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Raman studies of polyacetylene prepared by the Naarmann method
Synthetic Metals, 32 (1989) 253 - 256
253
Short Communication
Raman Studies of Polyacetylene Prepared by the Naarmann Method Y. FURUKAWA, Y. IMAI and M. TASUMI* Department of Chemistry, Faculty of Science, The University of Tokyo, Bunkyo-ku, Tokyo 113 (Japan) (Received April 14, 1989; accepted June 1, 1989)
Introduction Recently Naarmann and Theophilou [1] reported a m e t h o d of preparing polyacetylene (PA) which, upon doping, showed an electrical conductivity so high t h a t it is comparable to t h a t of iron. As far as we know however, it is n o t clear whether the molecular structure of the Naarmann PA is different from that of widely used PA which is obtained by the Shirakawa m e t h o d [2, 3]. To obtain information on this point, we have studied the Raman and infrared spectra of PA obtained by a m e t h o d based on that of Naarmann and Theophilou.
Experimental Polymerization of acetylene was carried out by a procedure which we believe is in line with that adopted by Naarmann and Theophilou [1]. Al(Et)3 and Ti(O-n-Bu)4 were mixed in silicone oil at about 40 °C under an argon atmosphere, and then kept at 120 °C for 2 h. The catalyst thus obtained was transferred to a vessel for polymerization, which was kept at about --78 °C with a mixture of dry ice and acetone. Acetylene was introduced into the vessel to an initial pressure of 200 Torr. A film of PA (cis c o n t e n t ~95%, thickness ~ 1 0 Izm) was obtained in 20 rain after the initiation of polymerization. The film was thoroughly washed with toluene and immersed in a concentrated alkaline solution for 5 rain to remove the remaining silicone oil. The density of the PA film thus obtained was 0.8 ~i cm-3. The as-polymerized cis-rich PA was thermally isomerized to the transrich PA under various conditions.
*Author to whom correspondence should be addressed. 0379-6779/89/$3.50
© Elsevier Sequoia/Printed in The Netherlands
254 Doping of the PA film was performed with gaseous I2 at room temperature. The electrical conductivity of the doped film was measured by the standard four-probe technique. Raman spectra were observed from the outer surface (facing the inner side of the vessel for polymerization) of the PA film at liquid-nitrogen temperature using the 457.9 and 514.5 nm lines of an Ar ion laser and the 632.8 nm line of a He-Ne laser for excitation. A Raman spectrometer consisting of a Spex 1877 Triplemate and an EG & G PAR 1421 intensified photodiode array detector was used. To minimize the effects of laser irradiation on the sample, the laser beam was used at low power (20 mW for the 457.9 and 632.8 nm lines and 30 mW for the 514.5 nm line), and the time of exposure for measuring one spectrum was limited to shorter than 10 min.
Results and discussion The electrical conductivity of the iodine-doped unstretched cis-PA film obtained in this study was 1800 S cm -1, which is comparable to the value reported by Naarmann and Theophilou [1]. The electrical conductivities of iodine-doped trans-PA films varied widely from 20 to 880 S cm -1, depending on the isomerization conditions. The Raman spectra of the as-polymerized cis-PA film observed with 457.9, 514.5 and 632.8 nm excitation are shown in Fig. 1. Apart from the changes of the bands due to the coexisting trans part, the Raman spectra of the cis-PA depend only slightly on the excitation wavelength, in accordance with the results previously reported for cis-PA [4, 5] obtained by the Shirakawa method. The peak wavenumbers in Fig. 1 coincide with the values in the literature within the limits of experimental error. Little difference is observed between the infrared spectrum (not shown) of the cis-PA obtained in this study and that reported previously [ 2]. These results indicate that the cis-PA obtained in this study is practically identical to the cis-PA prepared by the Shirakawa m e t h o d with respect to the length of ~r-electron conjugation. In Fig. 2 are shown the 457.9 nm excited Raman spectra of trans-PA films prepared under various isomerization conditions (see the caption). The bands at 1503 - 1499 cm -1 (whose intensities are designated as Is) and those at 1 1 2 7 - 1 1 2 1 cm -1 (Is') represent shorter trans-conjugated segments consisting of approximately 15 C=C bonds; the bands at 1461 - 1456 cm -1 (IL) and those at 1068 - 1064 cm -I (IL') arise from much longer segments [6]. It is evident in Fig. 2 that the relative intensities IL/Is and IL'/I s' greatly depend on the isomerization conditions. Therefore, the two parameters [6] R = IL/(IL + Is) and R ' = IL'/(IL' + IS') may be used as measures of the content of the longer segments. Values of R and R' calculated for the sample (isomerization temperature 190 °C, period 15 min) giving rise to the spectrum in Fig. 2(e) are 0.47 and 0.48, respectively. These values are slightly but definitely lower than the values (0.53 and 0.55) [6] obtained
255
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Fig. 1. Raman spectra of the as-polymerized cis-polyacetylene film. Shadowed bands are due to the coexisting trans-part. Excitation wavelength: (a) 457.9 nm; (b) 514.5 nm; (c) 632.8 nm. Fig. 2. Raman spectra of the trans-polyacetylene films obtained by isomerizing the cis-polyacetylene under different conditions. Isomerization temperature and period (electrical conductivity upon full doping in parentheses): (a) 230 °C, 60 min (20 S cm-1); (b) 190 °(3, 60 rain (300 S cm-1); (c) 160 °C, 60 rain (460 S cm-1); (d) 230 °C, 15 rain (290 S cm-1); (e) 190 °C, 15 min (880 S cm-1). Excitation wavelength, 4 5 7 . 9 nm.
for the Shirakawa PA under the same isomerization conditions. This probably means that there is some difference in the packing of chains between the cis-PA obtained in this study and the Shirakawa cis-PA. It has been reported [6] that the electrical conductivity of fully d o p e d Shirawawa trans-PA is proportional to the R or R' value of the same transPA before doping. The five trans-PA films giving rise to the Raman spectra in Figs. 2(a)- (e) show very different electrical conductivities after full doping. The plot of the observed electrical conductivities versus R and R' is shown in Fig. 3, where the relationship [6] reported for the Shirakawa trans-PA is also indicated by a broken line. Clearly, the electrical conductivity increases with increasing R or R' value. However, the doped trans-PA obtained in this study shows an electrical conductivity a b o u t ten times higher than that of the doped Shirakawa trans-PA at the same R or R' value of a b o u t 0.5. Thus, although a high content of long trans-conjugated segments increases the electrical conductivity upon doping, there are some other factors which are more effective in determining the electrical conductivity u p o n doping. In conclusion, the observations described above seem to suggest that intermolecular interactions (e.g., the packing of chains and the form of
256 10oo
>,.
i
AO
O
o
A
0.3
0,4
0.5
R , R'
Fig. 3. Plots of the observed electrical conductivities of the trans-polyacetylene films fully doped with iodine: (o) vs. R and (A) vs. R'.
fibrils) rather than the molecular structure are responsible for the high electrical conductivities of the polyacetylene samples prepared in this study.
References H. Naarmann and N. Theophilou, Synth. Met., 22 (1987) 1. H. Shirakawa and S. Ikeda, Polym. J., 2 (1971) 231. T. Ito, H. Shirakawa and S. Ikeda, J. Polym. Sci., Polym. Chem. Ed., 12 (1974) 11. I. Harada, M. Tasumi, H. Shirakawa and S. Ikeda, Chem. Lett., (1978) 1411. L. S. Lichtmann, E. A. Imhoff, A. Sarhangi and D. B. Fitchen, J. Chem. Phys., 81 (1984) 168. 6 T. Arakawa, Y. Furukawa, H. Takeuchi, I. Harada and H. Shirakawa, Chem. Lett., (1984) 1637.
1 2 3 4 5
253
Short Communication
Raman Studies of Polyacetylene Prepared by the Naarmann Method Y. FURUKAWA, Y. IMAI and M. TASUMI* Department of Chemistry, Faculty of Science, The University of Tokyo, Bunkyo-ku, Tokyo 113 (Japan) (Received April 14, 1989; accepted June 1, 1989)
Introduction Recently Naarmann and Theophilou [1] reported a m e t h o d of preparing polyacetylene (PA) which, upon doping, showed an electrical conductivity so high t h a t it is comparable to t h a t of iron. As far as we know however, it is n o t clear whether the molecular structure of the Naarmann PA is different from that of widely used PA which is obtained by the Shirakawa m e t h o d [2, 3]. To obtain information on this point, we have studied the Raman and infrared spectra of PA obtained by a m e t h o d based on that of Naarmann and Theophilou.
Experimental Polymerization of acetylene was carried out by a procedure which we believe is in line with that adopted by Naarmann and Theophilou [1]. Al(Et)3 and Ti(O-n-Bu)4 were mixed in silicone oil at about 40 °C under an argon atmosphere, and then kept at 120 °C for 2 h. The catalyst thus obtained was transferred to a vessel for polymerization, which was kept at about --78 °C with a mixture of dry ice and acetone. Acetylene was introduced into the vessel to an initial pressure of 200 Torr. A film of PA (cis c o n t e n t ~95%, thickness ~ 1 0 Izm) was obtained in 20 rain after the initiation of polymerization. The film was thoroughly washed with toluene and immersed in a concentrated alkaline solution for 5 rain to remove the remaining silicone oil. The density of the PA film thus obtained was 0.8 ~i cm-3. The as-polymerized cis-rich PA was thermally isomerized to the transrich PA under various conditions.
*Author to whom correspondence should be addressed. 0379-6779/89/$3.50
© Elsevier Sequoia/Printed in The Netherlands
254 Doping of the PA film was performed with gaseous I2 at room temperature. The electrical conductivity of the doped film was measured by the standard four-probe technique. Raman spectra were observed from the outer surface (facing the inner side of the vessel for polymerization) of the PA film at liquid-nitrogen temperature using the 457.9 and 514.5 nm lines of an Ar ion laser and the 632.8 nm line of a He-Ne laser for excitation. A Raman spectrometer consisting of a Spex 1877 Triplemate and an EG & G PAR 1421 intensified photodiode array detector was used. To minimize the effects of laser irradiation on the sample, the laser beam was used at low power (20 mW for the 457.9 and 632.8 nm lines and 30 mW for the 514.5 nm line), and the time of exposure for measuring one spectrum was limited to shorter than 10 min.
Results and discussion The electrical conductivity of the iodine-doped unstretched cis-PA film obtained in this study was 1800 S cm -1, which is comparable to the value reported by Naarmann and Theophilou [1]. The electrical conductivities of iodine-doped trans-PA films varied widely from 20 to 880 S cm -1, depending on the isomerization conditions. The Raman spectra of the as-polymerized cis-PA film observed with 457.9, 514.5 and 632.8 nm excitation are shown in Fig. 1. Apart from the changes of the bands due to the coexisting trans part, the Raman spectra of the cis-PA depend only slightly on the excitation wavelength, in accordance with the results previously reported for cis-PA [4, 5] obtained by the Shirakawa method. The peak wavenumbers in Fig. 1 coincide with the values in the literature within the limits of experimental error. Little difference is observed between the infrared spectrum (not shown) of the cis-PA obtained in this study and that reported previously [ 2]. These results indicate that the cis-PA obtained in this study is practically identical to the cis-PA prepared by the Shirakawa m e t h o d with respect to the length of ~r-electron conjugation. In Fig. 2 are shown the 457.9 nm excited Raman spectra of trans-PA films prepared under various isomerization conditions (see the caption). The bands at 1503 - 1499 cm -1 (whose intensities are designated as Is) and those at 1 1 2 7 - 1 1 2 1 cm -1 (Is') represent shorter trans-conjugated segments consisting of approximately 15 C=C bonds; the bands at 1461 - 1456 cm -1 (IL) and those at 1068 - 1064 cm -I (IL') arise from much longer segments [6]. It is evident in Fig. 2 that the relative intensities IL/Is and IL'/I s' greatly depend on the isomerization conditions. Therefore, the two parameters [6] R = IL/(IL + Is) and R ' = IL'/(IL' + IS') may be used as measures of the content of the longer segments. Values of R and R' calculated for the sample (isomerization temperature 190 °C, period 15 min) giving rise to the spectrum in Fig. 2(e) are 0.47 and 0.48, respectively. These values are slightly but definitely lower than the values (0.53 and 0.55) [6] obtained
255
i /~
i\
;I,~, ~
!::~.:~_~j
II ~
I~
1600
I
/\ ~
I
~_ .....
,
1400 1200 1000 WAVENUMBER/cm ~
~_
~,,~
1600
1400 1200 1000 WAVENUMBER/cm -~
Fig. 1. Raman spectra of the as-polymerized cis-polyacetylene film. Shadowed bands are due to the coexisting trans-part. Excitation wavelength: (a) 457.9 nm; (b) 514.5 nm; (c) 632.8 nm. Fig. 2. Raman spectra of the trans-polyacetylene films obtained by isomerizing the cis-polyacetylene under different conditions. Isomerization temperature and period (electrical conductivity upon full doping in parentheses): (a) 230 °C, 60 min (20 S cm-1); (b) 190 °(3, 60 rain (300 S cm-1); (c) 160 °C, 60 rain (460 S cm-1); (d) 230 °C, 15 rain (290 S cm-1); (e) 190 °C, 15 min (880 S cm-1). Excitation wavelength, 4 5 7 . 9 nm.
for the Shirakawa PA under the same isomerization conditions. This probably means that there is some difference in the packing of chains between the cis-PA obtained in this study and the Shirakawa cis-PA. It has been reported [6] that the electrical conductivity of fully d o p e d Shirawawa trans-PA is proportional to the R or R' value of the same transPA before doping. The five trans-PA films giving rise to the Raman spectra in Figs. 2(a)- (e) show very different electrical conductivities after full doping. The plot of the observed electrical conductivities versus R and R' is shown in Fig. 3, where the relationship [6] reported for the Shirakawa trans-PA is also indicated by a broken line. Clearly, the electrical conductivity increases with increasing R or R' value. However, the doped trans-PA obtained in this study shows an electrical conductivity a b o u t ten times higher than that of the doped Shirakawa trans-PA at the same R or R' value of a b o u t 0.5. Thus, although a high content of long trans-conjugated segments increases the electrical conductivity upon doping, there are some other factors which are more effective in determining the electrical conductivity u p o n doping. In conclusion, the observations described above seem to suggest that intermolecular interactions (e.g., the packing of chains and the form of
256 10oo
>,.
i
AO
O
o
A
0.3
0,4
0.5
R , R'
Fig. 3. Plots of the observed electrical conductivities of the trans-polyacetylene films fully doped with iodine: (o) vs. R and (A) vs. R'.
fibrils) rather than the molecular structure are responsible for the high electrical conductivities of the polyacetylene samples prepared in this study.
References H. Naarmann and N. Theophilou, Synth. Met., 22 (1987) 1. H. Shirakawa and S. Ikeda, Polym. J., 2 (1971) 231. T. Ito, H. Shirakawa and S. Ikeda, J. Polym. Sci., Polym. Chem. Ed., 12 (1974) 11. I. Harada, M. Tasumi, H. Shirakawa and S. Ikeda, Chem. Lett., (1978) 1411. L. S. Lichtmann, E. A. Imhoff, A. Sarhangi and D. B. Fitchen, J. Chem. Phys., 81 (1984) 168. 6 T. Arakawa, Y. Furukawa, H. Takeuchi, I. Harada and H. Shirakawa, Chem. Lett., (1984) 1637.
1 2 3 4 5