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Temperature-Dependent Behavior of Langmuir Monolayers of Octadecyl-Substituted Preformed Polyimides Hyun Yim,†,‡ Mark D. Foster,*,† Joachim Engelking,§ Henning Menzel,§ and Anna M. Ritcey| Maurice Morton Institute of Polymer Science, The University of Akron, Akron, Ohio 44325-3909, Institut fu¨ r Makromolekulare Chemie, Universita¨ t Hannover, Am Kleinen Felde 30, 30167 Hannover, Germany, and De´ partment de chimie and CERSIM, Universite´ Laval, Quebe´ c, Quebe´ c, Canada G1K 7P4 Received September 4, 1998. In Final Form: August 7, 2000
The behavior of Langmuir monolayers of octadecyl-substituted preformed polyimide molecules exhibits a strong dependence on temperature when the isotherms are measured using a Wilhelmy balance. This suggests a change in structure with temperature, though isotherms measured with a Langmuir balance change only modestly with temperature. Brewster angle microscopy images of the monolayer morphology are consistent with the notion that the monolayer is extremely rigid at 20-21 °C, while it has somewhat more fluidity at 28 °C. Polarization-modulated infrared reflection absorption spectroscopy results also indicate a qualitative difference between the characters of the monolayer at the two temperatures. The Wilhelmy plate method is sensitive to this difference, but its limited accuracy in the case of very rigid films results in a response that greatly amplifies the character of this change as compared to other means of probing the monolayer structure.
1. Introduction Hairy-rod polyimide molecules have been a focus of research due to their tremendous thermal stability and interesting physical properties.1,2 Several attempts to prepare polyimide Langmuir-Blodgett (LB) films have been reported,3-5 in which polyimide precursor monolayers were transferred and imidization was driven by thermal treatment in the thin film form. Another approach to prepare polyimide LB films is to use preformed polyimide molecules that contain side chains.6 This method is preferable to the previous one because it overcomes the problem of shrinkage that accompanies thermal treatment. The behavior of monolayers of alkyl side-chainsubstituted preformed polyimides on the water surface has been studied in detail in a previous paper.7 There it is shown that the monolayer behavior of the polyimide with octadecyl side chains is strongly dependent on temperature, with a sharp reduction in zero pressure area * To whom correspondence may be addressed. Phone: (330) 9725323. FAX: (330) 972-5290. E-mail:
[email protected]. † The University of Akron. ‡ Present address: Sandia National Laboratory. § Universita ¨ t Hannover. | Universite ´ Laval. (1) Mittal, K. L. Polyimides: Synthesis, Characterization, and Applications; Plenum Press: New York, 1984. (2) Myrvold, B. O. Liq. Cryst 1988, 3, 1255. (3) Uekita, M.; Awaji, H.; Murata, M. Thin Solid Films 1988, 160, 21. (4) Nishikita, Y.; Kakimoto, M.; Morikawa, A.; Imai, Y. Thin Solid Films 1988, 160, 15. (5) Tsukruk, V.; Mischenko, N.; Scheludko, E.; Krainov, I.; Tolmachev, A. Thin Solid Films 1992, 210/211, 620. (6) Yim, H.; Wu, H.; Foster, M. D.; Cheng, S. Z. D.; Harris, F. W. Langmuir 1997, 13, 3202. (7) Yim, H.; Foster, M. D.; McCreight, K.; Jin, X.; Cheng, S. Z. D.; Harris, F. W. Polymer 1998, 39, 4675.
occurring between 20 and 24 °C. This effect becomes negligible, however, when the number of carbon atoms in the side chain drops to seven. As the side chains become shorter, their role in the monolayer behavior becomes less pronounced. In this study, the strong dependence of the monolayers’ behavior on temperature is shown, using Brewster angle microscopy (BAM),10,11 to be connected with a change in monolayer morphology. Supporting evidence from polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS)8,9 also indicates a change in layer rigidity with temperature. 2. Experimental Section The octadecyl-substituted polyimide molecule, shown in Figure 1, was prepared by polymerizing 3,3′,4,4′-biphenyltetracarboxylic dianhydride and n-alkyl 4,4′-diamino-6,6′-dibromodiphenate in refluxing m-cresol. Substitution of the biphenyl with two bromine atoms and the two alkyl side chains renders the polymer soluble in THF, chloroform, and a few other common organic solvents.12 This makes it possible to readily spread the polyimides to form monolayers. Monolayers were prepared by spreading 0.2 mg/mL solutions in chloroform onto a Millipore-quality water subphase in a Nima 611 trough or a Lauda FW1 trough. Pressure-area isotherms were measured in the Nima trough with asymmetric compression at a rate of 20 cm2/min for various temperatures (15-32 °C). Surface pressure was measured by a Wilhelmy plate located at the opposite end of the trough from the barrier, and the plate was oriented perpendicular to the barrier, that is, parallel to the direction of movement of the barrier. Pressure(8) Buffeteau, T.; Desbat, B.; Turlet, J. M. Mikrochim. Acta 1988, 2, 23. (9) Buffeteau, T.; Pzolet, M. Appl. Spectrosc. 1996, 50, 948. (10) Ho¨nig, D.; Mo¨bius, D. J. Phys. Chem. 1991, 95, 4590. (11) He´non, S.; Meunier, J. Rev. Sci. Instrum. 1991, 62, 936. (12) McCreight, K. Ph.D. Thesis, The University of Akron, Akron, OH, 1998.
10.1021/la981167x CCC: $19.00 © 2000 American Chemical Society Published on Web 11/03/2000
Monolayers of Octadecyl-Substituted Polyimides
Figure 1. Chemical structure of the octadecyl-substituted polyimide molecule.
Figure 2. Pressure-area isotherms of the polyimide molecule measured with a Wilhelmy plate at a compression rate of 20 cm2/min at different temperatures. area isotherms measured in the Lauda trough were also measured in asymmetric compression at a compression speed of 20 cm2/ min. However, in that case the surface pressure was measured with a Langmuir balance which functions as a simple “floating barrier”. The morphologies of the polyimide monolayers were observed using a Brewster angle microscope (NFT MiniBAM) at various surface areas and various temperatures (16-28 °C) using the Lauda trough and the above-mentioned conditions. PM-IRRAS spectra of the covered (S(d)) and uncovered (S(0)) water surface were recorded with a Nicolet 850 FTIR spectrometer at 4 cm-1 resolution. A total of 500 scans were collected for each measurement at an angle of incidence of 76° with respect to the surface normal. Normalized difference spectra, ∆S/S, were considered, where ∆S/S ) (S(d) - S(0))/S(0). Surface pressures associated with the different PM-IRRAS spectra were measured using a Wilhelmy balance.
3. Results and Discussion The effect of temperature on the isotherm for the octadecyl-substituted polyimide molecule is shown in Figure 2. The zero pressure area per repeat unit decreases abruptly with temperature between 20 and 24 °C and remains nearly constant for still higher temperatures. This behavior was reproduced several times. In searching for the cause for this dramatic temperature effect, we considered how the isotherm might depend on the manner in which the surface pressure is measured. It has been reported13,14 that the Wilhelmy plate method is problematic if very rigid films are formed. In particular these reports showed that surface pressure measured by the Wilhelmy balance was smaller than that measured by the Langmuir balance. Furthermore, a deflection of the Wilhelmy plate can occur, though this can be avoided by orienting the plate perpendicular to the barrier. However, even in the geometry with the plate perpendicular to the barrier there may be problems due to the limited ability (13) Kumaki, J. Macromolecules 1988, 21, 749. (14) Tanizaki, T.; Hara, K.; Takahara, A.; Kajiyama, T. Polym. Bull. 1993, 30, 119.
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Figure 3. Schematic showing the difference in interaction of the monolayer with the pressure sensing device in the cases of a Wilhelmy plate and Langmuir balance.
Figure 4. Pressure-area isotherms of the polyimide measured with a Langmuir balance at different temperatures.
of the large rigid monolayer domains to follow the curvature of the menicus as shown in Figure 3. This difficulty with the menicus might cause inhomogeneities in the coverage of the water film at the plate. Since the Langmuir balance directly measures the surface pressure, one anticipates it may be more accurate for rigid monolayers. Isotherms measured at three different temperatures with the Langmuir balance are shown in Figure 4. The isotherms change modestly with temperature and show higher surface pressures than those obtained with the Wilhelmy plate. The differences in the isotherms measured with the two different methods were a first indication that the monolayers of polyimides were very rigid. Moreover, the strong temperature effect on the isotherms observed with the Wilhelmy plate can be attributed to a significant change in the rigidity of the monolayer in the temperature range investigated. The morphologies of monolayers at various temperatures and surface areas were observed with BAM, which is now a common technique for direct visualization of phase transitions and polymorphism in monolayers.10,15 The BAM images obtained at two different temperatures of 20 and 28 °C are shown in Figure 5 and Figure 6, respectively. The monolayer at 20 °C shows a distinct domain structure at high area per repeat unit. The domains are large (several hundred micrometers in diameter) and have sharp and clearly defined edges. The domains of the polyimide monolayer are observed right after spreading, which indicates that the polyimide molecules do not form a gas analogous phase, but directly a solid analogous phase. This behavior has also been observed for the hairy-rod polyglutamates.16 While some of the polyglutamates form soft domains that can coalesce upon compression, others form stiff domains that are pushed together like floes of ice as in the case of the polyimide molecules. Whether one (15) Ho¨nig, D.; Mo¨bius, D. Thin Solid Films 1992, 210/211, 64. (16) Menzel, H.; Rambke, B. Macromol. Chem. Phys. 1997, 198, 2073.
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Figure 5. Brewster angle microscopy images of a monolayer of the polyimide at 20 °C as a function of area per repeat unit (4 mm × 6 mm scale images): (a) 264; (b) 260; (c) 168; (d) 82 Å2.
sees coalescence or packing like that of ice floes depends on the stiffness of the molecules. Upon compression at 20 °C the domains are pushed together and form a coherent monolayer, but the domains still exist, as can be seen from the contrast variation at low surface area per repeat unit. Note that the BAM images are presented as a function of area per repeat unit rather than surface pressure because the isotherms observed with the two different balances show large differences in surface pressure. The morphology of the monolayer is markedly different at 28 °C. Large domains are formed by the polyimide, as was the case at lower temperature. However, at 28 °C the domains have edges that are less sharp and clearly defined and seem to be embedded in a matrix of more fluid material. The surface coverage appears to be higher. Upon compression, a certain fluidity is observed, for example when the coalescence of monolayer regions occurs. PM-IRRAS spectra obtained during compression show differences with temperature that likewise suggest a difference in rigidity with temperature. Changes suggestive of reorientation of some groups17,18 can be seen upon compression at 21 °C. No such reorientation is prompted by compression at 28 °C. PM-IRRAS measurements were made at four different temperatures (21, 22, 26, and 28 °C) and four pressures (3, 5, 7, and 9 mN/m) with the intent of probing directly the side-chain orientation. (17) Schmidt, A. Ph.D. Thesis, Johannes Gutenberg University, Mainz, 1992. (18) Clauss, J.; Schmidt-Rohr, K.; Adam, A.; Boeffel, C.; Spiess, H. W. Macromolecules 1992, 25, 5208.
Acquiring a bulk FT-IR spectrum of polyimide molecules, shown in Figure 7, allowed for identification of some bands. Strong CH2 symmetric and antisymmetric stretches are seen around 2850 and 2920 cm-1. Two CdO stretching peaks are also observed around 1728 and 1778 cm-1. It is known19 that the carbonyl groups in the imide backbone cause two symmetric and asymmetric CdO stretching peaks around 1720 and 1785 cm-1. The carbonyl group of the ester linkage in the side chain is expected to have a stretching peak at 1725 cm-1. Therefore, the CdO stretching peak appearing at 1728 cm-1 has contributions from two different carbonyl groups, one in the imide ring and the other in the ester linkage. There are two strong peaks in the bulk spectrum at 1442 and 1359 cm-1. These peaks seem to be associated with vibrations in the ester linkage. However, these vibrations are often highly coupled with C-C stretching peaks, and specific assignments are impossible. Portions of two PM-IRRAS spectra measured at 21 and 28 °C are shown in Figure 8. Neither spectrum shows bands for the CH2 symmetric and antisymmetric stretches around 2850 and 2920 cm-1 at any temperature or pressure. These bands should appear if the side chains are oriented. The bands would be positive if the side chains are oriented perpendicular to the surface and negative if the side chains are oriented flat on the surface. Two aspects of the monolayer structure dictate the intensities of these bands. One is the surface density of side chains and the second is the orientation of those chains.20 Since the density (19) Fayat, C.; Foucaud, A. Bull. Soc. Chim. Fr. 1970, 12, 4501. (20) Mao, L.; Ritcey, A. M.; Desbat, B. Langmuir 1996, 12, 4754.
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Figure 6. Brewster angle microscopy images of a monolayer of the polyimide at 28 °C as a function of area per repeat unit (4 mm × 6 mm scale images): (a) 262; (b) 236; (c) 126; (d) 88 Å2.
Figure 7. Bulk FT-IR spectrum of the polyimide in chloroform.
of the side chains is very low, it is possible that they could be oriented somewhat and that the degree of orientation changes with temperature but that this orientation is still not observed with PM-IRRAS. However, since the CH2 stretches are the strongest absorptions in the bulk spectrum, the possibility that the side chains are welloriented perpendicular to the water surface can be clearly eliminated. PM-IRRAS spectra of the monolayer at 21 and 28 °C are presented for four values of surface pressure in Figure 9. The observed intensities depend both on the number of molecules per unit area of surface and on the orientation of the transition moments responsible for the absorption. Therefore intensities normalized for the number of
Figure 8. Normalized PM-IRRAS spectra of the polyimide at a surface pressure of 9 mN/m: 21 °C (s), 28 °C (- - -).
molecules per unit area of surface are plotted and changes seen in intensity are due only to changes in orientation of the groups responsible for the various bands. (The broad dip at 1650 cm-1 is due to liquid water that is probably ordered at the interface.21) The signal-to-noise ratio is poor, but an important difference between the behavior at the two temperatures can be seen nonetheless. At 28 °C the spectra do not change appreciably with surface pressure. In contrast, the spectra obtained at 21 °C do change somewhat with surface pressure. Specifically, bands at 1728 and 1359 cm-1 increase with compression. (21) Blaudez, D.; Buffeteau, T.; Cornut, J. C.; Desbat, B.; Escafre, N.; Pezolet M.; Turlet, J. M. Thin Solid Films 1994, 242, 146.
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4. Conclusions For the octadecyl-substituted polyimide molecule a sharp reduction in zero pressure area occurs with increasing temperature when isotherms are recorded using a Wilhelmy plate as the pressure sensor. This suggests a change in the structure of the polyimide monolayers with temperature. However, if the surface pressure is sensed employing a Langmuir balance, the isotherms are different and in particular the temperature dependence is far less dramatic. BAM images of the layer morphology are consistent with the notion that the monolayer is extremely rigid at 20-21 °C, while it has slightly more fluidity at 28 °C. PM-IRRAS results also indicate a qualitative difference between the characters of the monolayer at the two temperatures. The Wilhelmy plate method is sensitive to this change, but its limited accuracy in the case of very rigid films results in a response that greatly amplifies the character of this change as compared to other means of probing the monolayer structure.
Figure 9. Normalized PM-IRRAS spectra divided by the number of repeat units per Å2, at two temperatures as a function of surface pressure: (a) 3; (b) 5; (c) 7; (d) 9 mN/m.
We conjecture that this may be related to a change in orientation of the carbonyls in the imide ring or in the ester linkages of the side chains.
Acknowledgment. The polyimide molecules were a gift from Professor F. W. Harris. We thank He´le`ne Bourque for assistance in using the PM-IRRAS instrument. Research support from the Army Research Office (Contract DAAH04-96-1-0018/Subcontract 95-0950-01) is gratefully acknowledged. LA981167X
