J. Phys. Chem. B 1998, 102, 6515-6520
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Infrared Study on Molecular Orientation and Phase Transition in Langmuir-Blodgett Films of an Amphiphilic Microgel Copolymer with the Branching of Octadecyl Groups Bing Zhao,*,†,‡ Hongbin Li,‡ Xi Zhang,‡ Jiacong Shen,‡ and Yukihiro Ozaki*,† Department of Chemistry, School of Science, Kwansei-Gakuin UniVersity, Uegahara, Nishinomiya 662-8501, Japan, and Key Laboratory of Supramolecular Structure and Spectroscopy, Jilin UniVersity, Changchun 130023, P. R. China ReceiVed: February 16, 1998; In Final Form: May 5, 1998
Molecular orientation, structure, and phase transition in Langmuir-Blodgett (LB) films of an amphiphilic polymer consisting of a flexible hydrophilic epichlorohydrin-ethylenediamine slightly cross-linking microgel and a number of hydrophobic stearic chains (ES-3) have been studied by infrared (IR) transmission and reflection-absorption (RA) spectroscopy. The infrared study indicates that the LB films of ES-3 possess crystalline, tightly packed methylene chains and extended interchain hydrogen bonds between the amide groups, which connect the alkyl chains with the microgel. A comparison of the band intensities between the transmission and RA spectra suggests that the hydrocarbon chains are highly oriented with a small tilt angle and that N-H and CdO bonds of the amide groups and the interchain hydrogen bonds between the N-H and CdO bonds are nearly parallel to the surface. It is indicated from infrared spectra of LB films of ES-3 prepared under different surface pressures that the alkyl chains are highly ordered even in the film deposited under nearly zero surface pressure, while the microgel core shows some mobility. The phase transitions have been observed in narrow temperature ranges near 65, 105, and 140 °C for multilayer LB films of ES-3. The three temperatures are very close to those for the phase transitions of bulk materials of ES-3, but the transitions occur more sharply for the LB films. Of note is that the transition temperatures of the LB films are markedly different from those of the self-assembling (SA) films previously reported. This may be due to structural differences in the alkyl chains between the two kinds of films; the SA films have partial interdigitation of the alkyl tails, while the LB films do not have it. In contrast to LB films of low molecular weight organic dyes, even a one-monolayer LB film of ES-3 shows clear phase transitions at slightly higher temperatures. Probably, the microgels prevent the direct interaction between the substrate and the alkyl chains in the first layer, providing the one-monolayer LB film with clear phase transitions.
Introduction In recent years Langmuir-Blodgett (LB) films of polymeric materials have drawn keen interest because they may offer superior mechanical properties, higher damage thresholds, and improved thermal stability compared with LB films of low molecular weight organic dyes.1-3 Since Tredgold and Winter4 first successfully fabricated polymeric LB films from preformed polymer, a number of research groups have been involved in the studies of polymeric ultrathin films.1-10 The polymeric LB films had one problem which the researchers had to overcome; the rearrangement and orientation of hydrophobic chains of prepolymerized amphiphiles at the air/water interface are disrupted by polymer main chains, often leading to polymeric LB films with low order. Laschewsky et al.5 and Ringsdorf et al.6 proposed an approach to decouple the interaction between the polymer main chains and side groups by introducing a “spacer group” into the prepolymerized amphiphiles 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, and vice versa.11-22 This kind of amphiphilic polymer can self-rearrange at the air/water interface and is readily * To whom correspondence should be addressed. † Kwansei-Gakuin University. ‡ Jilin University.
transferable onto solid substrates as so-called “duckweed” or “reversed duckweed” polymeric LB films.11-14 The term “duckweed” means 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 the water. The amphiphilic polymer we discuss in this paper is composed of a flexible hydrophilic epichlorohydrin-ethylenediamine slightly cross-linked network (microgel) and hydrophobic stearic grafting chains (denoted ES3; see Figure 1).15,16 This polymer, which has well-balanced hydrophilicity and hydrophobicity, can form a monolayer with a “reversed duckweed” structure.16 Although this sample lacks mesogens, it exhibits thermotropic liquid crystalline behavior possibly due to some semirigid rods formed by the collective strong interactions between the polymer’s alkyl chains.15 Furthermore, it has been found that ES-3 can be used as a matrix for assembling functional or composite ultrathin two-dimensional films.18 Detailed structural characterization and studies of thermal behavior is of great importance for practical applications as well as basic research of the polymeric ultrathin films.23,24 We have been investigating molecular orientation, structure, and thermal behavior of bulk ES-3 and its self-assembling (SA) film by
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Figure 2. π-A isotherm of ES-3 at 20 °C. Figure 1. Structure of the amphiphilic polymer (ES-3) and a schematic model for the “reversed duckweed” monolayer on the water subphase.
means of X-ray diffraction, infrared/attenuated total reflection (ATR) spectroscopy, and differential scanning calorimetry (DSC).15,16,21,22 These studies showed that both the bulk material and SA film of ES-3 have highly ordered layer structure and that only the SA film has partially interdigitated alkyl chains.21 The thermal behavior of the SA film is also quite different from that of the bulk material.15,21 The purpose of the present study is to investigate the structure, molecular orientation, their dependences on the surface pressure, and the thermal behavior of one- and multilayer LB films of ES-3. Infrared transmission and reflection-absorption (RA) spectroscopy has been employed to explore them. We have previously reported in conference proceedings fragmental results for the structure and molecular orientation in the multilayer LB films of ES-3,16,17 but this is the first time that we have ever described their detailed structural characterization. The conclusions reached for the LB films are compared with those for the bulk materials and SA films. Experimental Section Materials. The amphiphilic polymer (ES-3; Figure 1) was synthesized by the method described in detail in our previous paper.11 The molecular weight of ES-3 was 7840, and its alkyl chain content was 75.8%;12 one molecule contains 20.8 stearic side chains on average. The polymer was slightly cross-linked, and the cross-linking degree of ES-3, defined as the content of tertiary amine unit (dN-) in the polymers’ hydrophilic part, was determined to be 25% by XPS spectra.21 Preparation of Langmuir-Blodgett Films. A Kyowa Kaimen Kagaku model HBM-AP 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 1.0 × 10-3 M) of ES-3 was placed onto an aqueous subphase of water (pH 6.2). To prepare water for the aqueous subphase, water was passed through activated charcoal and reverse osmosis filters and then distilled. Finally, it was purified by a Ultrapure Water System model CPW-101 (Advantec, Japan). The resistance of the finally prepared water was larger than 18.1 MO, and its surface tension was 72.0 mN/ m. The temperature of the subphase was kept at 20 °C. After evaporation of the solvent, the monolayer was compressed at a constant rate of 20 cm2/min up to the given surface pressure. The monolayers were transferred by the vertical dipping method onto CaF2 plates for infrared transmission measurements and gold-evaporated glass slides for infrared reflection-absorption (RA) measurements at the given surface pressure. The substrates used had been subjected to ultrasonification in chloroform and then in distilled water. After drying they had been cleaned
by a homemade UV-ozone cleaner. The transfer ratio was found to be nearly unity throughout the experiments. Infrared Spectroscopy. Infrared spectra of the LB films were measured on a Nicolet Magna 550 FT-IR spectrometer equipped with a MCT detector. The spectra were taken at a 4 cm-1 resolution, and typically, 512 interferograms were coadded to yield spectra of high signal-to-noise ratio. For the infrared RA measurement, a reflection attachment (Spectra-Tech. FT80 RAS) was employed at the incident angle of 80°, together with a JEOL IROPT02 polarizer. To measure the infrared spectra at elevated temperatures, the CaF2 substrates on which the LB films had been deposited were inserted into a sample holder in a copper block that contained a ceramic heater. Temperature control was achieved by using an Omron E5T temperature controller. The temperature was monitored with a thermocouple connected with the sample holder and was raised by 1 °C min-1. Results and Discussion 1. Fabrications of the LB Films of ES-3. Figure 2 shows a surface pressure-area (π-A) isotherm of ES-3 on the subphase of pure water. It can be seen from the π-A curve that ES-3 forms a stable monolayer and has a high collapse surface pressure of 60 mN/m. The monolayer of ES-3 shows a typical condensed phase, and its limiting area is 4.10 nm2/ molecule. Based on the suggested “reversed duckweed” model,11,12 the area occupied by each alkyl chain is calculated to be 0.20 nm2. This area is very close to the limiting area of a stearic acid monolayer, suggesting that the alkyl chains are closely packed in the monolayer. The monolayer can be readily transferred onto a solid substrate with a transfer ratio of unity even at nearly zero surface pressure. To confirm the stability of the monolayer, we studied the compression and decompression cycles of the monolayer at 10, 20, and 30 mN/m. The p-A isotherm of the monolayer changed very little in the first cycle and showed almost no change in the second cycle. It was also found from the experiment of an isobar isotherm of the molecular area that the area of ES-3 at certain surface pressure does not change at least for several hours. These results suggest that the stability of the monolayer is very high. 2. Infrared Transmission and Reflection-Absorption Spectra of LB Films of ES-3. Figures 3 and 4 depict infrared transmission and RA spectra of one-, three-, and five-layer LB films of ES-3, respectively. The infrared transmission spectra were also measured for two-, seven-, nine-, and eleven-layer LB films (the spectra are not shown here). The assignments for bands in the infrared spectra are summarized in Table 1. Bands at 2917 and 2849 cm-1 are assigned to CH2 antisymmetric and symmetric stretching modes of the hydrocarbon
Amphiphilic Microgel Copolymer
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Figure 3. Infrared transmission spectra of (a) one-, (b) three-, and (c) five-layer LB films of ES-3 deposited on CaF2 plates.
Figure 4. Infrared reflection-absorption spectra of (a) one-, (b) three-, and (c) five-layer LB films of ES-3 deposited on gold-evaporated glass slides.
TABLE 1: Assignments for Infrared Bands of the LB Films of ES-3 wavenumber (cm-1)
assignment
3301 2961 2954 2931 2917 2874 2849 1697 1637 1564 1552 1465 1435 1380 1262 1249 1103 1038
NH stretch CH3 asymmetric stretch (in-plane) CH3 asymmetric stretch (out-of-plane) CH3 symmetric stretcha CH2 antisymmetric stretch CH3 symmetric stretch CH2 symmetric stretch CdO stretch amide I amide II amide II CH2 deformation CH2 deformation CH3 deformation amide III C-O stretch C-C stretch C-C stretch
a In Fermi resonance with the overtone of CH asymmetric deforma3 tion and crystalline transition.27
chains of ES-3, 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 (trans-zigzag) alkyl tail, while their upward shifts are indicative of the increase in conformational disorder, i.e., gauche conformers, in the hydrocarbon chain.25,26 The fact that the CH2 stretching bands appear at 2917 and 2849 cm-1 in the infrared spectra of Figures 3 and 4 suggests that the hydrocarbon chains of ES-3 are highly ordered, i.e., trans-zigzag in both the one- and multilayer LB
films. In the RA spectra, an additional band due to Fermi resonance of the CH3 mode appears at 2931 cm-1.27 This is further evidence of the highly ordered structure.27 Of note is that the spectra of one-, three-, and five-layer LB films are very close to each other except for band intensities, which increase with the number of layers. This observation suggests that the structure of the LB films changes little with the number of layers. According to the surface selection rule in infrared RA spectroscopy,28-30 vibrational modes with their transition moments perpendicular to the surface are enhanced in a RA spectrum. Therefore, the molecular orientation of the alkyl chains can be investigated by comparing the intensities of the CH2 antisymmetric and symmetric stretching bands between the infrared transmission and RA spectra. The intensities of the two CH2 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-3 are nearly perpendicular to the substrate surface in the LB films. This conclusion is in good agreement with the result obtained from the X-ray diffraction pattern of LB films of ES-3.16 The CH3 asymmetric stretching mode is inherently degenerate but splits into two by the intramolecular effects when the transzigzag conformation of a hydrocarbon chain has planar structure and its CH3 group does not rotate freely.31-34 The bands arising from the CH3 asymmetric stretching modes of the alkyl chains appear at 2954 and 2961 cm-1 in the spectra shown in Figures 3 and 4, respectively. The transition moments of the 2961 and 2954 cm-1 bands are in parallel with the x- and y-axes, respectively (here the x-axis is taken along the terminal C-H bond in the skeletal plane and the y-axis is perpendicular to the plane). It is noted that the band at 2961 cm-1 is strong in the RA spectra, while that at 2954 cm-1 is intense in the transmission spectra. Thus, it seems that the x-axis is nearly perpendicular to the substrate surface. The relative intensity of the two bands at 2961 and 2954 cm-1 follows an even-odd dependence on the number of methylene units present.33 The spectral pattern in the CH stretching region of RA spectra of the LB films of ES-3 is similar to that for n-alkanethiols containing even numbers of methylene groups.34,35 Other key bands in the infrared spectra of the LB films of ES-3 are as follows:36 3301 cm-1, NH stretching; 1697 cm-1, CdO stretching in the -C(dO)-N-C(dO) group; 1637 cm-1, amide I (monosubstituted amide); 1564 and 1552 cm-1, amide II; 1465 cm-1, CH2 scissoring mode; 1262 cm-1, amide III; 1380 cm-1, CH3 deformation; and 1249 cm-1, C-O stretching mode. The vibrational frequency of the NH stretching band suggests that the amide groups are in a trans configuration and involved in hydrogen bonds of medium strength in the LB films of ES3.30 The existence of hydrogen bonds between the adjacent amide groups is also indicated by the frequency of amide I (1637 cm-1), which is largely due to a CdO stretching mode.36 The N-H stretching band and amide I are stronger in the transmission spectra than in the RA spectra. Thus, both the N-H and CdO bonds seem to be nearly parallel to the substrate surface. A band due to amide II appears at 1552 cm-1 in the transmission spectra, while the RA spectra show another amide II at 1564 cm-1 very strongly. These observations also support the hydrogen bond structure described above. The amide III band, which is due to an in-plane C-N stretching coupled with N-H bending mode, is much stronger in the RA spectra than in the transmission spectra, indicating that the amide planes are nearly perpendicular to the substrate surface.
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Figure 5. Infrared transmission spectra of one-monolyer LB films of ES-3 fabricated under different surface pressures.
Figure 6. Infrared transmission spectra of an eleven-layer LB film of ES-3 measured at 30, 75, 110, and 150 °C.
The results from the infrared transmission and RA spectra lead us to conclude that the LB films of ES-3 possess crystalline, tightly packed methylene chains and extended interchain hydrogen bonds between the adjacent amide groups. We previously carried out low-angle X-ray diffraction measurement and polarized infrared/attenuated total reflection (ATR) experiments for multilayer LB films of ES-3.16 It was concluded from the studies that the alkyl chains are highly ordered, their average tilt angle is 28°, and that there is no interdigitation among the alkyl chains in the LB films. The present study has provided new insight into the hydrogen bonds of amide groups. In contrast to the LB films, it is likely that a SA film of ES-3 has partial interdigitation;21 in the case of the SA film prepared by slow evaporation of solvent under saturated vapor pressure, the solvophilic alkyl chains have enough time to undergo a selforganizing process which may be responsible for the interdigitation. The situation is quite different for the LB films. As suggested for the bulk ES-3,15 there may be strong interaction between the long alkyl chains and some semirigid rods are formed in the LB films. Two factors may be important for the highly ordered structure of the LB and SA films of this polymeric material; one is delicate balance between the hydrophilic and hydrophobic parts, and the other is flexibility of the network. 3. Dependence of Molecular Aggregation on the Surface Pressure. Figure 5 depicts infrared transmission spectra of onemonolayer LB films of ES-3 prepared under surface pressures of 0.1, 0.5, 5, and 10 mN/m. The CH stretching band region (the 3100-2700 cm-1 region) is almost unchanged with the surface pressure. Even in the spectrum of the LB film fabricated at the surface pressure of 0.1 mN/m, the CH2 stretching bands appear strongly at 2917 and 2849 cm-1. Thus, it seems that the alkyl chains assume a trans-zigzag conformation in the LB film prepared at the surface pressure of 0.1 mN/m. There are significant differences in the 3400-3200 and 1800-1300 cm-1 regions between the spectra of the LB films deposited at low and high surface pressures (Figure 5). The intensities of the NH stretching band and amide I are significantly weaker in the spectrum of the LB film fabricated at the surface pressure of 0.1 mN/m than those of the LB films prepared at higher surface pressures. It is therefore likely that the bond axes of the NH and CdO groups are titled with respect to the surface normal in the film prepared at 0.1 mN/m. However, the interchain hydrogen bonds do exist even in the LB film fabricated at the surface pressure of 0.1 mN/m, as evidenced by the frequencies of the NH stretching band and amide I. It is of note that the strength of the interchain hydrogen bonds changes little with the surface pressure. The bandwidth
of the NH stretching and amide I bands is significantly wider in the LB film fabricated at the surface pressure of 0.1 mN/m. Therefore, it seems that the mobility of the microgels increases in the LB film fabricated at low surface pressure. 4. Phase Transition in One- and Multilayer LB Films of ES-3. Figure 6 shows infrared transmission spectra of an eleven-layer LB film of ES-3 measured at room and elevated temperatures. The corresponding spectra were measured also for the one- and five-layer films, but the spectra are not shown here. The five-layer film shows temperature-dependent infrared spectral changes similar to the eleven-layer film, but there are appreciable differences in the temperature-dependent spectral variations between the one- and multilayer films. Figure 6 shows that the NH stretching band and the amide I band shift upward by 3-5 cm-1 between 75 and 110 °C and shift further at higher temperatures. These results indicate that the hydrogen bonds of the amide groups become weak with temperature. To monitor the temperature-dependent spectral changes in the CH stretching region, we plot the frequency and bandwidth of the CH2 antisymmetric stretching band against temperature for the one- and eleven-layer LB films. The results are shown in Figure 7A and B, respectively. It can be clearly seen from Figure 7A and B that the eleven-layer film gives sharp spectral changes in the narrow temperature ranges near 65, 105, and 140 °C, while the one-monolayer film shows gradual changes in slightly higher temperature ranges. The results for the eleven-layer LB film are in good agreement with our previous temperature-dependent infrared study of bulk ES3, for which a X-ray diffraction study for the thermal behavior was also performed.15 According to the X-ray diffraction study,15 the layer structure of the LB multilayer is maintained during the first two transitions, and only the layer spacing changes a little. In the third transition, the diffraction peak disappears, indicating that the layer structure is destroyed.8 Judging from the similarity in the temperature-dependent spectral changes between the LB film and bulk material, the thermal behavior of these two states may be very close. At room temperature, the alkyl tails of the eleven-layer LB film of ES-3 are in a highly ordered state, and they undergo the three transition processes from the highly ordered state into a melted disordered state through two meso-states. The first transition occurring near 65 °C may be ascribed to the conversion of the hydrocarbon chain from one ordered state to another one. It should be noted that the change in the bandwidth takes place gradually at lower temperature than that in the frequency. Therefore, it seem that the mobility of the alkyl chains becomes active before the first transition occurs. The second transition is probably due to the order-disorder transition of the alkyl
Amphiphilic Microgel Copolymer
J. Phys. Chem. B, Vol. 102, No. 34, 1998 6519 the first phase transition results just from the structural change in the alkyl chains. In the temperature range 30-100 °C, the structure of the microgel core and the amide groups seems to change little. The other two phase transitions are common for the alkyl chains and amide groups. Probably, the second and third phase transitions are related to the whole ES-3 molecule. In the second and third transition, the NH and amide I bands show an upward shift, indicating that the interchain hydrogen bonds become weak step by step. Conclusions
Figure 7. Temperature dependences of the frequency and bandwidth at half the peak height of a CH2 antisymmetric stretching band for (A) eleven- and (B) one-layer LB films of ES-3.
chains, and the third one is attributed to the melting of the alkyl chains into an isotropic state. The temperature-induced structural variations in the multilayer LB films bear close resemblance to those for the bulk materials, but the former gives shaper phase transitions than the latter. We infer that the LB films have more ordered layer structure, so that they can provide more concerted phase transitions. Of particular interest is that the LB films of ES-3 show quite different thermal behavior from its SA film; the former yields three transition temperatures, while the latter gives only two.21 The difference may be attributed to the difference in the interactions of the hydrophobic alkyl chains. The alkyl chains have partial interdigitation in the SA film, while they form semirigid rods in the LB films due to the strong lateral interaction among the long alkyl chains. In contrast to one-layer LB films of small organic dyes which do not show clear phase transition due to the interaction between the substrate and the first layer, the one-layer LB film of ES-3 shows clear phase transitions (Figure 7). There are flexible hydrophobic networks between the substrate and the long hydrocarbon chains in the LB films of ES-3. Therefore, the effect of the substrate on the hydrocarbon chains may be decreased largely by the microgel core of ES-3, although the interaction between the substrate and the first monolayer still exists. We also plotted the frequency and bandwidth at half the peak height of N-H stretching and amide I bands with temperature for an eleven-layer LB film of ES-3, respectively. A sharp frequency shift and a change in the bandwidth are observed for the N-H stretching and amide I bands near 105 and 140 °C, where the CH2 band also yields the transition. In contrast to the alkyl chains, the amide groups do not show a significant spectral change until 100 °C (Figure 6). This result means that
The infrared transmission and RA spectroscopy study has provided new insight into the molecular orientation, structure, and thermal behavior of the “reversed duckweed” type of polymeric LB films. The following conclusions have been reached for the structure and molecular orientation in the LB films. (i) The 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. (ii) 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. The temperature-dependent spectral changes in the infrared spectra of the one- and multilayer LB films of ES-3 have yielded the following conclusions. (i) 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 change corresponds to the order-disorder transition of the alkyl chains and the amide groups, and the last transition is attributed to the transition to the isotropic state. (ii) The LB films of ES-3 show quite different thermal behavior from its SA film; the former yields three transition temperatures, while the latter gives only two.21 The difference may be concerned with the difference in the interactions of the hydrophobic alkyl chains. (iii) 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 transitions. Acknowledgment. One of the authors (B.Z.) thanks the National Natural Science Foundation of China for financial support (Project 29633010). References and Notes (1) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991. (2) Fuchs, H.; Ohst, H.; Prass, W. AdV. Mater. 1991, 3, 10. (3) Tredgold, R. H. J. Mater. Chem. 1995, 8, 1095. (4) Tredgold, R. H.; Winter, C. S. J. Phys. D: Appl. Phys. 1982, 15, L55. (5) Laschewsky, A.; Ringsdorf, H.; Schreider, J. Angew. Makromol. Chem. 1986, 145/146, 1. (6) Ringsdorf, H.; Laschewsky, A.; Schmidt, G.; Schreider, J. J. Am. Chem. Soc. 1987, 109, 778. (7) Sun, F.; Caster, D. G.; Grainger, D. W. Langmuir 1993, 9, 3200. (8) Higashi, N.; Mori, T.; Niwa, M. J. Chem. Soc., Chem. Commun. 1990, 225.
6520 J. Phys. Chem. B, Vol. 102, No. 34, 1998 (9) Leuk, T. L.; Halmark, V. M.; Rabolt, J. F.; Haussliny, L.; Ringsdorf, H. Macromolecules 1993, 26, 1230. (10) Sun, F.; Grainger, D. W. J. Polym. Sci., Polym. Chem. Ed. 1993, 31, 1729. (11) Yin, R.; Cha, X.; Zhang X.; Shen, J. C. Macromolecules 1990, 23, 5158. (12) Cha, X.; Yin, R.; Zhang, X.; Shen, J. C. Macromolecules 1991, 24, 4985. (13) Shen, J. C.; Zhang, X.; Zhang, R. F. Thin Solid Films 1992, 210/ 211, 628. (14) Zhang, R. F.; Zhang, X.; Shen, J. C. Langmuir 1994, 10, 2727. (15) Li, H. B.; Zhang, X.; Zhang, R. F.; Shen, J. C.; Zhao, B.; Xu, W. Q. Macromolecules 1995, 28, 8178. (16) Zhang, R. F.; Zhang, X.; Li, H. B.; Zhao, B.; Shen, J. C. Polym. Bull. 1996, 36, 227. (17) Zhao, B.; Li, H. B.; Zhang, X.; Zhang, R. F.; Shen, J. C.; Ozaki, Y. Prog. Colloid Polym. Sci. 1997, 106, 144. (18) Zhang, X.; Zhang, R. F.; Shen, J. C.; Zou, G. T. Makromol. Rapid Commun. 1994, 15, 373. (19) Zhang, R. F.; Zhang, X.; Wang, J.; Shen, J. C. Thin Solid Films 1994, 248, 102. (20) 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. (21) Zhang, X.; Li, H. B.; Zhao, B.; Shen, J. C. Macromolecules 1997, 30, 1633. (22) Shen, J. C.; Li, H. B.; Xiong, H. M.; Zhang, X. Macromol. Symp. 1997, 118, 707.
Zhao et al. (23) Swalen, J. D. Annu. ReV. Mater. Sci. 1991, 21, 356. (24) Takenaka, T.; Umemura, J. In Vibrational Spectra and Structure; Durig, J. R., Ed.; Elsevier: Amsterdam, 1991; Vol. 19, p 215. (25) Umemura, J.; Cameron, D. G.; Mantsch, H. H. Biochim. Biophys. Acta 1980, 602, 32. (26) Sapper, H.; Cameron, D. G.; Mantsch, H. H. Can. J. Chem. 1981, 59, 2543. (27) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (28) Greenler, R. G. J. Chem. Phys. 1966, 44, 310. (29) Chollet, P. A.; Messier, J.; Rosilio, C. J. Chem. Phys. 1976, 64, 1042. (30) Umemura, J.; Kamata, T.; Kawai, T.; Takanaka, T. J. Phys. Chem. 1990, 94, 62. (31) Snyder, R. G.; Hsu, S. L.; Krimm, S. Spectrochim. Acta, Part A 1978, 34, 395. (32) MacPhail, R. A.; Snyder, R. G.; Strauss, H. L. J. Am. Chem. Soc. 1980, 102, 3976. (33) Tao, Y.-T. J. Am. Chem. Soc. 1993, 115, 4350. (34) Nuzzo, R.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (35) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y. T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152. (36) Colthup, N. B., Daly, L. H., Wiberley, S. E., Eds. In Introduction to Infrared and Raman Spectroscopy, 3rd ed.; Academic Press: San Diego, 1990; pp 289-325.
