Langmuir 1995,11, 4082-4088
4082
Monolayers and Langmuir-Blodgett Films of a Liquid-Crystalline Polysiloxane with a Schiff Base Mesogenic Unit in the Side Chain Xiao Chen,* Qing-Bin Xue, and Kong-Zhang Yang Institute o f Colloid and Interface Chemistry, Shandong University, Jinan, 250100, China
Qi-Zhen Zhang Department of Chemistry, Shandong University, Jinan, 2501 00, China Received December 5, 1994. I n Final Form: April 10, 1995@ Langmuir and Langmuir-Blodgett (LB)films of a newly synthesized side-chainliquid-crystallinepolymer (PSLC)and its blend with arachidic acid (AA)have been investigated by means of surface pressure-area isotherms, hysteresis curves, X-ray diffraction, and polarized IR and UV-Vis spectroscopies. The results show that AA matrices can enhance monolayer stability and improve the packing of side chains in the film. At high surface pressure, the mixed monolayer undergoes a phase transition from an ordered liquid condensed t o a solidlike phase, corresponding to the formation of aggregates or crystallites. Two types LB films (Y and X) can be obtained on the hydrophobic substrates under the condition of different deposition pressures and AA's proportion. A detailed analysis of the IR dichroism suggests that the axis of the all-trans hydrocarbon chains of AA or spacers in the side chain is less tilted t o the surface normal for larger AA's proportion and the axis of the aromatic Schiff base chromophore is nearly perpendicular to the substrate plane. The hypsoshift of UV-Vis absorption in the film compared with the spectrum in solution indicates that H aggregates form between the chromophore.
Introduction Liquid-crystalline polymers (PLCs)are ofwide interest due to their self-ordering capabilities, with the added advantage over low-molecular-weightcompounds of greater mechanical and thermal stabilities. PLCs have already been applied to optical storage,' nonlinear optical devices,2 fiber manufacture, and so f ~ r t h . Thus, ~ , ~ scientists are more interested in how to assemble PLC molecules to form monolayers or multilayers with defined thicknesses and structures, which are usually difficult to obtain in smectic liquid-crystalline systems. Using the Langmuir-Blodgett technique, it is convenient to form ordered layer structures by transferring PLC monolayers from water surface-to-solid substrate^.^,^ This ordered deposition offers potentially greater control over molecular orientation and inter-molecular packing for these materials, which could lead to more and more creations of new types of thin-film devices especially in the field of electronics and optics.',* Research works on the behavior of Langmuir and LB PLC films have been attracting increasing attention in recent y e a r ~ . ~ - Among ~O
them, side-chain liquid-crystalline polymers are more promising. Different functional mesogenic units can be incorporated into the side chains separated from the backbone by flexible spacers, which not only improve the deposition conditions for rigid and viscous polymer monolayers but also enhance the ultimate ordering in the builtup multilayer film and provide new functions. In this paper, we have chosen a newly synthesized sidechain liquid-crystalline polymer (denoted as PSLC) to investigate its monolayer behavior and LB multilayer preparation. The chemical structure of PSLC is shown in Figure 1together with its thermal bulk-phase behavior. Also shown in Figure 1is a monomer (labeled MLC) with a structure approximately the same as PSLC's side chain. The monolayer property of MLC has also been studied as a comparison. These two candidates are common in that they all contain the chiral center in the headgroups and can be induced to the smectic C* phase to exhibit ferroelectricity.21 Furthermore their para-substituted aromatic Schiff base chromophores may show NLO effe~ts.~~,~~,~~ ~~
Abstract published in Advance ACS Abstracts, September 1, 1995. (1)Eich, M.; Wendroff, J. J . Opt. SOC. Am. 1990, B7, 1428. (2)Assanto, G.; Neher, D.; Stegeman, G. I.; Tonuellas, W. E.; Margues, M. B.; Horsthuis, W. H. G.; Mohlmann, G. R. Mol. Cryst. Liq. Cryst. 1992,222, 33. (3) Nakamura, T.; Ueno, T.; Tani, C. Mol. Cryst. Liq. Cryst. 1989, 169, 167. (4) Sasaki, A. Mol. Cryst. Liq. Cryst. 1986, 139, 103. (5) Ulman, A. An Introduction to Ultrathin Organic Films; Academic: New York, 1991. (6) Roberts, G. G., Ed. Langmuir-Blodgett Films; Plenum: NewYork, 1990. (7) Tieke, B. Adu. Mater. 1990, 2, 222. ( 8 ) Sakuhara, T.; Nakahara, H.; Fukuda, K. Thin Solid Films 1989, 159, 345. (9) Adams, J.; Rettig, W.; Duran, R. S.; Naciri, J.; Shashidhar, R. J . Phys. Chem. 1993,97, 2021. (10)Adams, J.; Buske, A.; Duran, R. S. Macromolecules 1993, 26, 2871. (11)Thibodeaux, A. F.; Radler, U.; Shashidhar, R.; Duran, R. S. Macromolecules 1994, 27, 784. @
0743-746319512411-4082$09.0010
(12) Fadel, H.; Percec, V.; Zheng, Q.; Advincula, R. C.; Duran, R. S. Macromolecules 1993, 26, 1650. (13)Rettig, W.; Naciri, J.; Shashidhar, R.; Duran, R. S. Macromolecules 1991, 24, 6539. (14) Rettig, W.; Naciri, J.; Shashidhar, R.; Duran, R. S. Thin Solid Films 1992,210/211, 114. (15) Ou, S . H.; Percec, V.; Mann, J. A,; Lando, J. B.; Zhou, L.; Singer, K. D. Macromolecules 1993, 26, 7263. (16) Carpenter, M. M.; Prasad, P. N.; Griffin, A. C. Thin Solid Films 1988, 161,315. (17) Vandevyver, M.; Keller, P.; Rouillay, M.; Bourgoin, J.-P.; Barraud, A. J . Phvs. D: ADDI. Phvs. 1993. 26. 686. (18)Ali-Adib;Z.; Trdgold, R: H.: Hodge, P.; Davis, F. Langmuir 1991, 7, 363. (191Menze1, H.; Weichart, B.; Schmidt, A,; Paul, S.; Knoll, W.; Stumpe, J.; Fischer, T. Langmuir 1994, 10, 1926. (20) Menzel, H.; Weichart, B.; Hallensleben, M. L. Thin Solid Films 1993,223, 181. (21) Pfeiffer, S.; Shashidhar, R.; Fare, T. L.; Naciri, J.; Adams, J.; Duran, R. S. Appl. Phys. Lett. 1993, 63, 1285. (22) Takahashi, T.; Miller, P.; Chen, Y. M.; Samuelson, L.; Galotti, D.; Mandal, B. K.; Kumar, J.; Tripathy, S. K. J . Polym. Sci.: Part B 1993, 31, 165.
0 1995 American Chemical Society
Langmuir, Vol. 11: No. 10, 1995 4083
Monolayers and LB Films of Polysiloxane 60
A. Polymer PSLC:
,
I
150
200
IC
7 H3
\
(H3C)3SiO-Csi-O~;Ssi(CH3)3 c H3 ( C H 2 ) 1 0 C 0 0 0 C H = No O C H 2 i H C 2 H 5
glass---*SB 85'c
,
a
S A '-isotropic
b
(s: Smectic)
8. Monomer MLC:
CH3 CH2=CH(CH,),COOQCOO~CH=NQOCH2~HC*H,
3 il)
1
...
O L
0
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50
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Area (AZ/molecule) Figure 1. Chemicalstructures of compoundsused in this study. The asterisk designates the chiral atom, a,nd bulk transition temperatures are listed below each compound.
Pure and mixed monolayers were characterized by surface pressure-area isotherms and hysteresis curves to exploit the phase transition and the possible molecular arrangement. Multilayer structures were analyzed by small-angle X-ray diffraction, UV-visible absorption, and infrared polarization spectroscopic methods.
Experimental Section The synthesis and characterization of the materials under investigation are discussed in more detail elsewhere.24The monomer was purified by recrystallizationfrom ethanol,followed by silica gel chromatography. The polymer was purified by severalreprecipitationsfrom chloroformsolution into methanol. HPLC-grade arachidic acid (AA)was obtained from Sigma Chemical Co. and used as received. The chloroform used as the solvent was reagent grade, which was redistilled before use to remove any nonvolatile impurities. The experiments for monolayer spreading and deposition of LB films were performed on a commercially availableLangmuir trough NIMA 2000 (NIMA Technology) with computerized controls and two working modes (singleor alternate). A Wilhelmy balance was used as the surface pressure sensor. All measurements were carried out at room temperature (25f 1"C). Surface areas are reported in A 2 per molecule for the monomer and A2 per mesogenic repeating unit for the polymer. Monolayers were obtained by spreading chloroform solutions of the compounds, with concentrationsbetween 0.2 and 0.8 mg/ mL. The water used for the subphase was prepared from redistilled water (pH 6.2). After spreading, 10 min was allowed for solvent evaporation, and then the film was compressed at a barrier speed ranging from 2 to 10 &/(repeating unit-min) (typically= 4&/(repeating unit*min)).In hysteresisexperiments, the barrier movement was immediately reversed after the pressure reached the desired value. LB films were built up on glass or quartz plates by the vertical dipping method. The typical dipping rate was 10 mdmin. The substrates were hydrophobized with trimethylchlorosilane (Merck, Germany) after being rigorously cleaned with chromic acid. Small-angleX-ray diffraction patterns were obtained using a D/MAX-yBX-ray diffractometerwith Cu K a radiation (2 = 0.154 nm). The multilayers deposited on the microscope slides were examined.
Infrared polarization spectra were acquired by a Nicolet 710 FTIR spectrophotometer with the aid of a wire-grid polarizer. CaFz was used as the substrate. A Schimadzu recording spectrometer (Model UV-3000)was used to take the UV-visible absorption spectra. The bulk spectrumwas obtained from a 0.05 mg/mL solution (dilutedfrom (23)Chen, Y.M.; Rahaman, A. K. M.; Takahashi, T.; Mandal, B. K.; Lee, J. Y.; Kumar, J.;Tripathy, S. K. Jpn. J . Appl. Phys. 1991,30,672. (24)Zhang, Q. Z.; Xue, Q. B. Proceedings ofInternationa1 Conference on Liquid Crystalline Polymers (IUPAC), Beijing, China, 1994.
Figure 2. n-A isotherms of monolayers of MLC, at the airwater interface: (a) pure; (b and c) mixed 5:l and 1:l with arachidic acid (AA).
stock)of the PSLC in a fused silica cell with a path length of 1 cm. Hydrophobized fused silica plates were used for measurements of absorption of LB films.
Results and Discussion Although the materials used in our investigation were not specially designed as amphiphiles, all can be spread from a chloroform solution onto the water surface to form monomolecular films. Isothermsof Monomer MLC. The surface pressurearea isotherm of the monomer (Figure 2, curve a) shows larger compressibility; that is, the phase transition from a liquid-expanded to a liquid-condensed phase is not obvious. The collapse pressure (n,) of MLC is about 23 mN/m. The compression speed has little effect on the shape of the isotherms of the monomer. Hysteresis experiments reveal that large hysteresis appears when the reverse pressure is near the E , and, following compression after the pressure release, results in the reduction of the average surface area per molecule (Ad and a larger n,.We consider that it may be due to the formation of aggregates by MLC molecules in the monolayer at higher pressures, which are not spreadable even after the pressure release. The aggregation leads to a higher packing density and, hence, a larger n,. The results from the third compression also support the above opinion because the curve basically coincides with the second one, which indicates that the molecular packing is too dense to be compressed. The average surface area per molecule (Ao) obtained by extrapolation from the linear region (corresponding to the condensed film) to zero pressure is 31.9 A2, larger than the cross-sectional area of the aromatic Schiff base mesogenic unit with its long axis oriented perpendicular to the water surface.25 (It is concluded by comparison with the value (-25 Az)calculated from the structure model of the aromatic azo compound.) It implies that the chains twist and tilt to some extent in the monolayer. Parts b and c of Figure 2 are isotherms of blends in which the monomer is mixed with arachidic acids of different compositions. Because the materials in our research are not typical amphiphiles and usually form rigid and viscous monolayers which are difficult to transfer for LB film preparation, we incorporate MLC and PSLC molecules into matrices of AA to fabricate a stable and easily transferable film of these compounds. To do so can promote the orientation of the mesogenic side chains in (25)Nakahara, H.;Fukuda, K. J . Colloid Interface Sei. 1983,93, 530.
Chen et al.
4084 Langmuir, Vol. 11, No. 10, 1995 40
-
50 , 71 I
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I
1 1 :
1
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I
i
\
z
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- t:
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200
250
A 1- e a (F/m o IIo m e r Figure 3. Hysteresis curves of monolayers of MLC mixed 1:l with AA at a reverse pressures above the inflection region (. * *) and below the inflection region(-). the packing lattice as well. From Figure 2, it is clear that the collapse pressure increases notably when AA's proportion is larger. At a mole ratio for M L C of 1:1,a n inflection appeared on the isotherm which is speculated from the following hysteresis experiments as a phase transition from the liquid-condensed to a solidlike phase. Also the apparent surface area occupied by the monomer molecules goes up if more AA molecules are mixed. The A. values extrapolated before and after the inflection are 69.7 and 54.5 Az, respectively, all larger than those calculated by considering the cross-sectional areas of AA (20.5A2)and MLC with a completely extended chain ( < 3 0 A2).25From these results, we can get the initial images of the mixed monolayer: MLC molecules are distributed uniformly in the matrices ofAA. In this case, MLC chains cannot get close enough to each other as in their pure state. AA molecules prevent MLC chains from aggregating or tangling up effectively. Considering the poor amphiphilicities of MLC molecules,we should still assume the tilted orientation ofthe MLC chains in the AAmatrices. All these factors result in a larger area per molecule than that in the pure MLC monolayer. If MLC and AA molecules are separated into different phases, the MLC molecules will then twist, collapse, or aggregate to crystallites with raising the pressure because of their poor amphiphilicities. Such effects, however, will result in a negative deviation of the apparent surface area. Figure 3 shows hysteresis curves for the reverse pressure above and below the inflection region when the mole fraction of MLC is 0.5. It can be observed that there is only little hysteresis in the compression-expansion cycle before the inflection pressure, showingreversible behavior. Nevertheless, a large hysteresis of a n irreversible compression-expansion cycle occurs if the reverse pressure is higher than the inflection point. So we assert that a certain process occurs in the mixed film a t a n inflection region and causes moleculesto arrange in denser patterns. Comparing isotherms of pure and mixed films of MLC, we think this region may correspond to a phase transition from a liquid-condensed film to a solidlike film. Isotherms of Polymer PSLC. The isotherm of polymer PSLC (Figure 4) shows similarities with the monomer isotherm as well as dissimilarities. On one hand, it does not show any phase transition in the stable monolayer region but exhibits a fluidlike compressibility, which is typical for most polymeric substances on a n
20
40
60
a0
A (i7repeat u n i t )
Figure 4. n-A isotherm of the PSLC monolayer obtained by symmetrical compression.
aqueous subphase.26 On the other hand, the collapse pressure is significantly increased (-35 mN/m) and the isotherm shows some dependence on the compression speed, which indicates that the PSLC monolayer is more viscous. It should be noted that the PSLC monolayer is highly rigid and viscous. The reproducible isotherm can only be obtained with the Wilhelmy method if the Wilhelmy plate is positioned in the center of the trough and two barriers are compressed symmetrically from both sides. Otherwise, the plate may be squeezed out of its normal position easily by the rigid film. Furthermore, there are large gradients in the actual surface pressure along the length of the monolayer.27 These effects all produce large errors in pressure measurement. Extrapolations of the linear high-pressure region to zero pressure result in the compression-speed-dependent area per mesogenic repeating unit of 24-28 A2, approximately corresponding to the cross-sectional area of the mesogen unit oriented perpendicular to the airlwater interface.25 Compared to the monomer, this indicates that the siloxane backbone in the polymer is far more crucial in the monolayer packing of the mesogenic side chains. The extension of the siloxane backbone and its probable interaction with the water surface can reduce the possible twist, tilt, and other irregular arrangements usually occurring for monomer chains due to their poor amphiphilicities. These actions also prevent any extensive interaction between the side-chain mesogenic groups and lead to closer packing. That is one reason for the polymer to have a high nC. The results of the hysteresis experiments of the PSLC monolayer are very similar to that of the monomer, but with large hysteresis cycles. The followingcompressions show only a steep rise in pressure characteristic of solidlike packing. This behavior has been attributed before to rearrangement in the monolayer.28 Probably when the monolayer is initially spread a t the airlwater interface, the polymer molecules are uniformly distributed on the surface with their backbones a t the interface and the side chains tilted toward the air. As the pressure increases during the first compression, the side chains tilt further away from the water and start packing tightly. Since the side chains are longer, they may aggregate to form crystallites, and after releasing the pressure, they don't respread to the same state. Instead, they could remain (26)Gains, G. L.,Jr. Insoluble Monolayer ut Liquid Gas Interface; Wiley: New York, 1966;pp 172, 177,264. (27)Peng, J. B.;Barnes, G. T. Langmuir 1990,6,578. (28)Gains, G.L.,Jr. Langmuir 1991,7,834.
Langmuir, Vol. 11, No. 10, 1995 4085
Monolayers and L B Films of Polysiloxane
rlbr compresrton
before compnsrion
compression b hbh surface pressure
50
Figure 6. Proposed conformational changes of side chains in the PSLC monolayer during the compression.
100
150
02
Area ( A /repeat u n i t )
Figure 7. Hysteresis curves of PSLC/AA mixed monolayers at reverse pressures above the inflection region (* * .) and below the inflection region (-1. AA:PSLC = 1:l. Similar curves are obtained for other molar ratios. 100 h
5
m
1
o 0
50
100
150
200
Area (i2/repeat unit) Figure 6. n-A isotherms of PSLC/AA mixed monolayers.AA: PSLC = (a) 2:1, (b) 1:1,(c) 1:2, (d) 1:5.
Table 1. Apparent Areas Occupied by Mesogenic Repeating Units Extrapolated before and after the Inflection Region from the n-A Isotherms AA:PSLC 2: 1 1:1 1:2 1:5 &BEFORE A' 80 63 47 42 AOWTER,
76
52
39
on the water surface as islands ofcrystallites. This process is schematically shown in Figure 5 . As done in monomer monolayer studies, the PSLC molecules were mixed with AA to reduce the rigidity and viscosity of the film. Another important reason for mixing is to improve the deposition properties of PSLC because polymers with longer side chains (CU and above) form only rigid solid monolayers, which tend to crystallize after the initial compression and are difficult to transfer.29 Figure 6 shows isotherms for blends of PSLC with AA at different molar ratios. They exhibit a distinct inflection in the pressure-area response, and its pressure decreases with the reduction of the amount of AA. The apparent area taken by mesogenicrepeating unit (Ao)also decreases accordingly. Values of A0 extrapolated from the linear high-pressure regions before and after the inflection point are listed in Table 1. The results obtained before the inflection are larger than the sum of the cross-sectional area for each component calculated according to the composition, indicating the side chains don't reach the closest packing in AA matrices. Smaller values ofA0 after the inflection, however, can give us an enlightenment that the monolayer undergoes a phase transition at the inflection. In order to better understand the nature of this phase transition, we measured the hysteresis curves with the (29) Rodriguez-Parada,J. M.; Kaku, M.; Sogah,D.Y.Macromolecules 1994,27, 1571.
r
I
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I
40
*
i
6 0 L 0
5
10
15
20
Time (min)
Figure 8. A-t isobars of PSLUAA mixed monolayers for different deposition pressures. AA:PSLC = (a) 2:1(25mN/m), (b) 1:l (25mN/m),(c) 1:5 (15mN/m).
compression direction reversed before and after the inflection, respectively (see Figure 7), and obtained results very similar to that of mixed monolayers of the monomer. It is observed that little hysteresis and good reproducibility belong to the compression-expansion cycle below the inflection, but excessive hysteresis and irreversibility appear for compression to pressures higher than the inflection point. From this, we suggest that the PSLC exists as an ordered fluid phase in the mixed monolayer below the transition region. The siloxane backbone in the matrices ofAA can extend to some extent. Therefore, the mesogens cannot get close enough to each other to be dense packed normal to the plane of the monolayer and, must assume a tilted conformation. Above the inflection pressure, the mixed film is a highly incompressible solid film. At this time, we must assume that the mesogenic side chains are more perpendicular to the water surface and get close together from the two sides of the main chain to ensure denser packing of the mesogens (as seen in Figure 5 ) . Deposition of LB Films. In this section, the results from the polymer studies are discussed. To prepare LB films ofblends for PSLC and AA, we examined the surface area-time isobars at pressures below the transition region (Figure 8). It can be asserted from the isobars that the mixed film is stable enough to be deposited and the stability is proportional to the AA fraction. It should be remarked here that these isobars were obtained with the
Chen et al.
4086 Langmuir, Vol. 11, No. 10, 1995 3 0 0 0 ~ ,
2500 2000 v)
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,
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2
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4
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Table 2. Layer Spacings for PSLC/AA Mixed LB Films AA:PSLC 2: 1 1:1 1:2 1:5 spacing,A 50.7 51.9 50.1 34.5
U>
4
a 1 500 V
1000
1 000
500
500 0
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2
3
4
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Angle (degree)
Angle (degree)
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3
4
Angle (degree)
5
6
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3
4
5
6
Angle (degree)
Figure 9. X-ray diffraction patterns from LB films of PSLC/ AA blends (26 layers). AA:PSLC is the same as in Figure 6.
trough working in a n alternate mode; namely, the film is compressed unsymmetrically. Regular area-time curves could not be measured for the pure PSLC monolayer because the large rigidity ofthe film made the plate deviate from the normal position. Combining data from the n-A andA-t curves, we chose different pressures according to the proportion of AA to perform LB deposition. The transfers of monolayers were carried out a t 25 mN/m for molar ratios of AA and mesogenic repeating unit of at least 0.5, while a lower pressure of 15 mN/m was chosen for a smaller mole ratio (0.2). The reason for doing this is the siloxane backbones are flexible and easy to twine or twist without AA's support, which makes it difficult to reach high pressures (though it can be reached a t last, the PSLC has formed aggregates or crystallites, which are not suitabler to be deposited as LB films). All mixed films were transferred to the hydrophobic substrates a t a rate of 10 m d m i n . When hydrophilic substrates were used at the same pressure, the layer deposited on the upstroke would quantitatively "peel off' the substrate on the next downstroke. These observations may be related to the short delay time (3-4 min) after upward dipping, not long enough to dry the film which is often described as necessary for a successful transfer of a large number of layers.30 When layers are deposited a t 25 mN/m, we get Y-type films with a transfer ratio of about 1. However, a t 15 mN/m, the deposition type is changed from Y to X after the initial several layer are deposited, indicating the dependence of the molecular arrangement predominantly on film compositions(mixing ratios) and transfer pressure. These deposition types are also confirmed by the results of the X-ray diffraction measurements. X-ray Diffraction. Figure 9a-d shows X-ray diffraction patterns in the small-angle region for LB films (26 layers) with different mixing ratios. Several diffraction peaks are visible, exhibiting some layer orders in the direction of the substrate normal. Layer spacings are calculated according to the Bragg equation and are listed (30) Erdelen, C.; Laschewsky, A.; Ringsdorf, H.; Schneider, J.; Schuster, A. Thin Solid Films 1990,180, 153.
3100
2400 lf00 WAVENUMBER
1600
Figure 10. Polarized FT-IR transmission spectra of PSLC/AA mixed LB films at different incident angles. (1)Al(i=O"); (2) Aii(i=O");(3)All(i=6O0).(A) AA:PSLC = 1:l;(B)AA:PSLC= 1:5. in Table 2. For molar ratios ( M S L C ) from 2:l to 1:2, we get layer spacings of 50.7,51.9,and 50.1 respectively. Considering the chain length of AA (26.9 A) and the calculated length of the side-chain group including the siloxane backbone (31.8 A),31the data indicate that AA molecules dominate the layer spacing ofY-type films with their chains tilted on average with respect to the layer plane. But for X-type layers (AAPSLC = 1:5),the shorter spacing of 34.5 indicates that the controlling component may be the mesogenic side chains, which rearranges during deposition to form an interdigitated layer structure, much like that seen in the bulk of the smectic LC systems. Polarized Infrared Spectra. To further characterize the order ofthe structure for mixed LB films, we collected polarized infrared spectra of different deposition types (Y and Xtype). InVandevyver's c o n v e n t i ~ n s , ~ ~ A ~ ~(Al(i=O~) (i=Oo)) refers to the situation where the electric vector of light is in the film plane but perpendicular (parallel) to the dipping direction, andAll(i=6O0) refers to the situation where it forms an angle of 60" with the film normal but is perpendicular to the dipping direction. Figure 10shows the spectra of the multilayers a t different incidence angles. Linear dichroism can be clearly seen from Figure 10. Using these spectra data, we can obtain the average orientation
A,
A
(31)Weast, R. C., Ed. CRC Handbook of Chemistry and Physics; CRC: Boca Raton, FL, 1984; p. f-166. (32)Vandevyver, M.; Barraud, A,; Teixier, R.; Maillard, P.; Gianotti, C. J. Colloid Interface Sci. 1982, 85, 571.
Langmuir, Vol. 11, No. 10, 1995 4087
Monolayers and LB Films of Polysiloxane Table 3. Mejor Infrared Band Assignments and Orientation Angles for PSLClAA Mixed LB Films 8, dec! -
v. cm-'
assimment
1756 1701 1647 1600 1506 1471
AA:PSLC = 1:5
I
2917 CHZasymm 2849
AA:PSLC = 1:l
a = 70.5 y = 25.5 a = 63.3 y = 38.1
s t r e k h v,(CH2) /3 = 74.2 CH2 symm stretch v,(CHd C-0 stretch 72.8 asymm C-0 stretch 57.4 of C=O dimers C=N stretch in-plane bending 43.2 of benzene ring in-plane bending of benzene ring CH2 scissoring 6 (CH2) 57.4
/3 = 70.6
70.6 49.5 47.5 36.2 30.1 55.8
of the infrared transition moments of the AA and mesogenic side chains with respect to the substrate normal. The strong bands in Figure 10 at 2917 and 2849 cm-l are due to CH2 antisymmetrical and symmetrical stretching, respectively [Y,(CHZ) and v,(CH2)1. The CH2 scissoring is at 1468 cm-'. In addition, a lot of other major peaks exhibit in the spectra (see Table 3). Among these peaks, the band of 1647 cm-', corresponding to C-N stretching, is very weak and too weak to be measured in 1 : l mixed multilayers. The 1756-cm-l line is clearly assigned to the well-known C-0 stretching mode,33while the 1600cm-l and 1506-cm-' lines are assigned to in-plane vibrations of the benzene ring. On the other hand, the ester and ether vibration modes fall in the 1000-1300-~m-~ region.33 The 1701-cm-lline, however, is believed to result from the vibration of the dimers of carboxylic acids formed by the intramolecular hydrogen bonding.33 According to Vandevyver's method, the angles between the transition moments of va(CH2)or v,(CH2) and the film normal, a or p, have been calculated (the refractive index of the LB film is set as 1.50). Then, from the formula cos2 a cos2p cos2 y = 1, we can obtain the tilt angle y of the hydrocarbon chain. The calculated results are also listed in Table 3. It can be seen that the hydrocarbon chains are more perpendicular to the, substrate when increasing AA's proportion. But it should be mentioned here that the bands of v,(CHd and vACH2) include vibration contributions ofspacers in side chains, and they are assumed to take the same orientation as the hydrocarbon chains of AA. The average angles 8 between the transition dipole moments and the substrate normal for other bands have also been calculated in the same way (see Table 3). Comparing these values with the "magic angle" 54"44', for which the order parameter e = average [(3cos28 - 1)/ 21 vanishes, we conclude to a moderate out-of-plane order of the mixed multilayer, with the C-0 stretching modes preferentially oriented parallel to the substrate. The stretching of C-N in X-type films is in a tilted orientation (47.5"), which leads to the mesogenic cores preferentially perpendicular to the substrate. Figure 11 shows schematically a possible side-chain arrangement. The above results have shown that some weak axial order (in the direction of the substrate normal) does exist in these LB films. They also can be considered as supports to the conclusion we obtained in discussing monolayer behaviors: at pressures below the inflection, mesogenic side chains cannot be packed close enough to be perpendicular to the surface, but tilted. UV-Visible Spectra. Figure 12 shows the nonpolarized UV-visible spectra obtained from chloroform
+
+
(33)Colthup, N.B.,Daly, L. H., Wiberley, S. E., Eds. Introduction to Infrared a n d Raman Spectroscopy; Academic: New York, 1990.
I
I
Figure 11. Proposed LB film structures of PSLC and AA.
1 4
0.0
I '
200
I
1
250
300
,
350
1
400
Wavelength (nm)
Figure 12. UV-visible absorption spectra in chloroform solution for PSLC (-) and PSLC/AA blend (1:l)(- - -1 and in a LB film of the blend on a quartz substrate (. * -1.
solutions of the polymer and its blend with AA, together with that ofthe LB layers. For PSLC solution, there exist two strong absorption peaks a t 257 and 334 nm, corresponding to the n n* transition of the aromatic ring in the side chain and the n n* transition of the aromatic Schiffbase chromophore, respectively. However, only the absorption a t 257 nm is obviously shown in the solution of the PSLC/AA blend. The band at 334 nm decreases dramatically, which may be related to the possible interaction between the COOH and C=N groups. More work is necessary for the detailed explanation. A distinct hypsochromic shift is observed for the mixed LB multilayer compared with the solution. The band is shifted from 257 nm in the solution spectra to 245 nm for the LB films. The shiR is indicative of a linear aggregation of mesogenic units with their transition moments parallel to each other, arranged perpendicular o r slightly tilted to the stacking direction (H a g g r e g a t i ~ n )The . ~ ~ added band a t 211 nm is thought to be from the adsorption of the benzene ring (E band), which is sheltered by chloroform adsorption in the solution spectra. Figure 13 shows the absorption spectra for LB films with different layer numbers. It can be observed that the positions of the absorption peaks don't move and there is
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(34)Umemua, J.; Hishiro, Y.;Kawai, T.; Takanaka, T.; Gotoh, Y.; Fujihira, M. Thin Solid Films 1989,178,281.
Chen et
4088 Langmuir, Vol. 11, No. 10, 1995
0'25
7
005 -
000 I 200
I
I
I
I
250
300
350
400
Wavelength (nm)
Figure 13. W-visible absorption spectra for PSLC/AA (1:l) mixed LB films with different numbers of layers: (a)50, (b) 42, (c) 34, (d) 28. a nearly linear relationship of peak intensities to layer numbers. This, with the X-ray data, indicates homogeneous transfer and that no aggregation occurs between the molecules in different layers.
Conclusions From the above data and discussions, it is concluded that the materials under investigation can form monolayers a t the air-water interface. The polymer formed much more stable films than the monomer. For the rigid
61.
and viscous monolayers, symmetrical compression with the Wilhelmy plate in the center of the trough is recommended. Matrices of AA can improve the packing of side chains and enhance the film stability. At higher surface pressures, molecules either in pure or in mixed films aggregate or crystallite to form islands with solidlike stacking. The mixed films can be transferred onto hydrophobic substrates, where, depending on the AA proportion and applied surface pressure, Y-type andX-type depositions are observed. X-ray diffraction, polarized infrared spectra, and UV-visible spectra confirm that there indeed exist axial order and out-of-plane order in the mixed multilayers. Mesogenic chains take a tilted orientation. H aggregation occurred between molecules in the same plane. It must be pointed out that, however, the conclusion above should be further verified by other methods. It is necessary to observe the monolayer in situ, for example, by the BAM method, which will be helpful to judge the nature of the phase transition. In addition, from the above discussion, we have found that the LB technique is an efficient method of systematically manipulating the layer-to-layer stacking order of the liquidcrystalline polymer. Thus, constructed ordered structures will open wider fields for electrooptical property studies and applications.
Acknowledgment. This work was supported by the National Natural Science Foundation of China and State Major Basic Research Project. LA940960X
