Molecular Orientation and Phase-Transition Behavior of Langmuir

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Molecular Orientation and Phase-Transition Behavior of Langmuir-Blodgett and Casting Films of Reversed Duckweed Polymer ES-1 Studied by Infrared Spectroscopy Qiang Wang,† Bing Zhao,*,† Xi Zhang,† Jiacong Shen,† and Yukihiro Ozaki*,‡,§ Key Laboratory of Supramolecular Structure and Materials of Ministry of Education, Jilin University, Changchun 130023, P. R. China, and Department of Chemistry, School of Science and Technology, Kwansei-Gakuin University, Gakuen, Sanda, Hyogo 669-1337, Japan Received July 8, 2002. In Final Form: September 16, 2002 A kind of amphiphilic polymer, ES-1, is transferred onto solid substrates as so-called “reversed duckweed” polymeric Langmuir-Blodgett (LB) films. Their orientations, structures, and phase-transition behaviors were studied in detail by infrared (IR) transmission and reflection-absorption (RA) spectroscopy. The IR studies have revealed that the alkyl chains are nearly perpendicular to the film surface and assume highly ordered conformation. The first monolayer has a different orientation and arrangement from other layers in a multilayer ES-1 LB film deposited on a Au-evaporated glass slide because of the interaction between the gold surface and hydrophilic headgroup of ES-1. IR-RA spectra of an ES-1 LB film on a Au-evaporated glass slide at evaporated temperatures have shown that the order-disorder phase transition of the LB film takes place at about 60 °C after the clear pretransitional alterations. The packing density of alkyl chains decreases gradually with temperature after the order-disorder transitions in the LB films. The alkyl chains in ES-1 casting films on a CaF2 plate and a Au-evaporated glass slide are tilted considerably. The studies on the phase-transition behavior have revealed that the two casting films give rise to the order-disorder transition at about 75 °C after undergoing the pretransitional changes. For the casting film of ES-1 on the Au-evaporated glass slide, there is a phenomenon of reorientation that the alkyl chains change from more tilted orientation at room temperature to more perpendicular orientation at higher temperature before the order-disorder transition takes place and turn tilted again above the transition temperature.

Introduction Ordered polymeric ultrathin films have recently been the subject of increasing attention. When compared with organic ultrathin films, polymeric ultrathin films combine order with stability and provide higher antidamage thresholds and superior mechanical properties.1,2 Since Tredgold and Winter1 first successfully fabricated polymeric Langmuir-Blodgett (LB) films, many investigators have been involved in the studies of polymeric ultrathin films.2-5 Ringsdorf and co-workers2,3 have proposed a new idea of decoupling the interaction between polymeric main chains and side groups by introducing a “spacer group” into the prepolymerized amphiphilic to improve the order of polymeric LB films. We recently developed a new kind of amphiphilic polymer composed of a hydrophobic microgel and hydrophilic grafting chains (denoted ES; see Figure 1). This kind of amphiphilic polymer can self-rearrange at the air/ water interface and is readily transferable onto solid substrates as so-called “duckweed” and “reversed duckweed” polymeric LB films.6-20 The term “duckweed” means * To whom correspondence should be addressed. † Jilin University. ‡ Kwansei-Gakuin University. § Fax: +81-795-65-9077. E-mail: [email protected]. (1) Tredgold, R. H.; Winter, C. S. J. Phys. D: Appl. Phys. 1982, 15, L55. (2) Laschewsky, A.; Ringsdorf, H.; Schreider, J. Angew. Makromol. Chem. 1986, 145/146, 1. (3) Ringsdorf, H.; Laschewsky, A.; Schmidt, G.; Schreider, J. J. Am. Chem. Soc. 1987, 109, 778. (4) Higashi, N.; Mori, T.; Niwa, M. J. Chem. Soc., Chem. Commun. 1990, 225. (5) Sun, F.; Castner, D. G.; Grainger, D. W. Langmuir 1993, 9, 3200. (6) Yin, R.; Cha, X.; Zhang X.; Shen, J. C. Macromolecules 1990, 23, 5158.

that the hydrophobic microgels are floating on the surface of water and the hydrophilic grafting chains are projecting into the water, while “reversed duckweed” means that the hydrophilic networks extend downward into the water and the hydrophobic grafting chains are upward packing away from the surface of water. According to the structural analysis of synthetic compounds of ES, the average molecular weights are 12 100 and 7500 amu for ES-1 and ES-3, respectively. The contents of alkyl chain are 48.5% and 75.8%, respectively, indicating that each ES-1 and ES-3 molecule contains 20.6 and 20.8 stearic chains on average.12 (7) Cha, X.; Yin, R.; Zhang, X.; Shen, J. C. Macromolecules 1991, 24, 4985. (8) Shen, J. C.; Zhang, X.; Zhang, R. F. Thin Solid Films 1992, 210/ 211, 628. (9) Zhang, R. F.; Zhang, X.; Shen, J. C. Langmuir 1994, 10, 2727. (10) Zhang, X.; Zhang, R. F.; Shen, J. C.; Zou, G. T. Macromol. Rapid Commun. 1994, 15, 373. (11) Zhang, R. F.; Zhang, X.; Wang, J.; Shen, J. C. Thin Solid Films 1994, 248, 102. (12) Li, H. B.; Zhang, X.; Zhang, R. F.; Shen, J. C.; Zhao, B.; Xu, W. Q. Macromolecules 1995, 28, 8178. (13) Zhang, R. F.; Zhang, X.; Li, H. B.; Zhao, B.; Shen, J. C. Polym. Bull. 1996, 36, 227. (14) Zhao, B.; Li, H. B.; Zhang, X.; Zhang, R. F.; Shen, J. C.; Ozaki, Y. Prog. Colloid Polym. Sci. 1997, 106, 144. (15) Li, H. B.; Zhang, L.; Wang, Z. Q.; Zhang, X.; Shen, J. C.; Yang, Y. Q.; Fei, H. S. Chem. J. Chin. Univ. 1997, 18, 323. (16) Zhang, X.; Li, H. B.; Zhao, B.; Shen, J. C. Macromolecules 1997, 30, 1633. (17) Shen, J. C.; Li, H. B.; Xiong, H. M.; Zhang, X. Macromol. Symp. 1997, 118, 707. (18) Li, H. B.; Wang, Z. Q.; Zhao, B.; Xiong, H. M.; Zhang, X.; Shen, J. C. Langmuir 1998, 14, 423-428. (19) Zhao, B.; Li, H. B.; Zhang, X.; Shen, J. C.; Ozaki, Y. J. Phys. Chem. B 1998, 102, 6515. (20) Shen, J. C.; Wang L. Y.; Zhang, X. Colloids Surf., A 2000, 175, 235.

10.1021/la020608g CCC: $22.00 © 2002 American Chemical Society Published on Web 11/12/2002

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Figure 1. The syntheses (A) of the amphiphilic polymer (ES) and (B) a schematic mode of reversed duckweed LB film showing ES molecules (a) in solution, (b) on the surface of pure water, (c) on the surface of pure water after compression, and (d) after transfer onto a solid substrate.

Differential scanning calorimetry (DSC) was used to analyze the dust and cast film of ES. It was found that the sample ES-1 had only one transition peak corresponding to the melting process, T ) 87.4 K, ∆H ) 94.4 J/g, and ES-3 had three transition peaks, T1 ) 89.1 K, ∆H ) 75.5 J/g, T2 ) 108.9 K, ∆H ) 9.6 J/g, and T3 ) 130.9 K, ∆H ) 31.0 J/g. The thermotropic liquid crystalline behavior was found for a cast film of ES-3 because of strong interactions between long alkyl chain and some hydrophobic semirigid rods.12,13,16,17 With a good balance between the hydrophilic and hydrophobic parts, one can easily fabricate LB films of ES-3, and they show a higher order and orientation. That is the reason many researches have carried out investigations on ES-3.12-20 The “reversed duckweed” ES-3 LB films possess a well-ordered layer structure and highly oriented hydrophobic alkyl tails. The amide groups, which connect the alkyl tails with the microgel, form interchain hydrogen bonds along the substrate surface. The alkyl chains are highly ordered and well-oriented with a small tilt angle even in the LB film prepared at nearly zero surface pressure. The microgel core shows some mobility in the LB film deposited under the nearly zero surface pressure, and the strength of interchain hydrogen bonds changes little with the surface pressure.19 Three sharp phase transitions appear near 65, 105, and 140 °C for multilayer LB films of ES-3. The first transition is ascribed to the conversion of hydrocarbon chains from the highly ordered state to the slightly less ordered state. The second corresponds to the order-disorder transition of the alkyl

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chains and the amide groups, and the last transition is attributed to the transition to the isotropic state. The thermal behavior of the amide group is quite different from that of the hydrocarbon chain of LB films of ES-3; the former yields two transition temperatures, while the latter gives three. Even the one-monolayer LB film of ES-3 exhibits three clear phase transitions. The microgel greatly weakens the interaction between the substrate and the hydrocarbon chains, allowing the one-monolayer film to show the transition.19 Compared with ES-3, many characters are changed in ES-1 with the slightly different ratio of hydrophilic and hydrophobic groups in synthesis. Only one phase-transition point was found for an ES-1 cast film, and no liquid crystal character behavior appeared.12,13 Our previous study has revealed that ES-1 LB films can be selected as a matrix to form complex LB films with other compounds, such as PbS and C60.10 ES-1 can easily form the more stable complex LB films than ES-3 because of fewer alkyl chains and more inter- and intramolecular spaces. The difference in the composition also results in the difficulty in the fabrication process of ES-1 LB films.13 We previously found that it is difficult to transfer an ES-1 monolayer on pure water subphase onto a solid substrate to form a stable multilayer. However, Xiong et al.21 succeeded in fabricating an ES-1/Ca2+ LB multilayer with a good transfer ratio. Their study also showed that the introduction of Ca2+ ions into the subphase had a marked effect on the process of the organization of the amphiphilic polymer at the air/water interface due to the association of Ca2+ ions with the hydrophilic network and further led to the reorientation of the hydrophobic tails. The introduction of Ca2+ ions into the subphase makes the behaviors of mono- and multilayers of ES-1/Ca2+ closer to those of ES-3 and far from those of ES-1 itself. Although the fabrication of ES-1 LB films is much more difficult than that of ES-3 LB films, we need to understand the behaviors of pure ES-1 LB films and the structural differences between the LB films of ES-1 and ES-3. The structural differences should result in the differences in character between the two kinds of LB films. Recently, we found that LB films of ES-1 can be fabricated successfully on both CaF2 plates and Au-evaporated glass slides. Experimental details are described in the Experimental Section. The purpose of the present study is to compare the molecular structure, orientation, and phase-transition behaviors in LB and casting films between ES-1 and ES3. Infrared (IR) transmission and reflection-absorption (RA) spectroscopy have been employed to explore them. We previously reported an infrared study on the structure and molecular orientation in the multilayer LB films of ES-3,19 and thus, we can closely compare the present IR results for the LB films of ES-1 with the previous results for those of ES-3. Experimental Section Material. The amphiphilic polymer, ES-1, was synthesized by the method reported in our previous paper.16 The structure of the amphiphilic polymer (ES) was described in Figure 1. Preparation of Langmuir-Blodgett Films. A computercontrolled model Nima-200 Langmuir trough with a Wilhelmy balance was employed for the π-A isotherm measurements, as well as LB film preparation. A chloroform/ethanol (8:2) mixed solution (about 4 × 10-4 mol/L) of ES-1 was spread onto an aqueous subphase of pure water (>18.1 MΩ, pH ) 6.5), which (21) Xiong, H. M.; Li, H. B.; Wang, Z. Q.; Zhang, X., Shen, J. C.; Gleiche, M.; Chi, L. F.; Fuchs, H. J. Colloid Interface Sci. 1999, 211, 238.

Molecular Orientation and Phase-Transition Behavior

Figure 2. The π-A isotherm of (a) ES-1 and (b) ES-3 on the subphase of pure water at 20 °C. was doubly distilled from deionized water. Temperature of the subphase was kept at 20 ( 1 °C. After evaporation of the solvent (about 3 h), the monolayer was compressed at a constant rate of 18 cm2/min up to the given surface pressure of 25 mN/m (collapse surface pressure is 60 mN/m). The π-A isotherm revealed that the monolayer was a solid condensed film at this pressure.13 The film was allowed to equilibrate for an hour before being deposited. The monolayers were transferred by the vertical dipping method onto CaF2 plates for IR transmission measurements and Auevaporated glass slides for IR reflection-absorption (RA) measurements at the given surface pressure, the dipping speed was 2 mm/min, and an upper delay was 30 min for drying. The Auevaporated glass slides were annealed before use. The substrates used had been subjected to ultrasonication in chloroform and then in distilled water. Preparation of Casting Films. ES-1 in a chloroform/ethanol (8:2) mixed solvent with a concentration of about 4 × 10-4 mol/L was spread evenly onto a substrate (CaF2 plate or Au-evaporated glass slide), and then promptly, the solution-coated substrate was put into a sealed container that was full of saturated chloroform vapor. Under the saturated vapor pressure at room temperature for more than 48 h, the solvent was slowly evaporated, and then the substrate with the film was dried in atmosphere. By this method, the casting film of ES-1, a kind of ultrathin film, was prepared on the substrate. Spectroscopy. IR transmission and RA spectra were measured on a Bruker IFS-66V FTIR spectrometer equipped with a liquid-nitrogen-cooled MCT detector. The spectra were collected at a 4 cm-1 resolution, and typically 1000 interferograms were coadded to yield the spectra with a high signal-to-noise ratio. For the measurements of RA spectra, a homemade model CQ reflection attachment was employed with p-polarized light incident on the film surface at an angle of 80° from the surface normal. To measure the IR spectra at elevated temperatures, a substrate on which the ultrathin film had been deposited was inserted into a sample holder in the copper block that had a heater within. The temperature controller was homemade and could give a temperature stability of better than 0.1 °C when the sample was heated to each scheduled temperature, which was kept at least for 20 min prior to IR measurements to reach the thermal equilibrium. The temperature was monitored with a thermocouple connected with the sample holder and was raised at about 1 °C/min. The spectral data were processed and analyzed using GRAMS/ 32 software package (Galactic). The band area was calculated by integral or curve fit method or both.

Results and Discussion 1. Fabrications of the LB Films of ES-1. Curves a and b in Figure 2 show surface pressure-area (π-A) isotherms of ES-1 and ES-3 on the subphase of pure water, respectively. It can be seen from the π-A curves that both ES-1 and ES-3 form stable monolayers and have high collapse pressure of more than 60 mN/m. The monolayers of both ES-1 and ES-3 show typical condensed phases,

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Figure 3. IR transmission spectra of one-, three-, five- and seven-layer LB films of ES-1 deposited on a CaF2 plate. Insert figure shows the intensities of the CH2 (b) antisymmetric and (2) symmetric stretching bands of the ES-1 LB films as a function of the number of layers.

Figure 4. IR-RA of one-, three-, five-, and nine-layer LB films of ES-1 on a Au-evaporated glass slide at about 20 °C. Insert figure shows the intensity of the CH2 symmetric stretching band of the ES-1 LB films versus the number of layers.

and the limiting area is 4.10 nm2/molecule for ES-3 and 7.1 nm2/molecule for ES-1. The behavior of the pressurearea isotherm is quite different between ES-1 and ES-3. ES-3 gives a very sharp increase in the pressure with the decrease in the single molecular area; the surface pressure changes from zero to 60 mN/m with a very small change of less than 1.0 nm2 in the molecular area. On the other hand, the surface pressure of ES-1 increases gradually with the decrease in the molecular area from 14 to 6 nm2. On the basis of the suggested “reversed duckweed” model, the area occupied by each alkyl chain was calculated to be 0.20 and 0.34 nm2 for ES-3 and ES-1, respectively. The area of ES-3 is very close to the limiting area of a stearic acid monolayer, suggesting that the alkyl chains are closely packed in the monolayer of ES-3. In contrast, the area of ES-1 is much larger than the limiting area of a stearic acid monolayer, so it was expected that the order of the arrangement of alkyl chains of ES-1 is lower than that of the stearic acid or the other groups of ES-1 molecules occupy the space out of hydrocarbon chains. 2. Molecular Orientation in LB Films. Figures 3 and 4 show IR transmission and RA spectra of LB films of ES-1 with different numbers of monolayers, respectively. The frequencies and assignments of major IR bands are summarized in Table 1.19 Bands near 2917 and 2849 cm-1 are assigned to the CH2 antisymmetric and symmetric stretching modes of the hydrocarbon chains of ES-1 in the LB films, respectively. The frequencies of the CH2 stretching bands are sensitive to the conformation of a hydrocarbon chain; low frequencies (∼2918 and 2848 cm-1) of the bands are characteristic of a highly ordered transzigzag alkyl tail, while their upward shifts are indicative

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Table 1. The Wavenumber and Assignments of Infrared Bands of LB Films of ES-1 wavenumber (cm-1)

assignments

2962 2956 2933 2916, 2918 2875 2849, 2850 1687, 1697

νas (CH3), in-plane νas (CH3), out-of-plane νs (CH3) + Fermi resonance νas (CH2) νs (CH3) νs (CH2) ν (CdO and CdO in N,N-disubstituted amide) ν (CdO), amide I δ (N-H) + ν (C-N), amide II δ (CH2) δas (CH3) δs (CH3) ν (C-C)

1649 1544, 1537 1467 1464 1379 1101

of the increase in conformational disorder, that is, gauche conformers, in the hydrocarbon chain.22,23 The fact that the CH2 stretching bands appear near 2917 and 2849 cm-1 in the IR spectra of Figures 3 and 4 suggests that the hydrocarbon chains of ES-1 LB films on the two kinds of substrates are highly ordered with nearly all-trans zigzag conformation. A peak appearing at 2933 cm-1, which can be observed clearly in the IR-RA spectrum (Figure 4) of ES-1 LB film and assigned to the Fermi resonance mode, gives another evidence for the zigzag conformer of hydrocarbon chain of ES-1. There is no perceptible spectral change in the spectra shown in Figure 3, except for the peak intensities that increase almost linearly with increasing the number of monolayers. This is illustrated in the inset of Figure 3, which shows the relationship between the intensities of the CH2 antisymmetric and symmetric stretching bands and the number of monolayers. Therefore, the orientation and arrangement of the hydrocarbon chains of ES-1 in the LB films on the CaF2 plates are nearly identical irrespective of the number of the monolayer. The same examination was made for the LB films on the Auevaporated glass slides. It can be seen from Figure 4 that the RA spectra of the multilayer LB films are very close to each other except for band intensities that increase linearly with the number of monolayers. The inset in Figure 4 plots the intensity of the CH2 symmetric stretching band versus the number of monolayers. The observations in Figure 4 and its inset indicate that the molecular orientation changes little above the second monolayer in the multilayer LB films of ES-1. Of note in Figure 4 is that the RA spectrum of the one-layer LB film of ES-1 is significantly different from the spectra of the multilayer LB films in the relative intensities of CH3 stretching bands at 2956 and 2875 cm-1 and those of bands in the 1750-1650 and 1500-1350 cm-1 regions. These special changes in the spectrum of the one-layer LB film suggest the stronger interaction between the gold surface and ES-1 molecules. According to the surface selection rule in IR-RA spectroscopy,25-27 vibrational modes with their transition moments perpendicular to the surface of a substrate are enhanced in a RA spectrum. On the other hand, vibrational (22) Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy, Academic Press: New York, 1964. (23) Umemura, J.; Cameron, D. G.; Mantsch, H. H. Biochim. Biophys. Acta 1980, 602, 32. (24) Greenler, R. G. J. Chem. Phys. 1966, 44, 310. (25) Chollet, P. A.; Messier, J.; Rosilio, C. J. Chem. Phys. 1976, 64, 1042. (26) Umemura, J.; Kamata, T.; Kawai, T.; Takenaka, T. J. Phys. Chem. 1990, 94, 62. (27) Cameron, D. G.; Casal, H. L.; Gudgin, E. F.; Mantsch, H. H. Biochim. Biophys. Acta 1980, 596, 463.

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modes with their transition moments parallel to the surface of the substrate appear as strong peaks in a transmission spectrum. The transition moments of CH2 antisymmetric and symmetric stretching bands of an alkyl chain are all perpendicular to the chain axis, so the molecular orientation of the alkyl chain can be investigated by comparing the intensities of the CH2 antisymmetric and symmetric stretching bands between the IR transmission and RA spectra. In the present case, the intensities of the two stretching bands are much stronger in the transmission spectra than in the RA spectra (Figures 3 and 4). Therefore, it seems that the hydrocarbon chains of ES-1 are nearly perpendicular to the substrate surface in the LB films. If one wants to estimate the orientation of an alkyl chain quantitatively by IR transmission and reflection spectra, the optical model should be considered and the enhanced factor in grazing incidence IR spectra must be known or assumed. However, it is very hard for polymer LB films such as those of ES-1 to develop an optical model and to assume the enhanced factor. Therefore, in the present study, we just discussed the orientation qualitatively. The orientation of amide groups in the ES-1 LB films is quite different from that of ES-3 LB films. The amide I band of ES-3 is stronger in the transmission spectra than in the RA spectra. A band due to an amide II mode of ES-3 appears at 1552 cm-1 in the transmission spectra, while the RA spectra show another amide II at 1564 cm-1 very strongly. In the ES-3 LB films, the N-H and CdO bonds of amide groups are nearly parallel to the substrate surface.19 The NH stretching vibration is observed in neither IR transmission nor RA spectra of the LB films of ES-1, suggesting that no free N-H group exists in the LB films of ES-1. A band at 1649 cm-1 is assigned to the amide I mode, and bands at 1544 and 1537 cm-1 due to the amide II modes are weak both in the transmission and RA spectra, also suggesting that there are not many monosubstituted amides in the LB films. It can be seen that the amide I band of ES-1 appears at higher wavenumber than that of ES-3. On the other hand, the amide II of ES-1 is located at lower wavenumber than that of ES-3. These results from the IR transmission and RA spectra lead us to conclude that the LB films of ES-1 possess packed methylene chains and the interchain hydrogen bonds between the adjacent amide groups are weaker in the LB films of ES-1 than in those of ES-3. Possible formation of the hydrogen bonds is supported by the broad CdO stretching bands and the broad background in the low-frequency regions of Figures 3 and 4. A band at 1687 cm-1 in the transmission spectra and that at 1697 cm-1 in the RA spectra are mainly due to the CdO stretching modes. The CdO stretching mode of N,Ndisubstituted amide group may also contribute to the bands. The latter band is stronger than the former band. Therefore, it seems that the transition moment of CdO group is nearly perpendicular to the substrate surface. Meanwhile the intensity ratio of the amide I and II bands in Figures 3 and 4 changes little, suggesting that the Cd O bond in the amide group is titled. This result may be interpreted by assuming the reason as follows. ES-1 has fewer stearic side chains than ES-3 and relatively more CdO or C-O groups or both in the hydrophilic crosslinked networks than ES-3 in which free CdO group can be neglected. Hydrogen bonds may be formed between the hydrogen atoms and the oxygen atoms from the networks, leading to the nearly perpendicular orientation of the CdO groups to the substrate surface. 3. Phase-Transition Behavior in the LB Films of ES-1. Figure 5 depicts IR-RA spectra in the 3000-2800

Molecular Orientation and Phase-Transition Behavior

-1

Figure 5. IR-RA spectra in the 3000-2800 cm region of an 11-layer ES-1 LB film measured at various temperatures.

Figure 6. Temperature dependencies of the (b) frequency and ([) half bandwidth at half peak height of the CH2 symmetric stretching band of the 11-layer LB film of ES-1 on the Auevaporated glass substrate.

cm-1 region of an 11-layer LB film of ES-1 on a Auevaporated glass slide at various temperatures. It is noted that above 60 °C significant spectral changes occur. Frequencies and half bandwidths of CH2 antisymmetric and symmetric stretching bands are sensitive to the conformation and mobility of an alkyl chain, respectively, so they can be used as practical indicators for the phasetransition process of alkyl chains.23,24,28,29 To explore the temperature-dependent structural changes in the ES-1 LB films, the frequency and half bandwidth of the CH2 symmetric stretching band (the CH2 antisymmetric stretching band is overlapped by the band at 2933 cm-1 assignable to a Fermi resonance, so we do not use this band here) are plotted against temperature for the 11monolayer LB film. The results are presented in Figure 6. It can be readily seen from this figure that the frequency shifts upward by about 3 cm-1 at around 60 °C and reveals a clear pretransitional change between about 50 and 60 °C. The change in the bandwidth shows very similar tendency to that in the frequency. These results suggest that between about 50 and 60 °C the content of gauche conformers increases while that of trans-zigzag conformers still makes up the overwhelming majority and that upon elevating the temperature up to about 60 °C, the orderdisorder transition takes place in the LB films. The change in the bandwidth of the CH2 symmetric stretching band suggests that the mobility of the alkyl chains in the LB film begins to increase almost simultaneously with the conformational variations. (28) Mantsch, H. H.; Martin, A.; Cameron, D. G. Biochemistry 1981, 20, 3139. (29) Nakagoshi, A.; Wang, Y.; Ozaki, Y.; Iriyama, K. Langmuir 1995, 11, 3610.

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Figure 7. Temperature dependencies of the band area of the CH2 symmetric stretching band of the 11-layer LB film of ES-1 on the Au-evaporated glass substrate.

Figure 7 plots the band area of the CH2 symmetric stretching band as a function of temperature for the ES-1 LB film on the Au-evaporated glass slide. The temperature-dependent change in the band area proceeds almost in parallel with those in the frequency and bandwidth below 60 °C. It is of interest to note that the band area continuously increases up to 65 °C and after that it shows a gradual decrease. An alternation in the band area of a CH2 symmetric stretching mode of an alkyl chain results partly from a change in chain packing density and partly from a change in the orientation of a hydrocarbon chain. Therefore, it seems that below 65 °C the increase in the band area of the CH2 symmetric stretching band is mainly due to the change in the orientation of the hydrocarbon chain, from the nearly perpendicular orientation to tilted one with respect to the film surface while above 65 °C the decrease in the band area is ascribed to the decrease in the packing density or the change of isomer or both. The decrease in the packing density might be caused by the decomposition or evaporation or both of the ES-1 LB film. Temperature-induced spectral changes were also investigated for a 21-layer LB film of ES-1 on a CaF2 plate. It is found that the frequency and half bandwidth of the CH2 symmetric stretching band in the IR transmission spectra undergo similar changes to those illustrated in Figure 6 and the order-disorder transition occurs at about 60 °C as well. For both the ES-1 LB films on the CaF2 and those on Au-evaporated glass slide substrates, after the spectra had been measured at the elevated temperatures, the temperatures were lowered to 30 °C and then the subsequent spectra recorded. The results for the RA spectra are shown at the top of Figure 5. It is found in both the transmission and RA spectra that the frequencies of the two CH2 stretching bands recover nearly to their original values although their bandwidths and intensities do not recover. These observations lead us to the conclusion that the hydrocarbon chains in the ES-1 LB films become highly ordered again after the cyclic temperature treatment. The molecular orientation and mobility of the LB films change greatly probably because of the change in the structure of the cross-linked networks and the molecular running off; the monolayers migrate each other, making the LB films thin. 4. Molecular Orientation in Casting Films. Figure 8 shows IR transmission (trace a) and RA (trace c) spectra of casting films of ES-1 on a CaF2 and a Au-evaporated glass slide at room temperature, respectively. The spectrum of its bulk sample in KBr pellet is shown as trace b. It is noted that the relative intensities of some bands, for example, bands near 1685, 1648, and 1570 cm-1

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Figure 8. IR (a) transmission and (c) RA spectra of casting films of ES-1 on CaF2 plate and Au-evaporated glass substrate, respectively, and (b) an IR spectrum of the bulk sample in KBr pellet.

assigned to the CdO stretching mode of the N,Ndisubstituted amide, the CdO stretching mode of the monosubstituted amide, and amide II, respectively, are changed among the three spectra. The difference in the peak positions between spectra a and b in Figure 8 is rather small, but the band intensity change is clear. These changes imply that the molecular orientation in the casting films of ES-1 are different from that in its bulk sample, and the ordered films are formed after slow evaporation of the solvent.12,16,30 This result is similar to that of ES-3, in which the layer spacings of slow and natural evaporation of solvent measured by X-ray diffraction are 3.66 and 5.0 nm, respectively. The CH2 antisymmetric and symmetric stretching bands near 2919 and 2847 cm-1, respectively, suggest that the hydrocarbon chains assume trans-zigzag conformation in the ES-1 casting films, as well as in its solid state. A band progression from 1350 to 1180 cm-1 arising from the CH2 wagging mode is also indicative of the all-trans conformation of the hydrocarbon chains. Comparison of the IR transmission spectrum of the casting film of ES-1 (Figure 8) with those of the LB films (Figure 3) reveals that the relative intensities of the two CH2 stretching bands are weaker in the transmission spectrum of the casting film. Probably, the alkyl chains are titled considerably in the casting films in comparison with the LB film. The RA spectrum of the casting film of ES-1 is also quite different from the corresponding spectrum of the LB film (Figure 4) in terms of the relative intensities of many bands in the 3000-2800 and 1700-1500 cm-1 regions. We can discuss the molecular orientation in the casting film of ES-1 on the Au-evaporated glass plate from the relative intensity of two CH2 stretching bands. Figure 9 depicts the relationship between the molecular orientation of an alkyl chain and spectral pattern in the 3100-2700 cm-1 region of an IR-RA spectrum.29 If the CH2 antisymmetric stretching band is intense and its symmetric counterpart is rather weak, the hydrocarbon chain is tilted considerably with its carbon skeleton plane nearly parallel to the surface as shown in Figure 9a. If the intensity ratio of the two bands is reversed, in other words, the CH2 symmetric stretching band is stronger than the CH2 antisymmetric stretching band, the alkyl chain axis is nearly parallel to substrate surface with its carbon skeleton plane nearly perpendicular to the surface as shown in Figure 9b. If the two CH2 stretching bands give medium intensity bands (30) Wang Q. Master Degree Thesis, Jilin University, Changchun, P. R. China, 1998.

Figure 9. Relationship between the molecular orientation of an alkyl chain and the spectral pattern in the 3100-2700 cm-1 region of IR-RA spectrum.

with the antisymmetric one being slightly stronger than the symmetric one, the chain is neither perpendicular nor parallel with respect to the film surface, being in an intermediate direction and rotating along its long axis, as shown in Figure 9c. It may be concluded from comparison between Figure 8c and Figure 9 that the molecular orientation of the hydrocarbon chain in the ES-1 casting film on the Au-evaporated glass slide is in an intermediate direction as shown in Figure 9c. The molecular orientation in the casting films of ES-1 on both CaF2 and Au-evaporated glass slides is significantly different from that in the LB films. The reason may be that the orientation in the casting films is sensitive to the nature of the substrate surface while that in the LB films reflects more or less the orientation in the monolayer on a water-air interface. There might be another reason. In the casting films, it is rather difficult to get rid of some solvent molecules, for example, alcohol. They may interact with the hydrophilic cross-linked networks by hydrogen bonds, modifying the molecular orientation in the casting films. 5. Phase Transition Behavior in the Casting Films of ES-1. Figure 10 shows temperature-dependent changes in IR transmission spectra of a casting film of ES-1 on a CaF2 plate. It can be seen from Figure 10 that the spectrum almost remains unchanged in the temperature range from 33 to 63 °C. Above 63 °C, the intensities of bands in the whole spectral region, especially in the low-frequency region, become weak with the changes in the frequencies and bandwidths of some bands. The band progression in the 1350-1150 cm-1 region characteristic of the all-trans conformation of the hydrocarbon chain disappears at 72 °C. The bands at 1468 and 1413 cm-1 shift downward and bands at 1647, 1569, 1511, 1426, and 1081 cm-1 become so weak that we cannot recognize them any more at 76 °C. Temperature dependencies of the frequency and half bandwidth of the CH2 symmetric stretching band of the spectra in Figure 10 are shown in Figure 11. The frequency of the CH2 symmetric stretching band reveals that the

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Figure 12. IR-RA spectra of the casting film of ES-1 deposited on a Au-evaporated glass substrate measured at various temperatures. Figure 10. IR transmission spectra of the casting film of ES-1 on CaF2 plate at various temperatures.

Figure 13. Temperature dependence of the band area of the CH2 symmetric stretching band of the casting film of ES-1 deposited on a Au-evaporated glass substrate. Figure 11. Temperature dependences of the (b) frequency and ([) half bandwidth at half peak height of the CH2 symmetric stretching band of the casting film of ES-1 in the IR transmission spectra.

hydrocarbon chains in the film are in a highly ordered state up to 63 °C. However, the upward shift of the CH2 symmetric stretching band indicates that gauche conformers are introduced to the chains upon going from 63 to about 75 °C. The temperature dependence of the bandwidth of the CH2 symmetric stretching band of the ES-1 casting film is very similar to that of its frequency (Figure 11), indicating that the increase in the mobility proceeds in parallel with the increase in the conformational disorder. The CH2 scissoring band near 1467 cm-1, which reflects the intermolecular interaction of the alkyl chain, is also useful to explore the order-disorder transition of the chain. The frequency of this band remains near 1467 cm-1 up to about 75 °C and then shifts downward to 1457 cm-1 (Figure 10), suggesting that the subcell packing of the methylene chain is kept until about 75 °C in the ES-1 casting film on the CaF2 plate. This finding implies that the actual temperature at which the order-disorder transition of the ES-1 casting film on the CaF2 plate takes place is about 75 °C, though the increases in its mobility and conformational disorder begin below this temperature. In addition, it is noted that the top spectrum is quite different from the bottom one in the 1700-1000 cm-1 region in Figure 10 but similar to those in Figure 3 except for some fine differences in the 1700-1000 cm-1 region. Therefore, the structural changes after the annealing in the casting film of ES-1 may result from the adsorption of the solvent molecules with heating.

Figure 12 depicts temperature-dependent changes in IR-RA spectra of a casting film of ES-1 on a Au-evaporated glass slide. Only very small spectral changes can be seen until about 64 °C. With the further temperature increase up to about 75 °C, gradual spectral changes take place as in the case of the CaF2 plate in Figure 10. Near 75 °C occur sudden changes, the upward shifts of the two CH2 stretching bands and a large change in spectral appearance in the 1650-900 cm-1 region. In addition, it is noted that the casting film on the Au-evaporated glass slide shows very similar tendency for the changes in the frequency and bandwidth of the CH2 symmetric stretching band to that on the CaF2 plate in Figure 10. This tendency (not plotted for the ES-1 casting film on the Au-evaporated glass slide in this paper) suggests that the order-disorder transition of the film on the Au-evaporated glass slide also occurs at about 75 °C and there exists a clear pretransitional stage just below 75 °C. However, the intensity of the CH2 symmetric stretching band shows an unusual temperature-dependent change as shown in Figure 13. We can see that with the increase of temperature the intensity of the band decreases slowly below around 64 °C and then goes rapidly up to about 75 °C. When the film is heated above 75 °C, the intensity of the band begins to increase, but again it decreases after a moderate change between about 90 and 105 °C. The hydrocarbon chain is considerably tilted with respect to the surface normal in the casting film of ES-1 on the Au-evaporated glass slide as mentioned above. When the film is heated, the molecular orientation and structure in the casting film change with the increase in temperature. Summing up all the information from the temperature-dependent behavior of the CH2 symmetric

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stretching band, we can reach the following conclusion. In the temperature range from room temperature to about 64 °C, the hydrocarbon chain in the casting film shows a gradual reorientation from a tilted orientation to a nearly perpendicular one with the preservation of the all-trans conformation. On increase of the temperature up to about 75 °C, the hydrocarbon chain is oriented closed to the normal of the surface with a gradual increase of gauche conformers and mobility. When the film is heated above 75 °C, at which the order-disorder transition occurs, it appears that the orientation of the hydrocarbon chain turns tilted again owing to the increase in the degree of its disorder and mobility. However, the change in the orientation of the hydrocarbon chain nearly comes to a standstill between about 90 and 105 °C because the hydrocarbon chains assume random orientation to the utmost limit in this temperature range. Conclusions The present study has demonstrated that under properly prepared experimental conditions one can fabricate LB films of ES-1 on both CaF2 plate and Au-evaporated glass slide. The IR studies have revealed that the alkyl chains are nearly perpendicular to the film surface and assume highly ordered conformation. The first monolayer has different orientation and arrangement from other layers in the ES-1 LB films on the Au-evaporated glass slide probably because of the interaction between the gold surface and hydrophilic headgroup of ES-1. The IR

Wang et al.

transmission spectra of the ES-1 LB film on the CaF2 plate and the IR-RA spectra of the ES-1 LB film on the Au-evaporated glass slide at evaporated temperatures have shown that the order-disorder phase transitions of the two LB films take place at about 60 °C after the clear pretransitional alterations. It has been found that the packing density of alkyl chains decreases gradually with temperature after the order-disorder transitions in the LB films. The alkyl chains in the ES-1 casting films on both the CaF2 plate and Au-evaporated glass slide are tilted considerably. The studies on the phase-transition behavior have revealed that the two casting films give rise to the order-disorder transition at about 75 °C after undergoing the pretransitional changes. Moreover, for the casting film of ES-1 on the Au-evaporated glass slide, there is a phenomenon of reorientation that the alkyl chains change from more tilted orientation at room temperature to more perpendicular orientation at higher temperature before the order-disorder transition takes place and turn tilted again above the transition temperature. Acknowledgment. This study was supported by Ministry of Culture, Education, and Science and Technology, Japanese Government. Authors from Jilin University thank the supported from the NSFC Grants 20173019 and 29633010 and the Major State Basic Research Development Program Grant G2000078102. LA020608G