- Email: [email protected]
water interface
Thin Solid Films 327–329 (1998) 145–149
Mesogenic structural effects on monolayer behaviors of the liquid crystalline polysiloxanes at the air/water interface Xiao Chen a, Kong-Zhang Yang a ,*, Qing-Bin Xue a, Qi-Zhen Zhang b a
Institute of Colloid and Interface Chemistry, Shandong University, Jinan 250100, People’s Republic of China b Department of Chemistry, Shandong University, Jinan 250100, People’s Republic of China
Abstract The monolayer behavior of three newly synthesized side-chain liquid crystalline polysiloxanes and corresponding monomers have been investigated at the air/water interface. The surface pressure-area isotherms and Brewster angle microscopy (BAM) were used to study the influence by different mesogenic units in the side chains. The results indicate that difference in mesogenic unit structure, such as linkage bond and mesogen length, have marked effects on the liquid crystallinity and self-aggregation tendency of the molecular chains which determine the properties of the monolayers. 1998 Elsevier Science S.A. All rights reserved Keywords: Monolayers; Liquid crystalline polymers; Mesogenic unit; Brewster angle microscopy
face and can be deposited on substrates by means of LB technique [6–8].
1. Introduction Liquid crystalline polymers (LCPs) are of wide interest due to their potential use in electro-optical and information storage devices. Of interest from an applications point of view are self-organizing materials forming mono- and multilayer assemblies [1–5]. Using the Langmuir–Blodgett (LB) technique, it is convenient to prepare novel supermolecular structures not feasible in normal smectic LC systems. To do so, it is of great importance to get information about the structure of the underlying monolayer, which is normally very sensitive to changes in the chemical structure of the molecules. The mesogenic units, which play very important roles on the LCP properties, greatly affect the spreading behaviors of LCPs. But to our knowledge there has been no systematic investigation on such effects. Therefore, in this paper we chose a series of newly synthesized side-chain LCPs to investigate how the mesogenic structures influence monolayer behaviors. These candidates form monolayers when spread at an air/water inter-
* Corresponding author. Tel.: +86 531 8930446; fax: +86 531 8902167; e-mail: [email protected]
0040-6090/98/$ - see front matter PII S0040-6090 (98 )0 0617-8
2. Experimental details The chemical structures of three LCPs (denoted as P-n) and corresponding monomers (denoted as M-n) are shown in Fig. 1, which differ only in their mesogenic units. Their synthesis and characterization have been described elsewhere [9,10]. The experiments for monolayer spreading and deposition of LB film were performed on a commercially available Langmuir trough NIMA 2000 (NIMA Tech., Coventry, UK) with computerized control. A Wilhelmy balance was used as a surface pressure sensor. Monolayers were obtained by spreading chloroform solutions of the compounds, with concentrations between 0.2 and 0.8 mg/ml. The water used for the subphase was prepared from redistilled water. All measurements were carried out at room temperature (25 ± 1°C). All isotherms were run a minimum of three times with reproducibility errors of less than ±0.1 nm2. The details of the performance are given in [7]. The morphology of the monolayer at the air/water interface was observed by means of BAM-II microscope (Aca-
1998 Elsevier Science S.A. All rights reserved
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X. Chen et al. / Thin Solid Films 327–329 (1998) 145–149
demia Sinica) [11]. The p-polarized beam of a He–Ne laser (l = 632.8 nm) was directed at the Brewster angle (53.1°) at the pure air/water interface.
3. Results and discussion 3.1. Surface pressure (p)–area (A) isotherms Surface pressure–area isotherms for monolayers of monomers and polymers are shown in Fig. 2A,B, respectively. Monolayers of monomers on the pure water subphase showed large difference due to their variant phase transition temperatures. As shown in Fig. 2A, the M-I, which was in LC phase at room temperature, exhibited a more expanded monolayer isotherm and also it had been confirmed that the compression-expansion cycle was reversible [6]. This can be contributed to its high fluidity and self-organizing capability in LC phase. However, the same processes for two other monomers that were in crystalline states at room temperature showed more condensed isotherms and relatively large compression-expansion irreversibility due to higher rigidities and self-aggregation tendencies of the chains [7,8]. Monolayers for LCPs exhibited greater stabilities than those of the monomers, as can be seen from the collapse pressures in Fig. 2B. This is because the hydrophilicity and anchoring effect of the siloxane backbones could reduce the tangling of side chains and improve their orientation in the monolayer [7]. However, LCPs formed densely packed monolayers with larger irreversibility and aggregation tendency than those of monomers due to their inherent high viscosity and low diffusivity. The irreversibility is also resulted from the longer side chains which were easy to aggregate and form crystallites upon compression and after releasing the pressure they did not re-spread to the initial state. Instead they could remain on the water surface as islands of crystallites [12]. Though the difference of iso-
Fig. 2. Surface pressure-area isotherms for monolayers of investigated materials on the pure water subphase: (A) monomer; (B) polymer.
therms is not as clear as that of monomers, we can still observe that P-I (also in LC phase at room temperature) showed more fluid-like behavior than those of P-II and P-III. 3.2. BAM morphology observation
Fig. 1. Chemical structures of the investigated materials.
The dynamic changes of above monolayers during compression-expansion cycles were observed in situ by BAM. The direct images for spreading, surface stacking and monolayer rigidity could be used to effectively complement the results drawn from the isotherm analyses. Fig. 3 shows the representative BAM images of M-I and M-III monolayers selected at different stages of compression. It is clearly seen that M-I spread into the small fluid phase of the optically uniform texture (Fig. 3a). M-III, however, spread into more solid analogous islands due to reduced liquid crystallinity and strong self-aggregation (Fig. 3a′). On increase of the surface pressure during compression, the small fluid domains of M-I were changed gradually to liquid condensed phase and ‘melted’ together to form large domains which
X. Chen et al. / Thin Solid Films 327–329 (1998) 145–149
could be restored to their initial states after releasing pressure, showing good compression-expansion reversibility (Fig. 3b,c). This process is not the same for M-III because the initial islands coalesced and formed a complete rigid monolayer in response to the pressure increase (Fig. 3b′, c′). Such rigid monolayer showed larger incompressibility and concave lines with further compression. When it was expanded at this moment, only rigid fragments (aggregates) could be observed which were not spread again, showing large compression-expansion irreversibility. Compared with monomer monolayers, LCPs spread into domains or islands of higher density and thickness. Fig. 4 shows the BAM images of P-I and P-III monolayers at
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different stages of compression. As can be seen from Fig. 4a–c, domains of fluid-like characteristics were also shown after P-I spreading due to its high liquid crystallinity. Upon compression, a uniform and densely packed film was formed at the air/water interface, which could not be restored at once to the initial state, and only relaxation was shown upon the monolayer expansion. Similar to the monomer, flakes with irregular shapes but constant thickness were formed just after P-III spreading (Fig. 4a′). It seems that the P-III was ‘precipitated’ out of the spreading solution onto the water surface reflecting large self-aggregation tendency. During compression, these flakes were pushed together and ‘fused’ to produce a rigid and brittle monolayer which was
Fig. 3. BAM images of the M-I (a–c) and M-III (a′–c′) monolayers at (a) and (a′) spreading (p = 0 mN/m), (b) p = 4 mN/m, (b′) p = 2 mN/m, (c) and (c′) p = 6 mN/m.
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easy to break on further compression (Fig. 4b′). When decompressing, the monolayer was collapsed to small unrespreadable islands. It should be pointed that P-II monolayer only showed increased non-uniformity of morphology upon further compression at high pressures exhibiting more flexibility of its mesogenic side chains. 3.3. The Langmuir–Blodgett film deposition properties The LB film transfer properties for three LCPs were studied. Though the monolayers of investigated LCPs were stable, they showed quite different LB film deposition prop-
erties which were related to their monolayer flexibilities and rigidities [6–8]. Being on the LC phase at room temperature, the P-I monolayer could be uniformly deposited onto the solid substrate with a transfer ratio of about 1 due to its high flexibility. Monolayers of P-II and P-III, however, could not be quantitatively transferred because of their relatively high rigidities. Results from X-ray diffraction showed the side chains in P-I were rearranged during deposition to form LB film structures similar to that of smectic layers in bulk materials. Through mixing with AA, P-II and P-III could be successfully deposited because the monolayer rigidities were reduced [7,8].
Fig. 4. BAM images of the P-I (a–c) and P-III (a′–c′) monolayers at (a) and (a′) spreading (p = 0 mN/m), (b) and (b′) p = 5 mN/m, (c) and (c′) during monolayer expanding from high surface pressure (p = 20 mN/m).
X. Chen et al. / Thin Solid Films 327–329 (1998) 145–149
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to an enhancement of the thermal stability of the mesophase, i.e. less flexibility and larger rigidity at room temperature. Therefore it is clear that self-ordering capability and reversible monolayer behavior can only be exhibited when the LCPs are in their LC phases.
4. Conclusion
Fig. 5. Structural analyses of the mesogenic units.
3.4. The structure analysis of the mesogenic unit Difference in monolayer and LB film deposition properties for these LC compounds could be explained through analyzing their liquid crystallinities at room temperature which are related with their mesogenic unit structures. By comparison of the changes in the phase transition temperatures, linkage bonds and mesogenic unit length, the observed difference in fluidity, rigidities and self-aggregation could be well understood. Firstly, the values of phase transition temperature can exhibit the molecular interactions between mesogens. From I to III, Tiso (to isotropic state) increases gradually (monomer: 47.5 → 76 → 163°C, polymer: 128 → 155 → 225°C). Such increased thermal stability means reduced liquid crystallinity or self-organizing capability. This is why the M-I and P-I, being in LC states at room temperature, show fluid-like and flexible monolayer behaviors and good LB deposition qualities. Compounds of structures II and III, however, form more condensed and rigid monolayers because of not being in LC states. Secondly, analyses of the linkage bonds can help understand the effect of liquid crystallinity at the molecular level. As shown in Fig. 5, the linkage between the phenylene rings for I is the ester group, which means that additional chain motion is possible in the LC phase with the result that the transition temperature to LC phase is reduced [13]. The ester group also increases molecular flexibility. Besides the rotation of the phenylene ring along the long axis, crankshafttype motion exists, associated with the rotation of the ester group, which results in higher molecular conformational entropy, therefore reducing the chain rigidity. As to II, the ester linkage unit is substituted by methylenimine group (CH==N). The double bond restricts the freedom of rotation leading to a pronounced increase in melting temperature. Therefore it shows less liquid crystallinity than I at room temperature. Though III has similar structure to I, the substitution of the end carbonyl in I by a phenylene ring in III makes them differ greatly in monolayer behavior. On the one hand, such substitution results in a large steric effect which restricts the freedom of rotation. On the other hand, the increased mesogen length of III (compared to I and II) promotes the molecular interactions and results in the increase of the enthalpy changes in phase transition leading
Results of our experiments have given clear evidence that the mesogenic unit structure plays an important role on the monolayer and LB film properties of LCPs. The transition temperature to LC phase is directly determined by the linkage unit (bridge bond) and mesogen axial length. Molecules of LC phase at room temperature exhibit high fluidity and self-ordering capability which make it easy to form stable monolayers and to prepare LB films. Molecules with high transition temperatures, however, show large rigidities and self-aggregation effects which result in bad film-forming properties. Data and regularity obtained are very useful for preparation of functional ultrathin films from LCPs and are beneficial to application explorations.
Acknowledgements This work was supported by the National Natural Science Foundation of China and the State Major Basic Research Project. The authors thank the Key Laboratory for Colloid and Interface Chemistry of the State Education Commission for support in BAM experiments.
References [1] J. Adams, W. Rettig, R.S. Duran, J. Naciri, R. Shashidhar, J. Phys. Chem. 97 (1993) 2021. [2] H. Fadel, V. Percec, Q. Zheng, R.C. Advincula, R.S. Duran, Macromolecules 26 (1993) 1650. [3] W. Rettig, J. Naciri, R. Shashidhar, R.S. Duran, Thin Solid Films 210/211 (1992) 114. [4] R. Geer, S. Qadri, R. Shashidhar, A.F. Thibodeaux, R.S. Duran, Liq. Cryst. 16 (1994) 869. [5] W. Rettig, J. Naciri, R. Shashidhar, R.S. Duran, Macromolecules 24 (1991) 6539. [6] Q.B. Xue, X. Chen, K.Z. Yang, Q.Z. Zhang, Macromol. Chem. Phys. 196 (1995) 3243. [7] X. Chen, Q.B. Xue, K.Z. Yang, Q.Z. Zhang, Langmuir 11 (1995) 4082. [8] X. Chen, Q.B. Xue, K.Z. Yang, Q.Z. Zhang, Macromolecules 29 (1996) 5658. [9] Q.Z. Zhang, Q.B. Xue, K.Z. Yang, Polym. Adv. Technol. 7 (1996) 129. [10] Q.Z. Zhang, Q.B. Xue, K.Z. Yang, Polym. Adv. Technol. 7 (1996) 135. [11] J.A. Tang, X.C. Li, J.S. Yuan, L. Jiang, Funct. Mater. 24 (1993) 282. [12] J.M. Rodriguez-Parada, M. Kaku, D.Y. Sogah, Macromolecules 27 (1994) 1571. [13] A.M. Donald, A.H. Windle (eds.), Liquid Crystalline Polymers, Cambridge University Press, London, 1992, p. 59.
Mesogenic structural effects on monolayer behaviors of the liquid crystalline polysiloxanes at the air/water interface Xiao Chen a, Kong-Zhang Yang a ,*, Qing-Bin Xue a, Qi-Zhen Zhang b a
Institute of Colloid and Interface Chemistry, Shandong University, Jinan 250100, People’s Republic of China b Department of Chemistry, Shandong University, Jinan 250100, People’s Republic of China
Abstract The monolayer behavior of three newly synthesized side-chain liquid crystalline polysiloxanes and corresponding monomers have been investigated at the air/water interface. The surface pressure-area isotherms and Brewster angle microscopy (BAM) were used to study the influence by different mesogenic units in the side chains. The results indicate that difference in mesogenic unit structure, such as linkage bond and mesogen length, have marked effects on the liquid crystallinity and self-aggregation tendency of the molecular chains which determine the properties of the monolayers. 1998 Elsevier Science S.A. All rights reserved Keywords: Monolayers; Liquid crystalline polymers; Mesogenic unit; Brewster angle microscopy
face and can be deposited on substrates by means of LB technique [6–8].
1. Introduction Liquid crystalline polymers (LCPs) are of wide interest due to their potential use in electro-optical and information storage devices. Of interest from an applications point of view are self-organizing materials forming mono- and multilayer assemblies [1–5]. Using the Langmuir–Blodgett (LB) technique, it is convenient to prepare novel supermolecular structures not feasible in normal smectic LC systems. To do so, it is of great importance to get information about the structure of the underlying monolayer, which is normally very sensitive to changes in the chemical structure of the molecules. The mesogenic units, which play very important roles on the LCP properties, greatly affect the spreading behaviors of LCPs. But to our knowledge there has been no systematic investigation on such effects. Therefore, in this paper we chose a series of newly synthesized side-chain LCPs to investigate how the mesogenic structures influence monolayer behaviors. These candidates form monolayers when spread at an air/water inter-
* Corresponding author. Tel.: +86 531 8930446; fax: +86 531 8902167; e-mail: [email protected]
0040-6090/98/$ - see front matter PII S0040-6090 (98 )0 0617-8
2. Experimental details The chemical structures of three LCPs (denoted as P-n) and corresponding monomers (denoted as M-n) are shown in Fig. 1, which differ only in their mesogenic units. Their synthesis and characterization have been described elsewhere [9,10]. The experiments for monolayer spreading and deposition of LB film were performed on a commercially available Langmuir trough NIMA 2000 (NIMA Tech., Coventry, UK) with computerized control. A Wilhelmy balance was used as a surface pressure sensor. Monolayers were obtained by spreading chloroform solutions of the compounds, with concentrations between 0.2 and 0.8 mg/ml. The water used for the subphase was prepared from redistilled water. All measurements were carried out at room temperature (25 ± 1°C). All isotherms were run a minimum of three times with reproducibility errors of less than ±0.1 nm2. The details of the performance are given in [7]. The morphology of the monolayer at the air/water interface was observed by means of BAM-II microscope (Aca-
1998 Elsevier Science S.A. All rights reserved
146
X. Chen et al. / Thin Solid Films 327–329 (1998) 145–149
demia Sinica) [11]. The p-polarized beam of a He–Ne laser (l = 632.8 nm) was directed at the Brewster angle (53.1°) at the pure air/water interface.
3. Results and discussion 3.1. Surface pressure (p)–area (A) isotherms Surface pressure–area isotherms for monolayers of monomers and polymers are shown in Fig. 2A,B, respectively. Monolayers of monomers on the pure water subphase showed large difference due to their variant phase transition temperatures. As shown in Fig. 2A, the M-I, which was in LC phase at room temperature, exhibited a more expanded monolayer isotherm and also it had been confirmed that the compression-expansion cycle was reversible [6]. This can be contributed to its high fluidity and self-organizing capability in LC phase. However, the same processes for two other monomers that were in crystalline states at room temperature showed more condensed isotherms and relatively large compression-expansion irreversibility due to higher rigidities and self-aggregation tendencies of the chains [7,8]. Monolayers for LCPs exhibited greater stabilities than those of the monomers, as can be seen from the collapse pressures in Fig. 2B. This is because the hydrophilicity and anchoring effect of the siloxane backbones could reduce the tangling of side chains and improve their orientation in the monolayer [7]. However, LCPs formed densely packed monolayers with larger irreversibility and aggregation tendency than those of monomers due to their inherent high viscosity and low diffusivity. The irreversibility is also resulted from the longer side chains which were easy to aggregate and form crystallites upon compression and after releasing the pressure they did not re-spread to the initial state. Instead they could remain on the water surface as islands of crystallites [12]. Though the difference of iso-
Fig. 2. Surface pressure-area isotherms for monolayers of investigated materials on the pure water subphase: (A) monomer; (B) polymer.
therms is not as clear as that of monomers, we can still observe that P-I (also in LC phase at room temperature) showed more fluid-like behavior than those of P-II and P-III. 3.2. BAM morphology observation
Fig. 1. Chemical structures of the investigated materials.
The dynamic changes of above monolayers during compression-expansion cycles were observed in situ by BAM. The direct images for spreading, surface stacking and monolayer rigidity could be used to effectively complement the results drawn from the isotherm analyses. Fig. 3 shows the representative BAM images of M-I and M-III monolayers selected at different stages of compression. It is clearly seen that M-I spread into the small fluid phase of the optically uniform texture (Fig. 3a). M-III, however, spread into more solid analogous islands due to reduced liquid crystallinity and strong self-aggregation (Fig. 3a′). On increase of the surface pressure during compression, the small fluid domains of M-I were changed gradually to liquid condensed phase and ‘melted’ together to form large domains which
X. Chen et al. / Thin Solid Films 327–329 (1998) 145–149
could be restored to their initial states after releasing pressure, showing good compression-expansion reversibility (Fig. 3b,c). This process is not the same for M-III because the initial islands coalesced and formed a complete rigid monolayer in response to the pressure increase (Fig. 3b′, c′). Such rigid monolayer showed larger incompressibility and concave lines with further compression. When it was expanded at this moment, only rigid fragments (aggregates) could be observed which were not spread again, showing large compression-expansion irreversibility. Compared with monomer monolayers, LCPs spread into domains or islands of higher density and thickness. Fig. 4 shows the BAM images of P-I and P-III monolayers at
147
different stages of compression. As can be seen from Fig. 4a–c, domains of fluid-like characteristics were also shown after P-I spreading due to its high liquid crystallinity. Upon compression, a uniform and densely packed film was formed at the air/water interface, which could not be restored at once to the initial state, and only relaxation was shown upon the monolayer expansion. Similar to the monomer, flakes with irregular shapes but constant thickness were formed just after P-III spreading (Fig. 4a′). It seems that the P-III was ‘precipitated’ out of the spreading solution onto the water surface reflecting large self-aggregation tendency. During compression, these flakes were pushed together and ‘fused’ to produce a rigid and brittle monolayer which was
Fig. 3. BAM images of the M-I (a–c) and M-III (a′–c′) monolayers at (a) and (a′) spreading (p = 0 mN/m), (b) p = 4 mN/m, (b′) p = 2 mN/m, (c) and (c′) p = 6 mN/m.
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easy to break on further compression (Fig. 4b′). When decompressing, the monolayer was collapsed to small unrespreadable islands. It should be pointed that P-II monolayer only showed increased non-uniformity of morphology upon further compression at high pressures exhibiting more flexibility of its mesogenic side chains. 3.3. The Langmuir–Blodgett film deposition properties The LB film transfer properties for three LCPs were studied. Though the monolayers of investigated LCPs were stable, they showed quite different LB film deposition prop-
erties which were related to their monolayer flexibilities and rigidities [6–8]. Being on the LC phase at room temperature, the P-I monolayer could be uniformly deposited onto the solid substrate with a transfer ratio of about 1 due to its high flexibility. Monolayers of P-II and P-III, however, could not be quantitatively transferred because of their relatively high rigidities. Results from X-ray diffraction showed the side chains in P-I were rearranged during deposition to form LB film structures similar to that of smectic layers in bulk materials. Through mixing with AA, P-II and P-III could be successfully deposited because the monolayer rigidities were reduced [7,8].
Fig. 4. BAM images of the P-I (a–c) and P-III (a′–c′) monolayers at (a) and (a′) spreading (p = 0 mN/m), (b) and (b′) p = 5 mN/m, (c) and (c′) during monolayer expanding from high surface pressure (p = 20 mN/m).
X. Chen et al. / Thin Solid Films 327–329 (1998) 145–149
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to an enhancement of the thermal stability of the mesophase, i.e. less flexibility and larger rigidity at room temperature. Therefore it is clear that self-ordering capability and reversible monolayer behavior can only be exhibited when the LCPs are in their LC phases.
4. Conclusion
Fig. 5. Structural analyses of the mesogenic units.
3.4. The structure analysis of the mesogenic unit Difference in monolayer and LB film deposition properties for these LC compounds could be explained through analyzing their liquid crystallinities at room temperature which are related with their mesogenic unit structures. By comparison of the changes in the phase transition temperatures, linkage bonds and mesogenic unit length, the observed difference in fluidity, rigidities and self-aggregation could be well understood. Firstly, the values of phase transition temperature can exhibit the molecular interactions between mesogens. From I to III, Tiso (to isotropic state) increases gradually (monomer: 47.5 → 76 → 163°C, polymer: 128 → 155 → 225°C). Such increased thermal stability means reduced liquid crystallinity or self-organizing capability. This is why the M-I and P-I, being in LC states at room temperature, show fluid-like and flexible monolayer behaviors and good LB deposition qualities. Compounds of structures II and III, however, form more condensed and rigid monolayers because of not being in LC states. Secondly, analyses of the linkage bonds can help understand the effect of liquid crystallinity at the molecular level. As shown in Fig. 5, the linkage between the phenylene rings for I is the ester group, which means that additional chain motion is possible in the LC phase with the result that the transition temperature to LC phase is reduced [13]. The ester group also increases molecular flexibility. Besides the rotation of the phenylene ring along the long axis, crankshafttype motion exists, associated with the rotation of the ester group, which results in higher molecular conformational entropy, therefore reducing the chain rigidity. As to II, the ester linkage unit is substituted by methylenimine group (CH==N). The double bond restricts the freedom of rotation leading to a pronounced increase in melting temperature. Therefore it shows less liquid crystallinity than I at room temperature. Though III has similar structure to I, the substitution of the end carbonyl in I by a phenylene ring in III makes them differ greatly in monolayer behavior. On the one hand, such substitution results in a large steric effect which restricts the freedom of rotation. On the other hand, the increased mesogen length of III (compared to I and II) promotes the molecular interactions and results in the increase of the enthalpy changes in phase transition leading
Results of our experiments have given clear evidence that the mesogenic unit structure plays an important role on the monolayer and LB film properties of LCPs. The transition temperature to LC phase is directly determined by the linkage unit (bridge bond) and mesogen axial length. Molecules of LC phase at room temperature exhibit high fluidity and self-ordering capability which make it easy to form stable monolayers and to prepare LB films. Molecules with high transition temperatures, however, show large rigidities and self-aggregation effects which result in bad film-forming properties. Data and regularity obtained are very useful for preparation of functional ultrathin films from LCPs and are beneficial to application explorations.
Acknowledgements This work was supported by the National Natural Science Foundation of China and the State Major Basic Research Project. The authors thank the Key Laboratory for Colloid and Interface Chemistry of the State Education Commission for support in BAM experiments.
References [1] J. Adams, W. Rettig, R.S. Duran, J. Naciri, R. Shashidhar, J. Phys. Chem. 97 (1993) 2021. [2] H. Fadel, V. Percec, Q. Zheng, R.C. Advincula, R.S. Duran, Macromolecules 26 (1993) 1650. [3] W. Rettig, J. Naciri, R. Shashidhar, R.S. Duran, Thin Solid Films 210/211 (1992) 114. [4] R. Geer, S. Qadri, R. Shashidhar, A.F. Thibodeaux, R.S. Duran, Liq. Cryst. 16 (1994) 869. [5] W. Rettig, J. Naciri, R. Shashidhar, R.S. Duran, Macromolecules 24 (1991) 6539. [6] Q.B. Xue, X. Chen, K.Z. Yang, Q.Z. Zhang, Macromol. Chem. Phys. 196 (1995) 3243. [7] X. Chen, Q.B. Xue, K.Z. Yang, Q.Z. Zhang, Langmuir 11 (1995) 4082. [8] X. Chen, Q.B. Xue, K.Z. Yang, Q.Z. Zhang, Macromolecules 29 (1996) 5658. [9] Q.Z. Zhang, Q.B. Xue, K.Z. Yang, Polym. Adv. Technol. 7 (1996) 129. [10] Q.Z. Zhang, Q.B. Xue, K.Z. Yang, Polym. Adv. Technol. 7 (1996) 135. [11] J.A. Tang, X.C. Li, J.S. Yuan, L. Jiang, Funct. Mater. 24 (1993) 282. [12] J.M. Rodriguez-Parada, M. Kaku, D.Y. Sogah, Macromolecules 27 (1994) 1571. [13] A.M. Donald, A.H. Windle (eds.), Liquid Crystalline Polymers, Cambridge University Press, London, 1992, p. 59.