- Email: [email protected]
Studies of casting and LB films of ferroelectric liquid crystalline polysiloxane
Materials Science and Engineering C 10 Ž1999. 87–90 www.elsevier.comrlocatermsec
Studies of casting and LB films of ferroelectric liquid crystalline polysiloxane Qingbin Xue a
a,)
, Kongzhang Yang a , Hongguo Liu a , Xiao Chen a , Jianzhuang Jiang b, Qizhen Zhang b
Laboratory for Colloid and Interface Chemistry of State Educational Ministry, Shandong UniÕersity, Jinan 250100, China b Chemistry Department, Shandong UniÕersity, Jinan 250100, China Accepted 21 May 1999
Abstract Monolayers of side chain ferroelectric liquid crystalline polysiloxane have been successively transferred onto appropriate substrates by vertical dipping method to study the morphology and the orientation of the mesogens and the side chains by UV spectra and polarized FT-IR spectra and X-ray diffraction. The spectra for casting film and LB film are compared. The molecules in LB films are found to be more orientated along the normal of the substrate at an angle of 378 for the mesogens as the side chain as found by polarized FT-IR spectra and transmitting FT-IR spectra than casting film. Layer structure is also confirmed by X-ray diffraction results. The mesogens are found to form H-aggregates structure in bulk phase and LB films from UV spectra. Temperature-dependent UV spectra have revealed the change of the tilt angle of the mesogens inside the layer of both bulk phase and LB films. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Langmuir–Blodgett films; Ferroelectric liquid crystalline polysiloxane; Polarized FT-IR spectroscopy
1. Introduction Ferroelectric liquid crystalline polysiloxanes have attracted more and more attentions because of their potential application in different kind of flat panel display devices and fast electro-optical devices or nonlinear optical devices w1,2x. Ferroelectricity of liquid crystals which appeared when helix of the SUc phase is unwound can be obtained in different ways w3,4x including Surface-Stabilized Ferroelectric Liquid Crystals ŽSSFLC. involving the effect of boundary. But for these application, the prerequisite is that the ferroelectric liquid crystal molecules organize homogeneously to form uniform films with discrete thickness and orientation. For certain materials that spread on water surface to form a monolayer, the Langmuir–Blodgett technique can be used to produce organized films of controlled thickness w5–7x. Water surface can provide an unique microenvironment that induces formation of organized
monolayer of the molecules at the interface w8x. Transfer of these monolayers to solid surface can be performed to obtain films of microns thickness which show surfacestabilized effect. In this paper, we have studied transferred LB films of newly synthesized polysiloxane with low and broad Sm CU range by spectroscopy method in order to obtain information about the organization of the molecules in the monolayer after being transferred onto solid substrates.
2. Experimental
)
Corresponding author. Tel.: q86-531-8564750; fax: q86-5318565167; E-mail: [email protected]
The sample used in this study was synthesized as previously described w9x. The monolayer was obtained by spreading chloroform solution of the sample and com-
0928-4931r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 8 - 4 9 3 1 Ž 9 9 . 0 0 1 0 1 - 0
88
Q. Xue et al.r Materials Science and Engineering C 10 (1999) 87–90
pressed to target pressure and maintained for half an hour before transferred onto substrates in a vertical-dipping method on a commercially available computer controlled Langmuir trough NIMA 2000 with a Whilhelmy balance as pressure sensor. Details about the monolayer behavior of polysiloxane is published elsewhere w10x. The monolayers were deposited onto quartz slides to carried out the X-ray diffraction Žby a DrMAX-gB X-ray diffractometer with Cu K a radiation l s 0.154 nm. and UV absorption spectra Žby a Shimadzu recording spectrometer, Model UV-240.. The angular resolution of the 2 u scan is 0.028. Polarized FT-IR spectra is performed by Nicolet 710 FT-IR spectrometer for 200 scan with the aid of a wire-grid polarizer and CaF2 as substrate. Fused quartz plates are vigorously cleaned in chromic acid and 4% KOH aq. for 24 h and immersed into 10% trichloromethylsilanerCHCl 3 to hydrophobilize. CaF2 slides for the polarized FT-IR spectra have been cleaned by chloroform thoroughly.
3. Results and discussions
Fig. 2. The UV spectra of polysiloxane in CHCl 3 solution, casting film and 16-layer LB film.
3.1. Monolayer behaÕior and LB films deposition For some polysiloxanes, the deposition of LB films are difficult because of the large rigidity of the films especially for the polymer with high Tg. But PI showed low bulk liquid crystalline transition temperature due to its LC transition temperature is lower than the experimental temperature, the polysiloxane backbone is relatively flexibility, so film-forming property is excellent due to the high freedom of movement of polysiloxane backbone. The transfer is facile for hydrophobic substrate if keeping a slow dipping speed of 5 mmrmin and 20 min of drying time. The film behavior was found to show almost no
significant change in the isotherms in the range of 108C– 328C. The transfer ratios of polysiloxane monolayers of Y-type onto hydrophobic slide at 30 mNrm is 1.0 " 0.05 and the sample homogenous in appearance. 3.2. X-ray diffractometer measurement X-ray diffractometer results for both the casting films and LB films of polysiloxane are measured in the same conditions. The casting film is annealed at 1008C for half an hour to make the mesogenic side-chains well ordered. From Fig. 1, the intensity of the peaks for LB films are apparently higher than that for casting films. LB films showed a more ordered layer structure. The d-spacing for the LB films are also higher than that for casting films. The molecules in LB films are more extended along their main molecular axis in the monolayer at the airrwater interface and kept in LB films than in casting films, so the d-spacing for LB films are larger than that of casting films. 3.3. UV spectra of monolayer and LB films
Fig. 1. The X-ray diffraction patterns of polysiloxane for: Ža. 32-layers of Y-type; Žb. 16-Y-type LB films; Žc. 16-layers X-type LB film; Žd. LB films casting film after heating to 1008C.
The UV spectra for casting film and CHCl 3 solution and LB film are shown in Fig. 2. Only one peak at 246 nm appeared from the spectra for hexane solution correspond to the phenyl group conjugated with the carbonyl group. This peak shifts to 239 nm in casting film and 233 nm in LB film. The intensity of this peak increase linearly with the increasing of the number of layers of LB films. This hypsochromic shift indicates the formation of H-aggregates. In fact, the molecules can form smectic phase in which the dipoles of the molecules organized in a paral-
Q. Xue et al.r Materials Science and Engineering C 10 (1999) 87–90
leled manner. Larger hypsochromic shift has been found in LB film than in casting film, indicating the formation of more ordered films. The lower ratio of absorption of the peak at 233 nm to that of 200 nm in LB films than that ratio of corresponding peaks in casting film indicates that the dipole of the molecules tend to organize more parallel to the film normal of LB films. Maybe the LB film can be viewed as a tilted smectic C layer structure while the casting films are compose of polydomains of bulk Smectic C with different orientation due to the high viscosity of the bulk phase. The temperature-dependent UV spectra, shown in Fig. 3 for 10 layers of LB films are measured in situ when it was heated to above 1408C. It was found that the layer structure and the molecular order in the LB films changed when heating. The peak at 233 nm at room temperature moved to 239 nm at 1408C, almost the same value found in casting film, indicating that the angle of the dipole moments of the molecules decreased. Because the polysiloxane is in smectic CU liquid crystal state, this change apparently is caused by the tilt of the molecules main axes at higher temperature. 3.4. Polarized FT-IR spectra Further study on the order of the molecules in the layer of the LB films is performed by un-polarized and polarized FT-IR method w11x. The spectra of 32-Layer LB films with transfer ratios 1.0 " 0.05 on one side of CaF2 slide was measured. The definition of the polarized light followed Vandeveyver’s conventions w12x.
89
Fig. 4. The transmitting FT-IR of polysiloxane for casting film Ža. and 16-layers LB film Žb..
Fig. 4 shows the transmit FT-IR spectra of the casting film and that of 16-layers LB films. Frequencies of the –CH 2 – asymmetric and symmetric stretching bands are sensitive to the conformation of the alkyl chains. The bands appeared near 2918 and 2850 cmy1 for highly ordered Žall trans-zigzag conformation., and 2927 and 2856 cmy1 for highly disordered methylene chains w13– 17x, respectively. The bands appeared at 2921 and 2852 cmy1 , respectively for LB film of polysiloxane PI while appeared at 2924.9 and 2855 cmy1 for casting film, indicating that the packing of the molecules is more ordered in LB film than in casting film. Keeping the absorption of CH 3 stretch mode constant for both films, the absorption of the other groups for LB films are apparently higher than that of the casting films. It can be concluded that the projection of the dipole moments of the groups in the substrate surface for the casting film are larger than that for the LB film, especially for the peaks 1603 and 1503 cmy1 which correspond to the C5C stretch of Table 1 Major infrared-band assignment and the tilt angles of the groups respect to the film normal for PI
Fig. 3. The temperature-dependent UV spectra of polysiloxane at different temperature, the peak at 233 nm at room temperature moved gradually to 239 nm at 1408C.
Peak Žcmy1 .
Assignment of functional group
Orientation angle
2922 2851 1750 1738 1717 1603 1507 1465 1262 1203 1162 1096
CH 2 asymmetric stretch n as CH 2 symmetric stretch n s C5O stretch spacer C5O stretch end group C5O stretch bridge bond C5C stretch in benzene C5C stretch in benzene CH 2 deform. C–O–C stretch C–O–C stretch of benzene ester C–O–C stretch Si–O–Si
b s638, g s 37.7 a s668 648 598 608 318 358 498 378 418 378 548
90
Q. Xue et al.r Materials Science and Engineering C 10 (1999) 87–90
heating the sample, just the same as in bulk phase of Smectic CU of polysiloxane. For ferroelectric liquid crystalline materials the order in the smectic layer and the interaction between layers are important in the overall bulk properties. The polarization should be affected by LB technique which provide different layer order by different deposition method. It is also possible to optimize the spontaneous polarization of a ferroelectric liquid crystalline film without the use of electric or magnetic fields w19x. It is also found that the polarization switching in LB films are faster than in bulk. The characterization of the ferroelectricity of LB films of Liquid Crystal are in progress, this will give deep insight of the packing of the molecules. Fig. 5. The packing model of the polysiloxane molecules in LB films. The orientation angel are as shown in the scheme. Only one layer is shown.
benzene ring and that peaks at 1273, 1209, and 1165 cmy1 which correspond to the C–O–C stretch of the phenyl benzoate ester. The molecules in the casting film may form polydomains with isotropic orientation, while the molecules in the LB film tend to organized perpendicular to the film surface because they are constructed by depositing the monolayers in which the mesogenic side chains are oriented vertical to the surface of water. The orientation angle of the mesogenic unit in the LB films can be evaluated by polarized FT-IR spectra. The results are shown in Table 1. A ´ Ž i s 08, i s 608., A H Ž i s 08., and A ´ Ž i s 608. refer to the situation that the electric vector of the light is in the films plane but perpendicular Žparalleled. to the dipping direction and forms an angle with the film normal of 08 or 608, respectively w18x. We can get g s 37.78 which defined as the angle between the main axis of CH 2 chain and the film normal according to equation derived from Vandevyver’s by using the relation cos 2a q cos 2b q cos 2g s 1, where a , b are the angles formed by the symmetric vibration mode and asymmetric vibration mode of methylene with the film normal, respectively. The assignments and the orientation angles of the groups are listed in Table 1. The C5C stretch mode of the benzene ring is 318. The 5C–O stretch mode 378 is apparently consistent with the value of g . The carbonyl group oriented with an angle of 608 respect to the film normal. Considering the angles of the CH 2 chain, the benzene ring, and the C–O–C, we can obtain the layer structure model of the film as shown in Fig. 5, the Si–O–Si groups are anchored to the plane paralleled to the substrate surface, the Si–O–Si stretch mode formed an angle of 548 with the film normal. The film obtained by Langmuir–Blodgett technique is apparently uniform organized and with discrete thickness. The mesogens are found to be tilted which is similar to that in Sm CU phase, the tilt angle will change after
Acknowledgements The author thanks the financial support of the key Lab of Institute of Colloid and Interface Chemistry of Educational Ministry of China And the State Major basic Research Project.
References w1x P. LaBarney, J.C. Dubois, in: C.B. McArdle ŽEd.., Side Chain Liquid Crystal Polymers, Glasgow, 1989, p. 130. w2x S. Lagerwall, I. Dahl, Mol. Cryst. Liq. Cryst. 114 Ž1984. 151. w3x N.A. Clark, S. Lagerwall, Appl. Phys. Lett. 36 Ž1980. 899. w4x L.A. Beresnev, L.M. Blinov, M.A. Osipov, S.A. Pikin, Mol. Cryst. Liq. Cryst. 158A Ž1988. Ferroelctric Liquid Crystal. w5x G. Roberts, Langmuir–Blodgett Films, Plenum, New York, 1990. w6x G.L. Gaines, Jr., Insoluble Monolayers at liquidrgas Interface. Wiley, New York, 1966. w7x A. Ulman, An Introduction to Ultrathin Organic Films; From Langmuir–Blodgett Films to Self-Assembly, Academic Press, New York, 1991. w8x W. Rettig, J. Naciri, R. Shashidhar, R.S. Duran, Macromolecules 24 Ž1991. 6539. w9x Q.-Z. Zhang, Q.-B. Xue, K.-Z. Yang, Polym. Adv. Technol. 7 Ž1996. 129. w10x Q.-B. Xue, X. Chen, K.-Z. Yang, Q.-Z. Zhang, Macromol. Chem. Phys. 196 Ž1995. 3243. w11x H. Hui-Litwin, L. Servant, M.J. Dignam, M. Moskovits, Langmuir 13 Ž1997. 7211–7216. w12x M. Vandevyver, A. Barraud, R. Teixier, P. Millard, C. Gianotti, J. Colloid Interface Sci. 85 Ž1982. 571. w13x X. Zhang, H.-B. Li, B. Zhao, J.C. Shen, Z.M. Gao, X.S. Li, Macromolecules 30 Ž1997. 1633–1636. w14x C. Nasseli, J.F. Rabolt, J.D. Swalen, J. Chem. Phys. 82 Ž1985. 2136. w15x C. Nasseli, J.P. Rabe, J.F. Rabolt, J.D. Swalen, Thin Solid Films 134 Ž1985. 173. w16x M.D. Porter, T.R. Bright, D.L. Allara, C.E.D. Chidsey, J. Am. Chem. Soc. 109 Ž1987. 3559. w17x R.G. Nuzzo, F.A. Fusco, D.L. Allara, J. Am. Chem. Soc. 109 Ž1987. 2358. w18x Y. Hsu, L.P. Thomas, G. David, Whitten, Langmuir 10 Ž1994. 2757. w19x S. Pfeiffer, R. Shashidhar, T.L. Fare, J. Naciri, J. Adams, R.S. Duran, Appl. Phys. Lett. 63 Ž9. Ž1993. 1285.
Studies of casting and LB films of ferroelectric liquid crystalline polysiloxane Qingbin Xue a
a,)
, Kongzhang Yang a , Hongguo Liu a , Xiao Chen a , Jianzhuang Jiang b, Qizhen Zhang b
Laboratory for Colloid and Interface Chemistry of State Educational Ministry, Shandong UniÕersity, Jinan 250100, China b Chemistry Department, Shandong UniÕersity, Jinan 250100, China Accepted 21 May 1999
Abstract Monolayers of side chain ferroelectric liquid crystalline polysiloxane have been successively transferred onto appropriate substrates by vertical dipping method to study the morphology and the orientation of the mesogens and the side chains by UV spectra and polarized FT-IR spectra and X-ray diffraction. The spectra for casting film and LB film are compared. The molecules in LB films are found to be more orientated along the normal of the substrate at an angle of 378 for the mesogens as the side chain as found by polarized FT-IR spectra and transmitting FT-IR spectra than casting film. Layer structure is also confirmed by X-ray diffraction results. The mesogens are found to form H-aggregates structure in bulk phase and LB films from UV spectra. Temperature-dependent UV spectra have revealed the change of the tilt angle of the mesogens inside the layer of both bulk phase and LB films. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Langmuir–Blodgett films; Ferroelectric liquid crystalline polysiloxane; Polarized FT-IR spectroscopy
1. Introduction Ferroelectric liquid crystalline polysiloxanes have attracted more and more attentions because of their potential application in different kind of flat panel display devices and fast electro-optical devices or nonlinear optical devices w1,2x. Ferroelectricity of liquid crystals which appeared when helix of the SUc phase is unwound can be obtained in different ways w3,4x including Surface-Stabilized Ferroelectric Liquid Crystals ŽSSFLC. involving the effect of boundary. But for these application, the prerequisite is that the ferroelectric liquid crystal molecules organize homogeneously to form uniform films with discrete thickness and orientation. For certain materials that spread on water surface to form a monolayer, the Langmuir–Blodgett technique can be used to produce organized films of controlled thickness w5–7x. Water surface can provide an unique microenvironment that induces formation of organized
monolayer of the molecules at the interface w8x. Transfer of these monolayers to solid surface can be performed to obtain films of microns thickness which show surfacestabilized effect. In this paper, we have studied transferred LB films of newly synthesized polysiloxane with low and broad Sm CU range by spectroscopy method in order to obtain information about the organization of the molecules in the monolayer after being transferred onto solid substrates.
2. Experimental
)
Corresponding author. Tel.: q86-531-8564750; fax: q86-5318565167; E-mail: [email protected]
The sample used in this study was synthesized as previously described w9x. The monolayer was obtained by spreading chloroform solution of the sample and com-
0928-4931r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 8 - 4 9 3 1 Ž 9 9 . 0 0 1 0 1 - 0
88
Q. Xue et al.r Materials Science and Engineering C 10 (1999) 87–90
pressed to target pressure and maintained for half an hour before transferred onto substrates in a vertical-dipping method on a commercially available computer controlled Langmuir trough NIMA 2000 with a Whilhelmy balance as pressure sensor. Details about the monolayer behavior of polysiloxane is published elsewhere w10x. The monolayers were deposited onto quartz slides to carried out the X-ray diffraction Žby a DrMAX-gB X-ray diffractometer with Cu K a radiation l s 0.154 nm. and UV absorption spectra Žby a Shimadzu recording spectrometer, Model UV-240.. The angular resolution of the 2 u scan is 0.028. Polarized FT-IR spectra is performed by Nicolet 710 FT-IR spectrometer for 200 scan with the aid of a wire-grid polarizer and CaF2 as substrate. Fused quartz plates are vigorously cleaned in chromic acid and 4% KOH aq. for 24 h and immersed into 10% trichloromethylsilanerCHCl 3 to hydrophobilize. CaF2 slides for the polarized FT-IR spectra have been cleaned by chloroform thoroughly.
3. Results and discussions
Fig. 2. The UV spectra of polysiloxane in CHCl 3 solution, casting film and 16-layer LB film.
3.1. Monolayer behaÕior and LB films deposition For some polysiloxanes, the deposition of LB films are difficult because of the large rigidity of the films especially for the polymer with high Tg. But PI showed low bulk liquid crystalline transition temperature due to its LC transition temperature is lower than the experimental temperature, the polysiloxane backbone is relatively flexibility, so film-forming property is excellent due to the high freedom of movement of polysiloxane backbone. The transfer is facile for hydrophobic substrate if keeping a slow dipping speed of 5 mmrmin and 20 min of drying time. The film behavior was found to show almost no
significant change in the isotherms in the range of 108C– 328C. The transfer ratios of polysiloxane monolayers of Y-type onto hydrophobic slide at 30 mNrm is 1.0 " 0.05 and the sample homogenous in appearance. 3.2. X-ray diffractometer measurement X-ray diffractometer results for both the casting films and LB films of polysiloxane are measured in the same conditions. The casting film is annealed at 1008C for half an hour to make the mesogenic side-chains well ordered. From Fig. 1, the intensity of the peaks for LB films are apparently higher than that for casting films. LB films showed a more ordered layer structure. The d-spacing for the LB films are also higher than that for casting films. The molecules in LB films are more extended along their main molecular axis in the monolayer at the airrwater interface and kept in LB films than in casting films, so the d-spacing for LB films are larger than that of casting films. 3.3. UV spectra of monolayer and LB films
Fig. 1. The X-ray diffraction patterns of polysiloxane for: Ža. 32-layers of Y-type; Žb. 16-Y-type LB films; Žc. 16-layers X-type LB film; Žd. LB films casting film after heating to 1008C.
The UV spectra for casting film and CHCl 3 solution and LB film are shown in Fig. 2. Only one peak at 246 nm appeared from the spectra for hexane solution correspond to the phenyl group conjugated with the carbonyl group. This peak shifts to 239 nm in casting film and 233 nm in LB film. The intensity of this peak increase linearly with the increasing of the number of layers of LB films. This hypsochromic shift indicates the formation of H-aggregates. In fact, the molecules can form smectic phase in which the dipoles of the molecules organized in a paral-
Q. Xue et al.r Materials Science and Engineering C 10 (1999) 87–90
leled manner. Larger hypsochromic shift has been found in LB film than in casting film, indicating the formation of more ordered films. The lower ratio of absorption of the peak at 233 nm to that of 200 nm in LB films than that ratio of corresponding peaks in casting film indicates that the dipole of the molecules tend to organize more parallel to the film normal of LB films. Maybe the LB film can be viewed as a tilted smectic C layer structure while the casting films are compose of polydomains of bulk Smectic C with different orientation due to the high viscosity of the bulk phase. The temperature-dependent UV spectra, shown in Fig. 3 for 10 layers of LB films are measured in situ when it was heated to above 1408C. It was found that the layer structure and the molecular order in the LB films changed when heating. The peak at 233 nm at room temperature moved to 239 nm at 1408C, almost the same value found in casting film, indicating that the angle of the dipole moments of the molecules decreased. Because the polysiloxane is in smectic CU liquid crystal state, this change apparently is caused by the tilt of the molecules main axes at higher temperature. 3.4. Polarized FT-IR spectra Further study on the order of the molecules in the layer of the LB films is performed by un-polarized and polarized FT-IR method w11x. The spectra of 32-Layer LB films with transfer ratios 1.0 " 0.05 on one side of CaF2 slide was measured. The definition of the polarized light followed Vandeveyver’s conventions w12x.
89
Fig. 4. The transmitting FT-IR of polysiloxane for casting film Ža. and 16-layers LB film Žb..
Fig. 4 shows the transmit FT-IR spectra of the casting film and that of 16-layers LB films. Frequencies of the –CH 2 – asymmetric and symmetric stretching bands are sensitive to the conformation of the alkyl chains. The bands appeared near 2918 and 2850 cmy1 for highly ordered Žall trans-zigzag conformation., and 2927 and 2856 cmy1 for highly disordered methylene chains w13– 17x, respectively. The bands appeared at 2921 and 2852 cmy1 , respectively for LB film of polysiloxane PI while appeared at 2924.9 and 2855 cmy1 for casting film, indicating that the packing of the molecules is more ordered in LB film than in casting film. Keeping the absorption of CH 3 stretch mode constant for both films, the absorption of the other groups for LB films are apparently higher than that of the casting films. It can be concluded that the projection of the dipole moments of the groups in the substrate surface for the casting film are larger than that for the LB film, especially for the peaks 1603 and 1503 cmy1 which correspond to the C5C stretch of Table 1 Major infrared-band assignment and the tilt angles of the groups respect to the film normal for PI
Fig. 3. The temperature-dependent UV spectra of polysiloxane at different temperature, the peak at 233 nm at room temperature moved gradually to 239 nm at 1408C.
Peak Žcmy1 .
Assignment of functional group
Orientation angle
2922 2851 1750 1738 1717 1603 1507 1465 1262 1203 1162 1096
CH 2 asymmetric stretch n as CH 2 symmetric stretch n s C5O stretch spacer C5O stretch end group C5O stretch bridge bond C5C stretch in benzene C5C stretch in benzene CH 2 deform. C–O–C stretch C–O–C stretch of benzene ester C–O–C stretch Si–O–Si
b s638, g s 37.7 a s668 648 598 608 318 358 498 378 418 378 548
90
Q. Xue et al.r Materials Science and Engineering C 10 (1999) 87–90
heating the sample, just the same as in bulk phase of Smectic CU of polysiloxane. For ferroelectric liquid crystalline materials the order in the smectic layer and the interaction between layers are important in the overall bulk properties. The polarization should be affected by LB technique which provide different layer order by different deposition method. It is also possible to optimize the spontaneous polarization of a ferroelectric liquid crystalline film without the use of electric or magnetic fields w19x. It is also found that the polarization switching in LB films are faster than in bulk. The characterization of the ferroelectricity of LB films of Liquid Crystal are in progress, this will give deep insight of the packing of the molecules. Fig. 5. The packing model of the polysiloxane molecules in LB films. The orientation angel are as shown in the scheme. Only one layer is shown.
benzene ring and that peaks at 1273, 1209, and 1165 cmy1 which correspond to the C–O–C stretch of the phenyl benzoate ester. The molecules in the casting film may form polydomains with isotropic orientation, while the molecules in the LB film tend to organized perpendicular to the film surface because they are constructed by depositing the monolayers in which the mesogenic side chains are oriented vertical to the surface of water. The orientation angle of the mesogenic unit in the LB films can be evaluated by polarized FT-IR spectra. The results are shown in Table 1. A ´ Ž i s 08, i s 608., A H Ž i s 08., and A ´ Ž i s 608. refer to the situation that the electric vector of the light is in the films plane but perpendicular Žparalleled. to the dipping direction and forms an angle with the film normal of 08 or 608, respectively w18x. We can get g s 37.78 which defined as the angle between the main axis of CH 2 chain and the film normal according to equation derived from Vandevyver’s by using the relation cos 2a q cos 2b q cos 2g s 1, where a , b are the angles formed by the symmetric vibration mode and asymmetric vibration mode of methylene with the film normal, respectively. The assignments and the orientation angles of the groups are listed in Table 1. The C5C stretch mode of the benzene ring is 318. The 5C–O stretch mode 378 is apparently consistent with the value of g . The carbonyl group oriented with an angle of 608 respect to the film normal. Considering the angles of the CH 2 chain, the benzene ring, and the C–O–C, we can obtain the layer structure model of the film as shown in Fig. 5, the Si–O–Si groups are anchored to the plane paralleled to the substrate surface, the Si–O–Si stretch mode formed an angle of 548 with the film normal. The film obtained by Langmuir–Blodgett technique is apparently uniform organized and with discrete thickness. The mesogens are found to be tilted which is similar to that in Sm CU phase, the tilt angle will change after
Acknowledgements The author thanks the financial support of the key Lab of Institute of Colloid and Interface Chemistry of Educational Ministry of China And the State Major basic Research Project.
References w1x P. LaBarney, J.C. Dubois, in: C.B. McArdle ŽEd.., Side Chain Liquid Crystal Polymers, Glasgow, 1989, p. 130. w2x S. Lagerwall, I. Dahl, Mol. Cryst. Liq. Cryst. 114 Ž1984. 151. w3x N.A. Clark, S. Lagerwall, Appl. Phys. Lett. 36 Ž1980. 899. w4x L.A. Beresnev, L.M. Blinov, M.A. Osipov, S.A. Pikin, Mol. Cryst. Liq. Cryst. 158A Ž1988. Ferroelctric Liquid Crystal. w5x G. Roberts, Langmuir–Blodgett Films, Plenum, New York, 1990. w6x G.L. Gaines, Jr., Insoluble Monolayers at liquidrgas Interface. Wiley, New York, 1966. w7x A. Ulman, An Introduction to Ultrathin Organic Films; From Langmuir–Blodgett Films to Self-Assembly, Academic Press, New York, 1991. w8x W. Rettig, J. Naciri, R. Shashidhar, R.S. Duran, Macromolecules 24 Ž1991. 6539. w9x Q.-Z. Zhang, Q.-B. Xue, K.-Z. Yang, Polym. Adv. Technol. 7 Ž1996. 129. w10x Q.-B. Xue, X. Chen, K.-Z. Yang, Q.-Z. Zhang, Macromol. Chem. Phys. 196 Ž1995. 3243. w11x H. Hui-Litwin, L. Servant, M.J. Dignam, M. Moskovits, Langmuir 13 Ž1997. 7211–7216. w12x M. Vandevyver, A. Barraud, R. Teixier, P. Millard, C. Gianotti, J. Colloid Interface Sci. 85 Ž1982. 571. w13x X. Zhang, H.-B. Li, B. Zhao, J.C. Shen, Z.M. Gao, X.S. Li, Macromolecules 30 Ž1997. 1633–1636. w14x C. Nasseli, J.F. Rabolt, J.D. Swalen, J. Chem. Phys. 82 Ž1985. 2136. w15x C. Nasseli, J.P. Rabe, J.F. Rabolt, J.D. Swalen, Thin Solid Films 134 Ž1985. 173. w16x M.D. Porter, T.R. Bright, D.L. Allara, C.E.D. Chidsey, J. Am. Chem. Soc. 109 Ž1987. 3559. w17x R.G. Nuzzo, F.A. Fusco, D.L. Allara, J. Am. Chem. Soc. 109 Ž1987. 2358. w18x Y. Hsu, L.P. Thomas, G. David, Whitten, Langmuir 10 Ž1994. 2757. w19x S. Pfeiffer, R. Shashidhar, T.L. Fare, J. Naciri, J. Adams, R.S. Duran, Appl. Phys. Lett. 63 Ž9. Ž1993. 1285.