4754
Langmuir 1996, 12, 4754-4759
Evaluation of Molecular Orientation in a Polymeric Monolayer at the Air-Water Interface by Polarization-Modulated Infrared Spectroscopy Lijun Mao and Anna M. Ritcey* De´ partement de chimie and CERSIM, Universite´ Laval, Que´ bec, Que´ bec, Canada G1K 7P4
Bernard Desbat Laboratoire de Spectroscopie Mole´ culaire et Cristalline (URA 124 CNRS), Universite´ de Bordeaux I, F-33405 Talence Cedex, France Received September 8, 1995. In Final Form: June 11, 1996X Polarization-modulated reflection-absorption spectroscopy (PM-IRRAS) is employed to record infrared spectra of a monolayer of (dodecyl)cellulose spread at the air-water interface. Methylene symmetric and antisymmetric stretches of the alkyl side chains are clearly visible in the monolayer spectra as positive peaks at 2852 and 2922 cm-1. It is thus concluded that the side chains possess a net orientation perpendicular to the water surface. Bands characteristic of the cellulose backbone are also observed as positive PMIRRAS peaks, in the region 1160-980 cm-1. This result indicates that the rigid polysaccharide chain lies essentially parallel to the water surface. Spectra recorded as a function of molecular area reveal little change in molecular orientation during monolayer compression. Significant differences in orientation are however evident between dense films obtained by the compression of disperse monolayers and those prepared by direct spreading at low surface areas.
Introduction Cellulose derivatives containing alkyl side chains have been found to form stable monolayers at the air-water interface.1,2 Studies of these, and other rigid rodlike polymers,3 have suggested that under conditions of sufficient natural chain extension, a strong amphiphilic nature may not be required for the successful formation of Langmuir-Blodgett (LB) films. Such systems are of interest because they offer the possibility to combine the favorable material properties of polymers with the control of molecular organization available with the LB technique. With the introduction of appropriate chromophores, this ordering can be exploited for applications in the fields of nonlinear optics and microelectronics.4 Cellulose alkyl ethers are structurally very different from the fatty acids traditionally considered for LB film fabrication. These cellulose derivatives contain no strongly hydrophilic groups and the alkyl side chains are covalently fixed to the polymer backbone rather than existing as free small molecules. It is therefore impossible to predict if side chain orientation in the spread monolayers of such polymers will be analogous to the familiar well-characterized case of cadmium arachidate. Furthermore, the complete characterization of the molecular order in these systems must also include the determination of the orientation of the rigid cellulose backbone. Surface pressure-area isotherms of monolayers formed by cellulose ethers generally show a liquid analogous phase at molecular areas compatible with a model in which the rigid polymer backbone lies flat at the interface and the side chains are partially oriented perpendicular to the water surface.2 Upon compression beyond the liquid X
Abstract published in Advance ACS Abstracts, August 1, 1996.
(1) Mao, L.; Ritcey, A. M. Thin Solid Films 1994, 242, 263-266. (2) Basque, P.; de Gunzburg, A.; Rondeau, P.; Ritcey, A. M. Submitted for publication in Langmuir. (3) Wegner, G. Ber. Bunsenges. Phys. Chem. 1991, 95, 1326. (4) Basque, P.; Ritcey, A. M. Proceedings of the American Chemical Society, Division of Polymeric Materials: Science and Engineering; American Chemical Society: Washington, DC, 1994; Vol. 71, pp 488489.
S0743-7463(95)00745-1 CCC: $12.00
analogous phase, the surface pressure remains constant over a large range of molecular areas, resulting in the appearance of a distinct plateau. Differing interpretations of this plateau region have appeared in the literature. It has been attributed to the formation of bi- or multilayers,2,5 to the ordering of the alkyl side chains,6 and to the piling up of undefined films near the barriers.7 The purpose of the present work is to determine if side chain orientation can be achieved in spread monolayers of a cellulose alkyl ether. The orientation of the cellulose backbone is also studied. In order to clarify the origin of the surface pressure plateau typically observed in the monolayer isotherms of these rigid polymers, changes in molecular orientation during monolayer compression are investigated. These objectives are met by the application of the technique of polarized infrared spectroscopy. Infrared spectroscopy has proven to be well-suited to the characterization of Langmuir-Blodgett films. One important aspect of this method is the possibility of obtaining information about the orientation of specific parts of a molecule from characteristic group vibrations. The enhanced sensitivity of both attenuated total reflection8,9 and reflection-absorption measurements at metal surfaces10,11 has frequently been exploited in the determination of molecular orientation in films transferred to appropriate substrates. Recently, Buffeteau et al.12-14 have developed the technique of polarization-modulated infrared reflection (5) Matsumoto, M.; Itoh, T.; Miyamoto, T. In Cellulosics Utiliization, Inagaki, H., Phillips, G. O., Eds.; Elsevier: London, 1989; pp 151-160. (6) Kawaguchi, T.; Nakahara, H.; Fukuda, K. Thin Solid Films 1985, 133, 29. (7) Motschmann, H.; Reiter, R.; Lawall, R.; Duda, G.; Stamm, M.; Wegner, G.; Knoll, W. Langmuir 1991, 7, 2743. (8) Haller, G. L.; Rice, R. W. J. Chem. Phys. 1970, 74, 4386. (9) Ulman, A. An Introduction to Ultrathin Films; Academic Press: Toronto, 1991; p 6. (10) Rabolt, J. F.; Jurich, M.; Swalen, J. D. Appl. Spectrosc. 1985, 39 (2), 269. (11) Crooks, R. M.; Xu, C.; Sun, L.; Hill, S. L.; Ricco, A. J. Spectroscopy 1993, 8 (7), 28. (12) Buffeteau, T.; Desbat, B.; Turlet, J. M. Appl. Spectrosc. 1991, 45 (3), 380.
© 1996 American Chemical Society
Evaluation of Molecular Orientation by PM-IRRAS
Langmuir, Vol. 12, No. 20, 1996 4755 recorded at an angle of incidence of 76°. All spectra are reported as normalized difference PM-IRRAS, ∆S/S ) (S(d) - S(o))/S(o)).
Results and Discussion
Figure 1. Chemical structure of (dodecyl)cellulose.
Figure 2. Infrared spectra of (dodecyl)cellulose: (a) PM-IRRAS spectrum of a monolayer spread at the air-water interface and (b) normal FT-IR spectrum in chloroform solution.
absorption spectroscopy (PM-IRRAS), which enables spectra to be recorded directly from monomolecular layers spread at the air-water interface. It should be emphasized that this represents a significant challenge because of the strong absorption of infrared radiation by water. With this method the incident beam is rapidly modulated between polarizations parallel and perpendicular to the plane of incidence. Since the absorption by oriented species depends on the incident polarization, the resulting detector signal is also modulated. The selective detection of signals modulated at the appropriate frequency eliminates isotropic absorptions from the recorded spectrum. In addition, information concerning molecular orientation can be deduced from the sign and intensity of the recorded bands. Experimental Details (Dodecyl)cellulose was prepared from cellulose acetate (Aldrich, Mw 30 000, acetyl content 39.8%) and 1-bromododecane in DMSO/NaOH.2 The structure of this polymer is shown in Figure 1. Alkyl substitution is considered to be complete, with three dodecyl chains per anhydroglucose unit. This is supported by the absence of a significant hydroxyl absorption band in the IR spectrum of Figure 2b. PM-IRRAS spectra of the covered S(d) and uncovered S(o) water surface were recorded with a Nicolet 740 FTIR spectrometer at 4 cm-1 resolution and a total of 200 scans. Details of the instrument have been described elsewhere.12 Spectra were (13) Blaudez, D.; Buffeteau, T.; Cornut, J. C.; Desbat, B.; Escafre, N.; Pe´zolet, M.; Turlet, J. M. Thin Solid Films 1994, 242, 146. (14) Blaudez, D.; Buffeteau, T.; Cornut, J. C.; Desbat, B.; Escafre, N.; Pe´zolet, M.; Turlet, J. M. Appl. Spectrosc. 1993, 47 (7), 869.
Side Chain Orientation. Infrared spectra recorded by PM-IRRAS from a monolayer of (dodecyl)cellulose spread at the air-water interface show clear bands corresponding to the symmetric and antisymmetric CH2 stretching at 2852 and 2922 cm-1, respectively. As shown in Figure 2, the PM-IRRAS spectrum of the monolayer corresponds very closely in form and band position to the solution spectrum of the same sample, although a shift of ∼4 cm-1 to lower frequency is found for the monolayer. It is important to note that the two methylene vibrations appear as positive bands in the PM-IRRAS spectrum, indicating that the corresponding transition moments are preferentially oriented parallel to the water surface. In the case of an all-trans conformation, these transition moments are known to be perpendicular to the alkyl chain direction. It can thus be concluded that the side chains are at least partially oriented perpendicular to the surface. The methyl out-of-plane antisymmetric stretch is also evident in the monolayer spectrum at 2956 cm-1. Although difficult to quantify, the methyl stretch is clearly less intense, relative to the methylene bands, in the monolayer spectrum than in the corresponding solution spectrum. This is a further indication of side chain orientation. The C-H stretch transition moment of the methyl group lies at an angle of 54.5° with respect the alkyl chain direction, and the corresponding absorption band will therefore appear less intense in the PM-IRRAS spectrum of side chains oriented perpendicular to the water surface. In the case of phospholipid monolayers, the frequency of CH2 stretching bands has been correlated with the conformational order of the alkyl chains.15 Methylene stretches of all-trans conformations appear at lower frequencies than in less ordered systems. The shift of the corresponding absorptions in the spectra of Figure 2 is thus an indication that there are fewer gauche conformers within the alkyl side chain of the monolayer than in solution. This is in agreement with the above conclusion concerning side chain orientation. PM-IRRAS spectra of the spread monolayer are presented in Figure 3 as a function of compression. The intensity increases rapidly with decreasing molecular area, as plotted in Figure 4. The observed intensity of a PM-IRRAS spectrum depends both on the number of molecules per unit area of surface and on the orientation of the transition moments responsible for the absorption. If no changes in orientation occur, the PM-IRRAS intensity per unit molecular density should remain constant. That is
I ) constant n/A
(1)
where I is the PM-IRRAS normalized signal intensity, n is the number of spread molecules, and A is the trough area. The intensity at any surface area I(A) can thus be calculated from the observed intensity Io at a given surface area Ao
I(A) ) IoAo/A
(2)
The dashed lines in Figure 4 correspond to the variation in intensity calculated in this way from the intensity observed at 120 Å2/monomer. No changes in orientation (15) Mitchell, M. L.; Dluhy, R. A. J. Am. Chem. Soc. 1988, 110, 712.
4756 Langmuir, Vol. 12, No. 20, 1996
Figure 3. Normalized PM-IRRAS spectra of (dodecyl)cellulose spread as a monolayer at the air-water interface as a function of monolayer compression from 108 to 14 Å2 per anhydroglucose repeat unit. Spectral intensity increases with decreasing molecular area.
Figure 4. Normalized PM-IRRAS intensity recorded at 2852 cm-1 ([) and 2922 cm-1 (b) for a spread monolayer of (dodecyl)cellulose as a function of molecular area. The dashed lines correspond to the expected intensities calculated from the intensity observed at high molecular area and the increase in surface density upon compression.
are assumed. The observed behavior follows the calculated line very closely. This is perhaps more clearly illustrated in Figure 5 where the observed intensity per unit molecular density is plotted as a function of molecular area. This plot shows that there is no significant change during compression, except perhaps at very low surface areas. The error bars represent only the noise in the IR spectra; no uncertainty was attributed to the surface area. The
Mao et al.
Figure 5. Normalized PM-IRRAS intensity recorded at 2852 cm-1 ([) and 2922 cm-1 (b) divided by the number of monomers per Å2, plotted as a function of molecular area. The error bars represent only the noise in the IR spectra; no uncertainty was attributed to the surface area.
fact that the increase in intensity with compression is accounted for solely by the increase in the number of molecules per unit surface, indicates that side chain orientation does not vary much during compression. As mentioned above, the data presented in Figure 5 show an apparent decrease in the normalized intensity below molecular areas of 30 Å2. While it is possible that this decrease corresponds to a loss of orientation accompanying monolayer collapse, it is difficult to draw firm conclusions because errors in the surface area of the trough become increasingly important at high degrees of compression. This experimental difficulty will be corrected in future measurements by the simultaneous monitoring of surface pressure and barrier position during spectral acquisition. The observation that side chain orientation is independent of molecular area is somewhat surprising, especially when the pressure-area isotherm for this monolayer is considered. As shown in Figure 6, important changes in surface pressure occur during monolayer compression. A sharp rise in surface pressure is observed at a molecular area of 115 Å2. This is followed by an important transition region between 100 and 40 Å2 per repeat unit, over which the surface pressure remains relatively constant. As mentioned above, similar plateau regions have been attributed both to the formation of bilayers as molecules leave the water surface2,5 and to an ordering of alkyl side chains.6 In the present case it is clear that side chain order does not improve upon compression in this region, and the plateau can rather be attributed to bi- or multilayer formation. It is significant that this transition occurs without major modifications in side chain orientation. Since these polymers are not amphiphilic (the most strongly polar groups present are ether linkages), there is no reason to expect that a stable oriented bilayer, analogous to that found for lipids, for example, should exist. Furthermore, since the density of molecules on the surface, far from the barriers, increases regularly with compression through this transition area,
Evaluation of Molecular Orientation by PM-IRRAS
Langmuir, Vol. 12, No. 20, 1996 4757
Figure 7. Sketch depicting possible side chain orientation in a “hairy-rod” polymer.
Figure 6. Surface pressure-area isotherm of a spread monolayer of (dodecyl)cellulose recorded at 25 °C.
this monolayer does not seem to show the undefined piling up of molecules near the barriers that was found for synthetic polypeptides by ellipsometry measurements.7 A second important aspect of the above results is the observation that side chain orientation at molecular areas as large as 120 Å2 is similar to that present in highly compressed films. This is true despite the fact that the monolayer surface pressure falls to zero at this molecular area. It is significant that reasonably good side chain orientation exists at molecular areas larger than the 2025 Å2 typically quoted for the ordering of fatty acid molecules. Unfortunately, it is impossible to determine if the loss of PM-IRRAS intensity above 120 Å2 per repeat unit is the result of a loss of side chain orientation or simply because the molecular surface density becomes insufficient for detection. The degree of side chain orientation can be estimated by comparison to methylene stretching intensities obtained for small molecule systems that are known to be highly oriented. The sample studied here gives a normalized PM-IRRAS intensity at 2852 cm-1 of about 0.1 per monomer per Å2 (see Figure 5). When it is considered that each monomer contains 3 alkyl side chains comprising 11 methylene units each, the normalized intensity of 0.003 per CH2 per Å2 can be calculated. This compares very favorably with an intensity of 0.005 per CH2 per Å2 obtained for a highly oriented phospholipid monolayer.16 Several features of the molecular architecture of the sample studied here prevent the side chains from achieving the same degree of order as is possible in small molecule amphiphiles. Firstly, it is important to note that the alkyl substituents all occupy equatorial positions on the anhydroglucose ring. Furthermore, it is unlikely that neighboring rings within the polymer backbone lie within the same plane. It is therefore necessary to model the cellulose ether as a “hairy rod”,17 as sketched in Figure 7. Since the side chains are distributed at random about the central rod, they must turn away from the water surface before adopting a perpendicular orientation. A (16) Labrecque, J.; Pe´zolet, M.; Desbat, B. Unpublished results. (17) Seufert, M.; Fakirov, C.; Wegner, G. Adv. Mater. 1995, 7 (1), 52.
Figure 8. Infrared spectra of (dodecyl)cellulose: (a) PM-IRRAS spectrum of a monolayer spread at the air-water interface and (b) normal FT-IR spectrum in chloform solution.
relatively high degree of order can thus be expected only for side chain segments located further from the polymer backbone. Orientation of the Cellulose Backbone. IR absorption bands characteristic of the cellulose backbone can be identified in the region 1160-980 cm-1 of the PM-IRRAS spectrum, as shown in Figure 8a. Comparison of the monolayer spectrum with that obtained in solution indicates that, although the band positions are similar, major differences in relative intensity exist. The bands in this region of the spectrum arise primarily from C-O stretches. The exact assignment of the corresponding vibrations is extremely difficult because of the large number of atoms present and the inherent asymmetry of the glucose unit. Normal coordinate analysis for cellulose shows that vibrations in this region of the spectrum exhibit a high degree of molecular coupling and thus none of the modes arises from a single isolated molecular vibration.18 A strong absorption at 1163 cm-1, observed for both cellulose and many cellulose derivatives, is believed to be characteristic of the ring structure. Liang and Marchessault19 attribute this vibration to the antisymmetrical stretching of the bridge oxygen. Experimentally, all of (18) Cael, J. J.; Gardner, K. H.; Koenig, J. L.; Blackwell, J. J. Chem. Phys. 1975, 62 (3), 1145. (19) Liang, C. Y.; Marchessault, R. H. J. Polym. Sci. 1959, 39, 269.
4758 Langmuir, Vol. 12, No. 20, 1996
Mao et al.
Figure 9. Sketch illustrating the possible orientation of a ring transition moment with respect to the chain direction.
Figure 11. Normalized PM-IRRAS intensity recorded at 1028 cm-1 (b) and 1092 cm-1 ([) for a spread monolayer of (dodecyl)cellulose as a function of molecular area. The dashed lines correspond to the expected intensities calculated from the intensity observed at high molecular area and the increase in surface density upon compression.
Figure 10. Normalized PM-IRRAS spectra of (dodecyl)cellulose spread as a monolayer at the air-water interface as a function of monolayer compression from 87 to 11 Å2 per anhydroglucose repeat unit. Spectral intensity increases with decreasing molecular area.
the absorptions in the region from 1160 to 980 cm-1 show net polarizations parallel to the cellulose chain direction.20 Therefore the observation of positive bands in this region of the PM-IRRAS spectrum for the monolayer indictes that the cellulose backbone lies preferentially parallel to the water surface. As noted above, significant differences in relative intensities exist between the monolayer and isotropic solution spectra. These differences are further evidence of the orientation of the cellulose backbone. Even though all of the absorptions show net polarization along the chain direction, the corresponding transition moments are not strictly parallel to the polymer backbone. The intensity observed in the PM-IRRAS spectrum is determined by the projection of each transition moment onto the plane of the air-water interface. If it is assumed that the cellulose chain adopts a cylindrical symmetry, the relative intensity of each band observed in the PM-IRRAS spectrum of the monolayer will depend on the angle between the corresponding transition moment and the chain axis. This is sketched in Figure 9. The observed differences in intensity result simply from differences in the orientation of the various transition moments responsible for the absorptions. Variations in the relative intensities of the complex combination of bands in this region may also be responsible for the slight shift in peak positions observed in the monolayer relative to isotropic solution. These differences could also be related to changes in polymer or ring conformation upon interaction with the water surface. (20) Morohoshi, N. In Wood and Cellulosic Chemistry; Hon, D. N.-S., Shiraishi, N., Eds.; Dekker: New York, 1991; p 341.
Figure 12. Normalized PM-IRRAS intensity recorded at 1028 cm-1 (b) and 1092 cm-1 ([) divided by the number of monomers per Å2, plotted as a function of molecular area. The error bars represent only the noise in the IR spectra; no uncertainty was attributed to the surface area.
As shown in Figure 10, the intensity of the PM-IRRAS spectrum increases as a function of monolayer compression. The relative intensities of the major bands, however, remain constant, again suggesting that the orientation of the cellulose backbone does not significantly change during compression. Figure 11 illustrates that the variation in band intensity at both 1028 and 1092 cm-1 upon compression can be totally attributed to the increased mo-
Evaluation of Molecular Orientation by PM-IRRAS
Figure 13. Ratio of PM-IRRAS intensity recorded at 1028 cm-1 (b) and 1092 cm-1 ([) to that of the methylene stretch at 2852 cm-1. The error bars represent only the noise in the IR spectra. The open symbols and dashed lines indicate the corresponding relative intensities observed for the sample polymer in chloroform solution.
lecular surface density. The intensities normalized to unit molecular density are plotted in Figure 12, and as was the case for the methylene stretches considered above, little variation is found over a large range of molecular area. Relative Intensities of the Two Spectral Regions. The ratio of the intensities of absorption bands characteristic of the cellulose backbone at 1028 and 1092 cm-1 to that of the methylene stretch at 2852 cm-1 is plotted in Figure 13, as a function of monolayer compression. The corresponding ratios from the solution spectra are also shown as the dashed horizontal lines. The relative intensity of the band at 1092 cm-1 is found to be the same in the spread monolayer as in isotropic solution. The band at 1028 cm-1 is, however, twice as intense in the monolayer spectrum, relative to the methylene stretch, as it is in solution. This difference in relative intensity again emphasizes the anisotropic nature of the spread monolayer. The increased intensity of the band at 1028 cm-1, relative to the methylene stretch, in the PM-IRRAS spectrum suggests that the polymer backbone is more highly oriented than are the side chains. As noted above, a quantitative analysis cannot be made because of the complexity of the band structure in this region of the spectrum. Effect of Initial Spreading Area. The PM-IRRAS spectral intensities of the spread monolayer depend on the way in which the film is formed. This is illustrated in Figure 14, where two spectra are given, both for films at molecular areas of 14 Å2 per repeat unit. The spectrum shown as in Figure 14a was recorded after spreading at
Langmuir, Vol. 12, No. 20, 1996 4759
Figure 14. PM-IRRAS spectra recorded for a monolayer of (dodecyl)cellulose: (a) spread at large surface area and then compressed to 14 Å2 per monomer and (b) spread directly at 14 Å2 per monomer.
zero surface pressure and subsequent compression. The spectrum shown in part b was obtained after direct spreading at the same final molecular area. The PMIRRAS intensity is significantly smaller in the latter spectrum, being reduced by a factor of 2.5 for methylene stretches and of 1.5 for the cellulose backbone vibrations. Since the molecular surface density is identical for the two spectra, these differences can only be attributed to differences in orientation. The film spread directly at low surface area is less ordered than the one obtained by compression of a highly dispersed monolayer. Furthermore, this difference in orientation is more important for the side chains than for the cellulose backbone. Conclusions The results presented above demonstrate the utility of the technique of polarization-modulated IR spectroscopy in the characterization of films spread at the air-water interface. PM-IRRAS spectra of a spread monolayer of (dodecyl)cellulose indicate that molecules of this polymer are oriented at the air-water interface. Alkyl side chains are found to have a net orientation perpendicular to the water surface, whereas the cellulose backbone lies flat at the interface. No significant change in molecular orientation occurs upon monolayer compression. The degree of order present in these films depends on the way in which the monolayer is formed, with maximum orientation being obtained by initial spreading at large areas followed by compression. Acknowledgment. The financial support of NSERC (Canada) and FCAR (Que´bec) is gratefully acknowledged. LA950745W
