Langmuir 1996,11, 3013-3017
3013
Self-Assembled Layers of an Aromatic Poly(ketone) and Poly(benzi1)on Gold and Copper Ulrich B. Steiner, Matthias Rehahn, Walter R. Caseri,* and Ulrich W. Suter Eidgenossische Technische Hochschule (ETH), Institut fur Polymere, CH-8092 Zurich, Switzerland, and Eidgenossische Materialprufings- und Forschungsanstalt (EMPA), Abt. 136, CH-8600 DubendorL Switzerland Received October 31, 1994. In Final Form: May 2, 1995@ Self-assembledlayers of a polflketone) and a poly(benzi1)withp-terphenylene units have been prepared fromorganic solutionson gold and copper. The layers formed were studied with contact angle measurements, ellipsometry, infrared spectroscopy at grazing incidence, X-ray photoelectron spectroscopy, and surface profilometry. Most layer thicknesses are in a range expected for monolayers,but, depending on the system, “thick”layers are also observed. The IR spectra strongly change upon adsorption, indicating a chemical interaction between polymer and substrate. The results are compared to those offilms ofrigid-rod polymers with similar structural elements in the main chain.
Introduction A number of studies deal with self-assembled layers of organic compounds on gold and, occasionally, on copper. Mostly, low molecular weight compounds with sulfur functionalities have been investigated,’-17 but other molecules have also been focused on, e.g., selenols,la nitriles,lg nitrogen, phosphorus, arsene, antimony, and bismuth compOunds,2°olefines and acetylenes,21arenes,22 and b1ides.2~Although many of the self-assembled layers have been characterized extensively, interactions between the film’s molecules and the substrate have been detected spectroscopically only in a few case^.^^,^^ Abstract published in Advance A C S Abstracts, June 15,1995. (1) Nuzzo, R. G.; Allara, D. L. J.Am. Chem. SOC.1983, 105, 4481. (2) Mielczarski, J.; Leppinen, J. Surf. Sci. 1987, 187, 526. (3) Bain, C. D.; Evall, J.; Whitesides, G. M. J . Am. Chem. SOC.1989, 111,7155. (4) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J.Am. Chem. Soc. 1989,111, 321. (5) Mielczarski, J.; Souninen, E.; Johansson, L.-S.; Laajalehto, K. Int. J . Miner. Process. 1989,26, 181. (6) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87. (7) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J.Am. Chem. SOC. 1990,112, 570. (8) Widrig, C. A.; Alves, C. A,; Porter, M. D. J.Am. Chem. SOC.1991, 113,2805. (9) 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. (10) Kwan, W. S. V.; Atanasoka, L.; Miller, L. L. Lungmuir 1991, 7, 1419. (11)Bryant, M. A,; Pemberton, J . E. J.Am. Chem. Soc. 1991,113, 8284. (12) Laibinis, P. E.; Whitesides, G. M. J . Am. Chem. SOC.1992,114, 1990. (13)Tao,Y.-T.; Lee, M.-T.; Chang, S.-C. J. Am. Chem. SOC.1993, 9547. 115. ~ ~ , (14) Sabatini, E.; Cohen-Boulakia, J.; Bruening, M.; Rubinstein, I. Lungmuir 1993,9, 2974. (15) Graham, R. L.; Bain, C. D.; Biebuyck, H. A.; Laibinis, P. E.; Whitesides, G. M. J . Phys. Chem. 1993, 97, 9456. (16) Leeeett. G. J.: Davies. M. C.:’ Jackson. D. E.: Tendler. S. J. B. J . Phys. e&m.’ 1993; 97, 5348. (17) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M.; Deutch, J . J. Phys. Chem. 1994,98, 563. (18) Samant, M. G.; Brown, C. A,; Gordon, J. G., 11. Lungmuir 1992, 8, 1615. (19) Steiner, U. B.; Caseri, W. R.; Suter, U. W. Lungmuir 1992, 8, @
~
~
2771 -.
(20) Steiner, U. B.; Neuenschwander, P.; Caseri, W. R.; Suter, U. W.; Stucki, F. Lungmuir 1992, 8, 90. (21) Steiner, U. B.; Caseri, W. R.; Suter, U. W. Langmuir 1993,9,
-. .. R77
(22) Steiner, U. B.; Caseri, W. R.; Suter, U. W.; Rehahn, M.; Rau, I.
U.Langmuir 1994, 10, 1164.
(23) Steiner, U. B.; Caseri, W. R.; Suter, U. W.; Rehahn, M.; Schmitz, L. Lungmuir 1993, 9, 3245.
On gold, monolayers have been observed exclusively; a thicker layer formed after exposure of a gold slide to BiPhs was attributed to polymeric or oligomeric oxidation products.20 On copper, however, not on1 monolayers but also layers with a thickness of 50- 1000 frequently form (“thick”l a y e r ~ ) . ~ , These ~ J ~ -thick ~ ~ layers probably consist of organometallic c0mp1exes.l~~~~ There are only a few reports on self-assembled polymers on gold. Most studies deal with polymers with sulfur functionalities on gold, where the sulfur functionalities can support the adsorption of p ~ l y ( s t y r e n e ) , poly~~-~~ (methyl metha~rylate),~’ poly(acrylates),28or poly(y-benz y l - ~ - g l u t a m a t e ) .Amphiphilic ~~,~~ copolymers containing disulfide moieties in side chains can be useful for influencing the wetting properties of gold surfaces.31The thicknesses of the above mentioned polymer films are typically in arange of 10-30 A, i.e., in the region expected for monolayers. On copper, only a few systems of self-assembled polymer layers have been investigated. Films of several rigid-rod polymers with flexible side chains have been described on copper and gold,22323and in one case the surface layers could be studied with atomic force microscopy (Al?M).32 These rigid-rod pol ers usually form layers with a thickness of 10-20 on copper and gold, but thick layers (thickness above 50A)have also been observed on copper.22 Spectroscopic evidence for polymer-substrate interactions is rare. In some self-assembled layers of substituted poly@-phenylenes)on gold and copper, polymer molecules seem to interact with the surface via r2- or q4-arene coordination.22 In contrast, rigid-rod aromatic poly(imides) are likely to interact with gold and copper mainly by electrostatic rather than coordinative interaction^.^^ The present study deals with self-assembled layers of a poly(ketone1 (PK) and a poly(benzi1) (PB) with p -
K
r
(24) Stouffer, J. M.; McCarthy, T. J. Polym. Prepr. 1986,27 (2), 242. (25) Stouffer, J. M.; McCarthy, T. J. Macromolecules 1988,21,1204. (26) Waldman, D. A.; Kolb, B. U.; McCarthy, T. J.;Hsu, S. L. Polym. Mater. Sci. Eng. 1988, 59, 326. (27) Lenk, T. J.; Hallmark, V. M.; Rabolt, J. F.; Haussling, L.; Ringsdorf, H. Macromolecules 1993,26, 1230. (28) Sun, F.; Castner, D. G.; Grainger, D. W. Lungmuir 1993, 9, 3200. (29) Enriquez, E. P.; Samulski, E. T. Polym. Prepr. 1993,34 (l), 794. (30) Enriquez, E. P.; Gray, K. H.; Guarisco, V. F.; Linton, R. W.; Mar, K. D.; Samulski, E. T. J. Vac. Sci. Technol. A 1992, 10, 2775. (31) Erdelen, C.; Haussling, L.; Naumann, R.; Ringsdorf, H.; Wolf, H.; Yang, J.; Liliey, M.; Spinke, J.;Knoll, W. Langmuir 1994,10,1246. (32) Steiner, U. B.; Rehahn, M.; Caseri, W. R.; Suter, U. W. Macromolecules 1994, 27, 1983.
0743-746319512411-3013$09.00/00 1995 American Chemical Society
3014 Langmuir, Vol. 11, No. 8, 1995
Steiner et al.
,c6H 13
I
/ C6H13
PK FsH13
/ C6H13
PB
Figure 1. Structure and acronyms of the poly(ketone) and the poly(benzi1) used i n this study. ,C12H25
of solvent, and dried in a n argon stream. For layer thickness measurements, the slides were immersed into the adsorption solution only to the middle and then removed from solution, rinsed with ca. 3 mL of solvent, and dried in a n argon stream. IR Spectroscopy. IR reflection spectra were recorded at grazing incidence (83") on a Nicolet 5SXC FTIR spectrometer equipped with a mercury-cadmium-telluride detector. The measurements were carried out in alternating cycles using a step-motor-driven translation stage as described p r e ~ i o u s l y . ~ ~ , ~ ~ Ellipsometry. Ellipsometric measurements were performed using a PLASMOS SD 2300 ellipsometer at 632.8 nm with a n incidence angle of 70". The detailed procedure has been described p r e v i o u ~ l y . ~ ~ ~ ~ ~ Contact Angles. Advancing contact angles were measured on a Ram6-Hart 100-00 goniometer as described previously.22~23 X-ray Photoelectron Spectroscopy. XPS were obtained on a KRATOS ES 300 spectrometer using Mg K a radiation. The calibration procedure and measuring conditions have been described p r e v i o u ~ l y . ~ ~ , ~ 3 Surface Profilometry. Surface profiles and surface roughnesses were determined on an Alphastep 200 surface profilometer (Tencor Instruments) with a vertical resolution of 5 A and a horizontal resolution of 400 A. A stylus of 1.5-2.5-pm radius was used, and a stylus force of 10 mg and a scan time of 40 s were selected.
Results PB and PK (number average molecular weight 11 500 and 8000, respectively) adsorb from CH2C12 on gold and copper within 6 h. Longer adsorption times do not yield significant changes in the measured characteristics ofthe layers (see below) with the exception of PB on cop er. Here, a complete dissolution of the copper film (2000 i f on chromium coated silicon) takes place within a few days. / / / The thickness of the adsorbed layers, determined by CH20CdHg R CBH13 ellipsometry, is 10 (h2)A for PK on both metals and for R=CsH13 : Pc PCO PB on gold. This thickness is consistent with the R=CH20C&lg : PO assumption of monolayers. PB on copper however, yields thicker layers in the range of 200-700 PB on copper +$++ N*o N + also differs in surface roughness from the other layers (measured with a surface profilometer over horizontal distances of 40 pm and 1 mm). The surface roughness of a PB la er on copper is 20-40 A and of the other layers n 10-15 , i.e., in the range of the "pure" substrates. 0 R 0 C12H25 C12H25 C1ZH25 The advancing contact angles of water on films of PB and PK on gold are in the range of the pure substrate R=H : H-PI after washing with ethanol, i.e., 63 (~t.2)"(the error limit R=Ph : Ph-PI refers to the maximum deviations). The value for PK on Figure 2. Structure a n d acronyms of some polymers used for is 82 (13)"and that for PB on copper 96"-125". ~~ t h e formation of self-assembled layers in previous s t ~ d i e s . ~ ~ , copper Contact angles in the range of 120"- 130"have also been observed &er adsorption of short-chain alkanethiolsg and terphenylene units in the main chain (Figure 1). The ~ a l e r o n i t r i l eon l ~ copper. These extremely high values results are compared with polymers containing similar are probably due to an increase in surface r 0 ~ g h n e s s . l ~ structural elements, i.e., oligo- or poly(p-phenylenes), in All contact angles on the layers on copper are above the the main chain, as displayed in Figure 2. value of pure substrates (30"-39"). Experimental Section Reflection infrared spectra were recorded for the films and transmission spectra (in KBr) for the bulk compounds Materials. CHzClz was purchased from Fluka (Buchs, for comparison (Figures 3 and 4). Positions of selected IR Switzerland). It was used without further purification. The signals are given in Table 1. The lines in the adsorbed preparation of the polymers is described in ref 33. The number state are broad (typical full width a t half-maximum average molecular weight is 11500 for PB and 8000 for PK, determined with vapor pressure osmometry. ca. 60 cm-') which may readily occur for adsorbed Surface Preparation. The gold and co per surfaces were molecules,34-36 prepared by thermal evaporation of 2000-f gold or copper on The C=O stretching vibration of PK disappears after chromium coated silicon(ll1) wafers. The detailed procedure adsorption on copper. This could be caused by a n has been described p r e v i o u ~ l y .The ~ ~ ~gold ~ ~ surfaces consist of orientation of the transition dipole moment of the C-0 polycrystalline A u ( l l 1 ) faces, as revealed by AFM.
A.
w
Adsorption Experiments. Freshly prepared gold or copper slides were immersed i n 0.1 mM solutions of the polymers (the concentration refers to constitutional repeat units). After 6 h, the slides were removed from the solution, rinsed with ca. 6 mL (33) Rehahn, M.; Schliiter,A. D.; Wegner, G.Makromol. Chem.,Rapid Commun. 1990,11,535.
(34)Hayden, B. E. In Vibrational Spectroscopy of Molecules on Surfaces;Yates, J. T., Jr., Madey, T. E., Eds.; Plenum Press: New York, 1987. (35) Bradshaw, A. M.; Schweizer, E. In Spectroscopy of Surfaces; Clark, R. J. H., Hester, R. E., Eds.; Advances in Spectroscopy,Vol. 16; John Wiley & Sons: Chichester, U.K., 1988. (36) Hoffmann, F. M. Surf. Sei. Rep. 1983,3,109.
Aromatic Poly(ketone) and - ( b e n d ) on Gold and Copper
Langmuir, Vol. 11, No. 8, 1995 3015 Table 1. Selected IR Frequencies (cm-l) of PB and PK before (in KBr) and after Adsorption on Gold and Copper bulk
on gold
PK
3124 3091 3053 2955 2926 2857
2924 2862 1677
on copper 3123 3086 3030 2963 2925 2860
1659 1604 1602 1567 3052 3037"
PB
1591
ca. 1 7 1 P 1679
1677
1671
ca. 1 6 5 P 1602 1618 a
--
3200 2800 2400 2000 1600 1200 850 Wavenumber [em-']
Figure 3. IR spectra of PK. a, bulk P K in KBr; b, P K on gold; c, P K o n copper. The markers indicate the bands listed i n Table 1.
a.
"I A
r
.
I
I
I
,
8
8
1
.
1
3200 2800 2400 2000 1600 1200 850 Wavenumber [cm.'] Figure 4. IR spectra of PB: a, bulk P B in KBr; b, P B on gold; c, P B on copper. The markers indicate t h e bands listed in Table
ca. 1 6 2 P
assimment C-H C-H C-H C-H C-H C-H C=O C=O C=C C-0 C=O C-H C-H C=O C=O C=O C=O C-C C=O
str, arene str, arene str, arene str, methyl str, asym., methylene str, sym., methylene str or C=C str str str, arene stla or C=C str: arene stla or C=C str: arene str, arene str, arene str, or C=C str str or C=C str str strC or C=C str str, arene st@or C-C str: arene
*
Shoulder. Coordinated. Free or coordinated.
numbers by 8-18 cm-l after adsorption. However, v2- or q4-coordinated arene moieties might also absorb in this region.22 A shift of 12 cm-' to higher wavenumbers has been observed after adsorption of poly(acrylates) on gold.28 This might be due to physical effects occurring in the reflection e ~ p e r i m e n t . ~ ~ - ~ ~ The new or shifted IR signals of PK on gold at 1567 cm-', of PB on gold at 1618 cm-l, and the shoulders of PB on copper at ca. 1620, 1650, and 1710 cm-l could be due to v2- or v4-coordinated arene moieties.22 In the case of q2- or y4-coordination, additional signals in the range of coordinated C-C double bonds and C-C single bonds were also e ~ p e c t e d . ~Because ' of extremely broad lines below 1600 cm-l, it is not clear if signals could be attributed to such species, although additional, extremely broad lines are observed below 1500 cm-'. Compared to the aliphatic C-H stretching vibrations, the aromatic C-H stretching vibrations are very weak in the bulk compounds but strong on the substrates, at least for both polymers adsorbed on gold and for PB on copper; this phenomenon has already been described for poly(pphenylenes) substituted with alkyl or alkoxy side chains.22 We cannot explain this observation in a satisfactory way. XPS measurements ofthe bulk and adsorbed compounds reveal binding energies a t 287.3-287.8 eV in the C(1s) spectra, due to the carbonyl groups, and at 285.0-285.3 eV, due to aliphatic and aromatic carbon atoms. The integrated intensity ratio of the carbonyl groups and the other carbon atoms after adsorption on gold or copper corresponds precisely to the expected values from the chemical composition of the polymers, i.e., elimination of carbonyl groups upon adsorption does not occur or is not a main reaction. The O(ls) signals at 533-534 eV stem from the carbonyl oxygen atoms, the weak signals a t 532.0-532.1 eV after adsorption on copper probably from copper hydroxide species,36and those a t ca. 531 eV from copper(1) oxide specie^.'^,^' The lack of satellites in the Cu(2p~pJsignals (932.8 eV) confirms that no significant copper(11) oxide species are present.9J9,21-23,38 Discussion
1.
On gold, similar layers seem to form with PB and PK. The thicknesses of the PB and PK layers are in a range
vibration parallel to the surface, or by a coordination of the carbonyl group to copper atoms. On gold, the C=O stretching vibrations seem to be shifted to higher wave-
(37) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications of Organotransition Metal Chemistry; University Science Books: Mill Valley, CA, 1987. (38)Turner, N. H. Anal. Chem. 1988,60,377R.
3016 Langmuir, Vol. 11, No. 8, 1995 expected for monolayers consisting of molecules assembled in a flat array on the surface. It has to be considered, however, that the values obtained with ellipsometry are averages. If a part of the substrate was not covered by organic molecules and if the adsorbed molecules formed loops or tails, a n average layer thickness in the observed range could also result. The contact angles of water on these layers cannot contribute to the solution of this problem since they are in the range ofthe pure substrates. This could imply that large gold areas are not covered by organic molecules or that carbonyl groups of adsorbed molecules are exposed to the surroundings. The two compounds PB and PK behave completely differently on copper. The layers are distinguished in thickness, surface roughness, and wetting properties. The distinction in wetting properties might be due to the distinction in surface roughness. The thickness of PK on copper is in the range of that expected for a monolayer with molecules lain flat on the surface. The IR spectra indicate that the carbonyl groups in PK are predominantely oriented parallel to the surface or coordinated to copper atoms. In the case of a q2coordination, marked shifts of the O(ls) and C(ls) signals to lower binding energies are expected. Such shifts have not been observed. Considering the broad signals, the IR frequency of +coordinated carbonyl groups could coincidence with aromatic C=C stretching vibrations, and a r,J-coordination might not easily be detected with XPS. Hence, the carbonyl groups are most likely either oriented parallel to the surface or +coordinated, Both cases imply a high order with respect to a single adsorbed chain; i.e., only a few chain segments are present in loops or tails, since the carbonyl groups were more or less randomly oriented in these cases. The high advancingcontact angle of water (82") on such layers indicates that the metal surface is covered to a high degree with organic molecules. On a noncrystalline hydrocarbon layer, a contact angle of 90"-95" is expected.Ig The 82" on the PKlayers indicates, therefore, that either carbonyl groups or a relatively small part of the copper surface are in contact with the surroundings. PB forms thick layers on copper. Such layers probably consist rather of organometallic complexesthan of ordered multilayers of organic molecules.21 The high scattering in advancing contact angles of PB indicates that the PB layers are not uniform. Since thick layers are not observed with PK under the experimental conditions used here, it seems that the carbonyl groups in PB promote the formation of thick layers. The carbonyl groups in PB can basically form chelating complexes,39and such (intermediate) complexes might be responsible for the thicklayers. On the other hand arene moieties could coordinate, or electrons could be transferred from the substrate to the polymer or reverse. The strong signal a t 1677 cm-I in the IR spectrum indicates that a significant portion of the carbonyl groups is not coordinated, and from X P S a y2coordination of carbonyl groups is not likely. A rlcoordination of a part of the carbonyl groups cannot be excluded from XPS measurements. The IR signals a t ca. 1620,1650, and 1710 cm-I would agree with the assumption of r2-or q4-coordinated arene moieties.22 It is not likely that the signal at ca. 1710 cm-' is due to $-coordinated carbonyl groups since a shift to lower wavenumbers was expected in this case. In spite of the broad signals, it can be said that strong changes in IR spectra below 1500 cm-I agree with the assumption of q2- or q4-arene coordination,22while a +coordination of (39)Walther, D. 2.Anorg. Allg. Chem. 1977, 431, 17.
Steiner et al. carbonyl groups or a n electron transfer alone is not expected to induce changes in the observed m a g n i t ~ d e . ~ ~ J ~ If the transfer of electrons from gold and copper to the polymer was the only type of polymer-substrate interaction, the IR spectra would be expected to be determined by the type ofpolymer alone, the substrate having a minor i n f l u e n ~ e .A ~ ~dependence of the PB spectra on the substrate indicates that an electron transfer alone is not the origin of all the spectral changes, while different spectra are expected for q2- or q4-coordination22,23 (or, of course, for +coordinated carbonyl groups). y6-Arene coordination might be involved in the film-substrate interaction only if the coordinated phenylene rings are coplanar or strongly distorted. However, adjacent substituted phenylene rings are not coplanar for steric reasons, they are twisted in structures similar to the ones described here by ca. 50",40and $coordinated arene rings tend to be flat.41 Because of the broad lines of the C=O and aromatic C=C stretching vibrations, it is not clear if r2- or y4-coordinationis likely to occur apart from PK on copper. The drastic qualitative changes observed in the IR spectra, even in the range of C-H stretching vibrations, imply dramatic changes in symmetry upon adsorption in all cases. Analogous to the discussion for PB on copper, a possible electron transfer is not considered to be the only type of interaction between film and substrate. The presence of a n underlayer with copper(1) oxide species is common for self-assembled layers prepared under experimental conditions similar to those used here.19,21-23 The coordination sphere of the copper atoms, however, is not known precisely, and differences could be present in the different systems. For instance, the ratio between copper and oxygen might vary (within limits). When atoms of the self-assembled layers coordinate to the surface, a t least the uppermost copper atoms will have different coordination spheres in the different systems.
Comparison with Polymers Containing Similar Structural Elements Self-assembled layers of aromatic poly(imides)and poly(p-phenylenes), such as those shown in Figure 2, have been described p r e v i o ~ s l y . ~These ~ , ~ ~polymers ,~~ also contain substitutedp-terphenylene or poly(ppheny1ene) units in the main chain. However, the backbone of the poly(imides1and poly(p-phenylenes)is expected to be more rigid than that of PB and PK.42 Only PO and PB form thick layers on copper, probably consisting of organometallic complexes involving copper(I) species. It seems that a "critical amount" of oxygen functionalities supports the formation of thick layers, but a coordination of oxygen atoms could not be proven by IR spectroscopy or X P S . There are some basic differences in the spectra of the polyimide layers compared to those of the layers of other compounds. The IR spectra of the poly(imides) do not depend on the substrate, and in XPS a new signal appears a t 284.2-284.4 eV. It seems that electron transfer from the substrate to the polyimide molecules is a n important factor in the polymer-substrate interaction. $- or r4arene coordination is likely for some poly(p-phenylenes). Conclusive evidence for a n arene coordination of poly(imides) does not exist, although changes in IR spectra between 1550 and 1700cm-l are observed. These changes, (40)Galda, P.Ph.D. Dissertation, University of Karlsruhe, Karlsruhe, 1994. (41)Crabtree, R. H.The Organometallic Chemistry ofthe Transition Metals; John Wiley & Sons: New York, 1994. (42)Crowley, J. I.; Balanson, R. D.; Mayerle, J. J. J . A m . Chem. SOC. 1988,105,6416.
Aromatic Poly(ketone) and -(benzil) on Gold and Copper however, do not depend significantly on the substrate, and the other spectral regions are not affected significantly. Except for the poly(imides), a (partial) oxidation of the adsorbed polymers cannot be excluded. The geometry of the molecules is not the only factor that determines the nature of the films. However, thickness and wetting properties of the films formed with the rigid-rod poly(imides1and the poly@-phenylenes) can strongly differ. Wetting properties and (average) film thicknesses of the more flexible molecules PB and PK are not intrinsically different from values of the rigid-rod molecules. It is obvious that not only physical but also chemical processes contribute to the interaction between the polymers and the metal. The type of interaction strongly depends on the chemical composition of the polymers.
Conclusions PB and PK form self-assembled layers on gold and copper, and the polymer molecules do not decompose on these metals under the experimental conditions used here.
Langmuir, Vol. 11, No. 8, 1995 3017 Compared to the bulk material, the symmetry of the molecules is drastically changed in all the films. PKyields thick layers on copper; in the other cases monolayers are formed. v2-or v4-coordinatedarene moieties are suggested to be involved in the polymer-substrate interaction a t least in the case of PK on copper. The molecules in a PB film lie flat on the copper substrate which is covered to a high extent or completely by polymer molecules. Average layer thickness and wetting properties of PB and PK do not intrinsically differ from the corresponding values of the rigid-rod polymers. Coordination is a n import factor in the polymer substrate interaction of PB, PK, and some poly@-phenylenes); while in poly(imides1 with p-terphenylene units electrostatic interactions are more relevant.
Acknowledgment. We gratefully acknowledge financial support by the Schweizerische Nationalfonds (NF, Sektion 11)and by the Deutsche Forschungsgemeinschaft (DFG). LA940852A
