J . Phys. Chem. 1990, 94, 87 15-87 I9
8715
Infrared Transmission and Reflection-Absorption Studies of Langmuir-Blodgett Thin Films Comprised of Poly(3-alkylthiophenes) and Cadmium Stearate I. Watanabe, J . H. Cheung, and M. F. Rubner* Department of Materials Science a n d Engineering, Massachicsetts Institute of Technology:v. Cambridge. Mmsnchirsetts 0 2 / 3 9 (Receired: May 4 , 1990) The level and type of molecular orientation present in mixed Langmuir-Blodgett thin films containing poly(3-alkylthiophene) and deuterated cadmium stearate molecules were examined by using transmission and reflection-absorption FTlR techniques. For films fabricated with polythiophenes comprised of butyl. hexyl. or octyl side chains, it was found that the cadmium stearate niolcculea prcfcr 10 assemble into wcll-ordered, phase-separated stacks with a molecular organization very similar to that observed in homogcncous L B films of this material. The polymer molecules, on the other hand, maintain a state of essentially random oricntation over the entire compositional rangc of the mixed films. The most highly ordered domains of cadmium stcaratc iiiolcculcs wcrc found in films containing low polymer contents. For LB films fabricated with poly(3-octadecylthiophene), on the other hand, a higher level of molccular mixing with the cadmium stearate-d molecules, encouraged primarily by the longer alkyl sidc chain of the polymer, strongly inhibited the formation of well-ordered domains of cadmium stearate molecules, particularly at high polymer loadings. In this case. the hydrocarbon groups of the polymer molecules were found to be more oricntcd LO the surfacc normal than the hydrocarbon tails of the cadmium stearate-d molecules, and the level of orientation actually incrcascd with increasing polymer content. In certain cases, dichroic ratios were used to estimate relevant orientation angles.
Introduction Currently. there is a great deal of interest in utilizing electroactive polymeric materials as active elements in optical and electronic devices. A key element to the realization of this goal is the development of processing techniques suitable for the manipulation of these materials into thin films with well-defined structures. At the present time, the Langmuir-Blodgett (LB) technique appears to be the best available method for the preparation of ultrathin organic films with controllable molecular architectures.’ Earlier work on the LB technique focused on the fabrication of multilayer thin films containing a single molecular species such as a long-chain fatty acid. It was soon recognized, however, that the greatest potential of this technique was its ability to fabricate complex molecular assemblies in which the various types of molecules present either in different layers (organic superlattices) or within the same layer (mixed monolayers) of the multilayer film served specific well-defined functions. The utilization of, for example, mixed monolayers as the primary building block of the multilayer structure allowed for the manipulation of non-surface-active molecules and hence the extension of this technique to electrically and optically active materials that would normally not be suitable for LB manipulation. The investigation of these mixed monolayer and multilayer systems continues to be a very active research area in LB films. A review of this very interesting aspect of LB science has recently been presented by Kuhn.2 The development of complex molecular assemblies of organic molecules. however, also provides a unique challenge to the investigator seeking to relate the molecular structure of a mixed LB film to its electrical and optical properties. In principle. one would like to examine the structure of the film a t both the molecular and supermolecular level and determine the order and level of molecular orientation of each of the species present in the film. Thc transinisrion :ind reflection-absorption FTlR technique has rcccntly cmcrgcd3” as a powerful method for the analysis of the type and level of molecular orientation present in LB films. Such information is obtained by measuring the spectrum of a film i n both the transmission and reflection mode and then comparing
the intensities of the absorption bands of key functional groups. Those groups whose transition dipole moments are found essentially in the plane of the film will exhibit stronger absorption bands in the transmission spectrum, whereas groups whose dipole moments are oriented nearly perpendicular to the film substrate will exhibit stronger absorption bands in the reflection spectrum. This information can then be used to quantitatively evaluate the type of molecular orientation present in the film. For many systems, it is also possible to selectively probe the orientation of the various molecular species present in a mixed LB film. In this paper, we apply the technique of transmission and reflection-absorption FTIR spectroscopy to the analysis of mixed LB films fabricated from electroactive poly(3-alkylthiophenes) and deuterated stearic acid. In this particular system, the surface-active molecules promote the spreading of the non-surfaceactive conjugated polymer a t the air-water interface and facilitate the transfer of this material into Y-type multilayer thin films. Using this approach, it is possible to manipulate polythiophenes with a variety of different alkyl side chain lengths. The electroactive polymer can then be doped by using suitable oxidizing agents to create an electrically conducting LB film.’ W e have already demonstrated that when such films are fabricated from electrically conductive polypyrroles,8 they exhibit unusually large dielectric constants and the electrical properties are highly anisotropic. Both of these effects are a direct consequence of the heterogeneous nature of the film and its multilayer structure. By use of stearic-d acid to fabricate the mixed LB films of the poly(3-alkylthiophenes), it is possible to independently examine the orientation of the cadmium stearate molecules and the alkyl side chains of the polymer molecules. Thus, valuable information about the molecular organization of these complex mixed LB films can be obtained with this approach.
Experimental Section The soluble poly(3-alkylthiophenes) (P3AT) poly(3-butylthiophene) (P-BT), poly(3-hexylthiophene) (P-HT), poly(3octylthiophene) (P-OT), and poly( 3-octadecylthiophene) (P-ODT) were synthesized as described in a previous paper.’ Details conccrning thc monolayer formation and manipulation of these matcrials can also be found in previous publications.’ Deuterated
( I ) For ii rcccnt rcvicw of LB films see: Roberts, G.C. Elecrronic and Photonic A p p l i m i o m of Poljnrers; Advances in Chemistry Series 21 8; Bowdcn. M..Turncr. S. R.. Eds.: American Chemical Society, Washington, DC. 1988; p 22.5. ( 2 ) K u h n , H . Thin Solid Filmi 1989. 178, I . (3) Allara. D. L . ; Swalcn. J . D. J . Phys. Chem. 1982. 86. 2700. (4) Rabolt. J . F.; Burns, F. C.: Schlotter, N . E.; Swalen, J. D. J . Chent. Ph),s. 1983. 78. 946. ( 5 ) Chollct. P. A,: Mcasicr. J.; Rosilio. C. J . Chem. Phys. 1976. 3. 1042. ( 6 ) Umcmura. J.; Kamata, T.: Kawai. T.; Takenaka, T. J . Ph.ys. Chem. 1990. 94. 62.
0022-3654/90/2094-87 15$02.50/0
( 7 ) (a) Watanabe, 1.; Hong. K.; Rubner. M. F.; Loh, I . H. Synrh. Met. 1989.28. C473. (b) Watanabe. 1.: Hong. K.; Rubner. M. F. J . Chem. Soc., Chent. Conintun. 1989, 2, 123. (c) Watanabe, 1.; Hong, K.; Rubner, M. F. ThinSolid Filnis 1989, 179, 199. (d) Watanabe. 1.; Hong. K.; Rubner, M. F. Langmuir 1990, 6, 1164. (8) ( a ) Hong. K.; Rubner, M. F. Thin Solid Films 1988, 160, 187. (b) Hong. K . ; Rubner, M. F. Thin Solid Films 1989, 179, 21 5. (c) Hong, K.; Rosner, R. B.; Rubner, M. F. Chem. Marer. 1990, 2, 82. (d) Rosner, R. B.;
Rubner. M. F. Proceedings of the Marerials Research Society, Symposium L. Y..Ed.; 1989; Vol. 173, p 363.
on Organic Solid Stare Marerials; Chiang.
b 1990 American Chemical Societv
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The Journal of Physical Chemistry, Vol. 94, No. 24, 1990
n
i
i
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T
c
Watanabe et al.
Lo
n
a
A
T R
3200
2800
t
n
a
R
2000
2400
Wavenumbers ( c m - l ) Figure I . Transmission (T) and reflectance ( R ) FTlR spectra of LB films fabricated from pure deuterated stearic acid (DSA), poly(hexy1thiophcne)/DSA ( l / 2 ) , and poly(hexylthiophene)/DSA (2/1).
3200
I
I
28 00
2400
2000
Wavenum bers ( c m - l ) Figure 3. Transmission (T) and reflectance ( R ) FTlR spectra of LB films fabricated from pure deuterated stearic acid (DSA), poly(octadecylthiophene)/DSA ( I /2), and poly(octadecylthiophene)/DSA (2/l).
i
-
T
L
A
I800
1600
1400
a
1200
P-ODT/DSA(2/1)
A
1000
Wavenumbers ( c m - ' ) Figure 2. Transmission (T) and reflectance ( R ) FTlR spectra of LB films fabricated from pure deuterated stearic acid (DSA), poly(hexy1thiophene)/DSA ( l / 2 ) , and poly(hexylthiophene)/DSA (2/1) stearic acid (Fluka Chemical) and cadmium chloride (Fluka Chemical) were used as received without further purification. The water used as the subphase was purified with a Milli-Q purification system ( Millipore Corp.). HPLC grade chloroform (Aldrich) was used for the preparation of spreading solutions. Monolayers were spread from chloroform solutions (typical concentration about 1 mg of total solute/mL) onto a purified aqueous subphase containing 2 X IO4 M CdCI2. The multilayers were built up by the vertical dipping method a t 15-28 m N / m and 20 "C. A dipping speed of 5 mm/min was used for the first dip and was increased to I O mm/min for subsequent dips. Drying times of at least 2 h were used between the first and second dips. This timc was reduced to 15 min for all subsequent dips. Either zinc sclenidc plates or platinum-coated slides were used as substratcb. Transfer ratios close to unity were observed for all monolayers. Infrared measurements were made with a Digilab ITS-40 FTlR spectrometer equipped with a room-temperature deuterated triglycinc sulfate (DTGS) detector. All measurements were recorded
I
I
I
-
-R
I
I
I
I
,
Results and Discussion B ~ n dA.rsignnwnrs. Figures 1-4 display the FTlR spectra of
multilayer thin films fabricated from various poly(3-alkylthiophene)/stearic-d acid mixed monolayers. For comparison, each figure also includes the transmission and reflection spectrum of a multilayer t h i n film of pure cadmium stearate-d. To separately examine the orientation of the hydrocarbon tails of the
The Journal of Physical Chemistry, Vol. 94. No. 24, 1990 8717
Langmuir-Blodgett Thin Films TABLE I: Band Assignments of Poly(3-alkyIthiophene)/DSA Mixed LB Films wavenumbers. cm-'
transm 2956 2926 2x55
2214 2193 2088 I703 1j37 I166
refl
assigt"
'958 2928
2857 1220 2193 2086 I703 I466 1414
O i l . strctching vibrational mode: A. deformation mode: a. asymmetric modc: s. symmetric modc.
cadmium stearate molecules and of the alkyl side groups pendent to the polythiophene backbone. the multilayer films were fabricated with deuterated stearic acid. Since the vibrational bands of the hydrocarbon tails of the deuterated stearic acid molecules are shifted to lower wavenumbers relative to the hydrocarbon side groups of the polythiophene molecules. it is possible to independently examine the orientation of both molecules. For example, the asymmetric and symmetric C D 2 and CD, stretching vibrations of the hydrocarbon tail groups of cadmium stearate-d are found in the 2000-2300-cm-' region, while the asymmetric and symmetric CH, and C H 3 stretching vibrations of the hydrocarbon tail group5 of thc various alkyl-substituted polythiophenes are found in thc 2800-3000-cm-' region. A complete list of relevant band assignments can be found in Table I . Orientation of the Cadmium Stearate-d Molecules. Figures 1 and 2 display the transmission and reflection-absorption spectra of multilayer thin films comprised of two different molar ratios of poly(3-hexylthiophene)/cadmium stearate-d. Looking first at the spectra generated from a multilayer thin film of pure cadmium stcarate-d (top spectra). it can be seen in Figure 1 that the asymmetric (2193 cm-') and symmetric (2088 cm-I) C D 2 stretching bands observed in the transmission spectrum exhibit significantly stronger intensities than the asymmetric (221 4 cm-]) and symmetric (2066 cm-I) C D 3 stretching bands. In sharp contrast, ihc syinmctric and asymmctric C D 2 strctching bands in thc reflectance spectrum are comparable in intensities to their CD, counterparts. These spectral differences are similar to those obscrved in spectra of multilayers of nondeuterated cadmium stearate, which has been shown to form highly ordered Y-type multilayer thin films. Such a polarization dependence is known to occur3+ when the fully extended hydrocarbon tails are oriented close to the normal of the substrate surface. Thus. this is a well-known signature of a multilayer thin film containing highly oricnted surface active molecules with their hydrocarbon tail groups oriented nearly perpendicular to the substrate surface. A high level of molecular orientation is also indicated by the spectra in Figure 2. which displays the fingerprint region of the infrared spectrum. In this region the vibrational bands mainly associated with the polar head groups of the cadmium stearate-d molecules are found. The presence of the weak band located at about 1700 cm-I. which is due to the C=O stretching vibration of dimerized carboxylic acid groups. implies that a small number of the acid head groups have not been converted into their carboxylate form. The stronger bands observed at 1542 (transmission spectrum) and 1432 cm-l (reflectance spectrum), on the other hand. are assigned to the asymmetric and symmetric carboxylate (COO-) stretching vibrations, respectively. The transition dipole moment of the symmetric COO- stretching vibration is parallel to the C - C 0 2 bond. whereas the transition dipole moment of the asymmetric COO- stretching vibration is perpendicular to the C - C 0 2 bond. Thus, the observation of a strong asymmetric carboxylate band only in the transmission spectrum and a strong symmetric carboxylate band only in the reflection spectrum suggests that the head groups of the cadmium stearate-d molecules arc also preferentially oriented within the LB film with the bisector
of the carboxylate group oriented close to the surface normal. A number of methods have been proposed to obtain more quantitative estimates of the orientations of the various molecular One such technique recently degroups present in LB scribed by Umemura and colleagues6 is based on an analysis of the dichroic ratios obtained from measurements of the transmission absorbance and the reflection absorbance. In their model, the ratio of thc transmission absorbance, AT. to the reflection absorbance. AR. is related to the orientation angle, 4, that a transition dipole moment makes with the surface normal in the following manner: A T / A , = sin2 $/(2m, cos2 4
+ n7, sin2 4)
(I)
Thc paramctcrs 111, and m, are intensity enhancement factors that arise due to the metal surface used in the reflection measurement. These intensity enhancement factors basically depend on five parameters: the frequency of the absorption band (u), the angle of incidence (e), the film thickness (h), and the refractive indexes of the LB film and substrate (n2 and n 3 ) . The appropriate values of m, can be determined by using the approach outlined in Umemura et al.'s paper.6 In our case, the experimental grazing incidence angle was 6 = 82O, the thickness of the LB film was h = 50 n m and its refractive index was assumed to be n2 = I .5, the refractive index of zinc selenide6 was n3 = 2.445, and the refractive index for platinum was obtained by suitable extrapolation of the data in a paper by Weaver.' Since the value of m, is very small as compared to m2,6 wc assumed m, to be 0 and simplified eq I to the following form: Using this approach and suitable values for ni,, we find that the transition dipole moment of the asymmetric carboxylate stretch is oriented a t an angle of 85 f '3 to the surface normal. The asymmetric and symmetric C D 2 vibrations, on the other hand, were calculated to be oriented a t angles of 83 and 82 f 3O to the surface normal, respectively. Given the fact that these latter vibrations are orthogonally polarized and are both contained within the plane that is perpendicular to the axis of the fully extended hydrocarbon chain, it is possible to determine the tilt angle that the chain axis makes with the surface normal via the following equation: cos2
CY
4- cos2
a 4- cos2 y = 1
(3)
In this equation, (Y and /3 are the angles between the surface normal and the transition dipole moments of the asymmetric and symmetric CD2 vibrations, respectively, and y is the tilt angle. With this equation, we find that the cadmium stearate-d molecules tilt with an angle of 10 f '3 from the surface normal in the neat multilayer thin films of this material. All of these values compare favorably with the results of Umemura et a1.,6 who reported a 7 O tilt angle for the hydrocarbon chain axis in multilayers comprised of cadmium stearate and a 83' orientation angle for the asymmetric carboxylate stretch. Similar values have also been obtained by other researchers working with LB films of long-chain fatty acid^.^-^ Thc small tilt angle observed in the multilayer film comprised solely of cadmium stearate-d is also consistent with the 50-A bilayer repeat distance determined for this system by using low-angle X-ray diffraction. I f we now consider the orientation of the cadmium stearate-d molecules in the presence of various molar amounts of poly(3hexylthiophene), it can be seen from Figure 1 that the polarization dependence of the asymmetric and symmetric C D 2 and CD3 vibrational bands is very similar to that observed in the multilayer thin film comprised solely of cadmium stearate-d molecules. In fact, in the film containing a 1/2 mole ratio of poly(3-hexylthiophene) to cadmium stearate-d the relative intensities of the CD2 to CD, stretching bands observed in the reflectance spectrum are essentially identical, whereas in the multilayer film containing a 2/1 mole ratio the relative intensities are seen to be only slightly larger in the mixed LB film as compared to the cadmium stea(9) Weaver, J . H . Phys. Reo. B 1975, 11, 1416
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The Journal of Physical Chemistry. Vol. 94, No. 24, 1990
r a t e d LB film. The polarization behavior of the carboxylate bands observed in Figure 2 for the three different LB films is also idcntical. indicating that the cadmium stearate-d molecules retain thcir normal moleculnr organimtion in the presence of the pol) (3-hcu) Ithiophcne) molccules. Previous work7 has shown that the multilayer thin films fabricated from poly(3-hexylthiophene) and cadmium stearate are heterogeneous systems comprised of domains o f polythiophene molccules dispersed throughout a matrix of well-ordered domains of cadmium stearate molecules. In this light it is not tcm surprising to find that the cadmium stearate-d molecules adopt the same level and type of molecular orientation in the mixed LB films as that found in a homogeneous LB film of cadmium stearate-d (SA-d). The small changes in relative intensities observed in the mixed LB films containing a 2/1 P-HT/SA-d mole ratio might suggest that the cadmium stearate-d molecules have a tendency to assume a slightly larger tilt angle in the higher polymer content films. Low-angle X-ray diffraction measurements,' however. clearly show that the bilayer d spacing of the cadmium stearate molecules present in the mixed LB films is the same as that observed in a pure LB film of cadmium stearate (about 50 A ) and is completely independent of film composition. Thus, the small changes in polarization dependence observed with increasing polymer content are due to the introduction of disorder as opposed to changes in the tilt angle of the cadmium stearate-d molecules present in separate domains. The trends described above were common to all mixed LB films fabricated from poly(3-alkylthiophenes) in which the alkyl group pcndent to the polymer backbone contained eight carbon atoms or less (i.e.. poly(3-octylthiophene), poly(3-hexylthiophene). and poly(3-butylthiophene)). In all of these cases, the cadmium stearate-d molecules achieve their normal state of molecular orientation regardless of the poly(3-alkylthiophene) content of the mixed film. Low-angle X-ray diffraction analysis also confirms7 that the cadmium stearate molecules stack with their usual bilayer repeat distance of 50 A over the entire compositional range. In addition, both X-ray and FTlR analysis show that the cadmium stearate-d molecules become increasingly more disordered with increasing polymer content particularly when the polymer content exceeds about 6 5 mol % ( 2 / 1 mole ratio of P3AT/SA). The mixed multilayer thin films fabricated from poly(3-hexylthiophene) are an exception to this trend as the cadmium stearate domains in these films remain remarkably well ordered even in films containing as much as 83 mol 5% of the polymer. The situation changes quite dramatically when films are fabricated from poly(3-alkylthiophenes) fitted with much longer alkyl side chains. For example, Figures 3 and 4 display the spectra generated from Y-type LB films containing poly(3-octadecylthiophene) and cadmium stearate-d. I n this case, it can be seen that the relative intensities of the CD2 and CD, vibrational bands in both the reflection and transmission spectra are about the same in the 1 / 2 mole ratio film and only slightly different i n the 2 / l mole ratio film. The absorbances of the CD2 asymmetric and symmetric Vibrational bands are also seen to be weaker in transmission than in reflection in the 1 / 2 mole ratio film but revert back to the more typical situation in which the transmission absorbances are greater than the reflection absorbances in the 2 / I mole ratio film. All of these observations suggest that the molecular organization of the cadmium stearate-d molecules is strongly influenced by the presence of the poly(3-octadecq.lthiophene) molecules. One might anticipate that the level of molecular mixing of the poly(3-alkylthiophene) molecules with the cadmium stearate molecules at the air-water interface and subsequently in the LB f i l m ~ o u l ddcpcnd btronglj on thc length of the alkql group attachcd to thc pol) mer backbone. For polythiophenes fitted with butyl. hexyl. and octyl side chains, the alkyl groups are simply too hhort to promotc iiny significant intermolecular interactions w i t h the cadmium stearate molecules. the net result being a well phase-separated system with each component adopting its characteristic molecu1;ir organi7otion. However, in thc cast o f the pol4(3-octadccqlthiophcnc) niixcd LB f i l m . thc hydrocarbon
Watanabe et al. chains attached to the polythiophene backbone are long enough to promote a more favorable interaction energy with the hydrocarbon tails of the cadmium stearate molecules. This in turn promotes a higher level of molecular compatibility and a less phase-separated molecular organization. The effects of this higher level of molecular mixing can be readily observed in the pressure-area isotherms' of this system and in the X-ray diffraction patterns of multilayer films, which reveal that the (001) reflections characteristic of phase-separated stacks of cadmium stearate molecules are signficantly less intense and more broad than those observed in films fabricated from polythiophcnes with shorter alkyl groups. Thus, it is more difficult for the cadmium stearate molecules to organize into separate domains in the presence of the poly(3-octadecylthiophene)molecules, particularly in films with high polymer contents. The exact effect that this greater molecular compatibility has on the orientation of the cadmium stearate-d molecules in the LB films is difficult to determine. The FTIR spectra suggest that the molecules adopt a larger average tilt angle; however, it is not possible to determine whether these changes in polarization behavior are due to variations in the tilt angle or are simply reflecting the higher level of disorder that is also present in the poly(3octadecy1thiophene)-based LB films. The changes that occur in the FTlR spectra as the mole ratio of P-ODTISA-d increases from I / 2 to 2 / I , however, do seem to indicate that the cadmium stearate molecules in LB films containing the higher polymer content are in a more ordered state. The reasons for this will become apparent in the next section. It is also interesting to note that the carboxylate head groups retain the same orientation in the mixed films based on poly(3-octadecylthiophene)as was found in the pure film of cadmium stearate-d. This is indicated by the similar polarization behavior observed in Figure 4 for all three of the LB films. Thus, even though the hydrocarbon tail groups of the cadmium stearate molecules are more disordered, the head groups still achieve a high level of orientation. Orientation of the Poly(3-alkylthiophenes). T o examine the level and type of molecular orientation exhibited by the poly(3alkylthiophene) molecules, we turn our attention to the 28003000-cm-' region of the spectrum, which contains the asymmetric and symmetric C H , and C H 3 stretching vibrations of the hydrocarbon side groups of the polymer. I n Figure 1 it can be seen that the relative intensities of the C H 2 and C H , asymmetric and symmetric vibrational bands of the poly(3-hexylthiophene) molecules are not strongly dependent on the mode of polarization (note that these relative intensities also reflect the fact that the hexyl chain has fewer C H 2 groups per chain than the octadecyl chain of SA-d). The intensities of all of the hydrocarbon stretching bands observed in reflectance, however, are slightly stronger than those observed in transmission. Similar results were also obtained from films fabricated with poly(3-octylthiophene), whereas films fabricated from poly(3butylthiophene) displayed no detectable polarization dependence in the hydrocarbon stretching region. These results indicate that the hydrocarbon chains of the polythiophene molecules are relatively disordered and essentially randomly oriented in the LB lilms with only a slight tendency toward a more parallel orientation to the substrate surface in the case of films fabricated from poly(3-hexylthiophene) and poly(3-octylthiophene). The small level of dichroism observed in films of these latter materials may be the signature of some side-chain crystallization of the alkyl groups within the phase-separated polymer domains. S u l f u r L-edge N EXAFS experimentsI0 have also shown that the polythiophene backbones are randomly oriented within the mixed LB films. Thus, the polymer domains formed in LB films fabricated (10) (a) Skotheim, T. A.; Yang, X. Q.; Chen. J.: Hale, P. D.; Inagaki, T.; Samuelson, I..: Tripathy, S.; Hong, K.; Rubner. M. F.; denBoer, M. L.;
Okamoto. Y . S,vnrh. Mer. 1989. 28. C229. (b) Yang, X. Q.; Chen. J.; Hale, P. D.: Inagaki. T.; Skotheim, T. A.; Okamoto. Y.; Samuelsen, L.; Tripathy, S.;Hong, K.; Watanabe, 1.; Rubner, M. F.; denBoer. M. L.Langmuir 1989, 5 , 1288. ( c ) Skotheim, T. A.; Yang. X. Q.; Chen, J.; Inagaki, T.; den Boer, M.; Tripathg, S.; Samuelson. L.; Rubner, M . F.; Hong, K.; Watanabe. 1.; Okanioto. Y . Thin Solid filnis 1989. 178, 2 3 3 .
Langmuir-Blodgett Thin Films from poly( 3-alkylthiophenes) with short alkyl groups are compriscd of randomly oriented chain segments most likely in the form of semirigid random coils. Thc results arc again quitc different when poly(3-octadecylthiophcnc) is used to fabricatc the mixed LB films. In Figure 3, it can be seen that the intensities of the C H I asymmetric and symnictric strctching bands are significantly greater in the transmission spectrum than their counterparts in the reflectance spcctrum for both the I / 2 mole ratio film and the 2/1 mole ratio film. I n the I / ? niolc ratio film thc dichroic ratios (,+/AR) of the CH, asymmetric and symmetric stretching bands are I .8 and 2 . I , rcspcctivcly. whercas in the 2/ 1 mole ratio film they are 3.1 and 2.6. Thus, as the polymer contcnt in the film incrcases the octadccyl sidc chains attached to the polymer backbone become more oriented toward the surfacc normal. The higher level of molecular orientation in the 2/ I molc ratio film is also cvidcnt in Figure 4, uhcrc it can bc sccn that the intensity of thc C H , deformation band found at about 1470 cm-' is significantly stronger in the transmission spcctruiii than in thc rcflcction spcctrum. This particular vibrational mode has its transition dipole moiiicnt oricntcd parallel to the H-C-H plane of the hydrocarbon chain and thcrcforc should cxhibit a similar polarization dependcncc a h the symmctric C H 2 stretch. The high level of molecular orientation in the 2/ I molc ratio PODT/SA film has also bccn confirmed by N E X A FS mcasurcnicnts.10 With the method outlinc earlier and appropriate values for m:, a tilt anglc of about 17' was calculated for the hydrocarbon tails of the polythiophcnc molecules present in the 1 / 2 mole ratio LB film, and an anglc of about 12' was obtained for the 2 / I mole ratio film. Thcsc tilt anglcs should only be vicwcd as rough cstimatcs of thc average tilt angle since the effects of disorder have not been x c o u n t c d for in thcsc calculations. In any cvcnt, it is clear that the polynicr sidc chains arc more oriented toward the surface noriiial in the ? / I molc ratio film than in thc 1/2 mole ratio film. From thcsc results. it appears that both the cadmium stearate-d molcculcs :ind the hydrocarbon tails of the polythiophene molecules bccoiiic iiiorc ordered and/or assumc smaller tilt anglcs as thc polymer contcnt in thc film increases. Apparently, at intermediate polytiicr loadings. the interaction between the cadmium stearate-d molecules and the hydrocarbon tails of the polythiophene molecules docs not h v o r ii tightly packcd niolccular organization, the nct result being cithcr :I morc disordered multilayer or an organization in which the tail groups assutiic, on average, a distribution of relatively h r g c tilt angles. This most likely reflects a competition bctwl.cn molccular mixing of the two components and the strong tendency of thc cadmium stcaratc molecules to want to crystallize into separate molecular domains. At higher polymer loadings, the cadmiuin stcarate iiiolcculcs csscntially becomc dissolved in the polymer phase, thereby facilitating the orientation and order of thc hydrocarbon side groups of the polymer. This latter effect would be needed to create a well-oriented molecular organization since thc configurational nrrangcment of the long alkyl groups along the pol) nicr backbonc would prcvent these appendages from achieving ;I densely packcd organbation. I n othcr words, thc
The Journal of Physical Chemistry, Vol. 94, No. 24, 1990 8719 cadmium stearate molecules fill the space that would normally exist in the polymer phase due to the spatial restrictions placed on the side groups by the configurational and conformational arrangement of the rigid polymer backbone.
Conclusions The molecular and supermolecular organization of heterogeneous multilayer L B films comprised of poly(3-alkylthiophene) molecules and cadmium stearate molecules depend strongly on the length of the alkyl side chain attached to the polymer backbone and the amount of polymer present in the film. For films fabricated from poly(3-alkylthiophenes) fitted with alkyl groups containing eight or fewer carbon atoms, the cadmium stearate molecules exhibit a strong tendency to assemble into well-ordered iiiolccular stacks in which the molecules assume a molecular organization very similar to that found in a homogeneous LB film of this material. In general, the most ordered cadmium stearate domains are found in films containing a low polymer content, although in thc case of films fabricated from poly(3-hexylthiophene). a high level of molecular orientation of the cadmium stearate molecules is retained up to very high polymer loadings. Another common feature of these films is the absence of any signficant molecular orientation of the alkyl groups pendent to thc polymer backbone over the entire compositional range of the mixcd films. There docs seem to be a slight tendency for these groups to align parallel to the substrate surface, although this effect is weak and is observed only when the alkyl chain is either a hexyl or octyl group. When thc mixed LB films are fabricated from poly(3-octadccylthiophene), a polymer that has an alkyl side chain equivalent i n length to the hydrocarbon tail group of cadmium stearate, the formation of well-ordered, separate domains of cadmium stearate molecules is strongly inhibited. This is in part attributed to the grcatcr lcvcl of molecular mixing experienced by the two components of the film caused by stronger nonbonded interactions established between the long alkyl groups of each species. In this case. the level of molecular orientation of the polymer side chains is actually greater than that achieved by the cadmium stearate molecules and increases significantly with increasing polymer content. The high polymer content films are therefore best described as heterogeneous organizations comprised of small, weakly ordcrcd domains of cadmium stearate molecules and mixed polymer-rich domains containing highly oriented hydrocarbon tails contributed from both the polymer and the cadmium stearate molecules. Tilt angle calculations indicate that the hydrocarbon tails in the most ordered films (2/1 mole ratio of PODT/SA) are oriented nearly perpendicular to the substrate surface with a small tilt anglc of about 12' from the surface normal.
Acknowledgment. Partial support of this research was provided by the National Science Foundation and the M I T Center for Materials Science and Engineering. In addition, we thank Hitachi Chemical of Japan for support of I.W. Registry No. P-BT. 98837-51-5; P-HT. 104934-50-1; P-OT, 104934-5 I - ? ; P-ODT. 104934-55-6; cadmium stcarate, 2223-93-0.
