Langmuir 1991, 7, 2743-2147
2743
Ellipsometric Characterization of Langmuir Monolayers of “Hairy-Rod”Polymers at the Air-Water Interface H. Motschmann, R. Reiter, R. Lawall, G. Duda, M. Stamm, G. Wegner, and W. Knoll* Max-Planck-Institut fur Polymerforschung, Ackermannweg 10,0-6500 Mainz, FRG Received April 26, 1991. I n Final Form: July 1,1991 Surface pressure versus area ( r A ) diagrams of various poly(y-methyl-L-glutmate-co-y-alkyl-L-glutamates) are presented together with their simultaneously recorded ellipsometric IA-AH~O~ isotherms. These materials are representatives of the class of hairy-rod polymers. Ellipsometry is very sensitive toward changes in the refractive index and the thickness of the monolayer and therefore provides valuable information about the physical properties of the film. The combination of both techniques makes it possible to identify the characteristic broad plateaus occuring in the r A diagrams. Furthermore, the lateral homogeneitiesof the monolayers in the various phases were investigated by scanning the laser spot across the Langmuir trough. All phases except the plateau region turned out to be laterally homogeneous. In this region the formation of an undefined monolayer piling up in front of the barrier was detected.
Introduction Langmuir-Blodgett-Kuhn films (LBK) are considered to play an important role for the miniaturization of optical and electronic modules.1-3 However, for technical applications LBK films prepared from classical amphiphilic molecules like fatty acids are thermally and mechanically too unstable, and furthermore they contain a lot of defect~.~~s One way to increase the stability of the films is the use of functionalized amphiphiles which can be polymerized after the layer structure has been built up. By proceeding in that way, the stability of the film can be increased but the defects still remain.617 Another more promising way is the use of prepolymerized materials. In the last years a new class of the socalled hairy-rod polymers was developed. These polymers form stable monolayers at the air-water interface and can be transferred onto a solid s u p p ~ r t .A~ particular *~ example of this class of polymers are the poly(y-methyl-L-glutamate-co-y-alkyl-L-glutamates).These copolymers form stiff a-helices with a statistical sequence of alkyl side chains. Their self-organization a t the air-water interface is mainly due to the stiffness and form anisotropy of the molecules. Multilayers of these polymers show remarkable properties. It turns out that their thermal stability is higher than for fatty acids,1° and waveguide experiments gave evidence for good lateral optical homogeneity.” The a-helices in the multilayers are oriented mainly parallel to the dipping direction during transfer as shown by FTIR studies,12 whereas the alkyl side chains are isotropi-
*
To whom correspondence should be addressed. (1) Sugi, M. J.Mol. Electron. 1985, 1, 3. (2) Swalen, J. D. J. Mol. Electron. 1986, 2, 155. (3) Stegeman, G. I.; Seaton, C. T.; Zanoni, R. Thin Solid Films 1987, 152, 231. (4) Rabe, J.; Swalen, J. D.; Rabolt, J. F. J. Chem. Phys. 1987,86,1601. (5) Lieser, G.;Tieke, B.; Wegner, G. Thin Solid Films 1980, 68, 77. (6) Ringsdorf, H.; Schlarb, B.; Venzmer, J. Angew. Chem. 1988,100, 117. (7) Wegner, G. 2.Naturforsch. 1969,24b, 824. (8)Biegajski, J. E.;Burzynski, R.; Prasad, P. N. Macromolecules 1986, 19, 2457. (9) Orthmann, E.;Wegner, G. Angew. Chem. 1986,98, 114. (10) Duda, G. Ph.D. Thesis, University of Mainz, Germany, 1987. (11) Hickel, W.; Duda, G.;Jurich, M.; Krbhl, T.; Rochford, K.; Stegeman, G.;Swalen, D.; Wegner, G.; Knoll, W. Langmuir 1990,6, 1403. (12) Duda, G.; Shouten, A. J.; Arndt, T.; Lieeer, G.; Schmidt, G. F.; Bubeck, C.; Wegner, G. Thin Solid Films 1988, 159, 221. 0743-7463/91/2407-2743$02.50/0
cally distributed and can be regarded as a covalentely attached inert solvent of the rods. Despite the structural anisotropy suggesting an optically biaxial system, only a very small optical anisotropy was found by a combination of waveguide and surface plasmon s p e c t r o s ~ o p y . ~ ~ J ~ Although there are a lot of methods available to characterize LBK multilayers on a solid support, the number of techniques available for the investigation of monolayers at the air-water interface is limited due to the experimental conditions. In the present study we combine ellipsometry with the usually applied pressurearea measurements to get insight into the early stages of the monolayer preparation as an important primary step in the LBK technique. Ellipsometry is a nondestructive optical method which measures the change of the light polarization during reflection a t a film-covered substrate. These changes can be expressed in terms of the ellipsometric angles A and \k, which are related to the overall reflectivity coefficients, RF and I?:’, parallel and perpendicular to the plane of incidence, respectively,14 which in turn depend on the interfacial architecture: tan *eiA = R ~ / R : (1) A detailed analysis of the relation between the ellipsometric angles and the refractive index nl and the thickness dl of monolayers at the air-water interface is given in ref 16. This study reveals that under the experimental conditions given at the water surface an exact determination of the film parameters nl and dl is impossible even when multiple wavelengths or multiple angle of incidence ellipsometry is applied. The reasons are that the effects in \k are very small and that the highly sensitive quantity A is a function of all film parameters which remain strongly coupled during these manipulations. Thus, the number of independent measurable quantities is less than the film parameters necessary to describe the monolayer. The only way to overcome this problem is to reduce the number of unknown film parameters by the use of other (13) Hickel, W. Ph.D. Thesis, University of Mainz, Germany,1989. (14) Azzam, R. M. A.; Bashara, N. M. Ellipsometry and Polarized Light; North Holland Publication: Amsterdam, 1979. (15) Motschmann,H.; S t a ” , M.;Toprakcioghlu,Ch.Macromolecules 1991, 24, 3681. (16) Reiter, R.; Motschmann, H.; Lawall, R.; Sta”, M.; Knoll, W. J. Opt. SOC.Am., submitted for publication.
0 1991 American Chemical Society
Motschmann et al.
2744 Langmuir, Vol. 7, No. 11, 1991 Table I. Characteristics of the Poly(y-methyl-L-glutamate-co-y-alkyl-L-glutamates) percentage of the long samole alkvl side chain. mol % n(alkv1) PM-co-DdLG 32 11 17 PM-CO-OLG 30 21 PM-CO-DcLG 25
techniques, e.g., obtaining the thickness of a film by means of X-ray reflectometry as done in ref 17. Therefore, ellipsometry is not a tool to characterize monolayers at the air-water interface quantitatively, but it provides nevertheless information about some physical properties of the film. During compression, the film is getting optically denser and due to the increasing interactions of the alkyl side chains it becomes thicker. Both effects lead to an increase of the relevant quanitity IA AH~o!which is the difference of the ellipsometric angle of the film-covered (A) and of the clean water surface (AH+)), respectively. An abrupt change in thickness or refractive index is reflected by an abrupt change in A. Thus, phase transitions should also be recorded by the IA - Aaol isotherms. The quantity A is extremely sensitive to small changes of the film parameters. T o get an idea about its sensitivity, let us assume a film of 18 A with a refractive index of n = 1.5 studied at a wavelength of X = 633 nm and an angle of incidence of I$ = 5 4 O . A change of the refractive index of 0.001 corresponds then to a change in A of 0.13O. A similar effect in A is caused by a change of the layer thickness of 0.2 A, which is easily resolved because the experimental accuracy in A is about 0 . 0 2 O . We use this high sensitivity to investigate the lateral homogeneity of the monolayer in all phases by scanning the laser spot across the trough water surface by means of a special film balance mounted on a translation stage. In contrary to fluorescence microscopy, it is not necessary to add any other materials. We get information about lateral homogeneity of the monolayer with millimeter resolution without disturbing the system. These informations are very useful when new polymers for the LBK technique are being investigated.
Experimental Section Polymers. The molecular characteristics of the poly(y-methyl-L-glutamate-co-y-alkyl-L-glutamates) used are given in Table I. The sequence of alkyl side chains is statistically preventing the ordering of the side chains. The syntheses of the polymers are described in ref 10. The structure formula is presented in Figure 1. Ellipsometer. Ellipsometric measurements are performed using a self-built computer-controlled Null ellipsometer in a vertical PCSA (polarizer, compensator, sample, analyzer) arrangement. A detailed description of the setup is given in ref 15. The angles A and 9 were determined by the average of zone I and zone I11 of the analyzer and polarizer extinction settings." The differences between the zones were usually less than 5 / looo. All ellipsometric isotherms were measured at a wavelength of X = 633 nm; the angle of incidence was chosen close to the Brewster angle since the highest sensitivity in A is given there. The exact angle of incidence 4 can be evaluated by the determination of 9 of the clean water surface. 9 is not affected by capillary waves or transition layers as shown in ref 16, and it is a function only of the known bulk refractive index of water and the angle 4. The quantity 9 changes strongly and in a linear manner with 4; for instance, a change in 4 of O.0lo causes a shift in 9 of 0.016O if I$ = 5 4 O . Therefore a determination of 9 by means of the two (17) Paudler, M.;Ruthe, J.; Alberti, B.; Riegler, H. Makromol. Chem., Macromol. Symp. 1991,46,401.
I
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Figure 1. Formulaof the poly(y-methyl-L-glutamate-co-y-al~1L-glutamates) used.
I
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Figure 2. Location of the laser spot in the Langmuir trough. zone measurements offers a convenient check for the internal adjustment of the setup. Film Balance. The measurements were carried out with two different Langmuir troughs, both equipped with a Wilhelmy balance and a motor-driven barrier. All isotherms discussed in this paper were first recorded with a film balance that allowed ellipsometric measurements at one fixed spot located about 3 cm near the margin of the trough as indicated in Figure 2. In order to get information about the lateral homogeneity of the monolayer in the various phases, additional measurementawere carried out on a special film balance which was mounted on a translation stage, thus allowing to scan the laser spot along 15 cm of the trough. Experimental Performance. All isotherms were recorded at a temperature of T = 20 0.1 "C and a velocity of Compression of 437 mm*/min. The dimensions of the Langmuir trough are 93 mm in width and 304 mm in length. Surface pressure-area and ellipsometric isotherms were measured simultaneously. As subphase we used freshly purified water (Millipore Corp., Bedford, MA) with a specific resistance of 18 MQ/cm. The monolayer materials were spread using solutions of the polymers in chloroform. The whole setup was fixed on a special vibration isolated table (M-RS46-8Newport).
Results and Discussion In Figure 3 the *-A diagram of PM-co-OLG is presented together with the simultaneously measured 1A - AH^( isotherm. During compression the *-A diagrams show four regions of different compressibility. The first onset of the surface pressure is detected at an area per repeating unit of about 0.26 nm2. Then a strong change in the surface pressure occurs. The point usually used to transfer the film onto a solid support is at about 24 mN/m just before a new range with a different compressibility is detected. At an area per repeating unit of the polymer between 0.14 and 0.05 nm2, a broad plateau with a surface pressure of roughly 30 mN/m is recorded in the isotherm, showing almost no change in the measured surface pressure in that region. On expansion a strong hysteresis is found. After complete decompression all data can be reproduced. The only difference is a small shift to lower area values which could be explained by a minor loss of material a t the barrier. The ellipsometric isotherm lA - AH~o(showsthe following characteristics. A t a coverage where no surface pressure is detected yet, ellipsometry turns out to be sensitive to the presence of the film as expressed by (A - AHBI = 6.2'
Langmuir, Vol. 7, No. 11, 1991 2745
Ellipsometric Characterization of Langmuir Monolayers
I
5
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15
20
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Arca in A* pcr polymer rcpcating unit
Figure 3. F A diagram and simultaneouslymeasured IA - A ~ p l isotherm of PM-co-OLG during compression and decompression. Measurements were carried out at T = 20 OC and X = 633 nm at an angle of incidence of I$ = 54.07".
5
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Area in A* per polymer repeating unit Figure 4. P A diagram and simultaneouslymeasured IA - A ~ p l isotherm of PM-co-OLGduringcompression and decompression. Explanations are given in the text. Conditions are the same as in Figure 3.
a t an area per repeating unit of 0.26 nm2. During and the ellipsometric isotherm. Both measurements are compression the film is getting denser and thicker, reflected not strictly linkedbecause of the inhomogeneous character ~ , the first phase transition in an increase of [A - A H ~ obut of the film which is clearly seen in the expansion run where detected by the Wilhelmy system is not recorded in the clods of multilayers are pushed in and out of the ellipellipsometric isotherm. sometric measuring spot. The optical system integrates All polyglutamates investigated show the mentioned over 1 mm2 and is therefore very sensitive for inhomobroad plateau in the T-A isotherm. This corresponds to geneities of the film. Apart from the heterogeneities in the broad plateau in the ellipsometric isotherm, implying front of the barrier, once again the stable expansion plateau that the optical properties of the local spot investigated is measured, and at large areas the original compression by ellipsometry do not change with further compression. data are reproduced. The bend into this plateau of the r - A diagram correIn order to get further insight into the nature of the sponds to the maximum compression of the monolayer. A hysteresis, we investigated the temporal evaluation of the further compression leads to the formation of a multiellipsometric angle A within the reproducibly formed layer but not homogeneously over the whole trough. The expansion and compression plateaus. The barrier was strongest disturbance of the system is introduced by the moved to a position in the middle of the plateau and kept barrier. Just in front of the barrier, which is remote from fixed when the data were recorded. The results are given the optical spot as indicated in Figure 2, a multilayer is in Figure 5 , showing that the plateau data do not vary formed. This multilayer in front of the barrier is not wellwith time. No change to a supposed equilibrium state defined and is detected by ellipsometry only when it is was found, but the data prove the good time stability of pushed into the laser spot, causing a strong increase in lA the setup as can be seen from Figure 5a,b. Figure 5c shows -AH~O a t~small areas. the corresponding measurement in the range of the inThe ellipsometric isotherms of the expansion runs (LP) homogeneities. The data can be interpreted as clods drifting in and out of the laser spot. Measurements of the are different from those of the corresponding compression During expansion a new lower lying plateau clean water surface overnight show that the presented runs (LmF). - Aerpl = 3.4' to the is found with a difference of lacomp effect is caused by the film and is not due to a change of corresponding compression plateau. At high expansion an experimental parameter. Surface pressure and ellipsometric measurements are the original compression data are measured. All A data completely reversible up to the first break in the P A are exactly reproducible when the same run is repeated. This is also valid for an independent experiment unless diagram (see Figure 6a). As soon as this first break in the r - A diagram is passed an onset of the hysteresis is found the range of multilayer formation is investigated where a in the surface pressure measurements as can be seen from random and inhomogeneous character appears. Figure 6b. Ellipsometric compression and expansion runs Figure 4 shows the isotherms obtained if an excess of remain completely reversible. Thus, the normally used monolayer material was spread at the water surface, thus pressure range to transfer the film onto a solid support ia generating some lateral pressure already before further in the completely reversible regime. compression. The purpose was to detect the known The simultaneously measured isotherms of PM-coincrease in the T-A diagram at small areas.10 The corDdLG, the polyglutamate with the shortest alkyl side ~ shows of course all the aboveresponding IA - A H ~ ocurve chains (Clz), are given Figure 7. The most important mentioned features, first a nearly linear increase in the deviation from the results discussed already is the absence isotherm without detection of the first phase transition in of the first phase transition in the r - A diagram. The the r - A diagram and broad plateaus at the same areas in maximum compression of the monolayer corresponding both isotherms. No definite relation can be found between the steep pressure increase at low areas in the r - A diagram to a plateau in both isotherms is reached without a change
Motschmann et al.
2746 Langmuir, Vol. 7, No. 11,1991
0
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5
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Figure 6. Time dependence of a few selected points of the isotherm of PM-co-OLG (a) A curve, plateau expansion; (b) A curve, plateau compression; (c) A curve, range of inhomogeneities.
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Figure 6. Reversibility of the compression of PM-co-OLG experimental conditionsas in Figure 3: (a) up to the first break in the r-A diagram; (b)passing the first break in the P A diagram.
Figure 8. P A dia am and simultaneouslymeasured lA - A ~ p l isothermof PM-co-gcLG during compressionand decompression. C and A = 633 nm Measurements were carried out at T = 20 ' at an angle of incidence of = 54.07'.
of the compressibility. One is tempted to interpret the maximum compression in terms of an interdigitation of the side chains. Thus, the first phase transition in the T-A diagram could be identified as a side chain effect. The corresponding isotherm of PM-co-DcLG, the polymer with the longest side chains (C~Z), is presented in Figure 8. For the first time, ellipsometry also detects two regions with different slopes in the IA - AH*oJisotherm which correspond to the first change in the compressibility of the T-A diagram. All other features are similar to those of the other materials investigated. The presented data show that it would be very useful to have information about the lateral homogeneity of the film in the different phases. We obtain this information using a specially built film balance mounted on a translation stage. The optical spot of 1 mm2 can be scanned
across the trough and allows measurements as close as 1.5 cm from the barrier. The maximum lateral displacement is limited to 15 cm, less than half the 40-cm length of the trough, and is situated at one end of the trough. Therefore, the minimal distance of the observable region of the film from the barrier depends on the position of the barrier. When the monolayer is highly expanded, the closest position of the laser spot to the barrier referes to a distance of about 8 cm as can be seen in Figure 9. Here the A values of PM-co-OLGare plotted versus the distance of the optical spot from the barrier. Different symbols are used for different phases as indicated in the inset. The angle of incidence was 50.15' for these measurements. It is remarkable that the A values recorded just below the first break at approximately 0.19 nm2 in the compression run
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Ellipsometric Characterization of Langmuir Monolayers
Langmuir, Vol. 7, No. 11,1991 2747
1
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Figure 9. Lateral homogeneity of PM-co-OLG in different phases as indicated in the inset, recorded at 4 = 50.15O ind X = 633 nm.
Figure 10. Lateral homogeneity of PM-co-DcLG in different phases as indicated in the inset, recorded at 4 = 50.15' and X = 633 nm.
and in the plateau of the expansion run are almost identical, even though the correspondingsurface pressures are quite different. The corresponding measurements of PM-co-DcLG are given in Figure 10. All phases except the plateau region turn out to be laterally homogeneous. Let us discuss for instance PM-co-OLG a t the transfer pressure. A small scatter of 0.3O in A around an average value is found which can be attributed to small packing inhomogeneities. Such an effect can be caused by a change in the refractive index of 0.002 or a thickness variation of 0.3 A. Thus, a t the transfer pressure a very homogeneous film is formed. Experimentally we have checked that the reported noise is not introduced by the translation of the film balance. Every measured point can be reproduced when measured once again on driving back the whole unit, even though about 30 min elapsed between those measurements. Figures 9 and 10 again give evidence of the already discussed piling up of an undefined multilayer in front of the barrier. As soon as the plateaus in the T-A isotherms
are reached, the formation of this multilayer begins. For inhomogeneitiesas found in this case the definition of a surface pressure is doubtful since different points correspond to different surface pressures.
Conclusion In ref 16 it is shown that it is in principle not possible todetermine both the refractive index nl and the thickness d l of surfactant monolayers a t the air-water interface simultaneouslyby ellipsometry. Neverthelessthe present study shows that the qualitative informations of the ellipsometric isotherms provide valuable insight into the physical properties of the films. Thus, we could identify the characteristic phase transitions of several poly(y-methyl-L-glutamate-co-y-alkyl-L-glutamates) during compression. Especially the possibility to scan the laser spot across the Langmuir trough is an excellent tool for investigating the lateral homogeneity of the monolayer a t every state of compression without disturbing the system.
