- Email: [email protected]
Controlling the molecular orientation of liquid crystalline polymer films deposited by polarized-laser chemical vapor deposition
Nuclear Instruments and Methods in Physics Research
B 121
(1997)415-418
NOMB
Beam Int~tions with Materials & Atoms
ELSEVIER
Controlling the molecular orientation of liquid crystalline polymer films deposited by polarized-laser chemical vapor deposition Toshiaki Itadani Murerial
Science Reseurch Luborurory.
Kururuy
*,
Koichi Saito
Co.. Ltd., 2045-l
Sukuzu. Kurushiki,
Okuyumu
710. Jupun
Abstract We investigated the polarized-laser chemical vapor deposition for seven kinds of monomers of cyanobiphenyl-type side chain liquid crystalline polymers. We used KrF excimer laser with glan laser prism to obtain linear polarization. The orientational order parameter increased with increasing deposition time. When second harmonics of Nd: YAG laser was used, the film showed no orientation. The orientational order parameter of the deposited films was related to the phase transitions and glass transition temperatures of corresponding polymers. The orientation could be attributed to anisotropic photochemical reactions induced by absorption of polarized light.
1. Introduction The molecular orientation of organic thin films influences such functions as films optical non-linearlity and electrical conductivity. However, it is difficult to control the molecular orientation of functional compounds because they have complicated chemical structures. This is one of the problems to be solved for the application of organic thin films as functional devices. Many attempts to control the molecular orientation have been carried out. For example, the vacuum evaporation growth of polymer films on specific substrates such as alkali halide crystals [I] and rubbed polymer films [2,3] has been investigated. Recently, new methods using polarized light to control the molecular orientation have been proposed for the application of data storage devices and liquid crystal (LC) orientation layer. They are based on anisotropic photochemical reactions induced by polarized light. Ichimura et al. [4] reported the control of LC alignment using photochromic film with azobenzen. Gibbons et al. [5] used a silicone polyimide copolymer with a diazodiamine dye as an aligning film of LC materials. Schadt et al. [6] showed LC alignment by photo polymerized layer with linearly polarized light. We reported the molecular orientation of side-chain-type liquid crystalline polymer (LCP) film deposited by a polarized-laser chemical vapor deposition (CVD) method [7]. The side-chain was oriented parellel to the electric vector of polarized-laser beam. However, we could not explain the mechanism of orientation but only proposed two mod-
* Corresponding
author. Fax: + 8 I-86-422-485
0168.583X/97/$17.00 Copyright PII SOI 68-583X(96)00550-2
I.
els. One is based on photochemical reactions induced by absorption of polarized light and the other is an orientation by electric fields of intense polarized light without absorption [8]. In this report, to clarify the condition for controlling the molecular orientation, the orientational order parameter of the deposited films was investigated on the changes effected by such experimental factors as monomer chemical structures, deposition time and laser wavelength at which the monomers had no absorption.
2. Experimental The chemical structures of monomers used in this work are shown in Table I. These monomers can be polymerized to give side-chain-type LCPs. The glass transition and phase transition temperatures of these polymers are also shown in Table 1. The temperature range for LC phase of polymers becomes wider with increasing the length of alkyl chain in the side-chains. The experimental setup is shown in Fig. I. The deposition chamber was evacuated to a pressure of lO-4 Pa with a turbo molecular pump. KrF excimer laser (A = 248 nm) was mainly used as a light source. Second harmonics (SH) of Nd : YAG laser (h = 532 nm) was applied to investigate the effects of excitation wavelength by use of monomer no. 6. Fluences of KrF excimer laser was set at 2 to 3 mJ/cm* at the window of the chamber and the repetition rate was fixed at 10 Hz. To obtain polarized laser beam, a glan laser prism was used. The monomers, which were powder at room temperature, were put into crucibles and were covered with copper mesh. Soda glass plates (5 X 5 cm*) were set onto the substrate holder tilted at 45” to the
0 1997 Elsevier Science B.V. All rights reserved V. LASER PROCESS/APPLICATIONS
T. Itadani, K. Saito/Nucl. Instr. and Meth. in Phys. Reu.B 121 (1997)415-418
416 Table
1
Monomer chemical structures and thermal properties of corresponding polymers prepared with conventional methods 19,101.
Tg(;)
~c” Monomer chemical structures
No.
Y
1
HzC=C-COO(CH~)~O
\
,
2
H2C=C-COO(Cn2),0 WCN \ ,
3
HsC=C-COO(CH~)~O WC” \
4
H2C=C-COO(CH2)60
Phase tryzion$‘C)
\
,
\
/
48
72
,
\
,
42
120
\ , WCN
\
,
35
127
5
\ / W=C-COWH2hQ WCN
\
,
30
145
6
\ , HzC=C-COO(CH~)~O WCN
\
,
51
114
7
H2C=:%OO(CH2),
,-,
45
128
‘;’ Y
‘;’ 7 93
,OWC” ,-,
laser beam. The temperature of the crucible was kept at 130- 132°C during the deposition. The films were deposited for 30 minutes with laser irradiation. For monomer no. 6, 10 and 20 min deposition were also performed to investigate the effects of deposition time. Phase transition temperatures of the deposited film were obtained from polarized optical microscopy during heating. Molecular orientations of these samples were observed by polarized JR spectroscopy in attenuation configuration using an JR spectrophotometer (JOEL JJR-5500). Thickness of films was measured with a stylus instrument (Sloan DEKTAK3030).
no. 1, where JR polarization is parallel (A,,) and perpendicular (A I) to the electric vector of the UV laser beam. The JR dichroism at the peak of 2225 cm -’ ascribed to CN stretching vibration suggests that the side chain of the polymer orients parallel to the electric vector of the laser beam [7]. The orientational order parameter was calculated from (R - l)/( R + 2), where R was the IR dichroic ratio of CN peak (2225 cm-‘) normalized by CO peak (1760 cm-‘).
3. Results and discussions 3.1. Dependence of orientation on chemical structures Fig. 2 shows the polarized IR spectra (A,, and A I> of the film deposited by polarized-laser CVD of the monomer Sublimation source Pblarizer \
I
.Substrate
Ouart z window
Vacuum
pump
Fig. 2. Polarized IR spectra of the films fabricated from monomer
I.
Spectra A,, and
A I are parallel
and perpendicular
Fig. 1. A schematic diagram of experimental apparatus for polar-
no.
ized laser chemical vapor chemical vapor deposition.
electric vector of the UV laser heam. respectively.
to the
T. Itua’uni. K. Suit0 /Nucl.
Instr. and Meth. in Phys. Res. B 121 (1997) 415-418
The phase transition temperatures and the orientational order parameters of the deposited films are shown in Table 2. The order parameters of the deposited films seemed to decrease with decreasing phase transition temperatures. These results can be explained by the fact that the order parameter of low molecular-weight liquid crystals approaches zero at the phase-transition temperatures. During the deposition process, the films would achieve an expected temperature, which was high enough for relaxation of molecular orientation but still below the phase transition temperatures because the orientation occurred in the LC phase. In the case of films having low phase transition temperatures, the film temperatures would be close to the phase-transition temperatures and the molecular orientation reduce rapidly to show low-order parameter. This speculation could be supported by the result that films deposited from the monomers 2, which has the lowest phase-transition temperature of the samples, leads to poor order parameter. On the other hand, the films deposited from monomer 5 showed a lower-order parameter in spite of the highest phase-transition temperature. This result is possibly explained by orientation relaxation of the polymer over the glass transition temperature. As shown in Table 1, the polymer 5 has the lowest glass transition temperature. The glass transition temperature of the deposited film could be lower than chat of the polymer prepared by chemical polymerization because of inhomogeneities of chemical reaction, and it could become close to the substrate temperature. 3.2. Deposition time dependence Fig. 3 shows orientational order parameter and thickness of films deposited from monomer no. 6 as a function of deposition time. The order parameter increases with increasing deposition time. These phenomena suggest that the order parameter of an upper layer in the deposited film would be higher than that of lower layers. The orientation of the upper layer is probably influenced by orientational anisotropy of the lower layer, and the liquid crystal could enhance the orientation of the lower layer. Thus, the
Table 2 Orientational order parameter of deposited films. No.
Phase transitions
Order parameter
(“0
I
104
0.11
2
73
0.02
3
112
0.13
4
120
0.42 a
5
143
0.06
6
I10
0.15
7
122
0.48 b
“b Deposition for 60 min. using CaFZ substrate, measurement.
Fig. 3. Orientational order
417
parameter and thickness of deposited
films as a function of deposition time (monomer: no. 6, laser:
KrF
excimer laser.)
orientation could come from accumulation of anisotropy of lower layers induced by photochemical reactions. 3.3. Deposition by using second harmonics of Nd: YAG laser
When SH of Nd: YAG laser was used, the deposited film was not polymerized probably because the monomer no. 6 has no absorption at 532 nm. The orientation was not observed in this case. When circular polarized KrF extimer laser was simultaneously applied to polymerize the monomer, the polymerized film was obtained but showed no orientation. These results suggest that the absorption of polarized light is necessary for molecular orientation. The orientation is probably attributed to the anisotropic photochemical reaction induced by a polarized laser beam.
4. Conclusion
The molecular orientation of deposited film was changed by chemical structures of monomers, increased with deposition time, and was not observed upon the irradiation with SH of Nd: YAG laser. It could be attributed to the thermal properties and anisotropic photochemical reactions of polymers. To obtain the higher molecular orientation, phase transitions and glass transition temperatures of the polymers would be taken into account and it would be necessary for polarized light to be absorbed by deposited materials.
Acknowledgement
IR transmittance
We thank Drs. H. Niino, Y. Koga and A. Yabe of National Institute for Materials and Chemical Research for helpful discussions. This work was conducted as part of the program: Advanced Chemical processing Technology, commissioned to the Advanced Chemical processing TechV. LASER PROCESS/APPLICATTONS
418
T. Imkmi.
K. Suite /Nucl.
Insrr. mul Meth. in Phys. Res. B 1.21 (1997) 415-418
nology Research Association from the New Energy and Industrial Technology Development Organization, carried out under the Industrial Science and Technology Frontier Program administered by the Agency of Industrial Science and Technology, the Ministry of International Trade and Industry.
[41 K. Ichimura, Y. Suzuki, T. Seki, A. Hosoi and K. Aoki,
Langmuir 4 (1988) 1214.
I51W.M. Gibbons, P.J. Shannon, S.T. Sun and B.J. Swetlin, Nature 351 (1991) 49.
[61M. Schadt, K. Schmitt, V. Kozinkov and V. Chigrinov, Jpn. J. Appt. Phys. 31 (1992) 2155. 171T. Itadani. K. Saito, T. Akiya, S. Nagata, H. Niino and A.
Yabe, Jpn. J. Appl. Phys. (1995) L566.
b31G.K.L. Wong and Y.R. Shen, Phys Rev. Lett. 30 (1973) 895. References
[91 V. Shibaev, S. Kostromin and N. Plate, Eur. Polym. J. 18 (1982) 651.
[II S. Isoda, Polymer 25 (1984) 615. Dl T. Kanetake, K. Ishikawa, T. Koda, Y. Tokura and K. Takeda, Appl. Phys. Lett. 51 (1987) 1957. 131J.S. Patel, SD. Lee, G.L. Baker and A. Shelbume, Appl.
Phys. Lett. 56 (1990) 131.
II01 V.P. Shibaev. Y.S. Freidzon and S.G. Kostromin, in: Liquid Crystalline and Mesomorphic Polymers, eds. V.P. Shibaev and L. Lam (Springer, New York, 1994) p. 85.
B 121
(1997)415-418
NOMB
Beam Int~tions with Materials & Atoms
ELSEVIER
Controlling the molecular orientation of liquid crystalline polymer films deposited by polarized-laser chemical vapor deposition Toshiaki Itadani Murerial
Science Reseurch Luborurory.
Kururuy
*,
Koichi Saito
Co.. Ltd., 2045-l
Sukuzu. Kurushiki,
Okuyumu
710. Jupun
Abstract We investigated the polarized-laser chemical vapor deposition for seven kinds of monomers of cyanobiphenyl-type side chain liquid crystalline polymers. We used KrF excimer laser with glan laser prism to obtain linear polarization. The orientational order parameter increased with increasing deposition time. When second harmonics of Nd: YAG laser was used, the film showed no orientation. The orientational order parameter of the deposited films was related to the phase transitions and glass transition temperatures of corresponding polymers. The orientation could be attributed to anisotropic photochemical reactions induced by absorption of polarized light.
1. Introduction The molecular orientation of organic thin films influences such functions as films optical non-linearlity and electrical conductivity. However, it is difficult to control the molecular orientation of functional compounds because they have complicated chemical structures. This is one of the problems to be solved for the application of organic thin films as functional devices. Many attempts to control the molecular orientation have been carried out. For example, the vacuum evaporation growth of polymer films on specific substrates such as alkali halide crystals [I] and rubbed polymer films [2,3] has been investigated. Recently, new methods using polarized light to control the molecular orientation have been proposed for the application of data storage devices and liquid crystal (LC) orientation layer. They are based on anisotropic photochemical reactions induced by polarized light. Ichimura et al. [4] reported the control of LC alignment using photochromic film with azobenzen. Gibbons et al. [5] used a silicone polyimide copolymer with a diazodiamine dye as an aligning film of LC materials. Schadt et al. [6] showed LC alignment by photo polymerized layer with linearly polarized light. We reported the molecular orientation of side-chain-type liquid crystalline polymer (LCP) film deposited by a polarized-laser chemical vapor deposition (CVD) method [7]. The side-chain was oriented parellel to the electric vector of polarized-laser beam. However, we could not explain the mechanism of orientation but only proposed two mod-
* Corresponding
author. Fax: + 8 I-86-422-485
0168.583X/97/$17.00 Copyright PII SOI 68-583X(96)00550-2
I.
els. One is based on photochemical reactions induced by absorption of polarized light and the other is an orientation by electric fields of intense polarized light without absorption [8]. In this report, to clarify the condition for controlling the molecular orientation, the orientational order parameter of the deposited films was investigated on the changes effected by such experimental factors as monomer chemical structures, deposition time and laser wavelength at which the monomers had no absorption.
2. Experimental The chemical structures of monomers used in this work are shown in Table I. These monomers can be polymerized to give side-chain-type LCPs. The glass transition and phase transition temperatures of these polymers are also shown in Table 1. The temperature range for LC phase of polymers becomes wider with increasing the length of alkyl chain in the side-chains. The experimental setup is shown in Fig. I. The deposition chamber was evacuated to a pressure of lO-4 Pa with a turbo molecular pump. KrF excimer laser (A = 248 nm) was mainly used as a light source. Second harmonics (SH) of Nd : YAG laser (h = 532 nm) was applied to investigate the effects of excitation wavelength by use of monomer no. 6. Fluences of KrF excimer laser was set at 2 to 3 mJ/cm* at the window of the chamber and the repetition rate was fixed at 10 Hz. To obtain polarized laser beam, a glan laser prism was used. The monomers, which were powder at room temperature, were put into crucibles and were covered with copper mesh. Soda glass plates (5 X 5 cm*) were set onto the substrate holder tilted at 45” to the
0 1997 Elsevier Science B.V. All rights reserved V. LASER PROCESS/APPLICATIONS
T. Itadani, K. Saito/Nucl. Instr. and Meth. in Phys. Reu.B 121 (1997)415-418
416 Table
1
Monomer chemical structures and thermal properties of corresponding polymers prepared with conventional methods 19,101.
Tg(;)
~c” Monomer chemical structures
No.
Y
1
HzC=C-COO(CH~)~O
\
,
2
H2C=C-COO(Cn2),0 WCN \ ,
3
HsC=C-COO(CH~)~O WC” \
4
H2C=C-COO(CH2)60
Phase tryzion$‘C)
\
,
\
/
48
72
,
\
,
42
120
\ , WCN
\
,
35
127
5
\ / W=C-COWH2hQ WCN
\
,
30
145
6
\ , HzC=C-COO(CH~)~O WCN
\
,
51
114
7
H2C=:%OO(CH2),
,-,
45
128
‘;’ Y
‘;’ 7 93
,OWC” ,-,
laser beam. The temperature of the crucible was kept at 130- 132°C during the deposition. The films were deposited for 30 minutes with laser irradiation. For monomer no. 6, 10 and 20 min deposition were also performed to investigate the effects of deposition time. Phase transition temperatures of the deposited film were obtained from polarized optical microscopy during heating. Molecular orientations of these samples were observed by polarized JR spectroscopy in attenuation configuration using an JR spectrophotometer (JOEL JJR-5500). Thickness of films was measured with a stylus instrument (Sloan DEKTAK3030).
no. 1, where JR polarization is parallel (A,,) and perpendicular (A I) to the electric vector of the UV laser beam. The JR dichroism at the peak of 2225 cm -’ ascribed to CN stretching vibration suggests that the side chain of the polymer orients parallel to the electric vector of the laser beam [7]. The orientational order parameter was calculated from (R - l)/( R + 2), where R was the IR dichroic ratio of CN peak (2225 cm-‘) normalized by CO peak (1760 cm-‘).
3. Results and discussions 3.1. Dependence of orientation on chemical structures Fig. 2 shows the polarized IR spectra (A,, and A I> of the film deposited by polarized-laser CVD of the monomer Sublimation source Pblarizer \
I
.Substrate
Ouart z window
Vacuum
pump
Fig. 2. Polarized IR spectra of the films fabricated from monomer
I.
Spectra A,, and
A I are parallel
and perpendicular
Fig. 1. A schematic diagram of experimental apparatus for polar-
no.
ized laser chemical vapor chemical vapor deposition.
electric vector of the UV laser heam. respectively.
to the
T. Itua’uni. K. Suit0 /Nucl.
Instr. and Meth. in Phys. Res. B 121 (1997) 415-418
The phase transition temperatures and the orientational order parameters of the deposited films are shown in Table 2. The order parameters of the deposited films seemed to decrease with decreasing phase transition temperatures. These results can be explained by the fact that the order parameter of low molecular-weight liquid crystals approaches zero at the phase-transition temperatures. During the deposition process, the films would achieve an expected temperature, which was high enough for relaxation of molecular orientation but still below the phase transition temperatures because the orientation occurred in the LC phase. In the case of films having low phase transition temperatures, the film temperatures would be close to the phase-transition temperatures and the molecular orientation reduce rapidly to show low-order parameter. This speculation could be supported by the result that films deposited from the monomers 2, which has the lowest phase-transition temperature of the samples, leads to poor order parameter. On the other hand, the films deposited from monomer 5 showed a lower-order parameter in spite of the highest phase-transition temperature. This result is possibly explained by orientation relaxation of the polymer over the glass transition temperature. As shown in Table 1, the polymer 5 has the lowest glass transition temperature. The glass transition temperature of the deposited film could be lower than chat of the polymer prepared by chemical polymerization because of inhomogeneities of chemical reaction, and it could become close to the substrate temperature. 3.2. Deposition time dependence Fig. 3 shows orientational order parameter and thickness of films deposited from monomer no. 6 as a function of deposition time. The order parameter increases with increasing deposition time. These phenomena suggest that the order parameter of an upper layer in the deposited film would be higher than that of lower layers. The orientation of the upper layer is probably influenced by orientational anisotropy of the lower layer, and the liquid crystal could enhance the orientation of the lower layer. Thus, the
Table 2 Orientational order parameter of deposited films. No.
Phase transitions
Order parameter
(“0
I
104
0.11
2
73
0.02
3
112
0.13
4
120
0.42 a
5
143
0.06
6
I10
0.15
7
122
0.48 b
“b Deposition for 60 min. using CaFZ substrate, measurement.
Fig. 3. Orientational order
417
parameter and thickness of deposited
films as a function of deposition time (monomer: no. 6, laser:
KrF
excimer laser.)
orientation could come from accumulation of anisotropy of lower layers induced by photochemical reactions. 3.3. Deposition by using second harmonics of Nd: YAG laser
When SH of Nd: YAG laser was used, the deposited film was not polymerized probably because the monomer no. 6 has no absorption at 532 nm. The orientation was not observed in this case. When circular polarized KrF extimer laser was simultaneously applied to polymerize the monomer, the polymerized film was obtained but showed no orientation. These results suggest that the absorption of polarized light is necessary for molecular orientation. The orientation is probably attributed to the anisotropic photochemical reaction induced by a polarized laser beam.
4. Conclusion
The molecular orientation of deposited film was changed by chemical structures of monomers, increased with deposition time, and was not observed upon the irradiation with SH of Nd: YAG laser. It could be attributed to the thermal properties and anisotropic photochemical reactions of polymers. To obtain the higher molecular orientation, phase transitions and glass transition temperatures of the polymers would be taken into account and it would be necessary for polarized light to be absorbed by deposited materials.
Acknowledgement
IR transmittance
We thank Drs. H. Niino, Y. Koga and A. Yabe of National Institute for Materials and Chemical Research for helpful discussions. This work was conducted as part of the program: Advanced Chemical processing Technology, commissioned to the Advanced Chemical processing TechV. LASER PROCESS/APPLICATTONS
418
T. Imkmi.
K. Suite /Nucl.
Insrr. mul Meth. in Phys. Res. B 1.21 (1997) 415-418
nology Research Association from the New Energy and Industrial Technology Development Organization, carried out under the Industrial Science and Technology Frontier Program administered by the Agency of Industrial Science and Technology, the Ministry of International Trade and Industry.
[41 K. Ichimura, Y. Suzuki, T. Seki, A. Hosoi and K. Aoki,
Langmuir 4 (1988) 1214.
I51W.M. Gibbons, P.J. Shannon, S.T. Sun and B.J. Swetlin, Nature 351 (1991) 49.
[61M. Schadt, K. Schmitt, V. Kozinkov and V. Chigrinov, Jpn. J. Appt. Phys. 31 (1992) 2155. 171T. Itadani. K. Saito, T. Akiya, S. Nagata, H. Niino and A.
Yabe, Jpn. J. Appl. Phys. (1995) L566.
b31G.K.L. Wong and Y.R. Shen, Phys Rev. Lett. 30 (1973) 895. References
[91 V. Shibaev, S. Kostromin and N. Plate, Eur. Polym. J. 18 (1982) 651.
[II S. Isoda, Polymer 25 (1984) 615. Dl T. Kanetake, K. Ishikawa, T. Koda, Y. Tokura and K. Takeda, Appl. Phys. Lett. 51 (1987) 1957. 131J.S. Patel, SD. Lee, G.L. Baker and A. Shelbume, Appl.
Phys. Lett. 56 (1990) 131.
II01 V.P. Shibaev. Y.S. Freidzon and S.G. Kostromin, in: Liquid Crystalline and Mesomorphic Polymers, eds. V.P. Shibaev and L. Lam (Springer, New York, 1994) p. 85.