poly(2,5-dimethoxy-p-phenylenevinylene) (MO-PPV) and poly(2,5-thienylene- vinylene) (PTV), because their polyion complexes were unstable in the solid state.
Thin Solid Films, 179(1989) I-8
PREPARATION OF HIGHLY ORIENTED POLYARYLENEVINYLENE THIN FILMS BY THE L A N G M U I R - B L O D G E T T TECHNIQUE MASANAO ERA, KOJI KAMIYAMA, KAZUNORI YOSHIURA, TATSUO MOMII, HIDEYUKI MURATA, SHIZUO TOKITO, TETSUO TSUTSUI AND SHOGO SAITO
Department of Materials Science and Technology, Graduate School of Engineering Sciences, Kyushu University, Kasuga-shi, Fukuoka816 (Japan) (Received April 25, 1989; accepted May 2, 1989)
We obtained three kinds of polyarylenevinylene(poly (p-phenylenevinylene), poly(2,5-dimethoxy-p-phenylenevinylene) and poly(2,5-thienylenevinylene)) thin films with the Langmuir-Blodgett (LB) technique. Stable polyarylenevinylene precursor-anionic amphiphile complex monolayers were formed at the air-precursor aqueous solution interface through adsorption of the precursors onto an amphiphilic monolayer. The polyion complex monolayers could be transferred on substrates by the LB technique. The polyarylenevinylene precursor LB films were converted to polyarylenevinylene films with a heat treatment under nitrogen flow. Extension of the mean n conjugation length and planar orientation of the conjugation sequences were realized in the poly (p-phenylenevinylene) and poly(2,5dimethoxy-p-phenylenevinylene) LB films.
1. INTRODUCTION
The "precursor route", which has been applied to polyarylenevinylene, polyacetylene and poly(p-phenylene) l-s, is one of the most useful procedures by which free-standing conjugated polymer films are fabricated. In the procedure, the 7t-conjugated polymer films are produced through the conversion of precursor polymer films by thermal elimination reactions. Hence, for the fabrication of ordered films with few defects in n-conjugated chains, it is important to introduce orientational orderliness of the precursor polymer chains in the films 6. We and Kakimoto et al. have reported that poly(p-phenylenevinylene) (PPV), a polyarylenevinylene, thin films could be prepared by a procedure using the deposition of the precursor-anionic amphiphile polyion complex monolayer followed by thermal elimination 7's. In our report, we have also demonstrated that a high degree of orientation of the n-conjugated polymer chains and an extension of the mean n-conjugation length could be achieved. In other words, diminution of defects in the n-conjugated system is realized in the PPV Langmuir-Blodgett (LB) film through the smooth progress of the thermal elimination in the ordered precursor LB film. The technique, however, was not applicable to other polyarylenevinylenes, i.e. 0040-6090/89/$3.50
© ElsevierSequoia/Printedin The Netherlands
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poly(2,5-dimethoxy-p-phenylenevinylene) (MO-PPV) and poly(2,5-thienylenevinylene) (PTV), because their polyion complexes were unstable in the solid state. The elimination in the polyion complexes progressed in the solid state even at room temperature and the complexes consequently became insoluble in the usual organic solvents. Thus we had no way to form the polyion complex monolayers at the air-water interface. Recently we have found that stable polyion complex monolayers were formed through the adsorption of the precursors onto a bilayer-forming amphiphile monolayer when the amphiphile monolayers are spread on the precursor polymer aqueous solution. In this paper, we shall report the preparation of polyarylenevinylene precursor LB films utilizing the procedure involving the formation of the polyion complexes at the air-precursor aqueous solution interface and the conversion to polyarylenevinylene in the LB film. Then the optical properties and the orientation structure in the converted LB films are also described. 2. EXPERIMENTS
The polyelectrolyte precursors of polyarylenevinyleneswere prepared according to the method of refs. 2 and 9. The precursor ofPTV 3 has tetrahydrothiophene as a pendant group (Fig. 1). The precursors were employed after purification by dialysis for a few days with a cellulose tube against deionized water. The bilayerforming amphiphile 4 was purchased from SOGO Pharmaceutical Co. Ltd.
--~C
R
R
H-CHz-~n
L~2H5C2H5BF"
!: R:H, PPV
2_.:R=CH30. MO-PPV
- - ' ~ H - C H 2 ~ n "~',--~-CH=CH n-)~3: PTV
C. CH IO CH CH~CH2")ITOCC-5OT3Na+ /4
Fig. 1. Chemical structures of polyarylenevinylenes and their precursors. 4 is a bilayer-forming amphiphile.
Measurements of the surface pressure-area isotherms and preparation of the precursor-anionic amphiphile polyion complex LB films were carried out by use of a Langmuir trough with a Wilhelmy-type film balance (Kyowa Kaimen Kagaku Co. HBM-AP). Monolayers of the bilayer-forming amphiphile 4 were spread on an aqeous solution of polyarylenevinylene precursors (repeating unit concentration ca. I0 4M). The monolayers were compressed at a surface pressure of 30 mN m -~
HIGHLY ORIENTED POLYARYLENEVINYLENE LB FILMS
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and were allowed to stand for several hours to adsorb the precursors. The polyion complex monolayers with the adsorbed precursors were transferred onto quartz substrates by the conventional LB technique. Finally, the polyion complex LB films were heated at 100-200°C for 1 h under nitrogen containing a slight amount of hydrogen chloride as a catalyst. 3. RESULTS AND DISCUSSION
3.1. Preparation of poly (p-phenylenevinylene) and poly ( 2,5-dimethoxy-pphenylenevinylene) precursor Langmuir-Blodgett films Figure 2 shows surface pressure-area isotherms of the bilayer-forming amphiphile (4) on the PPV precursor aqueous solution and on the MO-PPV precursor aqueous solution. Amphiphile (4) monolayers on both the PPV precursor solution and the M O - P P V precursor solution formed more expanded monolayers than 4 on pure water, and they have larger limiting areas of 80 A 2 molecule-1 on the PPV precursor solution and 93/~2 molecule 1 on the M O - P P V precursor solution. The behaviour of ,he isotherms is most likely to be caused by adsorption of the precursors onto the amphiphile monolayer.
"TE 50 40
0
0
20
40 60 80 100 Area //~.2molecule-1
120
Fig. 2. Surface pressure area isotherms o f a bilayer-forming amphiphile 4 on a PPV precursor aqueous solution (curve 1), on an M O - P P V precursor aqueous solution (curve 2), and on pure water (curve 3).
002
-•.0-,0 ....
(v u c
0
,.-0"
.....
O--
. . . . . . . . . . . . . . . . .
.It..
o.o~
8 $
Or
i 0
I
0
5
1
10 Time / hr
I
15
Fig. 3. Time dependences o f absorptions of the precursors, at 235 nm for PPV precursor (O) and at 300 nm for MO-PPV precursor (O), in the bilayer-forming amphiphile monolayers after compression.
M. ERA et al.
4
The adsorption process of the precursors onto the amphiphile (4) monolayer was studied from the change in absorption assigned to the arylene rings of the precursors, at 235nm for the PPV precursor and at 300nm for the M O - P P V precursor (Fig. 3). The monolayers were allowed to stand to adsorb the precursors after the monolayers were compressed at 3 0 m N m -1. The monolayers were deposited on quartz substrates at appropriate time intervals and the absorption spectra of the monolayers were measured. For both the PPV and the M O - P P V precursors, the absorptions of precursors appeared just after the compression; the precursors had already been adsorbed on the monolayer during the compression process. Furthermore, the adsorption of the precursors is found to be completed within several hours; the absorbances of the precursors reached constant values within several hours. Alter adsorption of the precursors for more than 5 h, the monolayers with the adsorbed precursors were successively transferred onto the quartz substrates, on which 11 monolayers of cadmium arachidate had been deposited beforehand. The transfer ratios were about unity both at upstroke and at downstroke.
3.2. Conversion to poly (p-phenylenevinylene ) and poly ( 2,5-dimethoxy-pphenylenevinylene ) Langmuir-BIodgett films and the optical properties The IR absorption spectra of the PPV precursor LB film (60 layers) before and after a heat treatment at 200°C are depicted in Fig. 4(a). After the heat treatment, absorptions assigned to trans-vinylene CH out-of-plane bending and trans-vinylene CH stretching were observed at 963cm -1 and at 3022cm -~ respectively. The appearance of the modes is evidence of the formation of the PPV structure by the thermal elimination of the sulphonium group in the LB film. Absorptions at 2922 and 2854 c m - 1, which are assigned to aliphatic chains of the amphiphile 4, do not disappear after the heat treatment. It turned out that the amphiphile is not removed from the LB film during the heat treatment. . . . . : before h e a t - t r e a t m e n t - a f t e:r h e a t - t r e a t m e n t
-
A
/',, ~-.
.
I
,
"-J i
4OOO
. . . . : before heat-treatment
"- ......
--....~,
, ~~"- , ~"", '-k,i' d
i i , I , I i i 2OOO 1500 30OO W o v e N u m b e r / c m -1
v -
~,
I
I
I,.j~l,
. . . .
1 I000
4000
3000
2000
1500
1000
W o v e Number Icm-I
Ca) Fig. 4. IR spectra of the polyarylenevinylene precursor LB films before and after the heat treatment: (a) PPV; (b) MO-PPV.
The absorption spectrum of the PPV LB film is shown in Fig. 5(a). The appearance of a strong absorption in the visible region after the heat treatment also demonstrated the formation of the rt-conjugated PPV structure in the LB film.
H I G H L Y O R I E N T E D P O L Y A R Y L E N E V I N Y L E N E LB FILMS
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Furthermore, one should note that the absorption peak wavelength of the PPV LB films is about 40 nm longer than that of a conventional PPV film obtained from the cast film of the precursor and that the absorption band of the PPV LB film is sharp and well resolved. The results clearly indicate that extension of the mean conjugation length and narrowing of the distribution of n conjugation lengths were realized in the LB films. I
i \, I I
i/
\~
aa
2OO
3OO
4O0
5OO
60O
200
(a)
300
40O
5O0
600
700 800
Wavelength / nm
Wavelength/nm
(b)
Fig. 5. Absorption spectra of the polyarylenevinyleneLB films (spectra I), the precursor LB films (spectra 2), and conventionalpolyarylenevinylenecast films(spectra 3): (a) PPV; (b) MO-PPV. The formation of the MO-PPV structure in the precursor LB film was also confirmed spectroscopically, trans-vinylene CH out-of-plane bending at 951 c m - 1 and trans-vinylene CH stretching at 3032 c m - 1 were observed in the IR (Fig. 4(b)). The absorption due to the n conjugated system (MO-PPV structure) was located at 512 nm, which was 10 nm longer than that of the conventional M O - P P V film (Fig. 5(b)). Extension of the 7t conjugation length is also demonstrated in the MOPPV LB film. The development in the PPV and MO-PPV ~ conjugation systems occurs as a result of the orientational orderliness of the precursor chains in the LB films. Since the precursor polymers are polyelectrolytes, the precursors should form expanded chains in the dilute aqueous solutions of the precursors. As a result, the precursors will exhibit the two-dimensional orientation of expanded chains at the air-precursor aqueous solution interface during the adsorption process onto the amphiphile monolayer. Then the polyion complex LB films, which consist of ordered polyion complex monolayers, would possess ordered aggregation structures of the precursor polymers. Conversion reactions should progress exquisitely in ordered films such as the LB films. Consequently, the chemical and conformational defects are expected to decrease in the PPV and MO-PPV LB films. Through the process described above, extension of the mean n conjugation length and narrowing of the distribution of the rr conjugation length are most likely to have been realized.
3.3. Orientation structure in poly (p-phenylenevinylene ) and poly ( 2,5-dimetho xy-pphenylenevinylene ) Langmuir-Blodgett [ilms Orientation structures in the PPV and M O - P P V LB films were evaluated with linear dichroism. The dichroic ratios Ap/A~ of the PPV and M O - P P V LB films
M. ERA el al.
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are plotted against the incident angle of polarized light in Fig. 6, where Ap and A~ are the absorbances of respectively p-polarized and s-polarized light at the peak wavelength, and the axis of p polarization coincides with the dipping direction. In the PPV LB film, Ap/A s decreased from 1.0 to 0.73 when the incident angle increased from 0 ° to 45 °. The result demonstrates that PPV chains assume a planar orientation in the LB film. The planar orientation originates in the two-dimensional orientation of the precursor in the monolayers. 1.5,
1.0
0.5
I
L t I 20 ~0 Incident angleldeg.
0
Fig. 6. The dependence of the dichroic ratio A~,/As of the PPV LB film (O) and the MO-PPV LB film (0) on the incident angle of polarized light.
The planar orientation in the MO-PPV LB film was also demonstrated; A p / A s decreased from 1.3 to 1.0 when the incident angle increased from 0 ° to 45 °. Furthermore, the value of 1.3 at 0 ° incidence suggested that MO-PPV chains possess a tendency to orient uniaxially along the dipping direction in the LB film.
3.4. Preparation of poly( 2,5-thienylenevinylene ) Langmuir-Blodgett films We obtained a PTV LB film by using the same procedure that was used for PPV and M O - P P V LB films. More than 60 layers of the PTV precursor monolayer could be deposited on quartz substrates as Z-type films. The conversion of the precursor to PTV structure in the LB film was demonstrated by the observation of trans-vinylene CH out-of-plane bending at 3010cm -~ and trans-vinylene CH stretching at 975 c m - ~ in the IR spectrum after the heat treatment at 100 °C.
E
.: i!
" ',7'
i
z"
,, /
//
'\ T ..........
200
.'X',,3 1 ~
/
L
400 600 W a v e l e n g t h I n rn
~
800
Fig. 7. Absorption spectra of the PTV LB film (spectrum 1) and the precursor LB film (spectrum 2). The absorption spectrum of a conventional cast film is also shown (spectrum 3).
HIGHLY ORIENTED POLYARYLENEVINYLENE LB FILMS
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Figure 7 shows the absorption spectrum of the PTV LB film. The absorption peak is located at a shorter wavelength (535 nm) than that of a conventional PTV LB film. In contrast with the PPV and MO-PPV LB films, extension of the n conjugation length was not realized in the PTV LB film.
c
4000
3000 2000 1000 Wavenurnber I c rn-1
500
Fig. 8. I R s p e c t r u m o f t h e PTV precursor after stirring for 3 d a y s i n water.
The IR spectrum of the PTV precursor after stirring for 3 days in water is shown in Fig. 8. Absorption due to the OH group, which did not appear in the PPV and MO-PPV precursors, was observed at around 1000 cm -1 in the PTV precursor. The result shows that the sulphonium groups o f the PTV precursor were substituted by OH groups through the reaction of the precursor with water as solvent. It is clear that the precursor which was used in the preparation of the PTV LB film was also partially substituted by OH groups during the dialysis and the deposition of the precursor monolayer. Since the precursor partially substituted by O H groups cannot form a 1:1 complex with the amphiphile 4, we would not have obtained LB films with oriented PTV precursor chains. It may be for this reason that extension of the r~ conjugation length was not realized in the PTV LB film. 4. CONCLUSION We succeeded in obtaining polyarylenevinylene LB films by depositing monolayers which adsorbed precursors at the air-precursor aqueous solution interface, followed by thermal elimination. Extension of the mean n conjugation length and planar orientation of the n conjugated chains are demonstrated in the PPV and MO-PPV LB films. We are now proceeding with detailed studies of the preparation of highly'oriented polyarylenevinylene LB films and the optical and electronic properties in these polyarylenevinylene LB films. REFERENCES 1 D.C. Bott, C. K. Chai, J. H. Edwards, W. J. Feast, R. H. Friend and M. E. Horton, J. Phys. (Paris), Colloq. C3, 44 (1983) 1443. 2 I. Murase, T. Ohnishi, T. Noguchi and M. Hirooka, Polym. Commun,. 25 (1984) 327. 3 D . G . H . Ballard, A. Courtls, I. M. ~/hirley and S. C. Taylor, J. Chem. Soc., Chem. Commun., (1983) 954.
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I. Murase, T. Ohnishi, T. Noguchi and M. Hirooka, Polym. Commun., 28 (1987) 229. S. Yarnada, S. Tokito, T. Tsutsui and S. Saito, J. Chem. Soc, Chem. Commun., (1987) 1488. D.C. Bott, C. S. Brown, J. N. Winter and J. Buker, Polymer, 28 (1987) 601. M. Era, H. Shinozaki, S. Tokito, T. Tsutsui and S. Saito, Chem. Lett., (1987) 1097. Y. Nishikata, M. Kakimoto and Y. Imai, J. Chem. Soc., Chem. Commun., (1988) 1040. R.L. Elsembaumer, K. Y. Jen, G. G. Miller, H. Eckhardt, L. W. Shacklette and R. Jow, Electronic Properties of Conjugated Polymers, Springer, Berlin, 1987, p. 400.
