for (a) and (b) plots, respectively. The long wavelength edge at 2750 nm is the same for all investigated polyimides and do not depend on their structure. The level of transmission usually reaches 85% for all colorless polyimides, and is distinctly lower for colored foils. In order to obtain the absorption coefficient a from the transmission data, the expression a ¼ ð1=dÞ lnð1=T Þ, where d is the foil thickness, has been used. The absorption edges and the Tauc plots, as a function of photon energy (for the same two samples as in Fig. 2) are presented in Figs. 3 and 4, respectively. The shape of both edges is very similar to the behavior proposed by Tauc for a typical amorphous semiconductor [11–13], but the level of absorption ð101 –103 cm1 Þ is lower than for amorphous, inorganic thin films, as for example in [14]. This means that the strength of absorption in the polymers studied is lower than in inorganic semiconductors, which is thought to be due to a smaller degree of bonding delocalization in polymer materials [15]. Each of the absorption edges from Fig. 3 exhibits two different exponential regions with different slopes and a saturation region for higher energy. Both exponential parts are described by the relation: a expðE=AÞ, where A becomes either EU in the high-energy exponential region or ET describing the low-energy exponential part of the absorption coefficient. The absorption
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Fig. 4. The Tauc dependence of polyimide foils (the same as in – colored one. Figs. 2 and 3); (a) j – transparent, (b)
edges for all investigated polyimides have been 2 found to follow the Tauc power law: a ðE EG Þ in the range where the photon energy is greater than EG , so this dependence was used to obtain the Tauc optical energy gaps EG , as it is shown in Fig. 4. This approach, typical for amorphous semiconductors, has been also applied by us earlier for amorphous semiladder polymer foils [16]. The values of the obtained optical parameters for all types of polyimides under study are gathered in Table 1, together with the values of the film thickness and the experimental uncertainties are also indicated. Looking at the optical parameters presented in Table 1, one may observe that most polyimides are transparent, i.e., the energy gap EG values are greater than 3.26 eV. However, for each group, with different ratio of diamines (Ar–Al), there exists one or more colored foils. Unfortunately, we cannot connect this fact with the type of aliphatic monomer or with the evenness of the y value. Also the methylene chain length does not determine the color of the investigated polyimides. The polymer color is a sensitive property which may be influenced by many different factors, as for example in [9], where air oxidation of the end group (aromatic amino group) results in colored by-products. Nevertheless, as is seen in Table 1, for all transparent foils the values of EG decrease with increasing aromatic part of diamine. The same tendency of
1060
B. Jarzaz bek et al. / Journal of Non-Crystalline Solids 299–302 (2002) 1057–1061
Table 1 Optical parameters of investigated polyimides Ar–Al
d ðlmÞ ð5 lmÞ
ET ðmeVÞ ð5 meVÞ
EU ðmeVÞ ð2 meVÞ
EG ðeVÞ ð0:01 eVÞ
y¼4
75:25 60:40 50:50 40:60
200 50 65 75
1052 844 614 1265
149 149 153 300
3.47 4.05 4.09 1.43
y¼6
75:25 60:40 50:50 40:60
105 55 45 90
748 587 1004 935
74 200 122 162
3.86 3.88 4.11 4.02
y¼7
75:25 60:40 50:50 40:60
85 50 65 50
1820 496 1007 1101
418 131 167 747
1.01 4.03 4.05 3.11
y¼9
75:25 60:40 50:50 40:60
130 40 65 50
1359 738 1066 747
511 149 145 120
2.23 4.03 4.11 4.09
y ¼ 10
75:25 60:40 50:50 40:60
70 35 70 45
833 686 1220 774
104 163 495 133
3.79 4.02 1.42 4.11
y ¼ 12
75:25 60:40 50:50 40:60
160 35 60 60
1021 1189 687 1189
83 444 142 818
3.75 3.00 4.14 2.50
*
Colored foils.
the energy gap is not very different for colored foils. This means, that a higher amount of aromatic monomer influences the intensity of the polymer color, up to deep brown, what is also shown in [4,5,9,17]. In general, the coloration of an aromatic polymer is due to its conjugated aromatic structure and/or the intermolecular and intramolecular charge-transfer complex (CTC) formation. In our polyimides, it may be the formation between alternating electron-donor (diamine) and electronacceptor (dianhydride) moieties, like in [18]. Using aliphatic monomers as co-monomers seems to be reasonable to reduce the chain–chain interaction and disrupt interaction between aromatic moiety, because aliphatic diamines are more flexible and then the transparency increases. However the ratio of diamines seems not to influence the EU and ET values both for the transparent foils and the colored ones, but these two parameters are distinctly
higher for colored foils. The values of the Urbach energy are in the range 74–200 meV for transparent polyimides, while for colored foils they increase from 300 to about 800 meV. Also the ET energies for colorless polyimide foils are lower than for colored ones, for which ET is greater than 1100 meV. These two parameters are related to the localized states induced by the polymer atomic structure. Such possible structure defects like the break, the abbreviation or the torsion of polymer chains seem to be responsible for low-energy absorption, described by the ET parameter.
4. Conclusions New amorphous aliphatic–aromatic polyimides, with different ratio of aliphatic to aromatic parts and different types of aliphatic diamine, seem to be
B. Jarzaz bek et al. / Journal of Non-Crystalline Solids 299–302 (2002) 1057–1061
very interesting materials, because of their optical properties. Most of these flexible foils are transparent, with transmission above 85% in the whole visible spectral range. A few of them are colored from pale yellow to brown and we have not found any detailed explanation of this fact based on their chemical structure. The absorption edges of all investigated foils were found to be similar to the typical edge for amorphous semiconductors, allowing us to obtain the optical parameters. Increasing the aromatic part of polyimides caused a decrease of the energy gap, both for transparent and color foils. The formation of CTC seems to be responsible for this tendency. Larger structural disorder in color polyimides may cause an increase of the Urbach energy and of the parameter ET describing the low-energy part of the absorption edge.
References [1] Y.T. Chern, J. Polym. Sci. A 34 (1966) 125. [2] M. Goyal, M. Inoue, T. Kakimoto, M. Imai, J. Polym. Sci. A 36 (1998) 39.
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[3] L. Litauszki, F.E. Karasz, K. Gr€ ungerg, W. Berger, J. Polym. Sci.: Part A: Polym. Chem. 30 (1992) 1227. [4] Ch.P. Yang, R.S. Chen, K.S. Hunk, Polymer 42 (2001) 4569. [5] Q. Jin, T. Yamashita, K. Horie, J. Polym. Sci. A 32 (1994) 503. [6] Z. Falcigno, S. Jasne, M. King, J. Polym. Sci. A 30 (1992) 1433. [7] T. Matsumoto, in: Polycondensation, Tokyo, Japan, 70, 2000. [8] T. Chern, H.Ch. Shiue, Macromolecules 30 (1997) 4646. [9] T. Matsumoto, T. Kurosaki, Macromolecules 30 (1997) 993. [10] E. Schab-Balcerzk, D. Sez k, A. Volozhin, T. Chamenko, B. Jarzaz bek, Eur. Pol. J., in press. [11] E.A. Davis, N.F. Mott, Philos. Mag. 22 (1970) 903. [12] J. Tauc, A. Menth, D.L. Wood, Phys. Rev. Lett. 25 (1970) 749. [13] J. Tauc, A. Menth, J. Non-Cryst. Solids 8–10 (1971) 569. [14] B. Jarzaz bek, J. Weszka, A. Burian, G. Pocztowski, Thin Solid Films 279 (1996) 204. [15] J.P. Lowe, in: Quantum Chemistry, 2nd Ed., Academic Press, Boston, MA, 1993, p. 530. [16] B. Jarzaz bek, T. Orlik, J. Cisowski, E. Schab-Balcerzak, D. Sez k, Acta Phys. Pol. A 98 (2000) 655. [17] T. Matsumoto, Macromolecules 32 (1999) 4933. [18] A. Dine-Hart, W.W. Wright, Macromol. Chem. 143 (1971) 189.