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Ultrathin Polyimide Films from Preformed Polymers Hyun Yim, Hong Wu,† Mark D. Foster,* Stephen Z. D. Cheng, and Frank W. Harris Maurice Morton Institute of Polymer Science, The University of Akron, Akron, Ohio 44325-3909 Received August 26, 1996. In Final Form: March 28, 1997X Multilayer films of preformed polyimide molecules have been obtained for the first time by the LangmuirBlodgett technique. The pressure-area isotherm shows that the polyimide monolayers are strongly dependent on compression rate. The zero-pressure area per repeat unit is about 1.7 nm2 when the film is compressed at 100 cm2/min. This value is higher than that expected (1.2 nm2) for a closely packed structure. Multilayers have been investigated with X-ray reflectometry. The thickness of a bilayer film is about 4.05 nm, which is between the average bilayer thickness of a interdigitated multilayer film and that of a bilayer film containing closely packed backbones and fully extended alkyl side chains. The multilayer films do not display a distinctively periodic structure. When the films are annealed for a few hours at 180 °C, the structure relaxes somewhat and thickness decreases slightly.
Introduction Rigid rodlike polymers with flexible alkyl chains designed for the construction of stable and homogeneous ultrathin films by the Langmuir-Blodgett (LB) technique have recently been studied.1-6 These “hairy-rod” molecules present unique physical properties for use in photonics, optoelectronics, and chemical sensors and show excellent transfer characteristics due to the fact that while the rigid backbone of the molecules provide stiffness, the alkyl side chains render the molecules stable in organic solvent. Polyimides are the focus of this work due to their tremendous thermal stability and interesting physical properties. One potential application of interest is a liquid crystal display in which LB multilayers can be used as alignment layers. Several attempts to prepare polyimide LB films have been reported, in which polyimide precursor monolayers were transferred and imidization was driven by thermal treatment in the thin film form.7-9 This approach requires a complex process and is accompanied by shrinkage, since van der Waals interactions are replaced by chemical bonds during the imide-ring closure in the thermal treatment. A byproduct, usually water, is also released during the imidization. It is much preferable to prepare LB films from preformed polyimide molecules that contain side chains, which also enables them to overcome the general problem of conventional amphiphilic polymers, i.e., the formation of loops and tails in the monolayers.10,11 * To whom correspondence should be addressed. † Present address: Tesa Tape Inc., 5901 Carnegie Boulevard, Charlotte, NC 28209. X Abstract published in Advance ACS Abstracts, May 15, 1997. (1) Orthmann, E.; Wegner, G. Angew. Chem. 1986, 98, 1114. (2) Sauer, T.; Arndt, T.; Batchelder, D. N.; Kalachev, A.; Wegner G. Thin Solid Films 1990, 187, 357. (3) Duda, G.; Schouten, A. J.; Arndt, T.; Lieser, G.; Schmidt, G. F.; Bubeck, C.; Wegner, G. Thin Solid Films 1988, 159, 221. (4) Duda, G.; Wegner, G. Makromol. Chem., Rapid Commun. 1988, 9, 495. (5) Mathauer, K.; Schmidt, A.; Knoll, W.; Wegner, G. Macromolecules 1995, 28, 1214. (6) Embs, F.; Wegner, G.; Neher, D.; Albouy, P.-A.; Miller, R. D.; Wilson, C. G.; Schrepp, W. Macromolecules 1991, 24, 5068. (7) Uekita, M.; Awaji, H.; Murata, M. Thin Solid Films 1988, 160, 21. (8) Nishikata, Y.; Kakimoto, M.; Morikawa, A.; Imai, Y. Thin Solid Films 1988, 160, 15. (9) Tsukruk, V.; Mischenko, N.; Scheludko, E.; Krainov, I.; Tolmachev, A. Thin Solid Films 1992, 210/211, 620. (10) Fang, J. Y.; Lu, Z. H.; Min, G. W.; Wei, Y. Liq. Cryst. 1993, 14, 1621.
S0743-7463(96)00839-6 CCC: $14.00
Figure 1. Schematic illustration of the preparation of the polyimide molecules.
Transfer of a single polyimide monolayer onto highly oriented pyrolytic graphite has been reported by Hayashi et al.12 It has been shown that LB multilayers which have a periodic double-layer structure may relax on annealing. Therefore, it is necessary to understand their arrangement in LB multilayers, both as deposited and after annealing because in many cases of technical interest a fabricated structure must be thermally and geometrically stable to substantial temperature fluctuations. In the present study the preparation of LB films from preformed polyimide molecules is reported for the first time, and the film structure and its change upon annealing are investigated using X-ray reflectometry. Experimental Section Materials. A polyimide bearing long, flexible side chains (III), shown in Figure 1, was prepared by polymerizing 2,2′-bis(3,4(11) Wegner, G. Ber. Bunsen-Ges. Phys. Chem. 1991, 95, 1326. (12) Hayashi, T.; Yamamura, H.; Nishi, T.; Kakimoto, M. Polymer 1992, 33, 3751.
© 1997 American Chemical Society
LB Films of Polyimides
Figure 2. Pressure-area isotherms of the polyimide molecules with different compression rates: 40 (s), 100 (s -), 150 (- - -) cm2/min.
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Figure 3. XR data for a bilayer film with a model curve corresponding to a bilayer thickness of 4.05 nm.
Results and Discussion
suggests that the polyimide monolayer on the water subphase is not perfectly oriented and/or closely packed. It is possible that the polyimide molecules do not exist in a condensed phase, but rather in an expanded phase, where the long alkyl side chains are not fully extended perpendicular to the water surface. This may be attributed to the lack of close packing of the side chains because the polyimide molecules have a much lower side chain density than do other “hairy-rod” polymers, such as polyglutamate copolymers,13-15 from which LB films have been made. The effect of compression rate on the isotherm has been studied in order to deduce the nature of the monolayer. Figure 2 shows that the isotherm is strongly dependent on the compression rate. At the smallest compression rate the isotherm is seen to be unstable even though the zero-pressure area is lowest. However, the zero-pressure area is again decreased when the compression rate is increased above 100 cm2/min, which may be explained by the fact that the monolayer has a liquid crystalline nature and hence its orientation is subject to the shear rate, i.e., compression rate. Figure 3 represents a semilogarithmic plot of X-ray reflectivity as a function of q ()4π(sin θ)/λ) for a bilayer film. As can be seen, the reflectivity profiles contain a series of weak maxima and minima. These fringes, sometimes referred to as “Kiessig fringes”,16 are indicative of the film’s overall thickness, and their frequency increases with increasing film thickness. The reflectivity data are interpreted by means of regression using model composition profiles. A fit of the X-ray reflectivity data results in a thickness of 4.05 nm for the bilayer film, which means that the bilayer adopts a thickness lower than that of a bilayer containing closely packed backbones and fully extended alkyl side chains (ca. 5.6 nm as estimated from molecular modeling). That is, the fairly low degree of substitution does not demand that the bilayer film have fully extended alkyl side chain packing. It is more likely that the side chains are in a fluid phase and no particular conformation may be assumed, as studies of similar models have revealed.13-15,17,18 Figure 4 represents the X-ray reflection curve of a fourlayer sample. Also shown is a simulated reflectivity curve
Figure 2 shows the pressure-area isotherms for the polyimide molecules. The shape of the isotherms matches well with those obtained by others for polyimide precursors. No characteristic phase transitions are observed before the inflection point around 13 mN/m in the isotherm with the compression rate of 100 cm2/min, which is ascribed to a collapse of the monolayer. The zero-pressure area per repeat unit obtained by extrapolating the slope of the isotherm to zero pressure is about 1.7 nm2. This value is larger than that expected (about 1.2 nm2), which
(13) Tsukruk, V. V.; Foster, M. D.; Reneker, D. H.; Schmidt, A.; Knoll, W. Langmuir 1993, 9, 3538. (14) Tsukruk, V. V.; Foster, M. D.; Reneker, D. H.; Schmidt, A.; Wu, H.; Knoll, W. Macromolecules 1994, 27, 1274. (15) Vierheller, T. R.; Foster, M. D.; Schmidt, A.; Mathauer, K.; Knoll, W.; Wegner, G.; Satija, S.; Majikrzak, C. F. Macromolecules 1994, 27, 6893. (16) Kiessig, H. Ann. Phys. 1931, 10, 715. (17) Wegner, G. Mol. Cryst. Liq. Cryst. 1993, 235, 1. (18) Schaub, M.; Fakirov. C.; Schmidt, A.; Lieser, G.; Wenz, G.; Wegner, G.; Albouy, P.-A.; Wu, H.; Foster, M. D.; Majikrzak, C. F.; Satija, S. Macromolecules 1995, 28, 1221.
dicarboxylphenyl)-1,1,1,3,3,3-hexafluoropropane dianhydride (I), and 4,4′-diamino-2,2′-bis(octadecancarboxy)biphenyl (II) in mcresol (10% wt/wt solid content) containing a catalytic amount of isoquinoline. Compound I was purchased from Hoechst Celanese Corp. and compound II was synthesized. The solution was stirred at room temperature for 1-2 h. Then it was heated to 180-200 °C and maintained at that temperature for several hours. After the solution was allowed to cool to ambient temperature, it was diluted with some m-cresol, and added into a large amount of vigorously stirred methanol or ethanol. The polymer that precipitated was collected by filtration, washed with more ethanol, and dried under reduced pressure at 150 °C for 24 h. The intrinsic viscosity was 2.12 dL/g when measured in NMP solution at 25 °C. Sample Preparation. Multilayers were deposited on 2 in. round silicon substrates (Semiconductor Processing Inc.). The substrates were cleaned by being treated, first in chloroform for 10 min and then in a hot 1:1:5 solution of NH4OH-H2O2-H2O for 30 min. After rinsing in Millipore water, the surfaces were made hydrophobic by dipping in an aqueous solution of HF for 1 min. Monolayers were prepared by spreading 0.1 mg/mL solutions in chloroform onto a Millipore-quality water subphase in a Nima 611 trough (18 × 29 cm2) and compressed with a barrier speed of 100 cm2/min before transfer. A Y-type deposition was performed at a surface pressure of 7 mN/m with a dipping speed of 10 mm/min. Since one dipping cycle (up and down) usually consumed the compressed monolayer on the subphase, an additional monolayer had to be spread and compressed prior to each dipping cycle. Transfer ratios ((0.2) for all the layers were around 0.8 but decreased slightly with increasing number of layers. Annealing of the films was done in a vacuum oven at 180 °C. The films were cooled to 4 °C except during measurements to avoid spurious annealing effects in the sample structure. Isotherms were also measured for other compression rates to consider the influence of compression rate on monolayer structure. X-ray Reflectometry. X-ray reflectometry measurements were performed on the reflectometer at the University of Akron using Cu KR radiation (λ ) 0.154 nm) selected with a pyrolytic graphite monochromator. The resolution of the wavelength (δλ/ λ) was 0.015 and the angular divergence ca. 0.0002 radian. All measurements, typically lasting several hours, were performed in air at room temperature and all data sets were corrected for background.
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Figure 6. Schematic drawings of bilayer and multilayer films illustrating the interdigitation of side chains: (a) bilayer, (b) multilayer.
Figure 4. XR data for a four-layer film with an unrealistic model curve based on the model b/V profile in the inset.
Figure 7. XR data for (a) six-layer and (b) eight-layer films with model curves.
Figure 5. XR data for a four-layer film with a model curve corresponding to the b/V profile in the inset.
based on a model scattering length density, b/V, profile which envisions distinct backbone rich and side chain rich regions with the theoretical b/V values of 0.114 × 10-2/ nm2 and 0.092 × 10-2/nm2, respectively. The model curve does not match the experimental data. In particular, the distorted shape of the second and third peak in the model curve, which is due to a periodic structure, is not seen in the experimental curve. In order to match the experimental data, one must use another density profile, as shown in the inset of Figure 5, which suggests a less distinctive structure. The average b/V value of each layer is lowered to 0.0780 × 10-2/nm2, which is about 76% of the theoretical b/V value averaged over backbones and side chains. This value is reasonable considering that the transfer ratios are around 0.8. From the fit one obtains two key features of the sample structure. First, we obtain for the four-layer sample a bilayer thickness of 3.25 nm, which is much lower than the thickness of a single bilayer sample. The difference between the bilayer thickness measured in a single bilayer film and the average bilayer thickness measured in multilayer films has also been observed for polyglutamate LB films. For that case the difference is explained by interdigitation of the side chains from adjacent bilayers.14,18 When a bilayer stands alone on a substrate, as sketched in Figure 6a, the opportunity for interdigitation is lost, while the multilayer film, as shown in Figure 6b, has the opportunity for alkyl chains in adjoining bilayers to interdigitate. The second feature observed is the indication of two frequencies in the oscillations in the curve. While the character of the curve near q ) 0.024 nm-1 is similar to that seen for the model curve from periodic structure, it is not necessary to postulate a periodic structure in order to attain a reasonable fit to the data. Introduction of a region of nearly constant b/V at the center
of the film, with the thickness of this region differing markedly from the film thickness, is sufficient. Atomic force microscopy measurements are underway to characterize the lateral uniformity of the film. X-ray data for six-layer and eight-layer films are shown in Figure 7. Note that the model curves shown here for comparison do not match the data very well. However, they reproduce some essential features seen in the data, particularly those dictated by the film thickness. Precisely matching the data proved to be extremely difficult. The fringes in the data are somewhat less distinct than those seen for the four-layer data, indicating a lesser thickness uniformity of these films. The bilayer spacings obtained from these data are 2.9 ( 0.2 nm and 2.8 ( 0.2 nm for six layers and eight layers, respectively. The overall film thickness increases by 2.4 nm when going from six to eight layers, which is the same as the difference of thickness between the bilayer and four-layer sample. Thus, there is consistency in the thickness of additional bilayers after the first. However, Bragg reflections which are expected of a periodic multilayer architecture are not observed. Six and eight layers are enough layer numbers to have Bragg reflections as seen in papers published by Tsukruk et al.,9,19 who showed that LB films with seven and nine layers had distinct Bragg reflections. The reason Bragg reflections are not observed for these films is that they do not have a periodic modulation of the electron density, but rather a flat density distribution as seen in the insets. This is reflected by the beating of two frequencies in the structure of the reflectivity seen in Figure 7. The Kiessig peaks may be seen at small q, almost disappear for intermediate q values, and then appear again at large q values. This flat density distribution, i.e., the absence of periodic modulation, in the multilayer may reflect a relaxation of the layer structure. It could be that the intralayer asymmetry imposed by sample deposition is easily relaxed through the delocalization of the backbone positions due (19) Tsukruk, V. V.; Bliznyuk, V. N.; Reneker, D. H. Thin Solid Films 1994, 244, 745.
LB Films of Polyimides
Figure 8. Comparison of the XR data for a six-layer film before and after annealing at 180 °C: (a) film as-deposited; (b) film after 2 h annealing; (c) film after 3 h annealing.
to the low degree of side chain substitution and the existence of some defects in the multilayer. Also, it should be remembered that the zero-pressure area per repeat unit is higher than that expected if the polyimide molecules are closely packed. The reflectivity for the eight-layer film drops off more rapidly than does that for the six-layer film, suggesting that the eight-layer film is rougher. The surface roughness of each film was obtained by X-ray analysis. The values for the six- and eight-layer films were 1.2 and 1.4 nm, respectively, and 0.9 nm for the four-layer film. Thus the polyimide films became rougher as the number of layers increased. This result is similar to that reported by Tsukruk et al.,19 in which LB films were prepared from complexes of poly(naphthoylenebenzimidazole) precursor (PNIB) and stearic acid. They showed that LB films consisting of one to five molecular layers possess flat surfaces, with a roughness of 0.5 ( 0.1 nm for a films containing five layers, but for LB films with six to nine layers the roughness rose to 1.3 ( 0.2 nm. Further study will show if refinement of the deposition procedure can provide a reduction in this roughness. The stability of the as-deposited structure to annealing was studied also. Initially, six- and eight-layer films were annealed under vacuum at 180 °C which is above Tg (about 160 °C), and then studied again with X-rays. The X-ray data for the six- and eight-layer films are shown in Figures 8 and 9, respectively. Two changes with annealing are observed in the data for both films. First, the oscillations, especially at large q values, diminish as annealing time increases, which indicates that the superposition phenomenon disappears because the flat density distribution in the middle of the density profile is extended to nearly
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Figure 9. Comparison of the XR data for an eight layer film before and after annealing at 180 °C: (a) film as-deposited; (b) film after 2 h annealing; (c) film after 4 h annealing.
the whole film thickness by relaxation. The second change seen in the structure of both films is a decrease in the overall film thickness. The positions of the first and second Kiessig minima shift to higher q values with annealing above Tg. From these annealing results, it may be concluded that annealing at this temperature causes significant changes in the detailed film structure. Conclusions Layers of preformed polyimide molecules, which may be thought of as a type of hairy-rod, have been transferred onto substrates by the LB technique. The zero-pressure area per repeat unit obtained from pressure-area isotherm is larger than that expected for closely packed polyimide molecules. The pressure-area isotherm is strongly dependent on compression rate. The film structure and its change upon annealing were investigated by X-ray reflectometry. A single polyimide bilayer has a thickness greater than that seen in multilayer LB films and adopts a thickness between that of an interdigitated multilayer structure and that of a bilayer containing closely packed backbones and fully extended alkyl side chains. Even the polyimide ultrathin film with eight layers shows no Bragg reflection characteristic of a periodic structure. When the multilayers are annealed at 180 °C, the structure appears to relax somewhat, but the first Kiessig fringe remains after annealing for several hours. Acknowledgment. This research was supported in part by the U.S. Army Research Office (Contract No. DAAH04-96-1-0018). LA960839L
