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Polymethylmethacrylate optical waveguides prepared in electrical field
Materials Letters 61 (2007) 953 – 955 www.elsevier.com/locate/matlet
Polymethylmethacrylate optical waveguides prepared in electrical field V. Švorčík ⁎, I. Huttel, P. Paláček Department of Solid State Engineering, Institute of Chemical Technology, 166 28 Prague, Czech Republic Received 28 February 2006; accepted 10 June 2006 Available online 30 June 2006
Abstract Thin layers based on polymethylmethacrylate (PMMA) were prepared by spin-coating method with and without electrical field and the effect of the field on refractive index n and waveguide properties of the layers was investigated. Application of the field leads to an increase of refractive index due to dipole orientation. Structure comprising two PMMA based layers, about 5 μm thick, was prepared exhibiting waveguiding properties. © 2006 Elsevier B.V. All rights reserved. Keywords: Polymer; Thin film; Refractive index; Waveguide properties
1. Introduction The rapid expansion of optical telecommunication technology increases the need for planar optical amplifiers that can be used to compensate the losses in splitters, multiplexers, switches, and other devices [1]. “Optical” polymers can be highly transparent at all the key communication wavelengths (633, 840, 1310 and 1550 nm). There are many methods to fabricate polymer waveguides, which include photocrosslinking, photobleaching, reactive ion etching, photolocking and laser/electron beam writing. There are many replication processes that are simple and easy to do for fabrication and can be used for mass productivity, such as hot embossing, UV-embossing, and micro-transfer molding method. But these methods have some problems to overcome, namely limited substrate and core material that can be used [2–7]. Polymeric layers for optical waveguide amplifiers were doped e.g. with rare earth compounds (Eu chelate) [8,9]. Semiconductors are often used as a support of the waveguiding structure. The adverse effect of their relatively high refractory index is eliminated by the layer of a skin dielectric, in the case of silicon by SiO2. It is also known that a DC electrical field may orient organic dipoles added to thin polymeric layers [10,11]. The orientation takes place in the course of layer preparation or at temperatures close to the temperature of glassy transition [10]. As a result of the dipole orientation in polymer based composites an ⁎ Corresponding author. Tel.: +420 22044 5149; fax: +420 220444330. E-mail address: [email protected] (V. Švorčík). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.06.022
increase of relative permittivity or refractive index [10,11] was observed. In this work the dipole orientation by electrical field during spin-coating preparation of thin film of polymethylmethacrylate (PMMA) is studied. The effects of electrical field on refractive index and waveguiding properties such as a number of modes and attenuation are examined. 2. Experimental Present experiments were performed on polymethylmethacrylate (PMMA) in optical purity supplied by Goodfellow. The glass transition temperatures of PMMA, determined by standard calorimetric method using DSC 2920 technique, was Tg = 112 °C, refractive index 1.49 (for λ = 632.8 nm) [12] to 1.50 (λ = 589.3 and 632.8 nm) [13], density 1.2 g cm− 3. For the experiments 100 ± 20 nm and 3.0 ± 0.3 μm thick films of pristine polymer were prepared by spin-coating method (750 and 1500 rpm) onto a silicon (crystallographic orientation (100), resistance 0.002 Ω cm, refractive index n = 3.505) and Si/SiO2 (nSiO2 = 1.390) substrates. The layers were prepared from 1 and 5 wt.% solution of PMMA in chloroform and toluene. The films were prepared without or with the assistance of a DC (directcurrent) electrical field. The electrical field 0–20 kV cm− 1 was applied during the film preparation on centrifuge. For stronger fields local discharges occur in the space filled with solvent vapors. Thin layers were intended for refractometry, thicker ones for possibility to couple more modes and to measure refractive
954
V. Švorčík et al. / Materials Letters 61 (2007) 953–955
Fig. 1. Refractive index as a function of electrical field strength applied during spin-coating preparation of PMMA layer 105 ± 20 nm thick.
index with higher precision. The film thickness was measured by a profilometer Talystep. Refractive index of films was determined in the spectral region of 250–750 nm using refractometer Avaspec 2048 as a mean from 6 independent measurements. With the aid of computer code AvaSoft Full 6.1, including code Spectra 3, the dependence of the refractive index (n) on the wavelength (λ) for films deposited on substrate was found. In this way the refractive index n, extrapolated to infinite wavelength, was determined. Optical parameters of thin films (refractive index, layer thickness, attenuation, mode number) were studied with using mode spectroscopy and measurement of attenuation of transmitted modes. The measurement was accomplished using glass prism with refractive index of n = 1.7. Refractive index of the prism should be greater than refractive index of wave guiding film in order to ensure light good coupling into the waveguide. As a light source a He–Ne laser with wavelength of λ = 632.8 nm was employed. 3. Results and discussion In Fig. 1 the dependence of refractive index on the electrical field strength for PMMA layer 105 ± 20 nm thick is shown. The refractive
Fig. 3. Dependence of refractive index on the time elapsed from the beginning of PMMA layer (105 ± 20 nm thick) preparation by spin-coating and the moment of application of electrical field with 8 kV cm− 1 strength.
index of pristine PMMA is n = 1.411 ± 0.015 and it increases with increasing field strength. The highest refractive index of n = 1.473 ± 0.017 was observed for 8 kV cm− 1 electrical field. Another electrical field increase does not affect the refractive index. Spectral dependence of the refractive index for pristine PMMA and PMMA film prepared under 8 kV cm− 1 field is shown in Fig. 2. It is seen that for wavelengths above 350 nm the film prepared with the assistance of electrical field exhibits higher refractive index in comparison to the film prepared without the field. From the depicted spectral dependence the above mentioned values of the refractive index were obtained by extrapolation to infinite wavelength. A waveguiding structure usually consists of two layers with different refractive indexes deposited on a substrate. The upper layer (core) has to have higher refractive index comparing to the layer in contact with the substrate (cladding). One attempt to prepare such two layer structure is documented in Fig. 3. In this case the electrical field was applied with some delay after beginning of the structure preparation. It is seen that with increasing delay the “effective” refractive index declines. It may be supposed that with increasing delay and due to solvent gradual evaporation the thickness of the bottom, unoriented layer with lower refractive index increases so that the total “effective” refractive index of the structure declines. For delays above 120 s no significant additional change of the refractive index is observed (see Fig. 3). The lowest value of the refractive index n = 1.432 is still higher than that of pristine PMMA n = 1.411. So that even for a delay of 150 s the electrical field can orient dipoles and in turn elevate the refractive index. As a substrate for preparation of waveguiding structure based on PMMA was chosen SiO2 layer on Si substrate because of favourable ratio of refractive indexes (nSiO2 < nPMMA). Basic parameters of the structures deposited onto this substrate with and without assistance of Table 1 Thicknesses, refractive indexes, attenuation and number of observed modes measured at 632.8 nm wavelength on PMMA layers deposited with and without the assistance of DC electrical field of 8 kV cm− 1 strength onto Si/SiO2 (nSiO2 = 1.390) substrate
Fig. 2. Spectral dependence of refractive index of PMMA layer (105 ± 20 nm thick) prepared without electrical field (PMMA/0) and with electrical field of 8 kV cm− 1 (PMMA/8).
Quantity
No electrical field
Electrical field
Refractive index Thickness (ìm) Attenuation (dB cm− 1) Mode number
1.482 ± 0.003 3.251 ± 0.390 1st vid 0.07; 6th vid 0.20
1.499 ± 0.006 3.310 ± 0.352 1st vid − 1.44; 6th vid − 2.48
6
6
V. Švorčík et al. / Materials Letters 61 (2007) 953–955 Table 2 Thicknesses, refractive indexes, attenuation and number of observed modes measured on double layered PMMA structure prepared on Si substrate Quantity
Cladding layer
Core layer
Refractive index Thickness (μm) Attenuation (dB cm− 1) Mode number
1.482 ± 0.003 1.895 ± 0.009 – –
1.507 ± 0.007 3.526 ± 0.189 1st vid − 1.54; 6th vid − 2.55 6
Cladding layer was prepared without the assistance of electrical field and core layer under the electrical field with strength of 8 kV cm− 1. The measurement was accomplished at 632.8 nm wavelength.
955
• on PMMA layers about 3 μm thick, prepared with and without assistance of electrical filed, optical modes were identified, • double layered, PMMA based structure was prepared exhibiting waveguiding properties. The above results prove that the double layer was prepared based on polar polymers with different refractory indexes. This is a basic requirement on cladding layer and core in the construction of planar optical waveguides. Acknowledgements
electrical field are summarized in Table 1. With knowledge of refractive index of silicon oxide (nSiO2) the layer thickness and refractive index of deposited PMMA were determined by means of mode spectroscopy. It can be seen from Table 1 that the assistance of electrical field results in an increase of refractive index as well as of attenuation in PMMA layer. In the layers prepared with and without electrical field 6 modes were identified. Simultaneous increase of refractive index and layer attenuation after application of electrical field is probably due to better orientation and higher arrangement of dipoles in the layer. From comparison of Table 1 data with Fig. 1 a significant difference between refractive indexes of thin (about 100 nm) and “thick” (about 3 μm) layers are seen. The measured refractive index of thin layers depends on their thickness [14] and the refractive index of “thick” layers is in agreement with published ones [15]. In Table 2 the basic characteristics of double layered PMMA are summarized. The structure was prepared on Si substrate using the following procedure. Bottom layer (cladding) was deposited from PMMA solution in chloroform without an assistance of electrical field (see also Table 1). Then next layer (core) was deposited under electrical field from PMMA solution in toluene. Since the refractive index of the cladding layer is well known, the thickness and refractive index of the core layer can be determined by mode spectroscopy (see Table 2). Since the core layer has higher refractive index the structure exhibits waveguiding properties. Six modes were identified and the attenuation is comparable with that of SiO2/PMMA structure (Table 1). It can be supposed that during the preparation of the core layer the cladding layer, deposited before, could be partially etched. Initial thickness of the cladding layer was 3.251 μm (Table 1) and after deposition of 3.526 μm thick core layer (Table 2) a total thickness measured by profilometer was 5.421 μm. From these data the thickness of the cladding layer on Si was calculated (Table 2). From comparison of data of Tables 1 and 2 it may be concluded that the partial etching of the cladding layer during preparation of the double layered system does not affect the system attenuation significantly.
4. Conclusion The results can briefly be summarized as follows: • DC electrical field applied during the preparation of PMMA based structures increases the refractive index of both thin (100 nm) and thick (3 μm) PMMA films,
The work was supported by the Grant Agency of the CR under the project Nos. 104-03-0385 and 102-06-0424, by Grant Agency of the AS CR under the project A 5011301 and Ministry of Education of the CR under Research program Nos. MSM 6046137302 and LC 06041. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
H. Ma, A.K.Y. Jen, L.R. Dalton, Adv. Mater. 14 (2002) 1339. A. Becker, K. Holger, W. Dietz, SPIE 3515 (1998) 177. B. Ferm, M. Paul, W. Shackjette, SPIE 4106 (2000) 1. C. Paul, E. Materi, L. Breen, J. Aizenberg, G.M. Whitesides, Appl. Phys. Lett. 73 (1998) 2893. D. Maruo, Y.Y. Bauer, H.D.W. Ehrfeld, M. Harder, T. Paatzsch, M. Popp, F. Smaglinski, Synth. Met. 115 (2000) 13. L. Eldada, L.W. Shacklette, IEEE J. Sel. Top. Quantum Electron. 6 (2000) 54. H.T. Kim, S. Lee, S. Kim, Y. Kwon, K.S. Seo, Electron. Lett. 39 (2003) 1661. H. Liang, Z. Zheng, Q. Zhang, H. Ming, B. Chen, J. Xie, H. Zhao, J. Mater. Res. 18 (2003) 1895. H. Liang, X. Sun, Q. Zhang, H. Ming, H. Ming, J. Cao, Z. Li, B. Chen, J. Xu, H. Zhao, J. Mater. Res. 19 (2004) 256. V. Švorčík, R. Gardášová, V. Rybka, V. Hnatowicz, J. Plešek, J. Appl. Polym. Sci. 91 (2004) 40. V. Švorčík, M. Prajer, I. Huttel, V. Rybka, J. Plešek, Mater. Lett. 59 (2005) 280. A. Tanaka, A. Takahara, Macromolecules 28 (1995) 934. S. Prosycevas, S. Tamulevicius, Thin Solid Films 453–454 (2004) 304. V. Švorčík, H. Tichá, V. Rybka, V. Hnatowicz, J. Mater. Sci. Lett. 19 (2000) 1679. D.W. van Krevelen, Properties of Polymers, Elsevier, Amsterdam, 1991.
Polymethylmethacrylate optical waveguides prepared in electrical field V. Švorčík ⁎, I. Huttel, P. Paláček Department of Solid State Engineering, Institute of Chemical Technology, 166 28 Prague, Czech Republic Received 28 February 2006; accepted 10 June 2006 Available online 30 June 2006
Abstract Thin layers based on polymethylmethacrylate (PMMA) were prepared by spin-coating method with and without electrical field and the effect of the field on refractive index n and waveguide properties of the layers was investigated. Application of the field leads to an increase of refractive index due to dipole orientation. Structure comprising two PMMA based layers, about 5 μm thick, was prepared exhibiting waveguiding properties. © 2006 Elsevier B.V. All rights reserved. Keywords: Polymer; Thin film; Refractive index; Waveguide properties
1. Introduction The rapid expansion of optical telecommunication technology increases the need for planar optical amplifiers that can be used to compensate the losses in splitters, multiplexers, switches, and other devices [1]. “Optical” polymers can be highly transparent at all the key communication wavelengths (633, 840, 1310 and 1550 nm). There are many methods to fabricate polymer waveguides, which include photocrosslinking, photobleaching, reactive ion etching, photolocking and laser/electron beam writing. There are many replication processes that are simple and easy to do for fabrication and can be used for mass productivity, such as hot embossing, UV-embossing, and micro-transfer molding method. But these methods have some problems to overcome, namely limited substrate and core material that can be used [2–7]. Polymeric layers for optical waveguide amplifiers were doped e.g. with rare earth compounds (Eu chelate) [8,9]. Semiconductors are often used as a support of the waveguiding structure. The adverse effect of their relatively high refractory index is eliminated by the layer of a skin dielectric, in the case of silicon by SiO2. It is also known that a DC electrical field may orient organic dipoles added to thin polymeric layers [10,11]. The orientation takes place in the course of layer preparation or at temperatures close to the temperature of glassy transition [10]. As a result of the dipole orientation in polymer based composites an ⁎ Corresponding author. Tel.: +420 22044 5149; fax: +420 220444330. E-mail address: [email protected] (V. Švorčík). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.06.022
increase of relative permittivity or refractive index [10,11] was observed. In this work the dipole orientation by electrical field during spin-coating preparation of thin film of polymethylmethacrylate (PMMA) is studied. The effects of electrical field on refractive index and waveguiding properties such as a number of modes and attenuation are examined. 2. Experimental Present experiments were performed on polymethylmethacrylate (PMMA) in optical purity supplied by Goodfellow. The glass transition temperatures of PMMA, determined by standard calorimetric method using DSC 2920 technique, was Tg = 112 °C, refractive index 1.49 (for λ = 632.8 nm) [12] to 1.50 (λ = 589.3 and 632.8 nm) [13], density 1.2 g cm− 3. For the experiments 100 ± 20 nm and 3.0 ± 0.3 μm thick films of pristine polymer were prepared by spin-coating method (750 and 1500 rpm) onto a silicon (crystallographic orientation (100), resistance 0.002 Ω cm, refractive index n = 3.505) and Si/SiO2 (nSiO2 = 1.390) substrates. The layers were prepared from 1 and 5 wt.% solution of PMMA in chloroform and toluene. The films were prepared without or with the assistance of a DC (directcurrent) electrical field. The electrical field 0–20 kV cm− 1 was applied during the film preparation on centrifuge. For stronger fields local discharges occur in the space filled with solvent vapors. Thin layers were intended for refractometry, thicker ones for possibility to couple more modes and to measure refractive
954
V. Švorčík et al. / Materials Letters 61 (2007) 953–955
Fig. 1. Refractive index as a function of electrical field strength applied during spin-coating preparation of PMMA layer 105 ± 20 nm thick.
index with higher precision. The film thickness was measured by a profilometer Talystep. Refractive index of films was determined in the spectral region of 250–750 nm using refractometer Avaspec 2048 as a mean from 6 independent measurements. With the aid of computer code AvaSoft Full 6.1, including code Spectra 3, the dependence of the refractive index (n) on the wavelength (λ) for films deposited on substrate was found. In this way the refractive index n, extrapolated to infinite wavelength, was determined. Optical parameters of thin films (refractive index, layer thickness, attenuation, mode number) were studied with using mode spectroscopy and measurement of attenuation of transmitted modes. The measurement was accomplished using glass prism with refractive index of n = 1.7. Refractive index of the prism should be greater than refractive index of wave guiding film in order to ensure light good coupling into the waveguide. As a light source a He–Ne laser with wavelength of λ = 632.8 nm was employed. 3. Results and discussion In Fig. 1 the dependence of refractive index on the electrical field strength for PMMA layer 105 ± 20 nm thick is shown. The refractive
Fig. 3. Dependence of refractive index on the time elapsed from the beginning of PMMA layer (105 ± 20 nm thick) preparation by spin-coating and the moment of application of electrical field with 8 kV cm− 1 strength.
index of pristine PMMA is n = 1.411 ± 0.015 and it increases with increasing field strength. The highest refractive index of n = 1.473 ± 0.017 was observed for 8 kV cm− 1 electrical field. Another electrical field increase does not affect the refractive index. Spectral dependence of the refractive index for pristine PMMA and PMMA film prepared under 8 kV cm− 1 field is shown in Fig. 2. It is seen that for wavelengths above 350 nm the film prepared with the assistance of electrical field exhibits higher refractive index in comparison to the film prepared without the field. From the depicted spectral dependence the above mentioned values of the refractive index were obtained by extrapolation to infinite wavelength. A waveguiding structure usually consists of two layers with different refractive indexes deposited on a substrate. The upper layer (core) has to have higher refractive index comparing to the layer in contact with the substrate (cladding). One attempt to prepare such two layer structure is documented in Fig. 3. In this case the electrical field was applied with some delay after beginning of the structure preparation. It is seen that with increasing delay the “effective” refractive index declines. It may be supposed that with increasing delay and due to solvent gradual evaporation the thickness of the bottom, unoriented layer with lower refractive index increases so that the total “effective” refractive index of the structure declines. For delays above 120 s no significant additional change of the refractive index is observed (see Fig. 3). The lowest value of the refractive index n = 1.432 is still higher than that of pristine PMMA n = 1.411. So that even for a delay of 150 s the electrical field can orient dipoles and in turn elevate the refractive index. As a substrate for preparation of waveguiding structure based on PMMA was chosen SiO2 layer on Si substrate because of favourable ratio of refractive indexes (nSiO2 < nPMMA). Basic parameters of the structures deposited onto this substrate with and without assistance of Table 1 Thicknesses, refractive indexes, attenuation and number of observed modes measured at 632.8 nm wavelength on PMMA layers deposited with and without the assistance of DC electrical field of 8 kV cm− 1 strength onto Si/SiO2 (nSiO2 = 1.390) substrate
Fig. 2. Spectral dependence of refractive index of PMMA layer (105 ± 20 nm thick) prepared without electrical field (PMMA/0) and with electrical field of 8 kV cm− 1 (PMMA/8).
Quantity
No electrical field
Electrical field
Refractive index Thickness (ìm) Attenuation (dB cm− 1) Mode number
1.482 ± 0.003 3.251 ± 0.390 1st vid 0.07; 6th vid 0.20
1.499 ± 0.006 3.310 ± 0.352 1st vid − 1.44; 6th vid − 2.48
6
6
V. Švorčík et al. / Materials Letters 61 (2007) 953–955 Table 2 Thicknesses, refractive indexes, attenuation and number of observed modes measured on double layered PMMA structure prepared on Si substrate Quantity
Cladding layer
Core layer
Refractive index Thickness (μm) Attenuation (dB cm− 1) Mode number
1.482 ± 0.003 1.895 ± 0.009 – –
1.507 ± 0.007 3.526 ± 0.189 1st vid − 1.54; 6th vid − 2.55 6
Cladding layer was prepared without the assistance of electrical field and core layer under the electrical field with strength of 8 kV cm− 1. The measurement was accomplished at 632.8 nm wavelength.
955
• on PMMA layers about 3 μm thick, prepared with and without assistance of electrical filed, optical modes were identified, • double layered, PMMA based structure was prepared exhibiting waveguiding properties. The above results prove that the double layer was prepared based on polar polymers with different refractory indexes. This is a basic requirement on cladding layer and core in the construction of planar optical waveguides. Acknowledgements
electrical field are summarized in Table 1. With knowledge of refractive index of silicon oxide (nSiO2) the layer thickness and refractive index of deposited PMMA were determined by means of mode spectroscopy. It can be seen from Table 1 that the assistance of electrical field results in an increase of refractive index as well as of attenuation in PMMA layer. In the layers prepared with and without electrical field 6 modes were identified. Simultaneous increase of refractive index and layer attenuation after application of electrical field is probably due to better orientation and higher arrangement of dipoles in the layer. From comparison of Table 1 data with Fig. 1 a significant difference between refractive indexes of thin (about 100 nm) and “thick” (about 3 μm) layers are seen. The measured refractive index of thin layers depends on their thickness [14] and the refractive index of “thick” layers is in agreement with published ones [15]. In Table 2 the basic characteristics of double layered PMMA are summarized. The structure was prepared on Si substrate using the following procedure. Bottom layer (cladding) was deposited from PMMA solution in chloroform without an assistance of electrical field (see also Table 1). Then next layer (core) was deposited under electrical field from PMMA solution in toluene. Since the refractive index of the cladding layer is well known, the thickness and refractive index of the core layer can be determined by mode spectroscopy (see Table 2). Since the core layer has higher refractive index the structure exhibits waveguiding properties. Six modes were identified and the attenuation is comparable with that of SiO2/PMMA structure (Table 1). It can be supposed that during the preparation of the core layer the cladding layer, deposited before, could be partially etched. Initial thickness of the cladding layer was 3.251 μm (Table 1) and after deposition of 3.526 μm thick core layer (Table 2) a total thickness measured by profilometer was 5.421 μm. From these data the thickness of the cladding layer on Si was calculated (Table 2). From comparison of data of Tables 1 and 2 it may be concluded that the partial etching of the cladding layer during preparation of the double layered system does not affect the system attenuation significantly.
4. Conclusion The results can briefly be summarized as follows: • DC electrical field applied during the preparation of PMMA based structures increases the refractive index of both thin (100 nm) and thick (3 μm) PMMA films,
The work was supported by the Grant Agency of the CR under the project Nos. 104-03-0385 and 102-06-0424, by Grant Agency of the AS CR under the project A 5011301 and Ministry of Education of the CR under Research program Nos. MSM 6046137302 and LC 06041. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
H. Ma, A.K.Y. Jen, L.R. Dalton, Adv. Mater. 14 (2002) 1339. A. Becker, K. Holger, W. Dietz, SPIE 3515 (1998) 177. B. Ferm, M. Paul, W. Shackjette, SPIE 4106 (2000) 1. C. Paul, E. Materi, L. Breen, J. Aizenberg, G.M. Whitesides, Appl. Phys. Lett. 73 (1998) 2893. D. Maruo, Y.Y. Bauer, H.D.W. Ehrfeld, M. Harder, T. Paatzsch, M. Popp, F. Smaglinski, Synth. Met. 115 (2000) 13. L. Eldada, L.W. Shacklette, IEEE J. Sel. Top. Quantum Electron. 6 (2000) 54. H.T. Kim, S. Lee, S. Kim, Y. Kwon, K.S. Seo, Electron. Lett. 39 (2003) 1661. H. Liang, Z. Zheng, Q. Zhang, H. Ming, B. Chen, J. Xie, H. Zhao, J. Mater. Res. 18 (2003) 1895. H. Liang, X. Sun, Q. Zhang, H. Ming, H. Ming, J. Cao, Z. Li, B. Chen, J. Xu, H. Zhao, J. Mater. Res. 19 (2004) 256. V. Švorčík, R. Gardášová, V. Rybka, V. Hnatowicz, J. Plešek, J. Appl. Polym. Sci. 91 (2004) 40. V. Švorčík, M. Prajer, I. Huttel, V. Rybka, J. Plešek, Mater. Lett. 59 (2005) 280. A. Tanaka, A. Takahara, Macromolecules 28 (1995) 934. S. Prosycevas, S. Tamulevicius, Thin Solid Films 453–454 (2004) 304. V. Švorčík, H. Tichá, V. Rybka, V. Hnatowicz, J. Mater. Sci. Lett. 19 (2000) 1679. D.W. van Krevelen, Properties of Polymers, Elsevier, Amsterdam, 1991.